[Alterations of cat papillary muscle mechanics induced by changes of preload under variation of physical and chemical parameters (author's transl)]

Abstract

A rapid change in length of cat papillary muscle induces two reciprocal diastolic and systolic processes: stretching causes a viscoelastic relaxation of the muscle. The opposite behavior is observed after abrupt releases when - subsequent to an increase in resting tension - diastolic force attains its new equilibrium. The stretch-induced process of relaxation is accompanied by a transient decrease in mechanogram amplitudes; a release is accordingly followed by a temporary increase in isometric muscle performance. The mechanograms of the steady state prove to be the function of the degree of stretch or release, whereas muscle contractions in the early phase of stress relaxation depend on the contractile state before the change in length. All interventions which augment developed tension (increased calcium, decreased potassium, increased or decreased sodium concentrations, strontium, postextrasystolic potentiation, sympathicomimetic agents, frequency potentiation) diminish the transient phenomena due to stretch, whereas low frequency or a lesser intensity of electromechanical coupling (Hexobarbital, Iproveratril, Desoxycorticosteron, Ryanodine) increases them. A decrease of bath temperature enhances the transient increase in force due to a release or an initial reduction of isometric tension after a sudden stretch, although the absolute forces increase. No substantial changes could be observed in reserpinized cats with beta-blocking agents or under hypoxia. It is supposed that alterations in the time course of the action potential could be related to post-stretch and post-release systolic phenomena. A final interpretation of mechanical and electrical events after stretch and release is not possible with the methods used in these experiments.