Receptor subtype involved in α1‐adrenergic receptor‐mediated Ca2+ signaling in cardiomyocytes1

Aim: The enhancement of intracellular Ca 2+ signaling in response to α 1 -adrenergic receptor (α 1 -AR) stimulation is an essential signal transduction event in the regulation of cardiac functions, such as cardiac growth, cardiac contraction, and cardiac adaptation to various situations. The present study was intended to determine the role(s) of the α 1 -AR subtype(s) in mediating this response. Methods: We evaluated the effects of subtype-specific agonists and antagonists of the α 1 -AR on the intracellular Ca 2+ signaling of neonatal rat ventricular myocytes using a confocal microscope. Results: After being cultured for 48 h, the myocytes exhibited spontaneous local Ca 2+ release, sparks, and global Ca 2+ transients. The activation of the α 1 -AR with phenylephrine, a selective agonist of the α 1 -AR, dose-dependently increased the frequency of Ca 2+ transients with an EC 50 value of 2.3 μmol/L. Blocking the α 1A -AR subtype with 5-methylurapidil (5-Mu) inhibited the stimulatory effect of phenylephrine with an IC 50 value of 6.7 nmol/L. In contrast, blockade of the α 1B -AR and α 1D -AR subtypes with chloroethylclonidine and BMY 7378, respectively, did not affect the phenylephrine effect. Similarly, the local Ca 2+ spark numbers were also increased by the activation of the α 1 -AR, and this effect could be abolished selectively by 5-Mu. More importantly, A61603, a novel selective α 1A -AR agonist, mimicked the effects of phenylephrine, but with more potency (EC 50 value =6.9 nmol/L) in the potentiation of Ca 2+ transients, and blockade of the α 1A -AR by 5-Mu caused abolishment of its effects. Conclusion: These results indicate that α 1 -adrenergic stimulation of intracellular Ca 2+ activity is mediated selectively by the α 1A -AR.