Mechanism of coronary vasodilation to insulin and insulin-like growth factor I is dependent on vessel size

Christine L. Oltman1, Neal L. Kane2, David D. Gutterman1,2, Robert S. Bar1,2, and Kevin C. Dellsperger1,2

1 Department of Veterans Affairs and 2 Department of Internal Medicine and the Cardiovascular Center, University of Iowa, Iowa City, Iowa 52246


    ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Insulin and insulin-like growth factor I (IGF-I) influence numerous metabolic and mitogenic processes; these hormones also have vasoactive properties. This study examined mechanisms involved in insulin- and IGF-I-induced dilation in canine conduit and microvascular coronary segments. Tension of coronary artery segments was measured after constriction with PGF2alpha . Internal diameter of coronary microvessels (resting diameter = 112.6 ± 10.1 µm) was measured after endothelin constriction. Vessels were incubated in control (Krebs) solution and were treated with Nomega -nitro-L-arginine (L-NA), indomethacin, or K+ channel inhibitors. After constriction, cumulative doses of insulin or IGF-I (0.1-100 ng/ml) were administered. In conduit arteries, insulin produced modest maximal relaxation (32 ± 5%) compared with IGF-I (66 ± 12%). Vasodilation was attenuated by nitric oxide synthase (NOS) and cyclooxygenase inhibition and was blocked with KCl constriction. Coronary microvascular relaxation to insulin and IGF-I was not altered by L-NA, indomethacin, tetraethylammonium chloride, glibenclamide, charybdotoxin, and apamin; however, tetrabutylammonium chloride attenuated the response. In conclusion, insulin and IGF-I cause vasodilation in canine coronary conduit arteries and microvessels. In conduit vessels, NOS/cyclooxygenase pathways are involved in the vasodilation. In microvessels, relaxation to insulin and IGF-I is not mediated by NOS/cyclooxygenase pathways but rather through K+-dependent mechanisms.

diabetes mellitus; coronary circulation; coronary microcirculation; dogs; potassium channels


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

INSULIN AND insulin-like growth factor I (IGF-I) are structurally related peptides capable of stimulating a variety of metabolic and mitogenic processes. Receptors for both hormones are heterodimeric transmembrane tyrosine kinase receptors that, when stimulated, activate multiple intracellular pathways, resulting in altered metabolic and growth regulatory processes (9). IGF-I has been proposed as a novel therapy for patients with type 1 diabetes mellitus and for severely insulin-resistant patients with type 2 diabetes. Previous studies examining the vascular effects of insulin and IGF-I have yielded conflicting results. Some studies have shown that insulin or IGF-I enhanced tone or caused constriction (18, 24). Other studies have shown vasodilation that is either nitric oxide synthase (NOS) dependent (3, 6, 10, 17, 19, 22-24) or nitric oxide (NO) independent (11). The disparate effects may be due to differences in species or vascular beds studied. For example, Juncos and Ito (7) showed that insulin caused dilation of efferent but not afferent arterioles of the rabbit kidney, demonstrating regional variation, even within an organ.

No previous study has examined the vascular effects of insulin and IGF-I on conduit and microvascular arteries of the heart, an organ that is impacted upon by multiple processes in diabetes. In this report, we tested the hypothesis that insulin and IGF-I have different responses and mechanisms of action in coronary conduit vs. microvascular vessels. We further explored potential mechanisms responsible for the hormonally induced vasodilation.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

All protocols were approved by the University of Iowa Animal Care and Use Committee and the Veterans Affairs Research Committee and conform to the Guide for the Care and Use of Laboratory Animals published by the United States National Institutes of Health (NIH publication no. 85-23, revised 1985).

Animals and tissue. Sixty-five adult mongrel dogs of either sex (5-8 kg) were killed with an overdose of pentothal sodium (50 mg/kg). Hearts were removed and immediately placed in cold (4°C), oxygenated (20% O2-5% CO2-75% N2) Krebs bicarbonate buffer solution (see Solutions and drugs). Coronary conduit arteries and ventricular microvessels were isolated and cleaned of fat and connective tissue.

Isolated vascular rings. Canine coronary arteries were studied using a standard isometric ring technique (13, 14). Segments of the right coronary, left circumflex, or left anterior descending coronary arteries (1-2 mm in diameter) were dissected and cut into 5-mm ring segments. In some ring segments, endothelium was removed by pulling silk suture through the vessel. Vascular rings were mounted on two stainless steel wire stirrups passed through the vessel lumen. One stirrup was attached to a force transducer (Grass FT03), and the other was attached to a micrometer microdrive to allow the vessel to be stretched by small increments. Each vessel apparatus was placed in a 10-ml jacketed organ bath containing Krebs buffer equilibrated at 37°C and aerated with 20% O2-5% CO2-75% N2. Isometric contractions and relaxations were measured on a physiological recorder. Coronary rings were individually stretched to the maximum of the length-developed tension relationship by repeated test exposures to 75 mM KCl at increasing vessel tension.

The coronary artery segments were allowed to stabilize 30 min before concentration-response curves in either control solution (normal Krebs) or Krebs solution with Nomega -nitro-L-arginine (L-NA, 100 µM) and indomethacin (Indo, 10 µM). PGF2alpha (6-10 µM) or KCl (30-40 mM) was used to constrict the vessels to 30-50% of their resting tension. After steady-state tension was achieved, a concentration-response curve to insulin (0.1-100 ng/ml) or IGF-I (0.1-100 ng/ml) was performed. A single dose of sodium nitroprusside (SNP, 10-4 M) was then administered to establish maximal smooth muscle vasodilation. Acceptable coronary artery ring experiments met the following criteria: 1) developed tension to PGF2alpha >1 g; 2) 80-150% dilation to SNP; and 3) <30% dilation to bradykinin (10-6 M) in denuded vessels.

Isolated microvessels. A standard in vitro pressurized arteriole preparation was used to study coronary microvessels (13, 14). Ventricular microvessels (75-175 µm intraluminal diameter and ~1 mm in length) were carefully removed from the myocardium and cleaned with the aid of a dissecting microscope (Olympus SZ-6045 stereo zoom microscope). Each end of the microvessel was cannulated with a glass micropipette and secured with 10-0 ophthalmic suture. The cannulated pipettes were attached to a hydrostatic pressure reservoir (20 mmHg) under conditions of no flow. The organ chamber was placed on the stage of an inverted microscope (Olympus CK40). Attached to the microscope were a video camera, a video monitor, and a calibrated video caliper (Boeckler Video encoder). The organ chamber was connected to a rotary pump that continuously circulated oxygenated Krebs buffer warmed to 37°C. An image of the microvessel was displayed on the video monitor, and intraluminal diameters were measured by manually adjusting the video micrometer. The resolution of the system allowed measurement of very small (1-2 µm) changes in vessel diameter.

Microvessels were allowed to equilibrate for 30 min at a distending pressure of 20 mmHg. KCl (50 mM) was added to the bath to test constrictor capacity. Microvessels were incubated for 30 min in Krebs buffer alone (control) or in the presence of one or more of the following inhibitors: Indo (10 µM), L-NA (100 µM), glibenclamide (1 µM), tetraethylammonium chloride (TEA, 1 mM or 10 mM), tetrabutylammonium chloride (TBA, 1 mM), 4-aminopyridine (4-AP, 0.5 mM), or combinations of L-NA and Indo, glibenclamide and TEA, or charbydotoxin (50 nM) and apamin (500 µM). To mimic the effects of euglycemic and hyperglycemic conditions, responses to insulin and IGF-I were studied using low (5.5 mM) and high (22 mM) glucose concentrations. The standard solution used in this study contained 11 mM glucose. KCl constriction was used in some vessels to examine the role of K+ channels on insulin- and IGF-I-induced dilation. Both hypertonic (addition of KCl to the bath) and isotonic (equimolar reduction of NaCl) solutions were used. Endothelin-1 (0.40-8.0 nM) was used to constrict microvessels to 30-60% of their resting diameter. Cumulative concentration-response relationships were evaluated for insulin or IGF-I (0.1-100 ng/ml) by adding the drug directly to the organ bath. A single dose of SNP (10-4 M) or papaverine (10-4 M) was given at the end of the experiment to determine maximal dilation. The following criteria were required for an acceptable microvessel experiment: 1) no obvious leaks; 2) constriction of >50% to 50 mM KCl and >30% to endothelin; and 3) dilation of >80% to 10-4 M SNP or 10-4 M papaverine.

Solutions and drugs. Krebs solution contained (in mM): 120.0 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2 MgSO4, 23.0 NaHCO3, 1.2 KH2PO4, 11 glucose, and 0.025 EDTA. Solutions were aerated with 20% O2-5% CO2-75% N2 and were warmed to 37°C with pH maintained at 7.4. Insulin was obtained from Eli Lilly (Indianapolis, IN) and was diluted in 0.01 normal HCl. IGF-I was obtained from Intergen (Purchase, NY) and was diluted in 0.05 M acetic acid. Insulin and IGF-I were stored at -20°C until ready to use. Endothelin-1 was purchased from Peninsula Laboratories (San Carlos, CA). All other inhibitors and vasoactive agents were purchased from Sigma Chemical (St. Louis, MO). All solutions and vasoactive agents were prepared fresh on the day of the experiment.

Statistical analysis. Results are expressed as percent dilation, with 100% representing the difference from the constricted value with endothelin, PGF2alpha , or KCl to the resting value (tension or diameter). All concentration-response curves were evaluated for changes in maximal responses and differences at each dose using ANOVA with repeated measures and the Fisher least-significant difference correction for multiple comparisons. Comparisons of percent vasodilation under different treatment conditions were performed. Significance of differences among mean values of resting conditions, maximal effect, and EC50 values was assessed with a one-way ANOVA. Data are expressed as means ± SE; n indicates the number of animals. Differences with P < 0.05 were considered significant.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Canine coronary conduit artery responses. Resting tension of conduit artery rings was 2.8 ± 0.1 and 2.7 ± 0.1 g in insulin and IGF-I control groups, respectively, and was not different among any of the treatment groups [P = not significant (NS)]. Developed tension to PGF2alpha was similar among groups [2.0 ± 0.2 and 2.6 ± 0.6 g in insulin and IGF-I control groups, respectively (P = NS)]. In canine coronary arteries constricted with PGF2alpha , both insulin and IGF-I produced relaxation (Fig. 1). The highest concentration of IGF-I (100 ng/ml) induced 68 ± 8% relaxation and was greater than the relaxation induced by 100 ng/ml insulin (33 ± 4%). Insulin and IGF-I did not produce relaxation in vessels that had endothelial cells removed (-1.4 ± 8.8% dilation for insulin and 11.3 ± 6.1% dilation for IGF-I at 100 ng/ml, n = 4). Cyclooxygenase (Indo, 10 µM) and NOS (L-NA, 100 µM) inhibition blocked the insulin-induced dilation and modestly attenuated (46%) the IGF-I-induced dilation (Fig. 1). When coronary rings were constricted with KCl (30-40 mM), relaxation to insulin and IGF-I was completely abolished (Fig. 1).


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Fig. 1.   Insulin- and insulin-like growth factor (IGF)-I-induced relaxation of canine coronary artery rings. Concentration-response curves were performed under control conditions (constricted with PGF2alpha ), in the presence of nitric oxide synthase (NOS) and cyclooxygenase inhibition, and after KCl-induced constriction. Left: insulin-induced relaxation, which was abolished by both the presence of Nomega -nitro-L-arginine (L-NA) and indomethacin (Indo), and KCl-induced constriction. IGF-I-induced relaxation (right) was slightly inhibited by L-NA and Indo. When the rings were constricted with KCl, IGF-I-induced relaxation was blocked; n, no. of animals. # P < 0.05, control vs. L-NA/Indo. * P < 0.05 PGF2alpha vs. KCl constriction.

Coronary microvascular responses. Baseline diameter was 107 ± 17 and 102 ± 16 µm (P = NS) for microvessels in the control groups for insulin and IGF-I, respectively. Baseline diameters were not different between the control and any of the treatment groups. Microvessels were constricted with endothelin (0.4-8.0 nM) to 43 ± 4 and 49 ± 5% resting diameter in the control insulin and IGF-I groups, respectively. Insulin and IGF-I produced concentration-related relaxation in canine coronary microvessels. The vasodilator responses to insulin and IGF-I were similar, with maximal relaxation of 69 ± 6% for insulin and 69 ± 7% for IGF-I (Fig. 2). Inhibition of NOS, cyclooxygenase, or both pathways did not alter insulin- or IGF-I-induced vasodilation in coronary microvessels (Fig. 3). Thus NOS/cyclooxygenase pathways appear not to be necessary in insulin/IGF-I-induced vasodilation in coronary microvessels, in contrast to their role in the coronary conduit arteries.


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Fig. 2.   Insulin- and IGF-I-induced relaxation of canine coronary microvessels. Insulin and IGF-I produced similar relaxation in coronary microvessels. Relaxation at 100 ng/ml was 69 ± 6% for insulin and 69 ± 7% for IGF-I; n, no. of animals. P = not significant (NS).



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Fig. 3.   Effect of cyclooxygenase and NOS pathways in insulin- and IGF-I-induced dilation of coronary microvessels. Vasodilator responses to insulin and IGF-I were performed in the presence of Indo (10 µM), L-NA (100 µM), or a combination of Indo and L-NA. Inhibition of these pathways did not alter vasodilation of insulin or IGF-I; n, no. of animals. P = NS.

Constriction with hypertonic KCl solutions (39 ± 4 mM for insulin and 40 ± 5 mM for IGF-I) abolished dilation to insulin and IGF-I, except at the highest dose of 100 ng/ml (Fig. 4). This concentration of KCl produced similar amounts of constriction as endothelin and should prevent dilation due to K+ efflux from smooth muscle cells. Similar responses were found using isotonic (NaCl replacement) solutions (data not shown). These results suggest that K+ efflux mechanisms play a role in insulin and IGF-I-induced vasodilation in coronary microvessels.


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Fig. 4.   Concentration-response curves for insulin and IGF-I in coronary microvessels constricted with KCl. The vasodilation response was determined after depolarization of the microvessels with KCl (35-45 mM). Vasodilation to insulin and IGF-I was essentially abolished with the exception of the highest dose. * P < 0.05, endothelin vs. KCl constriction.

To determine whether specific K+ channels are involved in insulin- and IGF-I-induced dilation, specific inhibitors of K+ channels known to be present in the coronary circulation were tested. TBA (1 mM), a nonspecific calcium-sensitive K+ channel inhibitor, significantly attenuated insulin- and IGF-I-induced dilation (Fig. 5) at 10-100 ng/ml for insulin and 30-100 ng/ml for IGF-I. Glibenclamide (1 µM), an ATP-sensitive K+ channel inhibitor, TEA (1 mM), a large-conductance Ca2+-activated K+ (KCa) channel inhibitor, and the combination of glibenclamide and TEA did not alter insulin- and IGF-I-induced dilation (Fig. 6). Tenfold higher concentrations of TEA (10 mM) also did not alter relaxation to insulin and IGF-I (response at 100 ng/ml: insulin: 65 ± 5, n = 4; IGF-I: 65 ± 11, n = 5). These data suggest that K+ channels other than ATP-sensitive and large-conductance KCa are involved in insulin- and IGF-I-induced vasodilation. Other inhibitors, including 4-aminopyridine (200 µM) to block voltage-dependent K+ channels or the combination of charybdotoxin (50 nM) and apamin (5 µM; large- and small-conductance K+ channel inhibitors, respectively) together did not alter dilation to insulin or IGF-I (Table 1).


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Fig. 5.   Effect of tetrabutylammonium chloride (TBA; 1 mM) on insulin- and IGF-I-induced vasodilation in canine coronary microvessels. When microvessels were incubated for 30 min in the presence of TBA, insulin- and IGF-I-induced vasodilation was significantly attenuated. This provides evidence that hyperpolarization via K+ channels plays a role in this dilation. * P <=  0.05 vs. control.



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Fig. 6.   Role of K+ channels in insulin- and IGF-I-induced dilation of canine coronary microvessels. Insulin- and IGF-I-induced relaxation was not altered when the microvessels were incubated in the presence of glibenclamide, tetraethylammonium chloride (TEA), or a combination of both inhibitors.


                              
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Table 1.   Effect of additional K+ channel inhibitors on 100 ng/ml insulin- or IGF-I-induced microvascular relaxation

Insulin- and IGF-I-induced dilation was also evaluated in "euglycemic" (5.5 mM, n = 5) and "hyperglycemic" (11 and 22 mM, n = 5) solutions. Vessels were in the modified glucose solutions for at least 1 h before the insulin or IGF-I curves were performed. Neither decreasing nor increasing the glucose concentration altered the vasodilation to insulin or IGF-I (data not shown).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In this study, insulin and IGF-I produced vasodilation in both canine coronary conduit arteries and microvessels. However, the mechanisms responsible for vasodilation differed in the two coronary vascular beds. In coronary conduit arteries, insulin produced less relaxation than IGF-I. Also, in conduit arteries, vasodilation induced by both hormones was endothelium dependent, was attenuated by NOS and cyclooxygenase inhibition, and was completely blocked when KCl was used to constrict the vessels. In canine coronary microvessels, relaxation to insulin was similar to IGF-I-induced relaxation. However, in contrast to conduit arteries, this relaxation was not mediated by NO, cyclooxygenase, ATP-dependent K+ channels, or small or large KCa channels; rather, hyperpolarization, via some other K+ channel(s), appeared to be involved in the vasodilation.

Previous studies that focused on the vasoactive properties of insulin and IGF-I yielded conflicting results (7, 10, 11, 17, 18, 22, 24). Differences among these studies include the vascular bed studied, vessel size examined, and methodological diversity. Few studies examined the effects of insulin and IGF-I in the coronary circulation. Recently, Hasdai et al. (5) showed that insulin and IGF-I produced 28 and 25% relaxation, respectively, of endothelin-constricted porcine coronary conduit arteries. This relaxation was endothelium independent and involved smooth muscle cell K+ channels. We also saw modest relaxation to insulin (32%) in canine coronary arteries. However, in our study, IGF-I produced a greater amount of vasodilation than insulin (66 vs. 32%). The current study also found that insulin- and IGF-I-induced relaxation was endothelial dependent in canine conduit arteries. Hasdai et al. did not investigate microvascular responses to insulin and IGF-I in the porcine coronary circulation.

Selective differences in insulin- and IGF-I-induced vasodilation between conduit coronary arteries and coronary microvessels were present. In this study, L-NA and Indo attenuated the dilation to insulin and IGF-I in conduit arteries; however, the same combination of inhibitors did not alter microvascular dilation. McKay and Hester (10) have shown that insulin-induced vasodilation in hamster cremaster muscle is NO dependent in second-order but not in third- or fourth-order arterioles. Kersten el al. (8) and Nagao et al. (12) reported that smaller blood vessels exhibited enhanced potency to aprikalim, an agonist that produces vasorelaxation through activation of ATP-sensitive K+ channels, suggesting that small arteries are more sensitive to the actions of hyperpolarizing vasodilators than are large arteries.

Our results also show that K+ channels mediate some of the insulin- and IGF-I-induced vasodilation in both coronary conduit arteries and microvessels. High extracellular K+ concentrations blocked insulin- and IGF-I-induced dilation in both coronary conduit arteries and microvessels. In coronary microvessels, the nonspecific K+ channel inhibitor TBA attenuated the response to insulin and IGF-I. However, when studies were performed to block a specific class of K+ channels, using glibenclamide (1 µM), an ATP-sensitive K+ channel inhibitor, or TEA (1 mM), a KCa channel inhibitor, normal vasodilation was observed to insulin and IGF-I. Comparable relaxation was also observed in the presence of the combination of glibenclamide and TEA or at a higher concentration of TEA (10 mM).

There are several possible explanations for the K+ channel findings of our study. The inhibitors chosen in this study may not completely inhibit their respective classes of K+ channels (4, 16). A distinct type of K+ channel may be involved in insulin- and IGF-I-induced dilation that is not blocked by the inhibitors used in our study. Insulin and IGF-I may produce vasodilation through redundant pathways or diverging signal cascades, which in turn affect K+ channel activity in different ways.

Limitations of the study. Our study did not address the reactivity of coronary vessels from diabetic animals in response to insulin and IGF-I. Previous investigations have demonstrated that some vascular dysfunction associated with diabetes can be mimicked using hyperglycemic solutions (15, 20, 21). However, in this study, neither high nor physiological glucose concentrations altered insulin- or IGF-I-induced vasodilation in coronary microvessels. The use of euglycemic and hyperglycemic solutions to assess the potential effect of varying levels of glycemia on insulin and IGF-I may mimic the hyperglycemia observed in diabetes; however, it does not produce the potential adverse effects on the vasculature that can result from chronic hyperglycemia. Studies using vessels from diabetic animals could produce different results.

Systemic circulating levels of insulin are ~0.2 ng/ml in the fasting state, which increase to 2 ng/ml after a glucose load. In the portal circulation, the concentration of insulin can be 10 ng/ml or higher. In patients with insulin resistance, plasma levels of insulin can be >5 ng/ml. IGF-I is present in the circulation in microgram quantities. However, >90% of plasma IGF-I is present as a 150,000 complex that is thought to be confined to the vascular compartment. The precise mechanisms by which circulating IGF-I gets to tissue sites of action are uncertain. Further compounding the difficulty in determining the extravascular concentrations of IGF-I are the recent findings that most tissues can synthesize both IGF-I and one or more of the six high-affinity IGF binding proteins (1, 2). The concentration range in this study for insulin and IGF-I was from 0.1 to 100 ng/ml. Effective relaxation was observed in this range in both coronary conduit and microvessels. Although complete relaxation to insulin and IGF-I was not achieved in this range, higher concentrations of these hormones would likely represent nonphysiological concentrations of insulin and IGF-I. Thus the range of doses used in these studies encompassed both physiological and supraphysiological concentrations of insulin and IGF-I.

In summary, insulin and IGF-I cause vasodilation in canine coronary conduit arteries and microvessels. In conduit vessels, endothelial-dependent NOS/cyclooxygenase pathways mediate insulin- and IGF-I-induced vasodilation. In coronary microvessels, relaxation to insulin and IGF-I is similar and is not mediated by NO/cyclooxygenase pathways, but rather by hyperpolarization through K+ channels.


    ACKNOWLEDGEMENTS

We thank Nam Yong Lee for technical assistance and Dr. Fausto Loberiza for statistical analysis.


    FOOTNOTES

This work was supported by National Institutes of Health National Research Service Award F32HL-09198 (to C. L. Oltman) and Grants RO1 HL-51308 (to K. C. Dellsperger), DK-25421-20 (to R. S. Bar), and DERC DK-25295-20 (R. S. Bar), the Veterans Affairs/JDF Diabetes Research Center (to D. D. Gutterman, R. S. Bar, and K. C. Dellsperger) and VA Merit Review (to D. D. Gutterman and R. S. Bar), and a Heartland Affiliate American Heart Association Grant-in-Aid (to K. C. Dellsperger).

Address for reprint requests and other correspondence: C. L. Oltman, Cardiovascular Research (151), VA Medical Center, Highway 6 West, Iowa City, IA 52246 (E-mail: coltman{at}blue.weeg.uiowa.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Received 1 November 1999; accepted in final form 10 February 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Bach, LA, and Rechler MM. Insulin-like growth factor binding proteins. Diabetes Rev 3: 38-61, 1995.

2.   Baxter, RC, and Martin JL. Binding proteins for the insulin-like growth factors: structure, regulation and function. Prog Growth Factor Res 1: 49-68, 1989[Medline].

3.   Chen, Y-L, and Messina EJ. Dilation of isolated skeletal muscle arterioles by insulin is endothelium dependent and nitric oxide mediated. Am J Physiol Heart Circ Physiol 270: H2120-H2124, 1996[Abstract/Free Full Text].

4.   Groschner, K, Graier WF, and Kukovetz WR. Activation of a small-conductance Ca2+-dependent K+ channel contributes to bradykinin-induced stimulation of nitric oxide synthesis in pig aortic endothelial cells. Biochim Biophys Acta 1137: 162-170, 1992[ISI][Medline].

5.   Hasdai, D, Rizza RA, Holmes DR, Richardson DM, Cohen P, and Lerman A. Insulin and insulin-like growth factor-1 cause coronary vasorelaxation in vitro. Hypertension 32: 228-234, 1998[Abstract/Free Full Text].

6.   Haylor, J, Singh I, and El Nahas AM. Nitric oxide synthesis inhibitor prevents vasodilation by insulin-like growth factor I. Kidney Int 39: 333-335, 1991[ISI][Medline].

7.   Juncos, LA, and Ito S. Disparate effects of insulin on isolated rabbit afferent and efferent arterioles. J Clin Invest 92: 1981-1985, 1993[ISI][Medline].

8.   Kersten, JR, Brooks LA, and Dellsperger KC. Impaired microvascular response to graded coronary occlusion in diabetic and hyperglycemic dogs. Am J Physiol Heart Circ Physiol 268: H1667-H1674, 1995[Abstract/Free Full Text].

9.   Le Roith, D. Insulin-like growth factors. N Engl J Med 336: 633-640, 1997[Free Full Text].

10.   McKay, MK, and Hester RL. Role of Nitric oxide, adenosine, and ATP-sensitive potassium channels in insulin-induced vasodilation. Hypertension 28: 202-208, 1996[Abstract/Free Full Text].

11.   Mimaki, Y, Kawasaki H, Okazaki M, Nakatsuma A, Araki H, and Gomita Y. Involvement of calcitonin gene-related peptide (CGRP) receptors in insulin-induced vasodilatation in mesenteric resistance blood vessels of rats. Br J Pharmacol 123: 1684-1690, 1998[Abstract].

12.   Nagao, T, Illiano S, and Vanhoutte PM. Heterogenous distribution of endothelium-dependent relaxations resistant to NG-nitro-L-arginine in rats. Am J Physiol Heart Circ Physiol 273: H1090-H1094, 1997[Abstract/Free Full Text].

13.   Oltman, CL, Gutterman DD, Scott EC, Bocker JM, and Dellsperger KC. Effects of glycosylated hemoglobin on vascular responses in vitro. Cardiovasc Res 34: 179-184, 1997[ISI][Medline].

14.   Oltman, CL, Weintraub NL, VanRollins M, and Dellsperger KC. Epoxyeicosatrienoinc acids and dihydroxyeicosatrienoic acids are potent vasodilators in the canine coronary microcirculation. Circ Res 83: 932-939, 1998[Abstract/Free Full Text].

15.   Pieper, GM, and Dondlinger L. Glucose elevations alter bradykinin-stimulated intracellular calcium accumulation in cultured endothelial cells. Cardiovasc Res 34: 169-178, 1997[ISI][Medline].

16.   Platts, SH, Mogford JE, Davis MJ, and Meininger GA. Role of K+ channels in arteriolar vasodilation mediated by integrin interaction with RGD-containing peptide. Am J Physiol Heart Circ Physiol 275: H1449-H1454, 1998[Abstract/Free Full Text].

17.   Scherrer, U, Randin D, Vollenweider P, Vollenweider L, and Nicod P. Nitric oxide release accounts for insulin's vascular effects in humans. J Clin Invest 94: 2511-2515, 1994[ISI][Medline].

18.   Schroeder, CA, Chen Y-L, and Messina EJ. Inhibition of NO synthesis or endothelium removal reveals a vasoconstrictor effect of insulin on isolated arterioles. Am J Physiol Heart Circ Physiol 276: H815-H820, 1999[Abstract/Free Full Text].

19.   Steinberg, HO, Brechtel G, Johnson A, Fineberg N, and Baron AD. Insulin-mediated skeletal muscle vasodilation is nitric oxide dependent. A novel action of insulin to increase nitric oxide release. J Clin Invest 94: 1172-1179, 1994[ISI][Medline].

20.   Tesfamariam, B, Brown ML, and Cohen RA. Elevated glucose impairs endothelium-dependent relaxation by activating protein kinase C. J Clin Invest 87: 1643-1648, 1991[ISI][Medline].

21.   Tesfamariam, B, Brown ML, and Cohen RA. Aldose reductase and myo-inositol in endothelial cell dysfunction caused by elevated glucose. J Pharmacol Exp Ther 263: 153-157, 1992[Abstract].

22.   VanVeen, S, and Chang PC. Prostaglandins and nitric oxide mediate insulin-induced vasodilation in the human forearm. Cardiovasc Res 34: 223-229, 1997[ISI][Medline].

23.   Walsh, MF, Barazi M, Pete G, Muniyappa R, Dunbar JC, and Sowers JR. Insulin-like growth factor I diminishes in vivo and in vitro vascular contractility: role of vascular nitric oxide. Endocrinology 137: 1798-1803, 1996[Abstract].

24.   Wu, YH, Jeng YY, Yue C, Chyu K-Y, Hsueh WA, and Chan TM. Endothelial-dependent vascular effects of insulin and insulin-like growth factor I in the perfused rat mesenteric artery and aortic ring. Diabetes 43: 1027-1032, 1994[Abstract].


Am J Physiol Endocrinol Metab 279(1):E176-E181