Phorbol ester-induced phosphorylation of a transmembrane glycoprotein (GP 180) in human blood platelets

Protein kinase C stimulates dense tubular Ca2+ uptake in the intact human platelet by increasing the Vm of the Ca(2+)-ATPase pump: stimulation by phorbol ester, inhibition by calphostin C

The effects of protein kinase C (PKC) on Ca2+ transport were investigated in human intact platelets. The indicator quin2 was used to measure the free cytoplasmic Ca2+ concentration ([Ca2+]cyt) and to search for possible PKC effects on the Ca(2+)-ATPase extrusion pump located in the plasma membrane. The Ca2+ indicator chlorotetracycline (CTC) was used to study PKC effects on the dense tubular Ca(2+)-ATPase uptake pump. The activity of PKC was stimulated by phorbol 12-myristate 13-acetate (PMA) and was inhibited with calphostin C. Neither PKC activation nor inhibition had any effect on [Ca2+]cyt or the Ca2+ extrusion pump. Substantial activation of the dense tubular pump was observed with PMA. In resting platelets bathed in 2 mM external Ca2+ giving [Ca2+]cyt = 102-106 nM, activation of PKC by PMA (100 nM) increases the rate and extent of dense tubular Ca2+ uptake to 1.62 +/- 0.35 and 1.25 +/- 0.3 times control value (respectively). The Vm of the dense tubular pump was measured by using ionomycin to manipulate [Ca2+]cyt. It is shown that PMA increases the Vm by a factor of 1.7 +/- 0.4 but has no effect on the Km value (= 180 nM). An unexpected finding was that PKC activity supports a portion of the basal activity of the dense tubular Ca2+ pump in resting platelets. Preincubation with the inhibitor calphostin C (100 nM) decreases the rate and extent of dense tubular Ca2+ uptake in resting platelets by 38 +/- 5% and 29 +/- 21% (respectively). This is due to a 28 +/- 9% decrease in the Vm of the dense tubular pump. This suggests that there is a low level of stimulation of dense tubular Ca2+ pump mediated by PKC in resting platelets.

The diacylglycerol kinase inhibitor, R59949, potentiates secretion but not increased phosphorylation of a 47 kDalton protein in human platelets

We have investigated the action of a novel inhibitor of DG-kinase, R59949. This agent was found to produce partial inhibition of formation of phosphatidic acid in human platelets challenged with thrombin, DC8 or OAG. However, this effect was not associated with enhanced phosphorylation of a 47 kDa protein, a known substrate for protein kinase C. We therefore believe that this compound does not represent a major advance on its earlier prototype, R59022.