The differential effects of midazolam and diazepam on intracellular Ca2+ transients and contraction in adult rat ventricular myocytes

We investigated the direct effects of midazolam and diazepam on cardiac excitation-contraction coupling in adult rat ventricular myocytes. Freshly isolated rat ventricular myocytes were loaded with fura-2/AM and field-stimulated at 28degreesC. Intracellular Ca2+ transients (340:380 ratio) and myocyte shortening (video edge detection) were simultaneously monitored in individual cells. Midazolam (3-100 muM) caused a dose-dependent decrease in both peak intracellular Ca2+ and cell shortening. Diazepam (30 and 100 muM) increased myocyte shortening and peak Ca2+ concomitant with a decrease in time to peak Ca2+. A larger concentration of 2 diazepam (>300 muM) nearly abolished intracellular Ca2+ and cell shortening. Midazolam (100 muM) and diazepam (300 muM) decreased the amount of Ca2+ released from intracellular stores in response to caffeine. Diazepam (30 muM), but not midazolam (10 AM), caused a downward shift in the dose-response curve to extracellular Ca2+ for shortening, with no concomitant effect on peak intracellular Ca2+ transient. These results indicate that midazolam and diazepam have different inotropic effects on cardiac excitation-contraction coupling at the cellular level, which is mediated by altering the availability of intracellular-free Ca2+. However, the benzodiazepines have no direct influence on excitation-contraction coupling in rat ventricular myocytes, except at very large doses. Inhibition of Ca2+ release from caffeine-sensitive intracellular Ca2+ stores may play some part in myocardial depression at the larger concentrations of benzodiazepines. Diazepam, but not midazolam, decreased myofilament responsiveness to Ca2+.