1 University of Nottingham, Division of Animal Physiology, School of Biosciences, Sutton Bonington Campus, Loughborough LE12 5RD, 2 University of Edinburgh, Department of Veterinary Clinical Studies, Easter Bush, Midlothian and 3 University of Glasgow, Department of Chemistry, Glasgow G12 8QQ, UK
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Abstract |
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Key words: endothermy/follicle/hydration/microcalorimetry/temperature
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Introduction |
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These observations are counter-intuitive: all tissues within an enclosed, thermally-conductive environment such as the abdomen would be expected to be of a similar temperature, with any local variations produced by transiently different rates of metabolism equilibrating rapidly. The existence of significant hypothermy, in particular tissues or regions of tissue, therefore requires a specific mechanistic explanation. Two possible mechanisms are apparent: counter-current heat exchange and biochemically-generated endothermy (Hunter et al., 1997). The counter-current hypothesis, though an attractive explanation for the regional heterothermy of the oviduct (David et al., 1972
; Hunter and Nichol, 1986
) or for temperature gradients between ovary and other viscera (Einer-Jensen, 1988
), lacks sufficient physiological evidence when applied to the individual follicle. Fundamentally, it would require the presence of an anatomically discrete heat sink within close range of the ovary, but no such site is apparent. Furthermore, although follicles have an external network of fine capillaries and lymphatics (Andersen, 1926
), it is difficult to envisage how the peri-follicular microcirculation could selectively maintain a significant local coolant effect on an avascular body of fluid whose volume may be very large (several millilitres in the maturing follicles of human and domestic species).
The aim of the present study was to examine the possibility that follicular hypothermy is biochemically generated. Our working hypothesis was that follicular fluid contains a component(s) which reacts endothermically to hydration. It is envisaged that such a component would be secreted continually within the follicle as part of the growth process, undergoing hydration by water derived from the circulation. We have tested this hypothesis in follicular fluids from four species, using two different laboratory approaches, and observed measurable enthalpic responses to aqueous dilution. We have also attempted to model mathematically the heat balance of a cool follicle so as to establish whether the measured amounts of enthalpy could account for the observed follicular phenomenon.
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Materials and methods |
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Bovine and porcine fluids were obtained from slaughterhouse material. Ovaries with medium to large healthy follicles but without active corpora lutea were selected according to standard visual criteria (Luck et al., 2000). Fluid was aspirated with a syringe and 25 G needle and either used immediately or stored frozen (20°C). Bovine follicles were treated individually but, for reasons of limited volume, pig samples consisted of the pooled fluids from all large follicles on individual ovaries. Size details are given in Results.
Equine follicular fluid was obtained from two horse mares undergoing oocyte recovery prior to gamete intra-Fallopian tube transfer (GIFT). Follicular fluid was aspirated during oestrus from two pre-ovulatory follicles (2935 mm diameter) in each mare. The follicles were accessed by flank laporotomy after administration of appropriate sedation and analgesia. The fluid was stored at 20°C.
Incubator studies
A sample of follicular fluid (500 µl) was placed in a central well of a 24 well polystyrene culture plate (Nunc, Denmark), held on the middle shelf of a humidified water-jacketed, fan-circulating laboratory incubator (size one; Gallenkamp, London, UK) maintained at 37 ± 0.3°C. The sample well was covered with a plastic test tube cap into which two holes had been bored. One hole facilitated the positioning of a fine copper-constantan thermistor junction (10 mV/°C; precision ± 0.05°C) within the fluid sample; the thermistor wire passed out of the incubator at the door seal and was attached via an amplifier to a pen recorder. The other hole allowed the suspension of a fine-bore plastic tube (internal radius 0.785 mm) just above the surface of the sample. This tube was filled with saline (0.9% w/v) and connected to a Hamilton syringe held outside the incubator. The length of tube inside the incubator was at least 0.5 m. The sample was stirred continuously by means of a short (~2 mm) home-made glass-covered stirring bar, maintained in motion by a water-driven magnetic stirrer positioned about 5 cm below the sample chamber. (Electrically driven stirrers were found to cause an unacceptable sine wave distortion of the thermistor trace.) The stirrer was driven by a warm (37 ± 1°C) water supply whose pipes entered and exited the incubator through a lagged access point some distance from the sample chamber.
All experiments were preceded by a long (at least 1 h) equilibration period during which both the indicated temperature of the incubator and the record trace produced by the thermistor were stable. Experiments were initiated by briefly opening the outer incubator door (the inner glass door remaining sealed) and causing a 50 µl aliquot of saline to drop into the sample by means of the externally held Hamilton syringe. The outer door was then kept shut for the remainder of the data collection period. Preliminary runs and control experiments (samples of saline instead of follicular fluid) demonstrated that this procedure caused no detectable change in incubator temperature or deviation in the thermistor trace. Prior to each day's experiments, the thermistor trace was calibrated over the 3835°C range against a mercury thermometer held in a beaker of water. The pen deflection was 22 mm/°C and occurred as a virtually instantaneous response.
Microcalorimeter studies
Absolute heats of dilution were determined at 25°C using sensitive isothermal titration calorimeters [Microcal Omega and VP-ITC systems; Microcal Inc., Northampton, MA, USA; (Wiseman et al. 1989). These computer-controlled instruments consist of an adiabatic chamber containing sample and reference cells, the former having a combined micro-injector and stirrer. Programmed aliquots of diluent are injected with constant stirring into the sample at 3 min time intervals. Any temperature difference between the sample and reference cells is constantly monitored and automatically corrected by internal Peltier feedback heaters to the appropriate cell. The enthalpy of the dilution reaction is recorded as the amount of heat adjustment required to maintain a zero temperature difference between the cells.
Follicular fluid samples for these experiments were prepared by overnight dialysis (Slide-A-Lyzer cassette®; Pierce, Rockford, IL, USA; Mr cut-off 3500) against phosphate buffered saline (PBS) (0.1 mol/l, pH 7.4) at 4°C. Preliminary experiments had shown that undialysed samples produced a large, artefactual exothermic response to PBS, probably due to salt equilibration. Experiments were performed on a sample volume of 1.4 ml (the volume of the ITC sample chamber), with repeated injections of 10 µl aliquots of the spent dialysis medium.
A number of human follicular fluid samples and a single bovine sample were subjected to additional treatment before ITC analysis. Half of each dialysed sample was concentrated by placing in a Slide-A-Lyzer® cassette and covering with Dialysorb® gel (Sigma, Poole, Dorset, UK) for 30 min. This reduced the volume of the sample by between 30 and 50%. A quantity of spent dialysis medium was treated in the same way for use as a control.
The specific heat of bovine follicular fluid was estimated using a Microcal® VP-DSC scanning microcalorimeter (Plotnikov et al., 1997).
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Results |
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Mathematical modelling
The thermal conditions of a growing follicle can be modelled using simple equations in order to determine the rates of enthalpy which would be required to produce a hypothermia of the magnitude observed in vivo. A cool follicle growing within a warm tissue environment will gain heat in two ways: (i) as a result of fluid accretion (assuming that the fluid taken up during growth is at ambient circulatory temperature), and (ii) by conduction from surrounding stromal tissue. The former will occur at the rate of increase in follicle volume. The latter will occur at a rate predicted by Newton's law (proportional to the temperature gradient) and will increase with the surface area of the growing follicle. The absence of capillary circulation within the follicle means that the follicular surface is the only site of heat entry and that heat can move within the antral fluid only by convection or conduction. The bulk of the avascular cellular components of an antral follicle (granulosa layer) are located just beneath its surface membrane; thus any metabolic energy produced by these cells can be conveniently included in the excess heat of the surrounding stromal tissue. The energy production of the cells of the cumulusoocyte complex is taken to be negligible. In the absence of any other metabolic effects, the temperature at the centre of the body of follicular fluid cannot be greater than that of any other region of fluid or that of the follicle surface.
The rate of heat entry (in Js1) by water accretion,
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The rate of heat entry by conduction,
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An initial simplification can be achieved by comparing the magnitudes of Qc and Qw. Given that
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A representative bovine pre-ovulatory follicle of 15 mm diameter grows at a rate of 5 mm diameter d1 (Savio et al., 1988) which is equivalent to a radial increase of 0.03 µm s1 or a volume increase of 0.03 µl s1. Taking the specific heats of water and follicular fluid to be identical, equation (3
) gives a value of 1.05x105 for the magnitude ratio of the heat sources. Thus, heat entry by fluid accretion is quantitatively negligible and can be ignored. Evaluating Qc at an instantaneous follicle radius of 7.5 mm (1.8 ml volume), in the presence of a 2°K temperature gradient, gives a rate of heat uptake by conduction of 17.7 J.s1. This result can be compared with the dilution-induced heat uptake detected in incubator and microcalorimetric experiments, as follows.
In the incubator studies, increasing the volume of the 500 µl fluid sample by 50 µl produced an average fall in temperature of 0.11°C, sustained for an average of 935 s (bovine values). This represents a total heat absorption of 0.23 J (sample volumexSwxtemperature difference). In the microcalorimeter studies, each injection of 10 µl into 1.4 ml gave instantaneous endothermic heat pulses approaching 100 µJ per injection. Using a heat capacity of 4.2 J degK1 ml1, this corresponds to a temperature decrease of 1.7x105°C per 10 µl injection. By proportion, the corresponding temperature change for injection of 50 µl into 500 µl would be approximately 2.4x104°C, which is significantly less than in the incubator experiments. The sustained nature of the incubator effect allows a rough estimation of a rate of heat uptake: assuming that heat was transferred to the cooling sample by conduction from the incubator air and that this took place predominantly at the open surface of the culture plate well (area 2 cm2), an adaptation of equation (2) provides a value of 0.24 J/s1. This is equivalent to a total energy uptake of 224 J over 935 s. (For convenience, this calculation assumes as previously that the heat transfer surface is a wet membrane of 50 µm thickness. Note also that by ignoring any heat transferred to the sample through the plastic walls of the plate well, the rate of energy absorption will tend to be underestimated). This value for heat transfer rate approaches an order of magnitude similar to that calculated for the representative follicle, given that the temperature gradient in the incubator was lower than that assumed to occur in vivo and that the sample volume was smaller.
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Discussion |
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Overall, the incubator method produced either a fall in temperature (half the human and bovine samples) or no response (porcine samples); exothermic effects were never seen. The format of the response profiles (a lag followed by sustained suppression of temperature), together with the lack of response in controls, indicates that the effects were responses to dilution and not procedural artefacts of the relatively crude equipment (for example, due to opening the external incubator door at the time of saline injection). In contrast, using the more sensitive microcalorimeter, all porcine and most bovine and samples exhibited endothermy. Equine and human samples showed both types of response.
The two types of experimental approach produced different types of response. The incubator studies, which used undialysed samples diluted by about 11% with saline, typically produced a delayed reduction in temperature which could be maintained for up to 25 min against the thermal gradient imposed by the circulating incubator air. In contrast, the microcalorimeter method, using dialysed samples hydrated with spent dialysis medium, produced near instantaneous falls in temperature (rapidly equilibrated by the machine) in response to repeated volume changes of about 0.6%. Thus, although both methods produced evidence of negative enthalpy, differences in response profile and magnitude suggest that more than one kind of biochemical response is being observed. This is also illustrated by the porcine samples, which showed no response in the incubator but uniform endothermy in the microcalorimeter. It is possible that the relative insensitivity of the incubator method hid more rapid, transient changes in heat balance occurring immediately after addition of diluent, or that the different basal temperatures of the two experimental approaches revealed different characteristics of the mechanism. Further experiments are needed to resolve these possibilities.
The bovine and porcine samples came from animals whose reproductive cycle stage was unknown, whereas the human and equine samples were from follicles in the final stages of stimulated (human) or natural (equine) maturity. All follicles were large enough to be considered as being either in a phase of active volumetric growth or at a stage of immediate pre-ovulatory maturity (when fluid accretion probably ceases and follicles exhibit flaccidity; Lenz, 1985; Espey and Lipner, 1994
; Hanna et al., 1994
; Fisch et al., 1996
; Kerban et al., 1999
). Using our hypothesis that hydration-induced endothermy is a growth-associated phenomenon, unrecorded differences in the exact stages of maturity may well explain the presence or absence of an endothermic response amongst the follicles tested. These conclusions are supported by the response to Dialysorb® treatment of the human and bovine fluids. The effect of post-dialysis volume reduction was either to amplify existing endothermy or to reverse the previous exothermy. This is consistent with there being a saturable hydration-dependent effect which is masked either by the completion of volumetric follicle growth under the IVF stimulation protocol, or by the experimental requirement for pre-calorimetric dialysis. We hypothesize that concentrating the fluid with Dialysorb® reverses the previous hydrational equilibrium of the sample, allowing a greater endothermic response to be seen.
The biochemical basis for the effect(s) we have observed remains to be elucidated. The follicular membrane is quite `leaky' to fluids and electrolytes and little or no hydrostatic or osmotic pressure can be detected between sampled fluid and blood (Gosden et al., 1988; Salustri et al., 1999
). We must therefore conclude that the molecules responsible are large and, in the case of the microcalorimetrically-detectable effect, larger than 3000 Mr. On this basis, the effects we have seen represent a residual amount of passive hydration (incubator experiments) or a forced over-hydration of the fluid (microcalorimeter experiments: samples previously equilibrated by dialysis). The viscosity of follicular fluid, though positionally and temporally variable (Kerban et al., 1999
), does not appear to change as a simple function of growth (Luck et al., 2000
; human, bovine, porcine), suggesting that high molecular weight components are secreted continuously as the follicle expands. The present data are consistent with the hypothesis that a large, hydratable molecule, such as a mucopolysaccharide (Zachariae, 1959
), is secreted within the follicle as part of the growth process and that this undergoes a rapid hydration with endothermic consequences. The continuous secretion of such a material during follicle growth would result in a constant uptake of heat energy from the surrounding tissue. Secretion at an increasing rate, to keep pace with the increasing rate of volume expansion associated with maturation (van Wezel and Rodgers, 1996
; Irving-Rodgers and Rodgers, 2000
), would be consistent with the greater temperature depression of larger follicles (Hunter et al., 1997
, 2000
).
Although the nature of the molecules responsible for this phenomenon have yet to be determined, it is known that the hydration of protein/proteoglycan complexes, as for example secreted by tracheal cells (Dodd et al., 1998) or in the cornea (Doughty, 2001), takes several seconds or minutes to complete. Such phenomena are multifactorial (Hatakeyama and Hatakeyama, 1998
) but the time course of the incubator responses in the present study (including the presence of a significant lag period) suggests that similar molecules may be involved.
In conclusion, we have evidence that follicular fluid from humans and domestic species is capable of responding endothermically to dilution. The mechanism(s) of the response is unknown but may provide a basis for understanding the hypothermy of large follicles observed in vivo. We propose that follicle growth involves the continuous secretion of a large molecule whose hydration results in the endothermic cooling of the fluid.
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Acknowledgements |
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Notes |
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References |
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Submitted on May 18, 2001; accepted on August 31, 2001.