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

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 (Vbl) and transepithelial (Vte) 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 Vbl and depolarized Vte. A similar response was seen with the addition of 1 mmol l−1 cyclic AMP; however, the apical membrane potential (Vap) then showed a hyperpolarization, whereas a depolarization of Vap was observed with Tenmo-DH37. Bafilomycin A1 (5 μmol l−1) inhibited fluid secretion of stimulated tubules and reversed the hyperpolarization of Vbl in response to Tenmo-DH37. In response to 100 nmol l−1 Tenmo-ADFa or 1 mmol l−1 cyclic GMP, Vbl and Vte depolarized, although cyclic GMP affected membrane potentials somewhat differently by causing an initial hyperpolarization of Vbl and Vte. In high [K+]–low [Na+] Ringer, 1 mmol l−1 amiloride decreased fluid secretion rates, and depolarized both Vbl and Vte. 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.