Length-dependent activation: its effect on the length-tension relation in cat ventricular muscle

In cat papillary muscles at 30 degrees C, bathed with Tyrode's solution containing 2.25 mM Ca2+, the effect of various inotropic interventions (varying the stimulus frequency and continual paired stimulation) on the shape of the steady state length-tension relation was examined at lengths from Lmax, where tension production is maximal, to 0.87 Lmax. The relative steepness of the length-tension curves for peak tension developed (DT) and for maximum rate of tension development (dT/dT) varied inversely with the degree of potentiation. Thus, during paired pulse stimulation the relative decline in DT and dT/dT for a given change in muscle length was significantly less than the decline observed during stimulation at 5 min-1. When a muscle was stretched DT did not reach its final steady level for several minutes, and this slow increase in DT contributed significantly to the steepness of the steady state length-tension relation. The half-time of the slow increase in DT exhibited beat-dependency, and conditions that reduce the transsarcolemmal influx of calcium (reduction in bathing [Ca2+] or the presence of verapamil) significantly prolonged the time course of the slow increase and reduced its magnitude. These results support the hypothesis (1) that there is length-dependence of the excitation-contraction coupling process, such that an increase in muscle length is accompanied by greater activation of the contractile system; and (2) that this is due at least in part to an increased influx of calcium into the muscle cells. The implication of this hypothesis is that the influence of muscle length on myocardial performance (the Frank-Starling relation) should not be regarded as fundamentally different in character from other inotropic interventions.