The Effect of Isoproterenol on Phospholamban-deficient Mouse Hearts with Altered Thyroid Conditions

The aim of the present study was to determine the effects of β -adrenergic stimulation in wild-type and phospholamban-deficient mouse hearts with altered thyroid conditions. Hypothyroidism was associated with significant decreases in heart/body weight ratio in wild-type and phospholamban-deficient mice, whereas hyperthyroidism was associated with significant increases in heart/body weight ratio in both groups. Hypothyroid hearts of wild-type and phospholamban-deficient mice exhibited similar increases in β -myosin heavy chain protein levels and decreases in α -myosin heavy chain protein levels. In hyperthyroidism, there were increases in the α -myosin heavy chain protein levels and these were similar in wild-type and phospholamban-deficient hearts. There were no detectable levels of β -myosin heavy chain protein in the hyperthyroid hearts. The relative tissue level of phospholamban in wild-type hearts was increased (133%,P <0.01) in hypothyroidism, and decreased (69%, P<0.01) in hyperthyroidism, when compared to euthyroid controls (100%). Similar increases and decreases in SR Ca2+-ATPase protein levels were observed between phospholamban-deficient and wild-type hearts in hyperthyroidism and hypothyroidism, respectively. The basal contractile state of wild-type and phospholamban-deficient hearts was significantly depressed in hypothyroidism. On the other hand, the basal contractile state of wild-type and phospholamban-deficient hearts was significantly increased in hyperthyroidism. Duringβ -agonist stimulation of wild-type hearts, the responses in the rates of contraction and relaxation were highest in the hypothyroid group, followed by the euthyroid, and lastly by the hyperthyroid groups. There was a close linear correlation between the magnitude of the contractile parameter responses and the phospholamban/SERCA2 ratios in these hearts. However, the phospholamban-deficient hypothyroid, euthyroid, and hyperthyroid hearts did not exhibit any responses to isoproterenol, indicating that the alterations in the thyroid states of these hearts do not influence the effects of isoproterenol on cardiac function. These findings suggest that phospholamban is an important regulator of the heartʹs responses to β -adrenergic stimulation under various thyroid states.