Mechanisms of K + uptake across the basal membrane of Malpighian tubules of Formica polyctena: the effects of ions and inhibitors

In the presence of 6 mmol l21 Ba2+, known to block the K+ channels in the basal membrane, a rise in bath [K+] ([K+]bl) induced an increase in intracellular K+ concentration ([K+]i) similar in amount and in time course to that obtained in the absence of Ba2+. The presence of active and passive (other than through K+ channels) K+ uptake mechanisms across the basal membrane was investigated in different bath K+ concentrations. Dihydro-ouabain (1023 mol l21), a blocker of the Na+/K+-ATPase, tested in low bath [K+], and Sch28080 (1024 mol l21), a K+/H+-ATPase inhibitor, were without effect on fluid secretion. Dihydro-ouabain was also without effect on electrical potential differences either in the absence or in the presence of Ba2+. Vanadate (1023 mol l21), in contrast, strongly reduced fluid secretion not only in control solution but also in high-K+, Na+-free medium and reduced the transepithelial and the apical membrane potential differences but not the basal membrane potential difference or [K+]i. Omitting Na+ from the bathing medium, replacing Cl2 by Br2 or applying bumetanide (1025 mol l21) inhibited fluid secretion only in a low-K+ (10 mmol l21) medium. In 51 mmol l21 [K+]bl, omitting Na+ was without effect and 1024 mol l21 bumetanide was needed to inhibi