K+ transport in Malpighian tubules of Tenebrio molitor L.: is a K-ATP channel involved?

Abstract

The presence of ATP-regulated K+ (K-ATP) channels in Tenebrio molitor Malpighian tubules was investigated by examining the effect of glibenclamide on both fluid secretion and basolateral membrane potentials (V-bl). Glibenclamide, a KATP channel blocker, slowed fluid secretion of Tenebrio tubules. In low bath K+ concentration (5 mmol 1(-1)), glibenclamide either hyperpolarized or depolarized V-bl, resembling the effect seen with Ba2+. Subsequent addition of 6 mmol 1(-1) Ba2+ caused a further hyper- or depolarization of V-bl. In control Ringer (50 mmol 1(-1) KCl, 90 mmol 1(-1) NaCl), glibenclamide had no visible effect on V-bl. The effect of ouabain was investigated in low bath [K+] in the presence of Ba2+. V-bl responded by a small but significant hyperpolarization from -51 +/- 4 mV to -56 +/- 4 mV (n=16, P<0.001) in response to 1 mmol 1(-1) ouabain. Repeating the experiments in the presence of both glibenclamide and Ba2+ resulted in a depolarization of V-bl when ouabain was added. In low bath [K+] (high Na+), the Na+/K+-ATPase is expected to function at a high rate. In the presence of Ba2+, replacing Na+ by K+ rapidly depolarized V-bl, but this was followed by a repolarization. Repeating the experiments in the presence of glibenclamide markedly reduced the depolarizing effect and abolished the repolarization, with a gradual decrease in the sensitivity of V-bl to the surrounding [K+]. These results suggest the presence of K-ATP channels in the basolateral membrane. Glibenclamide had no visible effect on V-bl in high K+ or in the absence of Ba2+, indicating that other highly conductive K+ channels may mask the effect on K-ATP channels. This is the first demonstration of the presence of K-ATP channels in an insect epithelium.