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 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.
Wiehart
U I M
UI
Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa.
Nicolson
S W
SW
Van Kerkhove
E
E
D000818:Animals; D001464:Barium / Q000494:pharmacology; D001693:Biological Transport, Active; D002034:Bumetanide / Q000494:pharmacology; D004563:Electrochemistry; D017136:Ion Transport / Q000187:drug effects; D008317:Malpighian Tubules / Q000378:metabolism* / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D012413:Rubidium / Q000494:pharmacology; D013712:Tenebrio / Q000378:metabolism* / Q000502:physiology
D015221:Potassium Channels; D002034:Bumetanide; D001464:Barium; D010042:Ouabain; D012413:Rubidium; D011188:Potassium