AVP AND ALDOSTERONE AT PHYSIOLOGICAL CONCENTRATIONS HAVE SYNERGISTIC EFFECTS ON NA+ TRANSPORT IN RAT CCD

These studies examined whether the previously reported synergism betwe en pharmacological doses of deoxycorticosterone (DOG) and arginine vas opressin (AVP) also occur at physiological concentrations of aldostero ne and AVP. We examined the dose-response of salt transport, as measur ed by the lumen-to-bath Na-22(+) flux (J(1-->b)) and transepithelial v oltage (V-T), and of osmotic water permeability (P-f), to AVP in isola ted perfused cortical collecting ducts (CCDs) from three groups of rat s: (1) implanted with a 1 mg d-aldosterone pellet, which produced a mo derately elevated, but physiologically relevant, plasma aldosterone co ncentration of 18.4 +/- 2.6 ng/dl: (7) implanted with a 2.5 mg DOC pel let (a high pharmacological dose); and (3) untreated rats. P-f reached the same maximal value in all three groups, and the concentration of AVP producing one-half the maximal P-f response (K-0.5) was not signif icantly different among the three groups, ranging from 5 to 10 pM. The re was a significantly greater increase in J(1-->b) and hyperpolarizat ion of V-T with increasing AVP in both groups of treated rats than in the untreated group. The maximum values of J(1-->) and V-T achieved at high AVP concentrations were not significantly different in CCDs from the aldosterone-treated and DOC-treated groups, but they were signifi cantly higher than in the CCDs from untreated rats. Although maximal V -T values achieved with DOC and aldosterone treatment were the same, t he AVP K-0.5 for V-T values were significantly lower in the DOC-treate d than in the aldosterone-treated group. Although not statistically si gnificant, the same trend was observed for J(1-->b). We conclude that AVP and aldosterone synergistically stimulate Na+ reabsorption at phys iological concentrations of birth hormones: however, V-T (and probably Na+ reabsorption, which is generally proportional to V-T) reaches max imum values at lower AVP concentrations when pharmacological doses of DOC are employed.