INDIRECT COUPLING OF CALCIUM-TRANSPORT IN CHROMAFFIN GRANULE GHOSTS TO THE PROTON PUMP

IN chromaffin granule ghosts, the Na+/Ca2+ exchanger in the granule me mbrane can provide a high affinity (K(m) 1-3 muM) and high capacity (V (max) 50-100 nmol mg min-2) mechanism for Ca2+ accumulation. The activ ity of the Na+/H+ antiporter can be used to couple Ca2+ uptake via Na/Ca2+ exchange to ATP-dependent proton translocation via the granule m embrane H+-ATPase. Therefore, Ca2+ uptake can be indirectly linked to the proton pump. However, under conditions designed to mimic the envir onment of a granule in the cytosol of a chromaffin cell, measured rate s of Ca2+ uptake are low, a free Ca2+ concentration of about 5 muM in the ghost matrix being attained. Under such circumstances, the granule s seem unlikely to play a major role in calcium homeostasis in the int act cell.