Length as a factor in excitation-contraction coupling in heart muscle: an investigation of mechanical transients in strontium-mediated contractions

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.

Length-induced changes in activation during contraction. A study of mechanical oscillations in strontium-mediated contractions of cat and frog heart muscle

The plateau phase of prolonged Sr-mediated contractions of preparations of cat and frog heart muscle was used to study the transient response to abrupt changes in load or length. An oscillatory response (total amplitude equal to or less than 5% Lmax) was obtained. Isotonic oscillations were less damped than their isometric counterparts, implying positive feedback and thus a causal role of the perturbation in length. Oscillation frequency was 2-3 HZ at 29 degrees C (Q 10, 2-3); it could be increased by epinephrine or caffeine, independently of their effects on extent of shortening; it otherwise changed as a generally constant function of the length at which the oscillation occurred, whether this was altered by changes in extracellular [Sr], frequency of contraction, or load (independent of the direction and magnitude of the preceding load step). Similar oscillatory responses were induced during Sr- or Ca-mediated contractures. Cat muscles showed an additional slower component to the oscillatory response. Transient augmentation of the velocity-length relationship after abrupt reduction in load, previously described for twitch contractions under certain conditions, appears to be analogous to the first phase of the oscillatory response studied here. Our findings indicate that the oscillation is not attributable to any mechanism intrinsic to myofilament interaction, but rather that it involves length-induced changes in the level of activation, probably mediated by Ca2+ or Sr2+. We conclude that length influences the level of activation during contraction of heart muscle.

Influence of caffeine and other inotropic interventions on the onset of unloaded shortening velocity in mammalian heart muscle. Time course of activation

The onset of maximum unloaded shortening velocity was measured in cat papillary muscles by analyzing velocity with respect to time and length after zero load clamps imposed at different times. Unloaded shortening velocity rose rapidly to a constant level by 20% of the time to peak force under all conditions studied except with application of 10 m M caffeine, which markedly slowed its onset. Load-clamping experiments showed that the force-velocity-length interrelationship which characterizes shortening remained constant throughout most of shortening except with caffeine, which delayed the onset of steady state. It is suggested that the time course of the force-velocity-length interrelationship usefully describes velocity active state , the onset of which can be measured independently of force generation at zero load. In afterloaded contractions, manifestation of velocity active state may be delayed by the slower time course of force generation which precedes shortening. It is concluded that velocity active state normally rises rapidly to a constant level which describes the contractile state. Caffeine slows the onset of velocity active state, and valid time-independent force-peak velocity curves cannot be obtained in the presence of this drug.

The treatment of Wegener's granulomatosis with immunosuppressive-cytotoxic drugs

Previous reports of 18 patients with Wegener's Granulomatosis treated with immunosuppressive and cytotoxic drugs are reviewed, and 4 new cases are reported. The prognosis in these patients is shown to be significantly better than that found in previous series where such drugs were not used

Fibrin degradation products in pre-eclamptic toxaemia and eclampsia

Serum levels of fibrinogen/fibrin degradation products, measured in African women, were significantly higher in pre-eclamptic toxaemia than in normal pregnancy, and were significantly higher with eclampsia than with toxaemia. These findings are in accord with the hypothesis that eclampsia and toxaemia are associated with disseminated intravascular coagulation, which may be responsible for certain clinical manifestations of these conditions.

Depression of myocardial contractility in rats by free fatty acids during hypoxia

Rat papillary muscles were used to study the influence of glucose (5 m M ), linoleate (1 to 1.75 m M ), octanoate (0.5 to 1.75 m M ) and pent-4-enoic acid (1 to 5 m M ) on mechanical performance under oxygenated, hypoxic and anoxic conditions. The buffer solution contained 0.3 m M albumin. Free fatty acids (FFA) (1.0 to 1.75 m M ) did not alter mechanical performance under oxygenated conditions. During hypoxia or anoxia, FFA (0.5 to 1.75 m M ) depressed contractility and increased resting force; glucose improved mechanical performance and modified the depressant effects of FFA. The depressant effect of the nonmetabolized FFA, pentenoic acid, was similar to that of other FFA. This suggests that the effect was mediated directly by FFA or acyl CoA derivatives rather than their metabolic products, and that it might be due to a detergent effect or calcium binding by FFA present in excess of intracellular FFA binding capacity at low pH. Force development during anoxia could be augmented by calcium, implying that the reduced ability of the myofilaments to contract could not be attributed entirely to a reduction of high energy stores.