The effects of endogenous diuretic and antidiuretic peptides and their second messengers in the Malpighian tubules of Tenebrio molitor: an electrophysiological study

Abstract

The Malpighian tubules of Tenebrio molitor provide a model system for interpreting the actions of endogenous diuretic and antidiuretic peptides. The effects of diuretic (Tenmo-DH37) and antidiuretic (Tenmo-ADFa) peptides and their respective second messengers (cyclic AMP and cyclic GMP) on basolateral (V-bl) and transepithelial (V-te) potentials of Tenebrio Malpighian tubules were determined using conventional microelectrodes. In the presence of 6 mmol l(-1) Ba2+, Tenmo-DH37 (100 nmol l(-1)) reversibly hyperpolarized V, and depolarized Y-te. A similar response was seen with the addition of I mmol l(-1) cyclic AMP; however, the apical membrane potential (V-ap) then showed a hyperpolarization, whereas a depolarization of Vap was observed with Tenmo-DH37Bafilomycin A(1) (5 mumol l(-1)) inhibited fluid secretion of stimulated tubules and reversed the hyperpolarization of V-bl in response to Tenmo-DH37, In response to 100 nmol l(-1) Tenmo-ADFa or 1 mmol l(-1) cyclic GMP, V-bl and V-te depolarized, although cyclic GMP affected membrane potentials somewhat differently by causing an initial hyperpolarization of V-bl and V-te. In high [K+-]-low [Na+] Ringer, 1 mmol l(-1) amiloride decreased fluid secretion rates, and depolarized both V-bl and V-te. Amiloride significantly decreased luminal pH in paired experiments, indicating the presence of a K+/nH(+) exchanger in tubule cells of Tenebrio. The results suggest that the endogenous factors and their second messengers stimulate/inhibit fluid secretion by acting on the apical V-ATPase, basolateral K+ transport, and possibly Cl- transport. (C) 2003 Elsevier Ltd. All rights reserved.