Stimulatory effect of calcitonin on Ca2+ inflow in isolated rat hepatocytes

The effects of synthetic [Asu1,7]eel calcitonin (CT) on the unidirectional inflow of Ca2+ were investigated in isolated rat liver cells by measuring the initial rate of 45Ca2+ uptake. CT increased Ca2+ inflow, with EC50 values (concentrations giving half-maximal effect) of 10(-10) M. The action of CT was in evidence within 15 s after the addition of 45Ca2+ to the cells. CT-activated Ca2+ inflow was completely blocked by the presence of the Ca2(+)-antagonist verapamil at a concentration of 10(-8) M. Meanwhile, epinephrine (10(-8) to 10(-4) M) or phenylephrine (10(-8) to 10(-4) M) increased Ca2+ inflow within 60 s after the addition of 45Ca2+ to the cells. Those hormonal effects were additively enhanced by the presence of CT (10(-8) M). Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, increased Ca2+ inflow at a concentration of 10(-9) to 10(-5) M. The presence of CT (10(-8) M) synergistically enhanced PMA-increased Ca2+ inflow at concentrations of 10(-7) to 10(-5) M. The present results suggest that CT can stimulate the rate of Ca2+ inflow in rat liver cells.