CALCIUM HOMEOSTASIS IN MOUSE FIBROBLAST CELLS - AFFECTED BY U-73122, A PUTATIVE PHOSPHOLIPASE C-BETA BLOCKER, VIA MULTIPLE MECHANISMS

1 The inhibitory effects of the putative phospholipase C-beta inhibitor, U-73122, on ligand-induced and thapsigargin-induced [Ca2+](i) transients were investigated in mouse fibroblast cells (the L line). 2 Ca2+ release from intracellular stores was stimulated either by ATP (and also by UTP or ADP) working through the activation of a P-2U receptor, or by lysophosphatidic acid, which elicited a more pronounced response. 3 U-73122 inhibited the Ca2+ mobilization produced by all the agonists in a dose-dependent manner, consistent with a mode of action involving phospholipase C inhibition. 4 In addition, however, U-73122 slowed the kinetics of intracellular Ca2+ release induced by the Ca2+-ATPase inhibitor, thapsigargin, and reduced the influx of Ca2+ across the plasma membrane, following stimulation of store-dependent influx by the latter. 5 We conclude that U-73122 has multiple sites of action, all of which can lead to a change in Ca2+ homeostasis. Thus, particular caution is recommended when employing this agent and when interpreting the results obtained.