Cardiac sarcoplasmic reticulum Ca2+-ATPase: heat production and phospholamban alterations promoted by cold exposure and thyroid hormone.

Short-term response to cold promotes a small but significant rise in serum T3 in euthyroid rabbits, where the heart is an important target of T3 action. In this work, we measured changes in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) and phospholamban (PLB) in hearts of hypo- and hyperthyroid rabbits and compared them with modifications induced by short- and long-term cold exposure. Short-term cold exposure promotes a small increase in T3 and, similar to hyperthyroidism, induces an increase of heart SERCA2a expression. The total PLB content does not change in hyperthyroidism, but short-term cold exposure promotes a significant decrease in total PLB and an increase in the ratio between phosphorylated and total PLB. The temperature of a given tissue depends on the balance between the heat provided by blood circulation and the rate of heat production by the tissue. In an attempt to evaluate the heat contribution of cardiac tissue, we measured mitochondrial respiration in permeabilized cardiac muscle and heat produced by cardiac sarcoplasmic reticulum (SR) during Ca(2+) transport. We observed that there was an increase in oxygen consumption and heat production during Ca(2+) transport by cardiac SR in both hyperthyroidism and short-term cold exposure. In contrast, both the mitochondrial respiration rate and heat derived from Ca(2+) transport were decreased in hypothyroid rabbits. The heart changes in oxygen consumption, SERCA2a-PLB ratio, and Ca(2+)-ATPase activity detected during short-term cold exposure were abolished after cold adaptation. We hypothesize that the transient rise in serum T3 contributes to the short-term response to cold exposure.