1. We used electrophysiological and binding techniques to assess the presence of beta 1- and beta 2-adrenoceptors (beta 1AR and beta 2AR) in rat cardiac myocytes and to determine their ratio during aging. Experiments were performed in left ventricular myocytes enzymatically dissociated from the heart of 3-(young) or 22-month-old (old) Wistar Kyoto rats. 2. In patch-clamp experiments, myocytes from old rats showed a prolonged action potential duration (at -20 mV: 41.7 +/- 3.6 vs 26.2 +/- 3.1 ms; at -60 mV: 154.4 +/- 17.7 vs 87.1 +/- 6.9 ms, P < 0.05) and an augmented membrane capacitance (an index of cell size) (271.7 +/- 20.2 vs 164.3 +/- 14.6 pF, P < 0.05) compared to young rats. beta 2AR stimulation, achieved by superfusing myocytes with the selective beta 2AR agonist, zinterol (10 microM) or with (-)-isoprenaline (1 microM) in the presence of the selective beta 1AR antagonist, CGP 20712A (0.1 microM), significantly increased L-type calcium current (ICa,L) in rat ventricular myocytes. The percentage increase was similar in both young and old rats, either with zinterol (26.9 +/- 3.6% and 24.2 +/- 2.8%, respectively) or isoprenaline plus CGP 20712A (30.4 +/- 3.7% and 22.4 +/- 4.1%, respectively). Isoprenaline alone (beta 1AR and beta 2AR stimulation) caused a much smaller increase in ICa,L in old rats (58.4 +/- 12.1%) than in younger ones (95.3 +/- 8.1%) (P = 0.067
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