Renal responses to oxytocin during the 4 days of the oestrous cycle in the rat.

Abstract

Oxytocin was administered to virgin female rats at doses of 25-200 pmol/min during 0.077 mol NaCl/l infusion at 150 microliters/min on each day of the oestrous cycle. The resultant rates of urine flow, glomerular filtration (GFR) and electrolyte excretion were determined. Oxytocin caused significant increases in urine flow (P < 0.001) and sodium excretion (P < 0.001); both responses being dose-dependent (P < 0.02 and P < 0.01 respectively). Significant variations in the renal responsiveness to the hormone occurred over the 4 days of the oestrous cycle. On oestrus the lowest dose of 25 pmol oxytocin/min produced a significant increase in urine flow (from 139.5 +/- 4.3 to 165.6 +/- 7.1 microliters/min, P < 0.005) and a dose of 50 pmol/min produced a significant increase in sodium excretion (from 10.6 +/- 0.1 to 14.5 +/- 0.7 mumol/min, P < 0.005). Significant increases in urine flow and sodium excretion were seen on pro-oestrus with hormone administration rates of 50 and 100 pmol/min respectively and on dioestrus day 2 with a rate of 100 pmol/min. On dioestrus day 1 no increase in urine flow or sodium excretion was seen over the dose range of oxytocin administration. A dose of 100 pmol oxytocin/min significantly increased GFR on pro-oestrus and dioestrus day 2, but not on the other 2 days of the cycle. The circulating hormone concentrations produced by oxytocin infusion were similar on each day of the cycle and so could not account for the differences seen. Therefore, these results suggest varying renal responsiveness to oxytocin during the reproductive cycle of the female rat.