Concerted action of cytosolic Ca2+ and protein kinase C in receptor-mediated phospholipase D activation in Chinese hamster ovary cells expressing the cholecystokinin-A receptor

Receptor-mediated activation of phosphatidylcholine phatidohydrolase or phospholipase D (PLD) was studied in Chinese hamster ovary (CHO) cells expressing the cholecystokinin-A (CCK-A) receptor. Cells were labelled with [H-3]myristic acid for 24 h and PLD-catalysed [H-3]phosphatidylethanol formation was measured in the presence of 1 degrees(degrees), (v/v) ethanol. Cholecystokinin-(26- 33)-peptide amide (CCK8) increased PLD activity both time- and dose-dependently. Maximal activation of protein kinase C (PKC) with 1 mu M PMA or sustained elevation of the cytosolic free Ca2+ concentration ([Ca2+](i)) with 1 mu M thapsigargin increased PLD activity to 50% and 70%, of the: maximal value obtained with CCK8 respectively. The stimulatory effects of CCK8, PMA and thapsigargin were abolished in cells in which PKC was downregulated or inhibited by chelerythrine. PMA/Ca2+-stimulated PLD activity was absent in a homogenate of PKC-downregulated cells but could be restored upon addition of purified rat brain PKC. CCK8-induced PLD activation was inhibited by 90% in the absence of external Ca2+, demonstrating that receptor-mediated activation of PKC in itself does not significantly add to PLD activation but requires a sustained increase in [Ca2+](i). Taken together, the results presented demonstrate that, in CHO-CCK-A cells, receptor-mediated PLD activation is completely dependent on PKC, but that the extent to which PLD becomes activated depends largely, if not entirely, on the magnitude and duration of the agonist-induced increase in [Ca2+](i).