Decreased expression of phospholamban is not associated with lower beta-adrenergic activation in rat atria

The aim of the study was to find out whether low phospholamban level in atr ia as compared with ventricles is associated with differences in sarcoplasm ic reticular Ca2+-uptake and contractile performance. Relationship between phospholamban and beta -adrenergic stimulation in rat left atria and papill ary muscles were examined by means of contractile measurements, sarcoplasmi c reticular oxalate-supported Ca2+-uptake, and Western blotting of phosphor ylated phospholamban. Phosphoprotein determination after beta -adrenergic s timulation demonstrated that the levels of Ser(16) and Thr(17) phosphorylat ed phospholamban in atria remained at about one-third of that in ventricles . However, comparison of sarcoplasmic reticular Ca2+-uptake in control and isoproterenol perfused preparations demonstrated that the effect of beta -a drenergic stimulation on sarcoplasmic reticular Ca2+-uptake was stronger in atrial preparations. Moreover, atria responded to isoproterenol with much larger increases in developed tension, contractility and relaxation rates t han papillary muscles. Thus, despite lower level of phospholamban, the beta -adrenergic activation of sarcoplasmic reticular Ca2+-uptake and contracti le indices are higher in atria.