1 Direct myocardial effects of the S(-)-and R(+)-enantiomers of bupivacaine were compared in the guinea-pig isolated papillary muscle by recording transmembrane action potentials with the standard microelectrode technique. 2 In 5.4 mm K+, at a stimulation rate of 1 Hz, the maximal rate of depolarization ( was reduced to 59.9 + 1.4% (n = 10) of control (mean+ s.e.mean) in the presence of 10puM R(+)-bupivacaine, and to 76.7 + 1.2% (n = 14) in the presence of the same concentration of S(-)-bupivacaine. This was mainly due to a difference in time constant at which block dissipated during the diastolic period. Recovery was slower in the presence of R(+)-bupivacaine. The slower recovery in the presence of R(+)-bupivacaine resulted also in a more pronounced frequency-dependent block of f,,,X.3 Time constants for recovery from use-dependent block became significantly faster for both enantiomers on hyperpolarization, while no significant change was observed at depolarization. At all membrane potentials recovery was slower in the presence of R( + )-bupivacaine. 4 The action potential duration (APD) was shortened to a greater extent in the presence of R(+)-bupivacaine over a large range of stimulation frequencies.5 We conclude that S(-)-bupivacaine affects VP"X and APD in the guinea-pig papillary muscle less than the R( + )-enantiomer at different rates of stimulation and resting membrane potentials.
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