Department of Physiology, Faculty of Medicine, The University of Hong Kong, Hong Kong, People's Republic of China
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ABSTRACT |
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In dogs anesthetized with pentobarbital sodium,
hilar venous pressure (Phv) and
secretion were measured from the submandibular gland receiving
spontaneous blood flow or vascular perfusion at the normal resting flow
rate. Parasympathetic nerve stimulation and ACh-induced secretion
increased Phv and its pulse
pressure; Phv also showed an
obvious arterial (or perfusion pressure)-like waveform. Vasoactive
intestinal polypeptide (VIP) exerted similar effects on
Phv but produced negligible
secretion. Sympathetic nerve stimulation, phenylephrine, and clonidine
did not induce secretion and had no significant action on
Phv, whereas isoproterenol provoked secretion and changed Phv
as with parasympathetic stimulation. Background or superimposed
sympathetic nerve stimulation reduced the parasympathetic nerve-induced
responses; the sympathetic inhibition was abolished by phentolamine and
yohimbine but not by prazosin and propranolol. The results suggest a
direct relationship between Phv
and secretion during parasympathetic salivation: the elevation in
Phv was primarily independent of
the concurrent blood flow response, mediated via muscarinic and
peptidergic mechanisms, and related to an opening of arteriovenous
anastomoses. Sympathetic inhibition of parasympathetic salivation may
be related to prevention of an increased
Phv exerted primarily via the
2-adrenergic mechanism.
parasympathetic salivation; - and
-adrenergic mechanisms; muscarinic and peptidergic receptors; arteriovenous anastomoses
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INTRODUCTION |
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PARASYMPATHETIC NERVE stimulation causes profuse secretion and an increase in blood flow in salivary glands of experimental animals (3, 16). We have found in the dog submandibular gland that salivary flow in response to a short period of parasympathetic nerve stimulation over a wide range of stimulus frequencies is independent of the change in blood flow, indicating that the concomitant blood flow response may not be necessary for supporting the secretory response of an actively secreting gland (12). It is now well known that electrolyte transport in one form or another in the acinar cells is fundamental to the formation of primary saliva (23, 30). However, we cannot rule out the fact that the actively secreting acinar cells must have an adequate amount of fluid supply. Fluid movement across the capillary walls depends on the total transcapillary pressure, including both hydrostatic and oncotic pressure; net filtration requires hydrostatic pressure to be larger than oncotic pressure (19). Fluid efflux from a capillary normally results in an increase in oncotic pressure along the vessel. It is therefore crucial to know the mechanism that maintains an adequate hydrostatic pressure in the capillaries for net fluid filtration during short periods of profuse salivary secretion when the oncotic pressure is concurrently increasing. Recent anatomic studies have shown that arteriovenous anastomoses are present in dog submandibular glands and that they are open when the glands are actively secreting (13). We have also found that parasympathetic nerve stimulation increases the venous pressure of the gland, irrespective of whether the gland receives spontaneous blood flow or controlled vascular perfusion at the normal resting flow rate. This study describes the action of autonomic nerves on venous pressure and salivary secretion in the dog submandibular gland as a first step in elucidating the vascular mechanism for maintaining a high hydrostatic pressure across the walls of blood vessels during copious salivary secretion.
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METHODS |
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The study was approved by the Committee on the Use of Live Animals for
Teaching and Research of The University of Hong Kong (CULATR no.
29-93 and 95-94). The experimental animals were supplied by
the Laboratory Animal Unit of The University of Hong Kong. Mongrel dogs
(17-20 kg body wt; n = 50) of
either sex were anesthetized with intravenous administration of
pentobarbital sodium (30 mg/kg); supplementary doses (10 mg · kg1 · h
1)
were given when necessary. Body temperature (rectal) was maintained at
37°C by means of an electric heating pad placed beneath the animal.
Ventilation was monitored via a pneumotachograph. A femoral artery was
cannulated for measurement of systemic arterial pressure. Heparin
(2,000 U) was introduced via a cannulated femoral vein before the
perfusion system was connected, and 1,000 U/h of heparin were given
thereafter. The doses given were within the recommended range for
initial dose (50-150 U/kg) and supplementary doses (25-400 U · kg
1 · h
1)
for total body perfusion in open heart surgery (29). There was no sign
of blood clotting or bleeding due to inappropriate administration of
heparin.
Measurement of submandibular arterial flow, venous flow, hilar venous pressure, pressure in the extraglandular segment of the outflow vein, and external jugular venous pressure. The glandular branch of the facial artery is the major artery supplying the dog submandibular gland (Fig. 1A). To determine arterial flow, an ultrasonic flow sensor (2S, Transonic System) was placed around the facial artery just proximal to the origin of the glandular artery (12). The facial artery that was distal to the origin of the glandular artery was retrogradely cannulated for intra-arterial administration of drugs into the gland.
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Vascular perfusion of the submandibular gland. The facial artery that was just distal to the origin of the glandular artery was retrogradely cannulated with a catheter (1.2 mm ID; Vygon). The facial artery proximal to the origin of the glandular artery was closed with a snare. The glandular artery was then perfused by means of a peristaltic pump, via the facial arterial catheter, with blood from a reservoir (50 ml) that was continuously replenished from the femoral artery via an inserted catheter (2 mm ID; Vygon). The perfusion pressure was measured by means of a pressure transducer from tubing (1.6 mm ID; Portex) connected to the side arm of a four-way stopcock that was placed between the facial arterial catheter and the tubing (2 mm ID; Norprene) of the peristaltic pump. The perfusion rate was adjusted to give a perfusion pressure approximating the systemic arterial pressure (12) (Fig. 1A).
Measurement of salivary secretion. The submandibular duct was retrogradely cannulated and the catheter was connected to a bottle in which the secreted saliva displaced a saline solution. Drops of saline, 0.025 ml in volume, displaced from the bottle were measured by means of a drop counter (92-100-70; E & M); salivary flow was calculated from the time interval between falling drops (12) (Fig. 1A).
Electrical stimulation of the autonomic nerves. In the dog, the preganglionic parasympathetic fibers to the submandibular gland follow the chorda tympani nerve and then the ramus communicans to synapse in the submandibular ganglion located in the hilum of the gland. The preganglionic sympathetic fibers pass to the cervical sympathetic trunk to synapse in the superior cervical ganglion. The ramus communicans to the submandibular ganglion, which courses along the submandibular duct, was exposed. The sympathetic trunk just cranial to the caudal cervical sympathetic ganglion was also exposed. The tied peripheral ends of both nerves were stimulated separately by bipolar platinum electrodes with varying frequency at fixed supramaximal voltage (5 V for parasympathetic nerve and 20 V for sympathetic nerve) and pulse duration (1 ms) according to the experimental protocol (12).
Drugs. Drugs were dissolved in saline solution and given intra-arterially by bolus injection in a volume of 0.1 ml or at an infusion rate of 0.1 ml/min into the gland, via either the inserted facial arterial catheter in preparations of spontaneous blood supply or the perfusion circuit in preparations with constant-flow vascular perfusion. They included ACh, vasoactive intestinal polypeptide (VIP), phenylephrine hydrochloride, clonidine hydrochloride, isoproterenol hydrochloride, yohimbine hydrochloride, and propranolol hydrochloride (all from Sigma), as well as phentolamine hydrochloride (Regitine, Ciba) and prazosin hydrochloride (Pfizer). Doses were expressed as the weight of the salt. Intra-arterial bolus injection (0.1 ml) or infusion (0.1 ml/min) of saline solution (drug vehicle) had no effect on any measured variable.
Experimental protocol.
Insertion of a narrow catheter or an ultraminiature catheter-tip
pressure transducer into the hilar venous system may not cause
significant hindrance to the total venous outflow or an increase in
hilar venous pressure under normal or low blood flow conditions, as the
venous vessels are compliant and venous blood can drain easily via
other outflow veins (Fig. 1B).
However, at times of high blood flow, there may be an increase in hilar
venous pressure as the venous vessels become less compliant, and the presence of a narrow catheter or a catheter-tip pressure transducer may
exaggerate the increase in hilar venous pressure caused by the high
blood flow. To estimate the magnitude of elevation of hilar venous
pressure under different blood flow conditions when pressure was being
monitored by either the narrow fluid-filled catheter or the
catheter-tip pressure transducer in the present preparation, the
relationship between the hilar venous pressure and blood flow was
studied in glands under vascular perfusion (n = 6). We found that the normal
resting blood flow to the submandibular gland was 0.5 ± 0.01 ml · min1 · g
1
(n = 50), and blood flow at a high
level of parasympathetic nerve stimulation (16 Hz) was increased to 3 ± 0.23 ml · min
1 · g
1
(n = 8). Hence, the control perfusion
blood flow rate was set at 0.5 ml · min
1 · g
1.
Blood flow was then altered by varying the perfusion flow rate in steps
of 0.5 ml · min
1 · g
1
until a flow rate of 3 ml · min
1 · g
1
was achieved. Each perfusion flow rate was maintained for a period of 1 min for recording the steady-state response of the hilar venous
pressure.
Data recording and analysis. All pressure and flow variables were recorded on magnetic tape (Store 14; Racal) and an oscillographic chart recorder (2800S; Gould). Gould P23ID transducers were used for arterial pressure measurement. All pressure transducers were zeroed to the atmospheric pressure and set at the level of the midchest. The flow sensors, which were precalibrated by the manufacturer, were connected to an ultrasonic flowmeter (T206; Transonic System). The zero baseline of the flow sensors was determined using stagnant saline solution before and at the end of each experiment. All values are given as means ± SE. Student's t-test for paired or unpaired data was used to test the statistical differences between two means. P < 0.05 indicated significant difference.
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RESULTS |
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Under resting conditions, the mean systemic arterial blood pressure was
110 ± 1.7 mmHg (n = 50),
submandibular glandular arterial inflow was 0.5 ± 0.01 ml · ml1 · g
1
(n = 50), and glandular venous outflow
(measured from the largest outflow vein) was 0.4 ± 0.02 ml · min
1 · g
1
(n = 14). Mean hilar venous pressure
was 12 ± 0.2 mmHg (n = 50), hilar
venous pulse pressure was 0.6 ± 0.01 mmHg
(n = 50), pressure in the
extraglandular segment of the outflow vein was 6 ± 0.6 mmHg
(n = 6), and pulse pressure in the
extraglandular segment of the outflow vein was 0.5 ± 0.01 mmHg
(n = 6). Mean external jugular venous
pressure was 5 ± 0.5 mmHg (n = 6),
external jugular venous pulse pressure was 0.5 ± 0.01 mmHg
(n = 6), and salivary secretion was
absent. The resting values of the submandibular arterial inflow and
venous outflow were not affected by the presence of a narrow catheter
or an ultraminiature catheter-tip pressure transducer in the hilar
venous system.
Hilar venous pressure and blood flow.
Increases in blood flow (via changes in the vascular perfusion rate)
were found to elevate the mean hilar venous pressure, whereas the hilar
venous pulse pressure was not significantly affected. Figure
2 shows the relationship between the mean
hilar venous pressure and blood flow in glands with vascular perfusion and with venous pressure monitored by two different methods, i.e., by
the narrow catheter or the catheter-tip pressure transducer. The two
parameters were found to be highly correlated
(r = 0.99, P < 0.001) in both preparations. The
regression coefficient for the change in blood flow on mean hilar
venous pressure was also similar, 2.6-2.7
mmHg · ml1 · min
1 · g
1.
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Parasympathetic stimulation. In dog submandibular gland with spontaneous blood flow or controlled vascular perfusion at the normal resting flow rate, parasympathetic nerve stimulation, apart from eliciting salivary secretion and an increase in spontaneous blood flow (or a decrease in arterial perfusion pressure), caused an increase in the hilar venous pressure of the gland (Fig. 3). The responses of the salivary flow and spontaneous blood flow (or the decrease in the perfusion pressure) were similar to results previously reported when measurements were made in glands without the insertion of a narrow catheter or a catheter-tip pressure transducer (12, 16). Figure 4 summarizes the steady-state response (at 60 s) of the mean hilar venous pressure and salivary secretion to short periods (1-2 min) of parasympathetic nerve stimulation (1-16 Hz) under both blood flow conditions and with hilar venous pressure monitored by two different methods, i.e., by the narrow catheter or the catheter-tip pressure transducer. The magnitude of all responses is in direct proportion to the frequency of stimulation. There was no significant difference between the group with spontaneous blood flow and the group with controlled blood flow with regard to the increase in salivary flow and hilar venous pressure in response to parasympathetic nerve stimulation. Similar results were obtained irrespective of the method of measurement of the hilar venous pressure. Figure 5 shows the relationship between salivary secretion and the mean hilar venous pressure during parasympathetic nerve stimulation at various stimulation frequencies. Salivary secretion and the mean hilar venous pressure were found to be highly correlated, with regression coefficients for the change in hilar venous pressure on salivary secretion being similar in glands with spontaneous blood flow and controlled vascular perfusion.
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Sympathetic stimulation. Sympathetic nerve stimulation was found to cause very little salivary secretion whether the gland received spontaneous blood flow or controlled vascular perfusion at the normal flow rate. In glands with spontaneous blood supply, although blood flow was significantly decreased as previously reported (12), the hilar venous pressure was not significantly affected, as in glands with constant-flow vascular perfusion (Fig. 7).
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Interaction between parasympathetic and sympathetic stimulation. In glands with spontaneous blood flow or controlled vascular perfusion at the normal resting flow rate, superimposed sympathetic nerve stimulation (20 Hz for 1 min) was found to inhibit the steady-state response (at 30-60 s) of both salivary secretion and the increase in hilar venous pressure caused by parasympathetic nerve stimulation (2-8 Hz). Against a background of continuous sympathetic nerve stimulation (20 Hz), salivary secretion and the increase in hilar venous pressure induced by parasympathetic nerve stimulation (2-8 Hz) were also found to be significantly reduced (Fig. 8). Even under conditions of superimposed sympathetic nerve stimulation or continuous background sympathetic discharge, the relationship between salivary secretion and hilar venous pressure during parasympathetic nerve stimulation still remained highly correlated, although the regression coefficient for the change in hilar venous pressure on salivary secretion was lower than the normal value (Fig. 9).
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DISCUSSION |
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This is the first study to monitor the in situ intraglandular venous pressure at the hilum region of the submandibular gland in experimental animals and to study its relationship to salivary flow during active secretion. Previous anatomic studies have suggested that arteriovenous anastomoses are present in the submandibular gland of the dog and that they are open during copious secretion caused by parasympathetic stimulation or application of a secretagogue (13). Shunting of arteriovenous anastomotic flow into the postcapillary venous segment would not only increase the mean venous pressure but would also impose a pulsatile wave onto the venous pressure. When venous pressure is measured by means of a conventional fluid-filled narrow catheter, the pulsatile nature of the venous pressure, if present, is significantly dampened. Our finding shows that when the hilar venous pressure is measured by a narrow catheter it is nonpulsatile not only at rest but even at a high level of parasympathtic nerve stimulation. To monitor the nondampened pulsatile wave of the venous pressure, in situ measurement of the hilar venous pressure is essential, and hence an ultraminiature catheter-tip pressure transducer, the smallest available at present for small animal research (1 mm OD), was chosen.
Does the presence of a narrow catheter or an ultraminiature catheter-tip pressure transducer affect the hilar venous pressure response to parasympathetic nerve stimulation in glands with controlled vascular perfusion at the normal flow rate? According to Bernoulli's principle on fluid energetics, if the total energy remains constant throughout a tube the total pressures at wide and narrow sections will not be different but the lateral pressure in the narrow section will be smaller than the lateral pressure in the wide section. The presence of a narrow catheter or a catheter-tip pressure transducer will decrease the cross-sectional area of the part of the venous system where it is located. This may be taken as equivalent to the narrow section of a tube. In glands with constant-flow vascular perfusion, i.e., the same total energy in the hilar venous system, the value of the hilar venous pressure measured when either the narrow catheter or the catheter-tip pressure transducer is present should be the same (for total pressure) or even smaller (for lateral pressure) than the value obtained in their absence. Hence, the presence of a narrow catheter or a catheter-tip pressure transducer would not affect the measurement of hilar venous pressure in glands with constant-flow vascular perfusion. It seems unlikely that the presence of either catheter will exaggerate the response of the hilar venous pressure to parasympathetic nerve stimulation in glands with controlled vascular perfusion at a normal resting flow rate.
In glands with spontaneous blood flow, parasympathetic nerve
stimulation induces a frequency-dependent increase in blood flow (12).
As mentioned in METHODS, the presence
of a narrow catheter or a catheter-tip pressure transducer in the hilar
venous system may exaggerate the rise in venous pressure under
conditions of high blood flow. Figure 2 shows that the regression
coefficient for the change in blood flow on hilar venous pressure is
the same (2.6-2.7
mmHg · ml1 · min
1 · g
1)
whether the narrow catheter or the catheter-tip pressure transducer was
used. Figure 4 also shows that the response of the hilar venous pressure to parasympathetic nerve stimulation is the same whether measurement was made by the narrow catheter or the catheter-tip pressure transducer. The findings suggest that the catheter-tip pressure transducer, which is bigger than the narrow catheter, does not
significantly enhance the response of the hilar venous pressure to
changes in blood flow or parasympathetic nerve stimulation compared
with the narrow catheter. Normal resting blood flow to the
submandibular gland is 0.5 ml · min
1 · g
1
on average, and blood flow during high levels of parasympathetic nerve
stimulation (16 Hz) is increased to an average of 3 ml · min
1 · g
1
in glands with spontaneous blood flow. According to the pressure-flow curve of the hilar venous system (Fig. 2), the predicted increase in
hilar venous pressure caused by the concomitant increase in blood flow
in the presence of a narrow catheter or a catheter-tip pressure
transducer during the same level of parasympathetic nerve stimulation
(16 Hz) is ~6 mmHg, which is small compared with the change in hilar
venous pressure (35-45 mmHg) that actually occurred (Fig. 4).
Hence, it seems unlikely that the presence of a narrow catheter or a
catheter-tip pressure transducer would have significantly exaggerated
the response of the hilar venous pressure to parasympathetic nerve
stimulation in glands with spontaneous blood flow.
Parasympathetic nerve stimulation caused, in a frequency-dependent manner, parallel increases in hilar venous pressure and salivary secretion, irrespective of whether the blood flow to the gland was allowed to increase spontaneously or under control by vascular perfusion at the normal flow rate (Figs. 3 and 4). The regression coefficient for the change in hilar venous pressure on salivary secretion was found to be similar under both blood flow conditions (Fig. 5). The result therefore suggests the existence of a direct relationship between salivary secretion and venous pressure in the submandibular gland during parasympathetic salivation. The finding also indicates that the concurrent blood flow response to parasympathetic nerve stimulation is not an absolute necessity for the change in venous pressure. Under resting conditions, the hilar venous pulse pressure was found to be small and almost nonpulsatile, as in the systemic veins. During moderate-to-high levels of parasympathetic nerve stimulation, the hilar venous pulse pressure was significantly increased, with a magnitude similar to that of the pulse pressure in small systemic arteries, e.g., 6-12 mmHg in cat mesenteric arteries 50-80 µm in diameter (32), and the waveform of the pulse pressure showed an obvious resemblance to that of the systemic arterial pulse pressure in glands with spontaneous blood flow or to that of the perfusion pulse pressure in glands with controlled vascular perfusion (Fig. 3). Parasympathetic nerve stimulation causes vasodilatation, and the changes in venous pressure were also observed in glands with controlled vascular perfusion. Is it possible that the high venous pressure and its arterial-like waveform are related to a direct transmission of the arterial pressure and its pulse pressure through the dilated arterioles and open capillaries? It is well known that the capillary network of any vascular bed contributes greatly to the degradation of hydrostatic pressure by virtue of the narrow caliber and large number of vessels. The pulsatile nature of the pressure waveform is greatly attenuated as the blood moves through the arteriolar and precapillary branching, and the capillary pulse pressure in most vascular beds is normally very small, e.g., 1-4 cmH2O in cat mesenteric capillaries (26, 31). The presence of obvious pressure oscillations in the veins may reflect some form of arteriovenous shunting (32). Fronek and Zweifach (7) studied the changes in hydrostatic pressure of the microvessels in response to systemic vasodilatation in skeletal muscle microcirculation, where arteriovenous shunt vessels are not commonly observed (28, 32). They found that, with maximal dilatation induced by papaverine, the increase in pressure in the smallest venules (8-15 µm in diameter) is ~4 mmHg and is negligible in veins larger than 80 µm in diameter (7). Maspers et al. (18) have also shown that the increase in postcapillary venular pressure (measured from a venule <10 µm in diameter) with maximal metabolic vasodilatation (induced by muscle exercise) is ~15 mmHg from the control value prevailing at normal intrinsic tone. We found in this study that high levels of parasympathetic nerve stimulation (8-16 Hz) were able to raise the hilar venous pressure, which was monitored from a venous vessel ~2-3 mm in diameter, by 30-40 mmHg (Fig. 4). In addition, the venous pulse pressure was increased by a magnitude of 8-12 mmHg (Fig. 3). Hence it is doubtful that such large increases in the mean pressure and pulse pressure in a large (hilar) vein could be caused by a direct transmission of pressure and pulse pressure from the arterial side to the venous side via dilated arterioles and open capillaries. However, further experiments on the measurements of micropressures in the salivary gland are required to justify this point.
Recent anatomic studies have confirmed the presence of arteriovenous anastomoses in the dog submandibular gland and demonstrated their opening during profuse salivation (13). Most of the arteriovenous anastomoses are seen draining into venules (<100 µm in diameter) devoid of valves (13). Fluid exchange is found to occur not only in the capillaries but also in the permeable postcapillary venules in some vascular beds, e.g., in the diaphragm and frog pial microvessels (22, 25). Measurements of hydraulic conductivity of walls of single mammalian capillaries, e.g., in rat intestine and cat mesentery, have shown that vessels at the venous end of the microcirculation usually have a higher hydraulic conductivity than those at the arterial end (19). If the postcapillary valveless venules of the salivary gland are permeable and have a higher hydraulic conductivity than the microvessels at the arterial end, as in other vascular beds, the pressure toward the venous end would then be the most important factor for fluid exchange. It is highly probable that during copious secretion, arteriovenous anastomoses open, allowing rapid transmission of the arterial pressure into the venules, and this immediately elevates the venular pressure, enhancing filtration. Arteriovenous anastomoses in some vascular beds have been shown to have a richer supply of cholinergic and peptidergic nerves than arteries of comparable size (21), suggesting that parasympathetic nerve stimulation may induce a larger dilatatory action on arteriovenous anastomoses than on the arterioles. If this happens, the predominant dilatatory response of arteriovenous anastomoses will not only result in a shifting of blood flow through arteriovenous anastomoses but also a direct transmission of the perfusion pressure to the venules, even in glands with controlled vascular perfusion at a normal resting flow rate. Hence, it is probable that the increases in venous pressure and pulse pressure and the arterial-like waveform observed during parasympahetic nerve stimulation are related to a direct transmission of the arterial pressure and pulse pressure through open arteriovenous anastomoses, irrespective of whether the gland receives spontaneous blood supply or controlled vascular perfusion. However, further studies on the measurements of the fenestral density and hydraulic conductivity of the postcapillary venules and arteriovenous anastomotic flow of the salivary gland, as well as their changes during parasympathetic salivation, are required to verify this point.
The arterial pressure and pulse pressure that are transmitted through opened arteriovenous anastomoses would be rapidly lost in the highly compliant venous vessels if there were no mechanism present for their rapid transmission and preservation in the venous system. The mean pressure and the pulse pressure of the extraglandular segment of the outflow vein were not affected during parasympathetic nerve stimulation, implying that the arterial pressure and pulse pressure transmitted through the opened arteriovenous anastomoses had already been lost in the extraglandular venous segment. Hence, the mechanisms responsible for preserving the transmitted pressures must be located within the gland. Histological studies have shown that smooth muscle cells are scarce in the venous blood vessels (13), suggesting that venoconstriction is unlikely to play an important role in preserving the transmitted arterial pressure and pulse pressure. However, dense connective tissue is found to enclose the ductal system and its accompanying structures (blood vessels, lymphatic vessels, and nerves), being most abundant in the hilum and diminished aborally (13). Moreover, the gland is encased by a strong fibrous capsule. On leaving the gland, all outflow veins must penetrate the fibrous capsule. Hence, the mechanisms responsible for preserving the high intraglandular venous pressure are probably the dense connective tissue surrounding the venous vessels and the fibrous capsule. This may explain why the pressure in the extraglandular segment of the outflow vein was not significantly affected during parasympathetic nerve stimulation, as the venous segment lies outside the fibrous capsule and is no longer surrounded by dense connective tissue.
ACh and VIP were found to induce not only an arterial-like waveform onto the hilar venous pressure but also to increase its level and the size of its pulse pressure in a dose-dependent manner, as with parasympathetic nerve stimulation (Fig. 6). It is interesting to note that ACh acts rapidly to cause an abrupt increase in venous pressure, whereas VIP acts slowly to raise this pressure. However, combined infusion of both agonists provoked a larger and more sustained change in venous pressure, indicating that the agonists act synergistically to bring about a very rapid and maintained increase in filtration pressure for copious salivary secretion. Parasympathetic nerve fibers supplying the arteriovenous anastomoses in many vascular beds have been shown to be positive for ACh and immunoreactive for VIP (9, 11). Hence, both parasympathetic neurotransmitters increase the venous pressure of the salivary gland, probably by activating dilatation of the arteriovenous anastomoses.
Sympathetic nerve stimulation was found to induce very little salivary
secretion and had no significant effect on the hilar venous pressure
whether the gland received spontaneous blood supply or controlled
vascular perfusion at the normal flow rate (Fig. 7). Administration of
phenylephrine and clonidine, 1-
and
2-adrenergic agonists,
respectively, did not provoke salivation and had no significant effect
on the hilar venous pressure, whereas isoproterenol injection caused
salivary flow and an elevated pulsatile venous pressure (Fig. 6).
Hence, the salivary flow in response to sympathetic nerve stimulation
is probably due to activation of the
-adrenergic receptors of the
secretory apparatus, as reported previously (6). We have previously
shown that the paucity of salivary flow in response to sympathetic
nerve stimulation is observed even in glands with controlled vascular
perfusion at the normal resting flow rate, indicating that the
phenomenon is unrelated to a reduced blood supply caused by
vasoconstriction, as traditionally believed (12). In this study, we
found that the hilar venous pressure was not significantly affected by
sympathetic nerve stimulation in glands with controlled vascular
perfusion, as in glands with natural blood supply. Sympathetic nerves
can act directly on the acinar cells, provoking slight secretion.
However, the paucity of salivary flow in response to sympathetic
stimulation may be related to a certain extent to the concurrent low
hilar venous pressure, a situation unfavorable for fluid filtration in
the salivary gland microcirculation. Arteriovenous anastomoses have been found to possess a denser sympathetic innervation than the arteries and veins of the same vascular bed (10, 20, and 21). Sympathetic nerve stimulation or infusion of norepinephrine or methoxamine has been shown to cause constriction of the arteriovenous anastomoses, resulting in blood flow redistribution to the capillaries in the dog hindpaw and the sheep hindlimb (1, 8). Sympathectomy or
infusion of phentolamine normally results in a redistribution of
capillary blood flow to the arteriovenous anastomoses in the muscle and
skin circulation of experimental animals (5, 8). In this study, the
resting hilar venous pressure of the salivary gland was found to be
rather nonpulsatile (Figs. 3 and 6), suggesting that, under normal
resting conditions, most of the arteriovenous anastomoses in the
salivary gland are probably closed, either by the basal sympathetic
discharge or circulatory catecholamines, with only a small number
remaining open. This may explain why sympathetic nerve stimulation did
not exert a significant action on the already low resting hilar venous
pressure, primarily because the number of open arteriovenous
anastomoses available for closure is meager.
Continuous background or superimposed sympathetic nerve stimulation has
been shown to inhibit steady-state parasympathetically induced salivary
secretion via an 2-adrenergic
mechanism (15). In this study, both modes of sympathetic stimulation
were found to depress in a parallel fashion the parasympathetically
induced elevation of venous pressure and salivary secretion (Fig. 8). It is therefore possible that the sympathetic inhibition of salivary secretion is related, to a certain extent, to the inhibition of the
vascular response, i.e., prevention of increased venous pressure. Under
sympathetic influence, the regression coefficient for the change in
hilar venous pressure on parasympathetic salivary secretion was found
to be smaller than the normal value (Fig. 9), suggesting a somehow
altered relationship between venous pressure and salivary secretion.
Hence we cannot rule out the possibility that the sympathetic inhibitory action on parasympathetic salivary flow may also be related
to a direct inhibitory action on the secretory apparatus. The
sympathetic inhibitory actions on both the salivary flow and the
increase in venous pressure were not significantly affected by the
selective
1-adrenergic
antagonist prazosin but were abolished by the selective
2-adrenergic antagonist
yohimbine (Fig. 8). Sympathetic control of the resistance of
arteriovenous anastomoses is exerted through a combination of
1- and
2-adrenergic mechanisms in the
dog hindpaw (2) but primarily by an
2-adrenergic mechanism in the
human finger (4). In the dog submandibular gland, the sympathetic
inhibition of parasympathetic salivation is very likely exercised
primarily through the
2-adrenergic mechanism, acting directly on the secretory cells as well as indirectly via a closure of
arteriovenous anastomoses.
This study has demonstrated for the first time that the volume of salivary flow during parasympathetic salivation is directly related to the venous pressure within the salivary gland. A plentiful supply of fluid to the secretory apparatus is a prerequisite for a copious and watery secretion. To ensure a massive fluid filtration across the walls of the microvessels in an actively secreting salivary gland when the oncotic pressure is concurrently increased because of fluid efflux, an extra high capillary hydrostatic pressure and/or an increased hydraulic conductivity is required. Studies on the rabbit submandibular gland have demonstrated that parasympathetic nerve stimulation does not lead to a change in capillary permeability (27). Whether this can be applied to the dog submandibular gland requires investigation. Studies in human skin have shown that a given increment in venous pressure will produce a much greater effect on capillary hydrostatic pressure than the same increment in arterial pressure (17, 24). If this is also true for the salivary gland, an increase in hydrostatic pressure at the venular end through opening of arteriovenous anastomoses will be more effective in elevating the capillary hydrostatic pressure than a similar increase in pressure at the arterial end through arteriolar dilatation. Of course both mechanisms may operate simultaneously to effect a high capillary hydrostatic pressure during copious secretion. The relative importance of each mechanism still awaits further investigation.
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ACKNOWLEDGEMENTS |
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I am grateful to Dr. J. C. C. Wang for helpful comments and K. K. Tsang for technical assistance, and to the Laboratory Animal Unit of The University of Hong Kong for the supply of experimental animals.
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FOOTNOTES |
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This study was supported by research grants from the Hong Kong Government (RGC 338/034/0017) and The University of Hong Kong (CRCG 337/034/0016 and 335/034/0060).
Address for reprint requests: M. A. Lung, Dept. of Physiology, Faculty of Medicine, Univ. of Hong Kong, Li Shu Fan Bldg., Sassoon Road, Hong Kong, China.
Received 15 October 1997; accepted in final form 28 April 1998.
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REFERENCES |
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