Actions of Three Local Anaesthetics: Lidocaine, Bupivacaine and Ropivacaine on Guinea Pig Papillary Muscle Sodium Channels (V̇max)

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Abstract
The new local anaesthetic ropivacaine (LEA 103) like lidocaine and bupivacaine used as references, blocked cardiac sodium channels in a use‐dependent fashion. At equimolar concentrations lidocaine had the lowest efficacy and bupivacaine the highest. The action potential was shortened and the plateau was depressed at high concentrations of each drug. Pacing a papillary muscle at 3.3 Hz in the presence of all three drugs resulted in a marked use‐dependent accumulation of block (P < 0.01). The accumulated block slowly dissipated after rest. At −90 mV holding (= resting) potential, and at a concentration of 10 μM, the time constant for recovery from block was 186 msec. in lidocaine (n = 4), 1.4 sec. in ropivacaine (n = 7), and 2.1 sec. in bupivacaine (n = 7). Lidocaine decreased V̇maxprogressively at high rates of stimulation, but not significantly at rates below 2 Hz. Ropivacaine progressively decreased V̇maxsignificantly at rates above 1 Hz, but to a lesser degree than bupivacaine. The use‐dependent action of the drugs was increased at more depolarized (less negative) holding potentials, whereas at more hyperpolarized potentials the block was diminished. Lidocaine and ropivacaine could be readily dissociated from the receptors at more hyperpolarized membrane potentials (−100 to −120 mV), whereas bupivacaine bound much harder. All three drugs blocked sodium channels more effectively after a long single conditioning pulse. Bupivacaine had the most prominent effect, and lidocaine was least effective. Bupivacaine and ropivacaine seem to interact with the inactivated state of the sodium channels, whereas lidocaine acted on both the open and on the inactivated state of the channels. The peak force of contraction was only moderately diminished after treatment with ropivacaine, whereas bupivacaine, even at 1 Hz, had a pronounced depressant action. Lidocaine at 10 μM had almost no effect on depolarization induced automaticity (DIA), whereas ropivacaine at 10 μM had a small depressant effect. Bupivacaine dampened the automaticity as well as peak force at 5 μM. At 10 μM no DIA could be initiated in the presence of bupivacaine.