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.