ELECTROPHYSIOLOGICAL EVIDENCE FOR THE PRESENCE OF AN APICAL H-ATPASE IN MALPIGHIAN TUBULES OF FORMICA-POLYCTENA - INTRACELLULAR AND LUMINALPH MEASUREMENTS()

Cellular and luminal pH of isolated ant Malpighian tubules were measur ed in different bath K+ concentrations using double-barrelled pH micro electrodes. The electrochemical gradient for H+ across the basolateral and the apical cell membranes was estimated. In control Ringer (51 mm ol/l K+) cell and luminal pH were alkaline with respect to the basolat eral solution: 7.77 and 7.36, respectively, versus 7.25. On lowering b asolateral 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 chang ed 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 cel l inwardly directed apical proton gradient; moreover, the apical membr ane 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 electr ogenic active proton pump at the apical membrane is facilitated in a h igh basolateral K+ concentration. The calculated electromotive force o f this pump is -159 mV. Comparing the proton with the K+ electrochemic al gradient, taken from another study in the same experimental conditi ons, we find that the apical proton electrochemical gradient can drive K+ extrusion into the lumen for each value of secretion rate.