Abstract
The possibility that diacylglycerol analogues might have a wider spectrum of intracellular targets than the well-known protein kinase C was investigated with vesicles containing the Ca2+-ATPase derived from the dense tubular system in platelets and from the sarcoplasmic reticulum of skeletal muscle. The diacylglycerol analogues PMA and 1-oleoyl-2-acetyl-rac-glycerol (OAG) inhibited Ca2+ accumulation by these vesicles, an effect that was antagonized by cyclosporin A. The inhibitory activity of PMA and OAG resulted from the uncoupling of the Ca2+-ATPase, characterized by a pronounced inhibition of Ca2+ uptake accompanied by a discrete decrease in ATPase activity and by the inhibition of the enzyme's phosphorylation by Pi, leading to both a decrease in ATP synthesis and an enhancement of Ca2+ efflux. The inhibition of Ca2+ uptake by PMA was found to decrease as the Ca2+ concentration in the medium was raised from 0.1 to 10.0 microM. This was observed with muscle, but not with platelet vesicles. In contrast, the ability of cyclosporin A to antagonize the inhibition of Ca2+ uptake by PMA also increased when the Ca2+ concentration in the medium was raised from 0.1 to 10.0 microM, but this was observed with both muscle and platelet vesicles. The fact that phospholipase C activity and products from the inositol metabolism have been described as localized in regions of the sarcoplasmic reticulum where Ca2+-ATPase and Ca2+ channels are found suggests a possible physiological role for these products in the regulation of cytosolic Ca2+ levels.