Pharmacologic evaluation of isometric contraction-relaxation coupling indexes in rabbit ventricular muscle

Investigations of the coupling between contraction and relaxation (contraction-relaxation [CRC] process) in isometric conditions are essential in determining whether pharmacologic interventions or cardiac diseases specifically modify isometric relaxation (intrinsic lusitropic effect) or change it in proportion with the accompanying changes in contractility (or inotropy). For this purpose, the CRC process is quantified by various indexes, derived from differentiation and/or curve fitting the whole or relaxation phase of the isometric twitch, one of the most used being tau, the time constant of the final iso(volu)metric phase of relaxation. Nevertheless, the possible redundancy and validity of such indexes have not been thoroughly investigated. Accordingly, we performed a pharmacologic evaluation of such indexes in isolated rabbit ventricular muscles isometrically contracting in vitro, using modifiers of either intracellular Ca(2)+ handling (nifedipine, ryanodine, 2,5-di-tert-butyl-benzohydroquinone, all negative inotropic compounds, and BAY K 8644, a positive inotropic drug), or myofibrillar Ca(2)+ sensitivity (CGP 48506, a Ca(2)+ sensitizer, and butanedione monoxime, a Ca(2)+ desensitizer, respectively positive and negative inotropic compounds). The isometric twitch in control conditions and in the presence of increasing concentration of each compound was analyzed to determine the classically used CRC and/or lusitropic indexes, derived either from single parameters such as the maximal rate or contraction and relaxation (+dT(max) and -dT(max), respectively), or from curve fitting of the whole, or part, of the twitch. As the rate of isometric relaxation is dependent on myofilament properties, we expected that compounds modifying myofibrillar Ca(2)+ sensitivity in an opposite direction (CGP 48506 vs butanedione monoxime) would be the only drugs exerting an intrinsic lusitropic and opposite effect on a validated CRC index. Results showed that (1) none of the tested compounds affected the slope of the linear relationship between peak twitch tension and dT(max), a previously assumed CRC index, sensitive only to myofibrillar Ca(2)+ sensitivity modifiers; (2) the lusitropic parameter B, derived from mathematical curve fitting of the whole isometric twitch, and the ratio +dT(max)/dT(max), exhibited similar drug- and dose-dependency, but no opposite sensitivity to CGP 48506 and BDM for either index; and (3) negative inotropic compounds dose-dependently slowed relaxation (and conversely for positive inotropes), whether the latter was quantified by the rate constant beta, derived from double exponential curve fitting of the whole relaxation phase, or by the time constants tau(L) and tau(E), derived from the curve fitting (logistic and monoexponential, respectively) of the final phase of relaxation. Nevertheless, the pharmacologicly induced changes in beta were statistically significant at lower concentrations and exhibited less individual variability, compared with the time constants. We demonstrate that intrinsic lusitropic changes can be quantified by the value of the slope of the relationship relating beta to peak isometric tension: the slope value was unchanged by Ca(2)+ handling modifiers, decreased by CGP 48506, and reversed by BDM (indicating number, negative, and positive intrinsic lusitropic effects respectively). Based on these data, we propose that the linear relationship between beta and peak isometric tension could be used a new method to assess whether pharmacologic interventions or cardiac diseases exert intrinsic effects on isometric relaxation.