RESUMO
Addition of histidine to the serosal bath of the toad bladder increases the hydrosmotic response of vasopressin in this tissue. Because this represents primarily the effect of the imidazole ring of histidine, which is a known inhibitor of the production of prostaglandins, we evaluated whether histidine increases the response to vasopressin through decreased prostaglandin production. Histidine increases the response to vasopressin much more than 10(-5) M naproxen, even though the latter was equipotent to histidine in reducing prostaglandin E2 (PGE2) production. Furthermore, histidine was additive to naproxen in increasing the hydrosmotic effect of vasopressin, without causing a further decrease in PGE2 production. These findings suggest that histidine has an effect over and above that due to inhibition of prostaglandin synthesis. Our results suggest that histidine enhances the permeability of the tissue beneath the luminal membrane, an effect not found with naproxen. We propose that histidine increases the hydrosmotic response to vasopressin through at least two distinct mechanisms: 1) it decreases prostaglandin synthesis and thus increases luminal permeability; 2) it decreases the resistance to water movement of the tissues beneath the luminal membrane.
