Heterogeneity of cAMP effect on endosomal proton transport

Abstract

The effects of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) on proton transport in endocytotic vesicles (i.e., endosomes) obtained from rabbit renal cortex were investigated. Endosomes were prepared from New Zealand White rabbits following the intravenous injection of horseradish peroxidase. Under high (106 mM) chloride conditions, 5 microM 8-BrcAMP stimulated proton transport by 11 and 12% in fractions 1 and 2, but inhibited proton transport by 10, 8, and 14% in fractions 4, 5, and 6. Two hundred and fifty micromolar 8-BrcAMP inhibited proton transport progressively from 20% in fraction 1 to 50% in fraction 6. Under conditions in which chloride entry was minimized, i.e., low (6 mM) external chloride concentration plus voltage clamping, 5 microM 8-BrcAMP consistently and significantly inhibited proton transport by 40-60% in all six fractions. Under the same low chloride conditions, 250 microM 8-BrcAMP inhibited proton transport by 80-90% in all six fractions. These studies indicate that 8-BrcAMP can modify endosomal transport in a heterogeneous manner, that 8-BrcAMP inhibits the H+-ATPase directly, and that this effect is significantly modified by chloride. Last, these studies suggest that the observed heterogeneity in proton transport arises from interaction of 8-BrcAMP and chloride.