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J Biol Chem, Vol. 274, Issue 40, 28344-28350, October 1, 1999

Ca²⁺ Release and Heat Production by the Endoplasmic Reticulum Ca²⁺-ATPase of Blood Platelets
EFFECT OF THE PLATELET ACTIVATING FACTOR

From the Instituto de Ciências Biomédicas, Departamento de Bioquímica Médica, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro 21941-590,-Brasil

Different sarco/endoplasmic reticulum Ca²⁺-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 Ca²⁺ gradient. Addition of platelets activating factor (PAF) to the medium promoted both a Ca²⁺ efflux that was arrested by thapsigargin and an increase of the yield of heat produced during ATP hydrolysis. The calorimetric enthalpy of ATP hydrolysis (H^cal) measured during Ca²⁺ transport varied between 10 and 12 kcal/mol without PAF and between 20 and 24 kcal/mol with 4 µM PAF. Different from platelets, in skeletal muscle vesicles a thapsigargin-sensitive Ca²⁺ efflux and a high heat production during ATP hydrolysis were measured without PAF and the H^cal varied between 10 and 12 kcal/mol in the absence of Ca²⁺ and between 22 up to 32 kcal/mol after formation of a transmembrane Ca²⁺ gradient. PAF did not enhance the rate of thapsigargin-sensitive Ca²⁺ efflux nor increase the yield of heat produced during ATP hydrolysis. These findings indicate that the platelets of Ca²⁺-ATPase isoforms are only able to convert osmotic energy into heat in the presence of PAF.

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