Collecting duct function in deoxycorticosterone acetate-escaped, normal, and salt-deprived rats. Response to hypervolemia.

The microcatheterization technique was used to study reabsorption of fluid, sodium, and potassium in the medullary collecting duct in chronically deoxycorticosterone acetate (DOCA)-treated and salt-loaded rats, as well as in normal and chronically salt-deprived (NaD) rats, before and after infusion of donor blood (33% of estimated circulating volume). Before expansion, urinary sodium excretion was highest in DOCA rats, intermediate in normal, and lowest in low salt rats. Significant collecting duct reabsorption was found in NaD, normal, and DOCA groups. In contrast to sodium, no net transport of potassium was found in any series. During intravascular expansion, increased renal excretion of fluid and sodium was observed uniformly in both DOCA and normal groups, whereas a diuretic response was found in five of seven rats, and a natriuretic response in four of seven rats of the NaD group. Natriuresis of DOCA rats was significantly greater than that of either normal or responding NaD rats. Diuresis and natriuresis in all three series were assocaited with complete inhibition of fluid and sodium reabsorption from the lumen of the medullary collecting duct, whereas such reabsorption persisted in nonresponding low salt rats. Increased sodium excretion in DOCA rats in comparison to the other two series could be explained by enhanced intratubular delivery of the ion to the medullary collecting system. I conclude that the renal response to acute blood volume expansion is due primarily to complete inhibition of both fluid and sodium reabsorption in the medullary collecting duct, but that differences in tubular delivery may modify the resulting diuresis and natriuresis.