Dopaminergic inhibition of vasopressin-stimulated water flow in the toad bladder: Evidence for local formation of dopamine

Abstract

Dopamine administration increases renal excretion of water and Na. It remains uncertain whether these effects of dopamine are the result of a hemodynamic effect or the consequence of a direct cellular action. We investigated the effect of dopamine on water transport by the isolated toad bladder in vitro. Dopamine failed to alter baseline water flow but caused a significant inhibition of arginine vasopressin (AVP) or cyclic adenosine monophosphate (AMP) stimulated water flow. The effect of dopamine on stimulated water flow was not due to activation of c~ adrenergic, /~ adrenergic, or cholinergic receptors. The selective antagonists of dopamine, metoclopramide and apomorphine, prevented the effect of dopamine on AVP-stimulated water flow. These observations suggest the existence of a dopaminergic receptor in the toad bladder. L-Dopa also inhibited AVP-stimulated water flow. The effect of L-Dopa could be prevented by metoclopramide, thus suggesting that L-Dopa is converted to dopamine by an aromatic amino acid decarboxylase present in the toad bladder. To investigate this possibility we measured the effect of the decarboxylase inhibitor, carbidopa, on the 14C02 production generated by decarboxylation of 14C L-Dopa in isolated toad bladder epithelial cells. Isolated toad bladder epithelial cells generated significant amounts of 14C0z from 14C L-Dopa. This effect could be blocked by carbidopa, thus suggesting the existence of an aromatic amino acid decarboxylase system in the toad bladder. Carbidopa also prevented the inhibitory effect of L-Dopa on AVP-stimnlated water flow, suggesting that L-Dopa needs to be converted to dopamine to inhibit water flow. These data suggest the existence of a dopaminergic receptor in the toad bladder. These data also suggest that dopamine can be formed locally in the toad bladder and can thus serve as a local modulator of water transport.

Effect of dopamine antagonists on the urine flow of rats infused with hypotonic saline

“…At this level 'Author for correspondence. dopamine seems to antagonize the hormone effect on water permeability (Arruda et al, 1982;Muto et al, 1985). It appeared, therefore, of interest to investigate whether or not dopamine antagonists would affect the water diuresis following hypotonic saline infusion.…”