In Vitro Negative Inotropic Effect of Low Concentrations of Bupivacaine Relates to Diminished Ca2+ Sensitivity but Not to Ca2+ Handling or β-Adrenoceptor Signaling

Abstract

Background: Systemic toxicity of local anesthetics is predominantly complicated by their myocardial toxicity. Especially long-acting local anesthetics exert a negative inotropic effect that has been described at lower concentrations than defined for blockade of myocardial ion channels. We evaluated the negative inotropic effect of bupivacaine at a concentration described for clinical toxicity testing the hypothesis that negative inotropy is a result of reduced Ca2+ sensitivity rather than blockade of ion channels. Methods: We simultaneously measured force development and action potentials in Guinea pig right papillary muscles (n = 5 to 7). L-type Ca2+ currents (n = 8 to 16) and Ca2+ transients (n = 10 to 11) were measured in isolated cardiomyocytes. Sensitivity of myofilaments to Ca2+ was assessed in skinned fibers (n = 10). Potential effects of bupivacaine on 3′,5′-cyclic adenosine monophosphate concentrations were measured using Förster Resonance Energy Transfer (n = 12 to 14) microscopy. Results: Bupivacaine reduced force in a concentration-dependent manner from 173 ± 119 μN at baseline to 28 ± 13 μN at 300 μM (mean ± SD). At concentrations giving half-maximum negative inotropic effects (5 μM), the maximum upstroke velocity of action potentials, as a surrogate of sodium channel activity, was unaffected. Maximum positive inotropic effects of isoprenaline were also reduced to 50%. Neither basal nor isoprenaline-induced 3′,5′-cyclic adenosine monophosphate accumulation, L-type Ca2+ currents, or Ca2+ transients were affected by 5 μM bupivacaine, but this concentration significantly decreased Ca2+ sensitivity of myofilaments, changing the negative logarithm of the half-maximum effective Ca2+ concentrations from 5.66 to 5.56-log[M]. Conclusions: We provide evidence that the negative inotropic effect of bupivacaine may be caused mainly by a reduction in myofilament sensitivity to Ca2+.