Follicular fluid responds endothermically to aqueous dilution

Abstract

BACKGROUND: Studies suggest that ovarian follicles are cooler than their surrounding tissues. The mechanism of this remarkable phenomenon is unclear. We postulate that endothermic reactions accompany the growth-associated hydration of follicular fluid. METHODS: We performed two types of experiment, using human and animal follicular fluids. In the first, saline (50 microl) was injected into follicular fluid (500 microl) held in an equilibrated incubator, with monitoring of sample temperature. In the second, an adiabatic microcalorimeter recorded thermal shifts after injection of buffer (10 microl) into previously dialysed samples (1.4 ml). The relevance of changes observed was assessed by mathematical modelling. RESULTS: In the incubator study, 9/17 bovine and 6/12 human fluids showed a temperature fall (0.05-0.2 degrees C). Cooling was delayed by up to 2 min but sustained for 7-25 min. Remaining fluids showed no change. In the microcalorimeter, 4/9 human, 4/6 bovine, 5/5 porcine and 1/4 equine samples showed an endothermic response. Remaining samples showed either no response (bovine) or exothermy (human, equine). Pre-concentration of human follicular fluid amplified the endothermy or reversed the exothermy. Modelling indicated that the incubator-type response was of appropriate magnitude to explain follicular hypothermy. CONCLUSION: Follicular fluid responds endothermically to aqueous dilution and may contribute to follicular cooling during growth.