Dopamine decreases fluid reabsorption in straight portions of rabbit proximal tubule.

The effects of DA and DA blockers on fluid transport, transepithelial potential difference (PD), and Na fluxes were studied in straight portions of the rabbit proximal tubule by the technique of microperfusion in vitro. DA (10(-6) M) added to the bath (rabbit serum) produced a significant decrease in fluid reabsorption (Jv, nl.min-2.mm-1) from 0.53 +/- 0.05 to 0.19 +/- 0.06 (n = 7, P less than 0.005). PD decreased from -2.7 +/- 0.5 to -1.2 +/- 0.4 mV (P less than 0.01). Drugs active on DA receptors were used to characterize the action of dopamine. Haloperidol (10(-8) M), lisuride (1.5 X 10(-9) M), and metoclopramide (10(-7) M) did not modify Jv by themselves but prevented the action of dopamine. Agonists of DA, Epinine and A 6,7 DTN, produced smaller or shorter decreases in Jv. A 6,7 DTN was active in the presence of alpha- and beta-adrenergic blockers. The serotonin antagonist methysergide decreases Jv by itself by 15%; in its presence DA was not effective. The effect of DA was accompanied by reductions of net Na flux and unidirectional lumen-to-bath and bath-to-lumen Na fluxes by approximately 25%. It is concluded that DA exerts a direct inhibitory effect on fluid transport by the pars recta. The evidence for the existence of DA receptor sites is not conclusive. The mechanism of action is uncertain, but an impairment of transcellular salt transport can be suggested.