Abstract 1971 Poster Session II, Sunday, 5/2 (poster 183)

The mechanisms involved in the attenuated inhibition of renal tubular sodium transport by dopamine, via D1 receptors, in the young were determined by examining D1 receptor and Na-H exchanger-3 (NHE-3) interaction during development in rats. Basal NHE activity (amiloride sensitive 22Na uptake, nmol 22Na/mg protein/min) in renal brush border membrane vesicles (BBMV) was least in 2 wk (1.32±0.24, n=3) compared with 4 wk (3.38±0.41, n=6) and 12 wk (2.82±0.38, n=6) old rats (P<0.05, ANOVA, Scheffe's test). D1 agonist (SKF81297, 5µM) inhibition of NHE activity in BBMV was also least in 2 wk (-1±9%, n=3) compared with 4 wk (-15±5%, n=6) and 12 wk (-65±4%, n=6) old rats (P<0.05, ANOVA, Scheffe's test). The decreased D1 effect is not due to any age-related differences in D1 receptor or NHE expression in BBM or proximal tubules (PT) but may be due to decreased generation of second messengers, e.g. cAMP. Indeed, we have reported that D1 receptor-mediated cAMP production increases with age (Am J Physiol 264: R726, 1993). However, D1 receptors can regulate NHE-3 independently of cAMP. Thus, G-αS, by itself, can mediate D1 receptor inhibition of NHE-3 activity (Am J Physiol 264: F1032, 1993). However, G-αS and G-αi3 protein expression in BBM or PT did not change with age. Moreover, inhibition of G-αi activity by pertussis toxin did not enhance the ability of D1 agonist to inhibit NHE activity in any age group. In the adult rat, G-β can increase NHE-3 activity and antagonize the inhibitory action of G-αS. Interestingly, G-β protein expression in BBM and PT was greatest in 2 wk old rats and decreased with age (2 wk=18±1 density units[DU], 4 wk=15±2DU, adult=11±2DU, n=5/group, P<0.05, ANOVA, Scheffe's test). To determine the influence of G-β on NHE activity, G-βcommon antibodies were introduced inside BBMV and NHE activity measured. In adult rats, the inhibition of NHE activity by SKF81297, 5µM (-67±4%, n=5) was slightly increased by G-βcommon antibodies (-75±4%, n=5). In contrast, in 4 wk old rats, the inhibition of NHE activity by SKF81297, 5µM (-20%, n=2) was markedly increased by G-βcommon antibodies (-75%, n=2). We conclude that the decreased inhibitory effect of D1 receptors on NHE activity in renal proximal tubules is due to decreased D1 receptor-mediated generation of second messengers as well as increased expression and activity of the G protein subunit, G-β. The modulation of NHE activity by G protein subunits provides another level of regulation during development.