INTRACELLULAR CA2- ROLE IN INACTIVATION OF NA-CA+K EXCHANGE( SEQUESTRATION AND RELEASE IN INTACT BOVINE RETINAL ROD OUTER SEGMENTS )

Intracellular Ca2+ sequestration and Ca2+ release was analyzed in inta ct rod outer segments (ROS) purified from bovine retinas. Ca2+ influx in Ca2+-depleted and fully bleached ROS was mediated exclusively by th e Na-Ca+K exchanger and was measured both as a rise in cytosolic free Ca2+ with fluo 3 and as a total transmembrane Ca2+ flux with Ca-45. Ca 2+ fluxes across the ROS plasma membrane were not completely reversibl e, in small part due to inactivation of the Ca2+ extrusion mode of the Na-Ca+K exchanger but mostly due to sequestration of cytosolic Ca2+ i nto the intradiskal space. Ca2+ release from the intradiskal space int o the cytosol could be induced by low concentrations of the Ca2+ ionop hore A23187 which gave rise to increases in cytosolic free Ca2+ by sev eral hundred nanomolar (low A23187 concentrations did not affect the C a2+ permeability of the ROS plasma membrane). We have used intracellul ar Ca2+ release induced by A23187 to examine inactivation of the rapid Ca2+ efflux mode mediated by the plasma membrane Na-Ca+K exchanger. I nactivation of Na-Ca+K exchange appeared to be dependent on intradiska l Ca2+ as it was abolished by selective Ca2+ permeabilization of the d isk membrane by A23187. We discuss possible physiological roles for Ca 2+ sequestration and release from ROS disks.