Hyperthyroidism affects the activity of a cAMP-dependent protein kinase and protein phosphorylation in heart sarcolemma.

We have investigated the effect of thyroxine administration on the cAMP related systems in heart sarcolemma. Hyperthyroidism produces an increase in the number of beta-receptors and enhances in situ the phosphorylation of some sarcolemmal proteins, in particular the 55 kDa peptide. On the other hand, the activity of a membrane-bound cAMP-dependent protein kinase is considerably lower in sarcolemma obtained from animals receiving the hormone. We suggest that the thyroxine-induced alteration in beta-receptors elevates the content of cAMP in the cell which, in turn, evokes a transient dissociation of the catalytic subunit of a cAMP-dependent kinase from the regulatory subunit embeded in the membrane.