Calcium homeostasis in mouse fibroblast cells: Affected by U-73122, a putative phospholipase C(β) blocker, via multiple mechanisms

1. The inhibitory effects of the putative phospholipase C(β) 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.