Crosstalk of β-Adrenergic Receptor Subtypes Through G i Blunts β-Adrenergic Stimulation of L-Type Ca 2+ Channels in Canine Heart Failure

Abstract

The mechanisms underlying the blunted contractile response to β-adrenergic receptor (β-AR) stimulation in heart failure (HF) are incompletely understood, especially with regard to β-AR subtype–specific regulation of L-type Ca 2+ channels. We evaluated the impact of HF induced by pacing tachycardia on β-AR regulation of L-type Ca 2+ channels in a canine model. To evaluate changes in the relative subcellular distribution of β-AR subtypes, left ventricular membranes enriched in surface sarcolemma and T-tubular sarcolemma were prepared. Radioligand binding using [ 125 I]cyanopindolol revealed that HF resulted in a comparable decrease in the density of β 1 -ARs in both surface and T-tubule sarcolemma (55±4%, n=7, P <0.001; and 45±10%, n=7, P <0.01, respectively), but no significant change in β 2 -AR density was observed. Whole-cell patch clamp studies demonstrated a markedly blunted increase in I Ca,L in response to saturating concentrations of the nonselective β-AR agonist isoproterenol (0.1 μmol/L) in failing myocytes compared with control (129±20%, n=11, versus 332±35%, n=7; P <0.001). Experiments testing β 1 -AR– and β 2 -AR–selective stimulation showed that the major component of the blunted response to nonselective β-AR stimulation in HF was caused by β 2 -AR activation, resulting in a pertussis toxin–sensitive, G i -mediated inhibition of the β 1 -AR–induced increase in I Ca,L . In conclusion, canine HF results in the following: (1) a uniform reduction in β 1 -AR density in surface and T-tubule membrane fractions without a change in β 2 -AR density; and (2) the emergence of distinct G i -coupling to β 2 -ARs resulting in accentuated antagonism of β 1 -AR–mediated stimulation of I Ca,L . These results have implications for optimizing the use of β-AR drugs in HF.