Contribution of endogenous oxytocin to sodium excretion in anaesthetized, surgically operated rats

In order to determine the possible role of endogenous oxytocin in controlli ng electrolyte and water excretion in animals whose renal function is being assessed by invasive techniques, rats were anaesthetized and subjected to micropuncture surgery. Clearance measurements were made in the presence and absence of the potent oxytocin receptor antagonist d(CH2)(5)[Tyr(Me)(2), T hr(4), Orn(8), Tyr(NH2)(9)]-vasotocin. ,In rats infused with vehicle alone, glomerular filtration rate (GFR), sodium excretion and urine flow rate rem ained stable. In contrast, in antagonist-treated rats GFR was modestly redu ced (P < 0.05), and there were large falls in both absolute and fractional sodium excretion (P < 0.01 in each case) and absolute and fractional water excretion (P < 0.05 in each case), indicating effects on both filtered load and fractional tubular reabsorption. The antinatriuresis was not accompani ed by a change in the fractional excretion of Lithium, suggesting that prox imal tubular function is unaffected by oxytocin receptor antagonism; nor wa s it accompanied by a change in the fractional excretion of potassium, sugg esting that the tubular effect:is located beyond the potassium secretory si te, i.e. downstream of the cortical collecting tubule. We conclude that cir culating plasma concentrations of oxytocin during anaesthesia and moderate surgery are sufficient to enhance GFR and reduce fractional tubular sodium and water reabsorption. This has important implications for the interpretat ion of invasive studies such as micropuncture.