The present study examines the effect of dopamine DA1-receptor agonists on the renal proximal tubular Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity and quantitates DA1 receptors and the coupled G proteins in Fischer 344 model of adult (6 mo old) and old (23 mo old) rats. Dopamine and the preferential DA1-receptor agonist, SKF-38393, produced a concentration-dependent inhibition of Na(+)-K(+)-ATPase activity in proximal tubules from adult rats, whereas the enzyme activity was unaffected by these agonists in the old rats. The binding of DA1-receptor antagonist [3H]Sch-23390 in the proximal tubular basolateral membranes showed a marked decrease (approximately 47%) in the receptor numbers in old compared with adult rats, whereas dissociation constant (Kd) values in old compared with adult rats were not significantly different. Dopamine and SKF-38393 stimulated 35S-labeled guanosine 5'-O-(3-thiotriphosphate) binding in adult rats, but there was no significant effect on the binding in the old rats. Quantification of G2 alpha and Gq/11 alpha using Western analysis revealed a significant increase in quantities of both the G proteins in old rats. The data suggest that a reduction in DA1 receptor number and subsequently reduced G protein activation may be the causative factors for the impairment in DA1 receptor-mediated inhibition of Na(+)-K(+)-ATPase activity in the proximal tubules of old rats.