BACKGROUND Myocardial contractility is regulated by intracellular concentration of free Ca2+ ([Ca2'],) and myofilament Ca2+ sensitivity. The objective of this study was to elucidate the direct effects of thiopental on cardiac excitation-contraction coupling using individual, field-stimulated ventricular myocytes. METHODS Freshly isolated rat ventricular myocytes were loaded with the Ca2+ indicator, fura-2, and placed on the stage of an inverted fluorescence microscope in a temperature-regulated bath. [Ca2+], (340/380 ratio) and myocyte shortening (video-edge detection) were monitored simultaneously in individual cells field-stimulated at 0.3 Hz. Amplitude and timing of myocyte shortening and [Ca2+l, were compared before and after addition of thiopental. Intracellular pH was measured with the pH indicator, BCECF (500/440 ratio). Real-time uptake of Ca2+ into isolated sarcoplasmic reticulum vesicles was measured using fura-2 free acid in the extravesicular compartment. One hundred thirty-two cells were studied. RESULTS Field stimulation increased [Ca2+]i from 85 + 10 nM to 355 + 22 nM (mean + SEM). Myocytes shortened by 10% of resting cell length (127 + 5 tlm). Times to peak [Ca2+], and shortening were 139 + 6 and 173 + 7 msec, respectively. Times to 50% recovery for [Ca2+], and shortening were 296 + 6 and 290 + 6 ms, respectively. Addition of thiopental (30-1,000 /lM) resulted in dose-dependent decreases in peak [Ca2+]i and myocyte shortening. Thiopental altered time to peak and time to 50% recovery for [Ca2+], and myocyte shortening and inhibited the rate of uptake of Ca2+ into isolated sarcoplasmic reticulum vesicles. Thiopental did not, however, alter the amount of Ca2+ released in response to caffeine in sarcoplasmic reticulum vesicles or intact cells. Thiopental (100 uM) increased intracellular pH and caused an upward shift in the dose-response curve to extracellular Ca2+ for shortening, with no concomitant effect on peak [Ca2+],. These effects were abolished by ethylisopropyl amiloride, an inhibitor of Na+-H+ exchange. CONCLUSIONS Thiopental has a direct negative inotropic effect on cardiac excitation-contraction coupling at the cellular level, which is mediated by a decrease in [Ca2+],. Thiopental also increases myofilament Ca2+ sensitivity via alkalinization of the cell, which may partially offset its negative inotropic effect.