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.
D000818:Animals; D001667:Binding, Competitive; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000097:blood / Q000494:pharmacology; D002147:Calmodulin / Q000037:antagonists & inhibitors*; D002462:Cell Membrane / Q000378:metabolism; D002746:Chlorpromazine / Q000494:pharmacology; D002794:Choline / Q000097:blood; D004983:Ethanolamines / Q000097:blood; D010743:Phospholipids / Q000097:blood*; D011817:Rabbits; D012694:Serine / Q000097:blood; D013449:Sulfonamides / Q000494:pharmacology; D014268:Trifluoperazine / Q000494:pharmacology
D002147:Calmodulin; D004983:Ethanolamines; D010743:Phospholipids; D013449:Sulfonamides; D014268:Trifluoperazine; D012694:Serine; C017967:W 7; C034700:N-(6-aminohexyl)-1-naphthalenesulfonamide; D002794:Choline; D002118:Calcium; D002746:Chlorpromazine