The increase in follicular diameter during maturation is accompanied by an increase in vascular supply. The oxygen tension (P(O2) in the follicular fluid of the growing follicle should therefore give an insight whether the oxygen delivery during this pertinent process is sufficient. Follicular fluid of 20 patients subjected to in vitro fertilization and embryo transfer were sampled in heparinized glass capillaries which were placed between the puncturing needle and the collecting reservoir. All patients were treated with human menopausal gonadotropin (HMG) and follicle-stimulating hormone (FSH). The P(O2), P(CO2) and pH of the follicular fluid were measured with an automatic gas analyser (Ciba Corning). Simultaneously, the same parameters were estimated in maternal capillary blood taken from the arterialized well perfused earlobe. Follicular size was measured by ultrasound. With growing follicular size (FS), the oxygen partial pressure (P(O2)) fell from 80 to 50-60 mmHg (P(O2) = 114.4-3.3 . FS; r = 0.79) while the carbon-dioxide partial pressure (P(CO2)) in the follicle increased from 35 to 50 mmHg (P(CO2) = 15.1 +/- 1.9 . FS; r = 0.71). In correlation to these results, the pH in follicular fluid fell from 7.40 to 7.30 (pH = 7.6 - 0.019 . FS; r = 0.62). The correlation between the P(CO2) and the pH demonstrates a deviation from the regression line if no metabolic alterations take place (DELTA-log P(CO2)/DELTA-pH = -0.69). The increase in ovarian blood flow during follicular maturation is obviously not sufficient to bridge the growing diffusion distance between the capillary network surrounding the follicle and its center. It is speculated that the falling P(O2) could be the trigger impulse for the generation of biochemical steps, e.g., the liberation of prostaglandins within the follicular fluid and/or the increase in activity of proteolytic enzymes directed to the lysis of the follicular wall.
