RENAL RESPONSES TO OXYTOCIN DURING THE 4 DAYS OF THE ESTROUS-CYCLE INTHE RAT

Oxytocin was administered to virgin female rats at doses of 25-200 pmo l/min during 0.077 mol NaCl/l infusion at 150 mu l/min on each day of the oestrous cycle. The resultant rates of urine flow, glomerular filt ration (GFR) and electrolyte excretion were determined. Oxytocin cause d significant increases in urine flow (P<0.001) and sodium excretion ( P<0.001); bath responses being dose-dependent (P<0.02 and P<0.01 respe ctively). Significant variations in the renal responsiveness to the ho rmone occurred over the 4 days of the oestrous cycle. On oestrus the l owest dose of 25 pmol oxytocin/min produced a significant increase in urine flow (from 139.5 +/- 4.3 to 165.6 +/- 7.1 mu l/min, P<0.005) and a dose of 50 pmol/min produced a significant increase in sodium excre tion (from 10.6 +/- 0.1 to 14.5 +/- 0.7 mu mol/min, P<0.005). Signific ant increases in urine flow and sodium excretion were seen on pro-oest rus with hormone administration rates of 50 and 100 pmol/nin respectiv ely and on dioestrus day 2 with a rate of 100 pmol/min. On dioestrus d ay 1 no increase in urine flow or sodium excretion was seen over the d ose range of oxytocin administration. A dose of 100 pmol oxytocin/min significantly increased GFR on pro-oestrus and dioestrus day 2, but no t on the other 2 days of the cycle. The circulating hormone concentrat ions produced by oxytocin infusion were similar on each day of the cyc le and so could not account for the differences seen. Therefore, these results suggest varying renal responsiveness to oxytocin during the r eproductive cycle of the female rat.