Ca super(2+) Release and Heat Production by the Endoplasmic Reticulum Ca super(2+)-ATPase of Blood Platelets: effect of the platelet activiting factor

Different sarco/endoplasmic reticulum Ca super(2+)-ATPases isoforms are found in blood platelets and in skeletal muscle. The amount of heat produced during ATP hydrolysis by vesicles derived from the endoplasmic reticulum of blood platelets was the same in the absence and presence of a transmembrane Ca super(2+) gradient. Addition of platelets activating factor (PAF) to the medium promoted both a Ca super(2+) efflux that was arrested by thapsigargin and an increase of the yield of heat produced during ATP hydrolysis. The calorimetric enthalpy of ATP hydrolysis ( Delta H super(cal)) measured during Ca super(2+) transport varied between -10 and -12 kcal/mol without PAF and between -20 and -24 kcal/mol with 4 mu M PAF. Different from platelets, in skeletal muscle vesicles a thapsigargin-sensitive Ca super(2+) efflux and a high heat production during ATP hydrolysis were measured without PAF and the Delta H super(cal) varied between -10 and -12 kcal/mol in the absence of Ca super(2+) and between -22 up to -32 kcal/mol after formation of a transmembrane Ca super(2+) gradient. PAF did not enhance the rate of thapsigargin-sensitive Ca super(2+) efflux nor increase the yield of heat produced during ATP hydrolysis. These findings indicate that the platelets of Ca super(2+)-ATPase isoforms are only able to convert osmotic energy into heat in the presence of PAF.