The role of cytoplasmic free calcium in the responses of quin2-loaded human platelets to vasopressin.
Responses to vasopressin were studied in human platelets loaded with the fluorescent Ca2+ indicator, quin2. In the presence of 1 mM external Ca2+, vasopressin caused a transient rise in [Ca2+]i from the basal level near 100nM to about 700 nM; peak [Ca2+]i was reached in a few seconds and the level then declined towards resting over several minutes. In the absence of external Ca2+ there was a much smaller rise of similar time-course, suggesting that vasopressin increases [Ca2+]i mainly by stimulated-influx across the plasma membrane but also by partly releasing internal Ca2+. Inhibition of thromboxane A2 formation somewhat reduced the peak [Ca2+]i in the presence of external Ca2+, but had no effect on the response attributed to release of internal Ca2+. With external Ca2+, vasopressin stimulated shape-change, secretion and aggregation. Secretion and aggregation were decreased by about half following blockage of thromboxane production. The ability of vasopressin to induce shape-change and secretion even at near basal [Ca2+]i suggests that activators other than Ca2+ are involved.
D000634:Aminoquinolines; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D016861:Cyclooxygenase Inhibitors; D006801:Humans; D008297:Male; D010974:Platelet Aggregation / Q000187:drug effects; D011451:Prostaglandin-Endoperoxide Synthases / Q000097:blood; D014667:Vasopressins / Q000494:pharmacology*
D000634:Aminoquinolines; D016861:Cyclooxygenase Inhibitors; D014667:Vasopressins; C035738:Quin2-acetoxymethyl ester; D011451:Prostaglandin-Endoperoxide Synthases; D002118:Calcium