Effect of isoproterenol on protein phosphorylation in myocardial ischaemia.

Perfused rat hearts prelabelled with 32P were made ischaemic by reducing the medium flow from 12 ml/min to 0.5 ml/min. There was a rapid decrease in the contractile performance, but no significant changes in the phosphorylation state of troponin I, myosin P-light chain, an 11 K protein or in the proportion of phosphorylase in the a form occurring up to 5 min of ischaemia. Control hearts stimulated with a bolus of isoproterenol showed a large increase in the contractile force and in the phosphorylation of troponin I, 11 K protein, and phosphorylase, respectively. These responses were progressively reduced by increasing periods of ischaemia. The reduction and loss of increased phosphorylation of these proteins on exposure to isoproterenol was parallelled with an inhibition of cyclic AMP accumulation in the ischaemic heart. Phosphorylation of the myosin P-light chain remained unchanged under all the conditions studied.