K+ transport in Malpighian tubules of Tenebrio molitor L.:: 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 1(-1) K+, 140 mmol 1(-1) Na+), V-bl was 24mV, cell negative, and V-ap was 48mV, 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 Yap being 60-77% that of Y-bl. A 10-fold drop in bath [K+] irreversibly decreased fluid secretion rates from 6.38+/-0.95 nl min(-1) (mean +/- S.E.M.) to 1.48 +/- 0.52 nl min-1 (N=8). In the presence of 6 mmol 1(-1) Ba2+, a blocker of basal K+ channels, fluid secretion rates reversibly decreased and the hyperpolarization of both V-bl and Yap seen in 50 mmol 1(-1) and 140 mmol 1(-1) K+ indicated a favourable electrochemical gradient for basal K+ entry. In 5 mmol 1(-1) K+, Ba2+ 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 mumol 1(-1) bumetanide reversibly decreased fluid secretion rates, with no detectable change in membrane potentials. Ouabair (1 mmol 1(-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 Ba2+. The results implicate K+ channels, the Na+/K+/2Cl(-) cotransporter and the Na+/K+-ATPase in basal K+ and fluid transport of Tenebrio tubule cells.