Summary
The addition of 0.1 μM ionophore A23187 to washed platelets incubated in citrated
saline caused massive release of stored serotonin accompanied by intracellular accumulation
of inosine monophosphate, but produced no detectable influx of externally added calcium
or abnormal structural alterations. With increasing ionophore concentration there
was a significant influx of calcium and a drastic alteration in the platelet ultrastructure.
The increase in ionophore concentration was accompanied by the conversion of the major
part of metabolic adenine nucleotides to inosine monophosphate and an almost complete
blockage of further conversion to inosine and hypoxanthine. The metabolic changes
were accentuated by the addition of calcium at concentrations less than 1/10 of the
citrate concentration. In the presence of Ca++, or when citrate was omitted, there was a substantial leakage of cytoplasmic material,
which at times suggested complete exchangeability between cytoplasm and extracellular
medium. Our findings are consistent with the hypothesis that the platelet release
reaction is triggered by intracellularly bound calcium. They also suggest that the
application of high ionophore concentration has a toxicologic rather than a physiologic
effect on platelets, and that a weak chelator added during incubation with the ionophore
can in the absence of divalent cations prevent cell destruction, but not the toxic
effect on cell metabolism.