Body sodium in rats: response to DOCA, adrenalectomy, changes in salt intake, and a salt load

Body Na was studied by an isotope method in rats on Na-free diet plus a choice of H2O and 0.5% or 0.1% NaCl. Two groups (1 on 0.5%, 1 on 0.1% NaCl) had Silastic deoxycorticosterone acetate (DOCA) implants, two similar groups were sham operated, and a fifth group (on 0.5% NaCl) underwent adrenalectomy (ADX). Saline consumption increased in DOCA-treated and ADX rats. Body Na was increased by DOCA and by drinking 0.5% NaCl compared with 0.1% NaCl. Body Na after intraperitoneal NaCl loading (which raised body Na 8-10%) and withdrawal of NaCl drinking fluids was analyzed by use of the model y = Ae-a(t-d) + Be-bt, where y is body Na at time t and d is delay before fast rate constant a is established; d was greater on the lower Na intake. Rate constant a was not reduced by chronic DOCA treatment. Coefficient B of the slow exponential, representing the basal level to which body Na falls on zero Na intake, and equivalent to Hollenberg's "set-point," was higher in DOCA-treated rats. This analysis makes use of Hollenberg's set-point concept, but the findings suggest that the set-point is related to mineralocorticoid activity and is thus presumably variable.