Electrophysiological evidence for the presence of an apical H(+)-ATPase in Malpighian tubules of Formica polyctena: intracellular and luminal pH measurements.
Cellular and luminal pH of isolated ant Malpighian tubules were measured in different bath K+ concentrations using double-barrelled pH microelectrodes. The electrochemical gradient for H+ across the basolateral and the apical cell membranes was estimated. In control Ringer (51 mmol/l K+) cell and luminal pH were alkaline with respect to the basolateral solution: 7.77 and 7.36, respectively, versus 7.25. On lowering basolateral K+ concentration to 5 mmol/l or increasing it to 113 mmol/l, luminal pH and to a lesser extent cell pH followed: luminal pH changed to 7.14 and 7.43 and cell pH to 7.69 and 7.82, respectively. In all conditions a cell inward electrochemical gradient for protons across both membranes was observed. Increasing basolateral K+ concentration, which was positively correlated with secretion rate, decreased the cell inwardly directed apical proton gradient; moreover, the apical membrane potential difference decreased as well, from -93 mV in 5 mmol/l K+ to -65 mV in 113 mmol/l K+. Therefore the turnover rate of the electrogenic active proton pump at the apical membrane is facilitated in a high basolateral K+ concentration. The calculated electromotive force of this pump is -159 mV. Comparing the proton with the K+ electrochemical gradient, taken from another study in the same experimental conditions, we find that the apical proton electrochemical gradient can drive K+ extrusion into the lumen for each value of secretion rate.
Zhang
S L
SL
Department Medische Basiswetenschappen, Limburgs Universitair Centrum, Diepenbeek, Belgium.
Leyssens
A
A
Van Kerkhove
E
E
Weltens
R
R
Van Driessche
W
W
Steels
P
P
D000818:Animals; D017920:Antiporters / Q000378:metabolism; D001000:Ants / Q000201:enzymology*; D002462:Cell Membrane / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004563:Electrochemistry; D004594:Electrophysiology; D006863:Hydrogen-Ion Concentration; D008317:Malpighian Tubules / Q000201:enzymology*; D008564:Membrane Potentials / Q000502:physiology; D008839:Microelectrodes; D011188:Potassium / Q000378:metabolism; D027961:Potassium-Hydrogen Antiporters; D006180:Proton-Translocating ATPases / Q000378:metabolism* / Q000502:physiology
D017920:Antiporters; D027961:Potassium-Hydrogen Antiporters; D006180:Proton-Translocating ATPases; D011188:Potassium