A novel positive inotropic substance enhances contractility without increasing the Ca²⁺ transient in rat myocardium

We have investigated the mechanism of action of a novel positive inotropic agent, the thiadiazinone derivative 5-(1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydrochinolin-6-yl)-6-methyl-3, 6-dihydro-2H-1,3,4-thiadiazin-2-on (EMD 53998). This subsance inhibits phosphodiesterase III and, in skinned myocardial fibers, it increases myofilament sensitivity to Ca²⁺. However the effects of EMD 53998 on intact myocardial preparations are still undefined. In isolated rat hearts EMD 53998 (0.5 to 5 μm) had a dose-dependent effect to increase left ventricular systolic pressure. In isolated left ventricular myocytes loaded with the ester derivative of the Ca²⁺ probe indo-1, EMD 53998 (0.5 to 5 μm) enhanced twitch amplitude without increasing the associated indo-1 transient. The myofilament responsiveness to Ca²⁺ was assessed as the relationship between twitch and the indo-1 transient amplitudes as the latter is varied by altering the bathing [Ca²⁺], or stimulation pattern. EMD 53998 reversibly shifted this relationship to the left which indicates that for indo-1 transients of the same amplitude in the absence and presence of the drug, twitch amplitude was enhanched by EMD 53998. In isolated myocytes studied in the absence of electrical stimulation, EMD 53998. (1.5 to 5 μm) had a concentration-dependent effect to markedly and reversibly decrease cell length without increasing indo-1 fluorescence ratio. Thus, the cellular basis for the positive inotropic action of EMD 53998 in rat myocardium is related to the unique effect of this substance to enhance myofilament responsiveness to Ca²⁺ and not to an increase in the indo-1 transient amplitude.