Overexpression of human beta2-adrenergic receptors increases gain of excitation-contraction coupling in mouse ventricular myocytes.

This study investigated cardiac excitation-contraction coupling at 37 degrees C in transgenic mice with cardiac-specific overexpression of human beta2-adrenergic receptors (TG4 mice). In field-stimulated myocytes, contraction was significantly greater in TG4 compared with wild-type (WT) ventricular myocytes. In contrast, when duration of depolarization was controlled with rectangular voltage clamp steps, contraction amplitudes initiated by test steps were the same in WT and TG4 myocytes. When cells were voltage clamped with action potentials simulating TG4 and WT action potential configurations, contractions were greater with long TG4 action potentials and smaller with shorter WT action potentials, which suggests an important role for action potential configuration. Interestingly, peak amplitude of L-type Ca2+ current (I(Ca-L)) initiated by rectangular test steps was reduced, although the voltage dependencies of contractions and currents were not altered. To explore the basis for the altered relation between contraction and I(Ca-L), Ca2+ concentrations were measured in myocytes loaded with fura 2. Diastolic concentrations of free Ca2+ and amplitudes of Ca2+ transients were similar in voltage-clamped myocytes from WT and TG4 mice. However, sarcoplasmic reticulum (SR) Ca2+ content assessed with the rapid application of caffeine was elevated in TG4 cells. Increased SR Ca2+ was accompanied by increased frequency and amplitudes of spontaneous Ca2+ sparks measured at 37 degrees C with fluo 3. These observations suggest that the gain of Ca(2+)-induced Ca2+ release is increased in TG4 myocytes. Increased gain counteracts the effects of decreased amplitude of I(Ca-L) in voltage-clamped myocytes and likely contributes to increased contraction amplitudes in field-stimulated TG4 myocytes.