The Thermogenic Function of the Sarcoplasmic Reticulum Ca2+-ATPase of Normal and Hyperthyroid Rabbit
After formation of a Ca2+ gradient, the amount of heat released during the hydrolysis of each mol of ATP cleaved (?Hcal) varies depending on the Ca2+-ATPase isoform expressed by the muscle cell. In vesicles derived from the sarcoplasmic reticulum of white muscle (SERCA 1) most of the ATP cleaved is not coupled to Ca2+ transport, and the ?Hcal varies between ?20 and ?22 kcal/mol. In contrast, in vesicles derived from red muscle (SERCA 2a) the hydrolysis of ATP is coupled with Ca2+ transport, and the ?Hcal varies between ?12 and ?14 kcal/mol. Hyperthyroidism increases the rate of heat production by the Ca2+-ATPase fourfold in white muscle and 40-fold in red muscle. In hyperthyroid rabbits, the amount of sarcoplasmic reticulum protein recovered from white and red muscle is four- to fivefold greater than that obtained from control rabbits. Hyperthyroid red muscle expresses SERCA 1, and the vesicles derived from these muscle hydrolyze ATP through a catalytic route that is not coupled to Ca2+ transport, thus increasing the amount of heat released during ATP hydrolysis, the ?Hcal varying between ?20 and ?22 kcal/mol.
ca2+-atpase hyperthyroidism ca2+ transport heat production atp hydrolysis thermogenesis