['We investigated whether morphine alters intracellular Ca(2+) concentration ([Ca(2+)](i)), left ventricular pressure (LVP), and myofilament Ca(2+) sensitivity under physiologic conditions in intact guinea pig beating hearts and whether delta(1), delta(2), and kappa opioid stimulations are related to the direct cardiac effects of morphine. Transmural LV phasic [Ca(2+)](i) was measured from fluorescence signals at 385 nm and 456 nm. The Ca(2+) transients during each contraction were defined as available [Ca(2+)](i). The hearts were perfused with modified Krebs-Ringer solution containing morphine in the absence and presence of delta(1) (BNTX), delta(2) (NTB), and kappa (nor-BNI) antagonists, while developed LVP and available [Ca(2+)](i) were recorded. Morphine (1 microM) decreased available [Ca(2+)](i) by 44 +/- 12 nM without decreasing developed LVP at 2.5 mM of [CaCl(2)](e) (P < 0.05). Morphine (1 microM) caused a leftward shift in the curve of developed LVP as a function of available [Ca(2+)](i) (P < 0.05). BNTX (1 microM), but not nor-BNI (1 microM) or NTB (0.1 microM) blocked morphine (1 microM) effects to decrease available [Ca(2+)](i). Morphine decreases available [Ca(2+)](i) but not LVP, and it enhances myofilament Ca(2+) sensitivity under physiologic conditions at clinical concentrations in intact isolated beating guinea pig hearts. The delta(1) opioid stimulation modifies the effects of morphine on Ca(2+) transients and myofilament Ca(2+) sensitivity.'] Does morphine enhance myofilament CA ( 2+ ) sensitivity in intact guinea pig beating hearts?