The relationship between plasma renin activity and renal electrolyte excretion in the fetal sheep

In eight fetal sheep the factors influencing renal sodium excretion were determined and related to the activity of the renin-angiotensin system. There was a direct relationship between glomerular filtration rate (GFR) and the amount of sodium reabsorbed (P less than 0.001) suggesting that glomerular tubular balance plays an important role in regulation of fetal sodium excretion. There was also a direct relationship between GFR and urinary sodium excretion (P less than 0.05). Since the percentage of the filtered sodium load that was reabsorbed was inversely related to the urinary Na/K ratio, (P less 0.001), the distal convoluted tubule is actively involved in the regulation of renal sodium excretion. There was no relationship between plasma renin activity and plasma sodium, and plasma osmolality or mean arterial pressure. There was however an inverse relationship between urinary sodium excretion and plasma renin activity, similar to that seen in adult animals (Vander & Miller, 1964) and between log plasma renin activity and GFR (P less than 0.001). These experiments do not distinguish between a possible effect of angiotensin on GFR and hence renal sodium excretion, and the possible effects of a variable GFR on distal tubular sodium delivery and hence renin secretion. However they do show that the fetal renin-angiotensin system like the adult renin-angiotensin system is closely linked to those renal factors that influence renal sodium excretion.