5-HYDROXYTRYPTAMINE REGULATES THE (NA-PROLIXUS - EVIDENCE FOR INVOLVEMENT OF G-PROTEIN AND CAMP-DEPENDENT PROTEIN-KINASE(+K+)ATPASE ACTIVITY IN MALPIGHIAN TUBULES OF RHODNIUS)

5-Hydroxytryptamine (5-HT, serotonin) acts as a diuretic hormone in Rh odnius prolixus, where it increases to 0.1 mu M in the haemolymph duri ng feeding and stimulates the fluid secretion in isolated Malpighian t ubules. The ouabain-sensitive (Na++K+)ATPase activity present in homog enates 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 competi tive inhibitor of G-protein GTPase activity. GTP gamma S, a nonhydroly sable analog of GTP, and cholera toxin, a G(s)-protein activator, also inhibit the ouabain-sensitive (Na++K+)ATPase activity, while pertussi s toxin, a G(s)-protein inhibitor, has no effect. The (Na++K+)ATPase a ctivity is inhibited 55% by 0.4-100 mu M dibutyryl-cAMP in the presenc e of IBMX, a phosphodiesterase inhibitor, which also potentiates the e ffect of a low concentration of 5-HT. The cAMP-dependent protein kinas e inhibitor peptide abolishes the 5-HT effect. These data suggest that the (Na++K+)ATPase activity in Malpighian tubules is inhibited by 5-H T through activation of G(s)-protein and a cAMP-dependent protein kina se. Inhibition of the Na++K+ pump would contribute to the diuretic eff ect of 5-HT. (C) 1997 Wiley-Liss, Inc.