5-Hydroxytryptamine regulates the (Na++K+)ATPase activity in Malpighian tubules of Rhodnius prolixus: Evidence for involvement of G-protein and cAMP-dependent protein kinase

5-Hydroxytryptamine (5-HT, serotonin) acts as a diuretic hormone in Rhodnius prolixus, where it increases to 0.1 mu M in the haemolymph during feeding and stimulates the fluid secretion in isolated Malpighian tubules. The ouabain-sensitive (Na++K+)ATPase activity present in homogenates of Malpighian tubules from unfed Rhodnius prolixus is inhibited 60% by 0.01 mu M 5-HT. This inhibition is reversed by ketanserin, a 5-HT2 receptor antagonist in mammals, and also by GDP beta S, a competitive inhibitor of G-protein GTPase activity. GTP gamma S, a nonhydrolysable analog of GTP, and cholera toxin, a G(s)-protein activator, also inhibit the ouabain-sensitive (Na++K+)ATPase activity, while pertussis toxin, a G(s)-protein inhibitor, has no effect. The (Na++K+)ATPase activity is inhibited 55% by 0.4-100 mu M dibutyryl-cAMP in the presence of IBMX, a phosphodiesterase inhibitor, which also potentiates the effect of a low concentration of 5-HT. The cAMP-dependent protein kinase inhibitor peptide abolishes the 5-HT effect. These data suggest that the (Na++K+)ATPase activity in Malpighian tubules is inhibited by 5-HT through activation of G(s)-protein and a cAMP-dependent protein kinase. Inhibition of the Na++K+ pump would contribute to the diuretic effect of 5-HT. (C) 1997 Wiley-Liss, Inc.