A regulatory calcium-binding site for calcium channel in isolated rat hepatocytes

Loading isolated rat hepatocytes with high concentrations of the fluorescent Ca2+-chelator quin-2 in the absence of extracellular Ca2+ decreases by about 3-fold the cytosolic Ca2+ concentration ([Ca2+]i). In these low [Ca2+]i cells, the initial 45Ca2+ uptake rate, assumed to represent the Ca2+ influx, is stimulated to a level close to that promoted by maximal doses of vasopressin and angiotensin II in control cells. The subsequent addition of Ca2+ to the quin-2-loaded hepatocytes results in a rapid increase in [Ca2+]i and a return of Ca2+ influx towards the basal level usually observed in nonloaded cells. This indicates that the Ca2+ influx is dependent on [Ca2+]i but not on the quin-2 load itself. In the low [Ca2+]i cells, both the apparent Km and the apparent Vmax of the Ca2+ influx are increased as compared to the controls, indicating that the properties of the channels activated by lowering [Ca2+]i are apparently identical to those initiated by the hormones (Mauger, J.-P., Poggioli, J., Guesdon, F., and Claret, M. (1984) Biochem. J. 221, 121-127). It is proposed that in the isolated rat hepatocytes there is an inverse relationship between the Ca2+ influx and [Ca2+]i. Under resting conditions, [Ca2+]i might be high enough to partially inhibit the Ca2+ influx via a Ca2+ binding to an inhibitory site presumably located at the inner membrane surface. The role of the site in the hormonal action is discussed.