Ethanol acutely and reversibly suppresses excitation-contraction coupling in cardiac myocytes

We used adult rat cardiac myocytes to examine the acute effects of 0.1-5.0% (vol/vol) ethanol (ETOH) on 1) the cytosolic [Ca²⁺] (Ca(i)) transient measured as the change in indo 1 fluorescence at 410/490 nm and contraction elicited by electrical stimulation of single cells and 2) the sarcoplasmic reticulum (SR) Ca²⁺ content in cell suspensions. During stimulation at 1 Hz, clinically relevant ETOH correlations (0.1-0.15% [vol/vol]) caused a 10-15% decrease in the contraction amplitude, measured by myocyte edge tracking, without decreasing the Ca(i) transient that initiates contraction. At higher ETOH concentrations (1-5% [vol/vol]), ETOH caused profound contractile depression and also reduced the magnitude of the Ca(i) transient. These effects were reversed within minutes of ETOH washout. Addition of norepinephrine (10 μM) to the bathing solution or an increase in bathing [Ca²⁺] in the continued presence of ETOH could also reverse its effects. The relation of the amplitude of the Ca(i) transient to the contraction amplitude measured across a range of bathing [Ca²⁺] was shifted by ETOH, such that for a given Ca(i) transient a marked reduction in contraction amplitude occurred. In unstimulated myocyte suspensions, ETOH (1-5% [vol/vol]) caused a concentration-dependent depletion of SR Ca²⁺ content, manifested as a diminution in the Ca(i) increase elicited by caffeine in the presence of extracellular EGTA and no added Ca²⁺. Thus, in rat cardiac myocytes a reduction in the myofilament Ca²⁺ response, possibly due to a decrease in myofilament Ca²⁺ sensitivity, is a mechanism for contractile depression due to clinically relevant ETOH concentrations. Higher concentrations of ETOH cause further contractile depression, in part, by inducing SR Ca²⁺ release and depleting the SR of Ca²⁺, leading to an attenuation of the Ca(i) transient elicited by electrical stimulation, and by further depressing the myofilament length-Ca²⁺ relation.