Crosstalk of β-adrenergic receptor subtypes through Gi blunts β-adrenergic stimulation of L-type Ca2+ 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 Ca2+ channels. We evaluated the impact of HF induced by pacing tachycardia on β-AR regulation of L-type Ca2+ 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 [125I]cyanopindolol revealed that HF resulted in a comparable decrease in the density of β1-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 β2-AR density was observed. Whole-cell patch clamp studies demonstrated a markedly blunted increase in ICa,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 β1-AR and β2-AR-selective stimulation showed that the major component of the blunted response to nonselective β-AR stimulation in HF was caused by β2-AR activation, resulting in a pertussis toxin-sensitive, Gi-mediated inhibition of the β1-AR-induced increase in ICa,L. In conclusion, canine HF results in the following: (1) a uniform reduction in β1-AR density in surface and T-tubule membrane fractions without a change in β2-AR density; and (2) the emergence of distinct Gi-coupling to β2-ARs resulting in accentuated antagonism of β1-AR-mediated stimulation of Ica,L. These results have implications for optimizing the use of β-AR drugs in HF. © 2005 American Heart Association, Inc.