Sustained β-Adrenergic Stimulation Increased L-Type Ca²⁺ Channel Expression in Cultured Quiescent Ventricular Myocytes

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Abstracts: Abundance of voltage-gated L-type Ca²⁺ channel is altered by β-adrenergic receptor (β-AR) stimulation and by elevation of the intracellular Ca²⁺ concentration in cardiac myocytes. In whole animal, chronic β-AR stimulation or pacing heart results in various changes in abundance of the channel, but reduces the β-AR responsiveness of the L-type channel. Because β-AR stimulation facilitates the L-type calcium channels, it is difficult in the whole animal to study the effects of β-AR and Ca²⁺ influx on the up-regulation of the L-type channel independently of one another, which makes cultures of quiescent (non-beating) adult myocytes an attractive model. We found that culturing quiescent adult rabbit ventricular myocytes with isoproterenol (ISO, 2 µM) for 72 h or more caused a significant increase in expression of mRNA coding for the L-type channel α_1C subunit by approximately 2-fold as compared to time-matched controls, and it was followed by a 1.8-fold increase in the Ca²⁺ current density at 96 h. Somewhat surprisingly, acute application of 1 µM ISO increased the current amplitude even in ISO-treated cells. The increase in the current density, induced by sustained β-AR stimulation, was blocked by a β-AR antagonist, propranolol (10 µM), but not by a Ca²⁺ antagonist, nitrendipine (10 µM). In addition, the effects were reproduced by forskolin (10 µM) but not by a Ca²⁺ agonist, Bay-K 8644 (2 µM). Taken together, these results suggest that sustained β-AR stimulation up-regulates L-type channel expression via PKA activation, but does not alter the β-AR responsiveness of the channel in quiescent myocytes.