Electrolyte Metabolism During the Oestrous Cycle in the Rat

Abstract

In the present study renal electrolyte function and adrenal cortical activity were determined during the rat oestrous cycle, and in ovariectomised animals given oestradiol and progesterone therapy. Initial studies showed a variation in the sodium content of uterus; kidney, liver, plasma. heart and skeletal muscle during the oestrous cycle, with higher levels of sodium at oestrus than at dioestrus. The water and potassium content of the tissues examined showed a less marked variation during oestrous cycle. The variation in tissue electrolytes during the oestrous cycle was paralleled by a variation in renal electrolyte function. Urine flow, urinary sodium concentration and excretion rate were significantly lower at oestrus, when ovarian hormones are raised, than at dioestrus. Urinary potassium concentration and excretion rates were significantly higher at oestrus than at dioestrus. No significant variation in glomerular filtration rate was observed during the oestrous cycle. Spironolactone blocked the increased sodium and water retention and potassium excretion observed at oestrus, but had little effect on renal electrolyte function in dioestrous or ovariectomised animals. Spironolactone has been shown to inhibit the action of mineralocorticoids on the distal tubules of the kidney. This, therefore, suggested that the adrenal cortex was involved in the variation in renal electrolyte function observed during the oestrous cycle, with an increase in adrenal activity at oestrus when ovarian hormones are also raised. In order to investigate further the role of ovarian hormones in the observed variation in renal electrolyte function during the oestrous cycle, oestradiol and progesterone were administered to ovariectomised rats. Prolonged administration of oestradiol, progesterone and oestradiol + progesterone all significantly reduced urine flow, urinary sodium concentration and excretion rate and increased potassium excretion, but had no effect on glomerular filtration rate in ovariectomised animals. Spironolactone administration blocked the action of ovarian hormone administration on renal electrolyte function, again implicating adrenal cortical action. A single, physiological dose of oestradiol resulted in a progressive increase in sodium and water retention and potassium excretion 5 to 8 hours after hormone administration. Spironolactone was found to block the action of oestradiol on renal function. Twelve hours after oestradiol administration renal electrolyte function was not significantly different to that observed in control ovariectomised animals. In contrast to the effect of oestradiol administration on renal electrolyte function, it was found that administration of physiological doses of progesterone had little effect on renal electrolyte function compared with control ovariectomised rats. The evidence obtained from investigation of renal electrolyte function during the oestrous cycle suggested a variation in adrenal activity at this time. A significant variation in aldosterone and corticosterone secretion rates were found during the rat oestrous cycle, the highest secretion rates being observed at pro-oestrus when oestrogen levels were also raised. It is suggested, therefore, that the action of ovarian hormones on renal electrolyte function is a result of an initial stimulation of the adrenal cortex by the ovarian hormones, primarily oestrogens.