A PRIMARY CATION-TRANSPORT BY A V-TYPE ATPASE OF LOW SPECIFICITY

The enzyme involved in outward K+ transport in insect epithelia belong s to the family of V-ATPases. Evidence has been reported relating the generation of the K+ gradient to a primary electrogenic proton transpo rt via a distinct electrophoretic nH(+)/K+ antiport. The subject of th is paper is the transport of K+ at a thread hair sensillum of the cock roach in situ, We recorded changes in the voltage and resistance of th e ion-transporting membrane and of shifts in pH caused by inhibition o f energy metabolism and by putative inhibitors of a proton/cation exch anger. The results are supplemented by previous determinations of the K+ activities in the same preparation. 1. In cockroach hair sensilla, the ion transport generates a membrane voltage of 105 mV. We found tha t the transport rendered the positive output compartment alkaline,vith respect to the cytoplasm by 1.0 pH unit compared with the pH at equil ibrium distribution, and we infer that proton transport cannot be the process that energizes the generation of the K+ gradient. 2. The ion t ransport created an electrochemical potential difference for protons, Delta(eta H), of approximately 4.5 kJ mol(-1), while the potential dif ference for K+, Delta(eta K) amounted to approximately 11 kJ mol(-1). Both potential differences are directed to the cytosol. It follows fro m Delta(eta K)/Delta(eta H) that an antiport would have to be electrop horetic to drive K+ by Delta(eta H) and it should, therefore, contribu te to the membrane conductance, Amiloride and harmaline did not signif icantly change the pH in the adjacent spaces and did not affect the vo ltage or the resistance of the transporting membrane, Previous determi nations of the impedance have shown that the ATP-independent conductan ce of this membrane is small, supporting the conclusion that it lacks an electrophoretic antiport. From these results, we deduce that K+ tra nsport in cockroach sensilla is not secondary to a proton transport an d an electrochemical proton gradient, The phenomena observed match the performance of a primary, electrogenic, cation-translocating ATPase o f the type deduced from analyses of the short-circuit current at the m idgut epithelium of lepidopteran larvae. The validity of the H+ transp ort/antiport hypothesis is discussed.