Calcium modulates the generation of inositol 1,3,4-trisphosphate in human platelets by the activation of inositol 1,4,5-trisphosphate 3-kinase.
We observed that more total inositol trisphosphate (InsP3) was formed when human platelets were stimulated with agonists (15-hydroxy-9,11-azo-prosta-5,13-dienoic acid or thrombin) in the presence of extracellular Ca2+ than in its absence. Analysis of the InsP3 by h.p.l.c. indicated that the increased InsP3 formed in the presence of extracellular Ca2+ was primarily the 1,3,4-trisphosphate [Ins(1,3,4)P3]. In addition, more inositol 1,3,4,5-tetrakisphosphate (InsP4) was formed in the presence of extracellular Ca2+. Experiments conducted with electrically permeabilized platelets demonstrated that conversion of [3H]Ins(1,4,5)P3 to [3H]InsP4 in platelets was Ca2+-dependent, with half-maximal conversion observed at approx. 2.5 microM-Ca2+. By contrast, dephosphorylation of [3H]InsP4 to [3H]Ins(1,3,4)P3 was not activated by Ca2+. A partially purified preparation of Ins(1,4,5)P3 3-kinase from human platelets was found to be insensitive to Ca2+, but addition of calmodulin restored Ca2+-sensitivity to the kinase, increasing its activity about 5-fold. These results show that in human platelets the metabolism of Ins(1,4,5)P3 is regulated by Ca2+-calmodulin, and suggest that the metabolites of Ins(1,4,5)P3 may also have important second-messenger functions in platelets, and are consistent with the hypothesis that the activation of phospholipase C is not dependent on extracellular Ca2+.
Daniel
J L
JL
Temple University Medical School, Pharmacology Department, Philadelphia, PA 19140.
Dangelmaier
C A
CA
Smith
J B
JB
D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000097:blood*; D010770:Phosphotransferases / Q000097:blood*; D017853:Phosphotransferases (Alcohol Group Acceptor); D011463:Prostaglandins H / Q000494:pharmacology; D013403:Sugar Phosphates / Q000097:blood*; D013917:Thrombin / Q000494:pharmacology
D007295:Inositol Phosphates; D011463:Prostaglandins H; D013403:Sugar Phosphates; C081655:inositol-1,3,4,5-tetrakisphosphate; C013824:azo analog II; D015544:Inositol 1,4,5-Trisphosphate; D010770:Phosphotransferases; D017853:Phosphotransferases (Alcohol Group Acceptor); C051038:Inositol 1,4,5-trisphosphate 3-kinase; D013917:Thrombin; D002118:Calcium