Receptors linked to polyphosphoinositide hydrolysis stimulate Ca2+ extrusion by a phospholipase C-independent mechanism

In A7r5 cells with empty intracellular Ca2+ stores in which the cytosolic free Ca2+ concentration ([Ca2+](i)) had been increased by capacitative Ca2+ entry, stimulation of receptors linked to phospholipase C (PLC), including those for Arg(8)-vasopressin (AVP) and platelet-derived growth factor (PDGF), caused a decrease in [Ca2+](i). This effect was further examined in a stable variant of the A7r5 cell line in which the usual ability of hormones to stimulate non-capacitative Ca2+ entry is not expressed. In thapsigargin-treated cells, neither AVP nor PDGF affected capacitative Mn2+ or Ba2+ entry, but both stimulated the rate of Ca2+ extrusion, and their abilities to decrease [Ca2+](i) were only partially inhibited by removal of extracellular Nai. These results suggest that receptors linked to PLC also stimulate plasma membrane Ca2+ pumps. Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu, 1 mu M) also caused a decrease in [Ca2+](i) by accelerating Ca2+ removal from the cytosol; the effect was again only partially inhibited by removal of extra-cellular Na+. An inhibitor of PKC, Ro31-8220 (10 mu M), abolished the ability of PDBu to decrease [Ca2+]i, without affecting the response to maximal or submaximal concentrations of AVP. Similar experiments with PDGF were impracticable because Ro31-8220, presumably by inhibiting the tyrosine kinase activity of the PDGF receptor, abolished all responses to PDGF. U73122 (10 mu M)- an inhibitor of PLC, completely inhibited PDGF- or AVP-evoked Ca2+ mobilization, without preventing either stimulus from causing a decrease in [Ca2+](i). We conclude that receptors coupled to PLC, whether via G-proteins or protein tyrosine kinase activity, also share an ability to stimulate the plasma membrane Ca2+ pump via a mechanism that does not require PLC activity.