| Abstract: | 5-Hydroxytryptamine (5-HT, serotonin) acts as a diuretic hormone in Rhodnius prolixus, where it increases to 0.1 μM 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 μM 5-HT. This inhibition is reversed by ketanserin, a 5-HT2 receptor antagonist in mammals, and also by GDPβS, a competitive inhibitor of G-protein GTPase activity. GTPγS, a nonhydrolysable analog of GTP, and cholera toxin, a Gs-protein activator, also inhibit the ouabain-sensitive (Na++K+)ATPase activity, while pertussis toxin, a Gi-protein inhibitor, has no effect. The (Na++K+)ATPase activity is inhibited 55% by 0.4–100 μM 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 Gs-protein and a cAMP-dependent protein kinase. Inhibition of the Na++K+ pump would contribute to the diuretic effect of 5-HT. Arch. Insect Biochem. Physiol. 36:203–214, 1997. © 1997 Wiley- Liss, Inc. |
