Calcium-dependent regulation of phospholipid methylation in rabbit platelets

Stimulatory effect of calmodulin antagonists on phospholipid base-exchange reactions in rabbit platelet membranes

The properties of Ca2+-dependent incorporation of [3H]serine, [3H]ethanolamine and [3H]choline into the corresponding phospholipids mediated by base-exchange enzymes in rabbit platelet membranes were studied in the presence or absence of the calmodulin antagonists chlorpromazine, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), all of which markedly activate three base-exchange reactions. The base-exchange activities were dependent on Ca2+ both in the presence and absence of the drugs. Other metal ions tested did not stimulate the base-exchange reactions, even in the presence of the drugs. Apparent Km values for serine, ethanolamine and choline were not affected significantly by the concentration of Ca2+, with or without the drugs. [3H]Serine incorporation into phospholipid was competitively inhibited by ethanolamine and choline, [3H]choline incorporation was competitively inhibited by serine and ethanolamine, whereas [3H]ethanolamine incorporation was competitively inhibited by serine and noncompetitively by choline. These competitive and noncompetitive relations between each base were also not affected by the drugs. The amount of 45Ca2+ binding to platelet membranes was decreased by the drugs dose dependently. A weaker calmodulin antagonist, N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5), only slightly stimulated the base-exchange reactions, but did clearly inhibit 45Ca2+ binding to the membranes, in the same manner as that of the other calmodulin antagonists used. The concentration of chlorpromazine, trifluoperazine, W-7 and W-5, required to produce half-maximal inhibition of Ca2+ binding, was approximately 30 microM. These results suggest that the calmodulin antagonists used activate the base-exchange reactions only in the presence of Ca2+ without changing the affinity of each free base to base-exchange enzymes. The activation of the base-exchange reactions was not due to the increase in free Ca2+ caused by the drug-induced inhibition of Ca2+ binding to platelet membranes.

Effects of calmodulin antagonists and calmodulin on phospholipid base-exchange activities in rabbit platelets

Effects of various calmodulin antagonists and calmodulin on the incorporation of serine, ethanolamine and choline into the corresponding phospholipids, such as phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine by Ca2+-stimulated base-exchange reactions in rabbit platelet membranes were studied. Under a Ca2+-EGTA buffer system, the incorporation of three bases were stimulated by Ca2+ in a biphasic manner. Minimum requirement of free Ca2+ for the reactions was found to be around 0.5 microM and maximal incorporation took place at high Ca2+ concentrations (3-5 mM). Various calmodulin antagonists such as chlorpromazine, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, not only activated the three reactions but also greatly enhanced their sensitivity to Ca2+ (K0.5, 0.1-0.3 microM). In the absence of Ca2+, however, the drugs did not show any effect on the reactions. The concentrations of the drugs required for half maximal stimulation were approx. 30-40 microM. Although platelet membranes contained endogenous calmodulin (0.3-0.6 microgram/mg of membrane protein), the addition of exogenous calmodulin inhibited choline exchange activity but had no or little effect on serine or ethanolamine exchange activity. The results suggest that in the presence of low Ca2+ concentrations, these drugs markedly stimulate base-exchange activities, and choline exchange activity may be regulated by calmodulin.