Low concentrations of the stable prostaglandin endoperoxide U44069 stimulate shape change in quin2-loaded platelets without a measurable increase in [Ca 2+] i

Abstract

Dose-response relationships for raised cytoplasmic free calcium concentration, [Ca 2+] i, and shape change were measured simultaneously in quin2-loaded human platelets. With the calcium ionophore ionomycin the threshold [Ca 2+] i for shape change was 300 nM with a maximal response at 800 nM. With 1 mM external Ca 2+ the U44069 concentrations required to stimulate half-maximal shape change and an increase in [Ca 2+] i were 2 and 41 nM, respectively. For PAF these values were 8.7 and 164 pg ml , respectively. Low concentrations of U44069 and PAF evoked substantial shape change without any rise in [Ca 2+] i. In the absence of external Ca 2+, U44069 stimulated half-maximal shape change at 2 nM, and half-maximal elevation of [Ca 2+] i at 69 nM: here, increased [Ca 2+ i never reached the threshold [Ca 2+ i for shape change derived with ionomycin. These results suggest that some transduction mechanism other than elevated [Ca 2+] i, as yet unidentified, can cause shape change. U44069 Ionomycin Ca 2+ Shape change Platelet Platelet-activating factor