Evidence for two pathways of receptor-mediated Ca2+ entry in hepatocytes

Receptor-mediated Ca2+ entry was studied in fura-2-loaded isolated hepatocytes. Emptying of internal Ca2+ stores by treatment with either the Ca(2+)-mobilizing hormone vasopressin or the inhibitors of the microsomal Ca2+ pump, 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) or thapsigargin, stimulated Ca2+ entry, as indicated by a rise in the cytosolic free Ca2+ concentration after Ca2+ was added to cells suspended in nominally Ca(2+)-free medium. The enhancement of Ca2+ entry was proportional to the degree of depletion of the intracellular Ca2+ pool and occurred also after removal of vasopressin from its receptor. In contrast, the stimulation of Mn2+ entry by vasopressin required the continuous presence of the agonist, since it was prevented by the addition of vasopressin receptor antagonist. This effect was observed under conditions where refilling of the agonist-sensitive pool was prevented by using nominally Ca(2+)-free medium. Unlike vasopressin, tBuBHQ or thapsigargin did not stimulate Mn2+ entry. These results suggest the existence of two pathways for receptor-mediated Ca2+ entry in hepatocytes, a 'capacitative' pathway that is sensitive to the Ca2+ content in the Ins(1,4,5)P3-sensitive Ca2+ pool and does not allow Mn2+ entry, and a second pathway that depends on receptor occupation, seems to require a second messenger for activation, and permits influx of Mn2+.