K+ transport in Malpighian tubules ofTenebrio molitorL.: a study of electrochemical gradients and basal K+uptake mechanisms

Malpighian tubules of the mealworm Tenebrio molitor were isolated for intracellular measurement of basolateral (V(bl)) and, indirectly, apical (V(ap)) membrane potentials. In control Ringer (50 mmol l(-1) K(+), 140 mmol l(-1) Na(+)), V(bl) was 24 mV, cell negative, and V(ap) was 48 mV, cell negative with reference to the lumen. Ion substitution experiments involving K(+) and Na(+) indicated that both V(bl) and V(ap) were sensitive to the bathing K(+) concentration, with the change in V(ap) being 60-77% that of V(bl). A 10-fold drop in bath [K(+)] irreversibly decreased fluid secretion rates from 6.38+/-0.95 nl x min(-1) (mean +/- S.E.M.) to 1.48+/-0.52 nl x min(-1) (N=8). In the presence of 6 mmol l(-1) Ba(2+), a blocker of basal K(+) channels, fluid secretion rates reversibly decreased and the hyperpolarization of both V(bl) and V(ap) seen in 50 mmol l(-1) and 140 mmol l(-1) K(+) indicated a favourable electrochemical gradient for basal K(+) entry. In 5 mmol l(-1) K(+), Ba(2+) induced two different responses: V(bl) either hyperpolarized by approximately 10 mV or depolarised by approximately 14 mV, according to the electrochemical gradient for K(+), which was either inward or outward in low bath [K(+)]. Rubidium, a 'permeant' potassium substitute, caused a hyperpolarization of V(bl), indicating the specificity of K(+) channels found in Tenebrio tubule cells. Other possible K(+) uptake mechanisms located in the basolateral membrane were investigated. Blocking of the putative electroneutral Na(+)/K(+)/2Cl(-) cotransporter by 10 micromol l(-1) bumetanide reversibly decreased fluid secretion rates, with no detectable change in membrane potentials. Ouabain (1 mmol l(-1)), an Na(+)/K(+)-ATPase inhibitor, irreversibly decreased fluid secretion rates but had no effect on electrical potential differences either in the absence or presence of Ba(2+). The results implicate K(+) channels, the Na(+)/K(+)/2Cl(-) contransporter and the Na(+)/K(+)-ATPase in basal K(+) and fluid transport of Tenebrio tubule cells.