Thyroid hormone regulation of phospholamban phosphorylation in the rat heart

Thyroid hormone exerts predictable effects on the contractile performance o f the heart in part by regulating the transcription of genes encoding speci fic calcium transporter proteins. In a rat model of hypothyroidism, left ve ntricular (LV) contractile function as measured by ejection fraction was de creased by 22% (P < 0.05), and this was returned to control values with T-3 treatment. In confirmation of prior studies, LV phospholamban (PLB) protei n content was significantly decreased by 25% and 40% compared with hypothyr oid LV when the animals were treated with T-3 at two doses, 2.5 and 7.0 mu g/day, respectively. The ratio of sarcoplasmic reticulum calcium adenosine triphosphatase (SERCA2) to PLB protein content was thus increased by 171% a nd 207%, respectively (P < 0.01). Resolution of the phosphorylated PLB pent amers by SDS-PAGE showed that T-3 infusion at 2.5 and 7.0 mu g/day decrease d (P < 0.001) the amount nonphosphorylated pentamers by 82% and 95%, respec tively, in a dose-dependent manner. T-3 treatment produced an increase in t he proportion of highly phosphorylated PLB pentamers (more than five phosph ates) when expressed as a fraction of total pentameric molecules (P < 0.05) . Site-specific antibodies showed that the T-3-induced increase in phosphor ylated PLB pentamers was the result of an increase in both serine 16 and th reonine 17 phosphorylation. We conclude that thyroid hormone, in addition t o regulating the expression of cardiac PLB, is able to alter the degree of PLB phosphorylation, which correlates with enhancement of LV contractile fu nction. These studies suggest that T-3 may augment myocyte calcium cycling via changes in both cAMP- and calcium/calmodulin-dependent protein kinase a ctivities.