Endothelin-1 does not phosphorylate phospholamban and troponin I in intact beating rat hearts.

To determine a role of phosphorylation of specific cardiac regulatory proteins in the positive inotropic effect of endothelin-1, we examined phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in perfused beating rat hearts treated with endothelin-1. In parallel experiments, the effects of isoprenaline and phorbol-12,13-dibutyrate (PDB) on protein phosphorylation were also tested. In 32Pi-labeled hearts, perfusion with isoprenaline (100 nM) caused 4.4- and 10.4-fold increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum and of troponin I in myofibrils, respectively. In contrast, neither endothelin-1 (100 nM) nor PDB (1 microM) significantly changed the phosphorylation state of these proteins. These findings provide evidence that phosphorylation of major cardiac regulatory proteins is not responsible for the positive inotropic action of endothelin-1.