Effects of Isoproterenol on Twitch Contraction of Wild Type and Phospholamban-deficient Murine Ventricular Myocardium

Ablation of the gene for phospholamban (PLB), a transmembrane peptide regulator for the cardiac sarcoplasmic reticulum Ca2+pump, in mice brings about a complete loss of the myocardial responses toβ-adrenergic agonists (e.gLuoet alCirc. Res.1994;75: 401). We have evaluated the functional significance of PLB-independent mechanisms in the myocardial responses toβ-adrenergic stimulation in isolated intact ventricular myocardium. We compared the effects of (−)-isoproterenol (ISO) on isometric twitch contraction of paced right ventricular muscle strips of wild type (WT) and PLB-deficient (PLBKO) mice. At 37°C, frequent spontaneous contractions in both types of muscles required the inclusion of lidocaine, an antiarrhythmic, in the bathing medium. Thus the experiments were also performed at two lower temperatures, 30°C and 25°C, at which lidocaine was not needed. Under three conditions, in the absence of ISO, PLBKO ventricular muscles exhibited substantially shortened time to peak tension (TPT) and half relaxation time (TR1/2), compared with the WT muscles. In both WT and PLBKO muscles ISO increased the peak developed tension and decreased TPT and TR1/2in a dose-dependent manner although the effects were generally smaller in PLBKO than in WT muscles. One micromolar ISO caused TPT and TR1/2to decrease by 7.3±1.2% (mean±SEM) and 7.5±1.2% in PLBKO vs. 22.8±0.7% and 29.1±1.7% in WT at 37°C; by 13.5±0.4% and 14.1±1.2% in PLBKO vs. 31.3±0.8%, and 44.8±1.3% in WT at 30°C; by 15.0±2.3% and 21.1±4.9% in PLBKO vs. 25.8±1.9% and 54.0±1.9% in WT at 25°C. These findings strongly suggest that PLB-independent mechanisms play a significant role in mediating the positive inotropic and lusitropic effects ofβ-adrenergic agonists on ventricular myocardium.