Logistic Time Constant of Isometric Relaxation Force Curve of Ferret Ventricular Papillary Muscle. Reliable Index of Lusitropism.

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We have found that a logistic function fits the left ventricular isovolumic relaxation pressure curve in the canine excised, cross-circulated heart more precisely than a monoexponential function. On this basis, we have proposed a logistic time constant (τL) as a better index of ventricular isovolumic lusitropism than the conventional monoexponential time constant (τE). We hypothesize in the present study that this τL would also be a better index of myocardial isometric lusitropism than the conventional τE. We tested this hypothesis by analyzing the isometric relaxation force curve of 114 twitches of eight ferret isolated right ventricular papillary muscles. The muscle length was changed between 82 and 100% Lmax and extracellular Ca2+ concentrations ([Ca2+]o) between 0.2 and 8 mmol/l. We found that the logistic function always fitted the isometric relaxation force curve much more precisely than the monoexponential function at any muscle length and [Ca2+]o level. We also found that τL was independent of the choice of the end of isometric relaxation but τE was considerably dependent on it as in ventricular relaxation. These results validated our present hypothesis. We conclude that τL is a more reliable, though still empirical, index of lusitropism than conventional τE in the myocardium as in the ventricle.<br>

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