Abstract
The plasma membrane Ca
2+ carrier system involved in receptor-mediated Ca
2+ entry was studied. Using the Ca
2+ readdition protocol, the rate of cytosolic free Ca
2+ concentration ([Ca
2+]
i) increase in vasopressin-pretreated hepatocytes was significantly higher than in thapsigargin- or 2,5-di(
tert-butyl)hydroquinone-pretreated cells. The addition of Mn
2+ to unstimulated hepatocytes resulted in a biphasic quench of fura-2 fluorescence. After an initial phase that was fast in rate but of short duration, the rate of fura-2 quench by Mn
2+ became much slower and lasted until all the cellular fura-2 was quenched. Pretreatment of the cells with vasopressin only accelerated the rate of the latter phase but not of the initial one. In agonist-stimulated cells, acidification of the extracellular medium or the presence of ruthenium red, econazole or SK&F 96365 decreased the rates of both [Ca
2+]
i increase and Mn
2+ entry upon addition of the respective cation. By contrast, neomycin and
N-
tosyl-
l-phenylalanine
chloromethyl ketone markedly decreased the rate of [Ca
2+]
i increase upon Ca
2+ readdition but had no effect on vasopressin-stimulated Mn
2+ entry. None of the treatments affected the ability of vasopressin and thapsigargin to mobilize the internal Ca
2+ store. It is concluded that in hepatocytes the two pathways of receptor-mediated Ca
2+ entry control two distinct yet pharmacologically related cation carriers.
