Constitutive β₂-adrenergic signalling enhances sarcoplasmic reticulum Ca²⁺ cycling to augment contraction in mouse heart

1. Transgenic overexpression of the β₂-adrenergic receptor (β₂AR) in mouse heart augments baseline cardiac function in a ligand-independent manner, due to the presence of spontaneously active β₂AR (β₂AR*). This study aims to elucidate the mechanism of β₂AR*-mediated modulation of cardiac excitation-contraction (EC) coupling. 2. Confocal imaging was used to analyse Ca²⁺ sparks and spatially resolve Ca²⁺ transients in single ventricular myocytes from transgenic (TG4) and non-transgenic (NTG) littermates. Whole-cell voltage- and current-clamp techniques were used to record L-type CA²⁺ currents (I(Ca)) and action potentials, respectively. 3. In the absence of any β₂AR ligand, TG4 myocytes had greater contraction amplitudes, larger Ca²⁺ transients and faster relaxation times than did NTG cells. 4. The action potentials of TG4 and NTG myocytes were similar, except for a prolonged endstage repolarization in TG4 cells; the I(Ca) density and kinetics were nearly identical. The relationship between peak Ca²⁺ and contraction, which reflects myofilament Ca²⁺ sensitivity, was similar. 5. In TG4 cells, the frequency of Ca²⁺ sparks (spontaneous or evoked at -40 mV) was 2-7 times greater, despite the absence of change in the resting Ca²⁺ sarcoplasmic reticulum (SR) Ca²⁺ content, and I(Ca). Individual sparks were brighter, broader and lasted longer, leading to a 2.3-fold greater signal mass. Thus, changes in both spark frequency and size underlie the greater Ca²⁺ transient in TG4 cells. 6. The inverse agonist ICI 118,551 (ICI, 5 x 10^-7 M), which blocks spontaneous β₂AR activation, reversed the aforementioned β₂AR* effects on cardiac EC coupling without affecting the sarcolemmal I(Ca). However, ICI failed to detect significant constitutive β₂AR activity in NTG cells. 7. We conclude that β₂AR*-mediatecl signalling enhances SR release channel activity and Ca²⁺-induced Ca²⁺ release in TG4 cardiac myocytes, and that β₂AR* enhances EC coupling by reinforcing SR Ca²⁺ cycling (release and reuptake), but bypassing the sarcolemmal I(Ca).