Evidence that 2-aminoethyl diphenylborate is a novel inhibitor of store-operated Ca2+ channels in liver cells, and acts through a mechanism which does not involve inositol trisphosphate receptors

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Abstract
The compound 2-aminoethyl diphenylborate (2-APB), an inhibitor of Ins(1,4,5)P(3) receptor action in some cell types, has been used to assess the role of Ins(1,4,5)P(3) receptors in the activation of store-operated Ca2+ channels (SOCs) [Ma, Patterson, van Rossum, Birnbaumer, Mikoshiba and Gill (2000) Science 287, 1647-1651]. In freshly-isolated rat hepatocytes, 2-APB inhibited thapsigargin- and vasopressin-stimulated Ca2+ inflow (measured using fura-2) with no detectable effect on the release of Ca2+ from intracellular stores. The concentration of 2-APB which gave half-maximal inhibition of Ca2+ inflow was approx. 10 microM. 2-APB also inhibited Ca2+ inflow initiated by a low concentration of adenophostin A but had no effect on maitotoxin-stimulated Ca2+ inflow through non-selective cation channels. The onset of the inhibitory effect of 2-APB on thapsigargin-stimulated Ca2+ inflow was rapid. When 2-APB was added to rat hepatocytes in the presence of extracellular Ca2+ after a vasopressin-induced plateau in the cytoplasmic free Ca2+ concentration ([Ca2+](cyt)) had been established, the kinetics of the decrease in [Ca2+](cyt) were identical with those induced by the addition of 50 microM Gd(3+) (gadolinium). 2-APB did not inhibit the release of Ca2+ from intracellular stores induced by the addition of Ins(1,4,5)P(3) to permeabilized hepatocytes. In the H4-IIE rat hepatoma cell line, 2-APB inhibited thapsigargin-stimulated Ca2+ inflow (measured using fura-2) and, in whole-cell patch-clamp experiments, the Ins(1,4,5)P(3)-induced inward current carried by Ca2+. It was concluded that, in liver cells, 2-APB inhibited SOCs through a mechanism which involved the binding of 2-APB to either the channel protein or an associated regulatory protein. 2-APB appeared to be a novel inhibitor of SOCs in liver cells with a mechanism of action which, in this cell type, is unlikely to involve an interaction of 2-APB with Ins(1,4,5)P(3) receptors. The need for caution in the use of 2-APB as a probe for the involvement of Ins(1,4,5)P(3) receptors in the activation of SOCs in other cell types is briefly discussed.