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 to beta-adrenergic agonists (e.g., Luo et al., Circ. Res. 1994; 75: 401). We have evaluated the functional significance of PLB-independent mechanisms in the myocardial responses to beta-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 degrees 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 degrees C and 25 degrees 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 TR1/2 in a dose-dependent manner although the effects were generally smaller in PLBKO than in WT muscles. One micromolar ISO caused TPT and TR1/2 to 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 degrees 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 degrees 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 degrees C. These findings strongly suggest that PLB-independent mechanisms play a significant role in mediating the positive inotropic and lusitropic effects of beta-adrenergic agonists on ventricular myocardium.