title: Length as a factor in excitation-contraction coupling in heart muscle: an investigation of mechanical transients in strontium-mediated contractions.
abstract: Sr-mediated contractions have been used to provide an analogue of the normal twitch contraction but with a plateau phase of steady-state activity. An abrupt change of load during the plateau induced an isotonic oscillatory response whose frequency was generally related to the length at which it occurred but could be independently increased by adrenaline or caffeine. The response was not specific for Sr, being seen also in Sr- or Ca-activated K-contractures. The isotonic oscillatory response was less damped than the corresponding isometric transient, and spontaneous isotonic oscillation often occurred following the initial shortening phase, suggesting that the change in length rather than the change in load was the causal perturbation. In cat but not frog preparations a second slower oscillation, also influenced by adrenaline, could be induced. Analysis of velocity--length relations indicates that the isotonic oscillations observed during the plateau phase are analogous to the velocity transients previously described in twitch contractions, but then only in three states characterized implicitly by a relatively poor capacity for internal sequestration. These findings provide evidence that length influences the level of activation of the contractile proteins during shortening in heart muscle, probably through changing Sr/Ca availability.
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002415:Cats; D004548:Elasticity; D004837:Epinephrine / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects*; D009991:Oscillometry; D010210:Papillary Muscles / Q000502:physiology*; D013268:Stimulation, Chemical; D013324:Strontium / Q000494:pharmacology*; D016276:Ventricular Function
chemical_list: D002110:Caffeine; D002118:Calcium; D004837:Epinephrine; D013324:Strontium