Enhanced basal contractility but reduced excitation-contraction coupling efficiency and β-adrenergic reserve of hearts with increased Cav1.2 activity

Cardiac remodeling during heart failure development induces a significant increase in the activity of the L-type Ca2+ channel (Cav1.2). However, the effects of enhanced Cav1.2 activity on myocyte excitation-contraction (E-C) coupling, cardiac contractility, and its regulation by the β-adrenergic system are not clear. To recapitulate the increased Cav1.2 activity, a double transgenic (DTG) mouse model overexpressing the Cavβ2a subunit in a cardiac-specific and inducible manner was established. We studied cardiac (in vivo) and myocyte (in vitro) contractility at baseline and upon β-adrenergic stimulation. E-C coupling efficiency was evaluated in isolated myocytes as well. The following results were found: 1) in DTG myocytes, L-type Ca2+ current ( ICa,L) density, myocyte fractional shortening (FS), peak Ca2+ transients, and sarcoplasmic reticulum (SR) Ca2+ content (caffeine-induced Ca2+ transient peak) were significantly increased (by 100.8%, 48.8%, 49.8%, and 46.8%, respectively); and 2) cardiac contractility evaluated with echocardiography [ejection fraction (EF) and (FS)] and invasive intra-left ventricular pressure (maximum dP/d t and −dP/d t) measurements were significantly greater in DTG mice than in control mice. However, 1) the cardiac contractility (EF, FS, dP/d t, and −dP/d t)-enhancing effect of the β-adrenergic agonist isoproterenol (2 μg/g body wt ip) was significantly reduced in DTG mice, which could be attributed to the loss of β-adrenergic stimulation on contraction, Ca2+ transients, ICa,L, and SR Ca2+ content in DTG myocytes; and 2) E-C couplng efficiency was significantly lower in DTG myocytes. In conclusion, increasing Cav1.2 activity by promoting its high-activity mode enhances cardiac contractility but decreases E-C coupling efficiency and the adrenergic reserve of the heart.