Influence of deuterium oxide on calcium transients and myofibrillar responses of frog skeletal muscle.

Abstract

SUMMARY1. The influence of substituting D20 for H20 on calcium transients and on contraction was studied in intact single skeletal muscle fibres injected with aequorin and in mechanically skinned fibres from frogs. Most experiments were carried out at loOC.2. Experiments performed in vitro established that the calcium concentration-effect curve for aequorin is depressed and the rate of change of aequorin light emission after sudden changes of [Ca2+] is decreased when D20 is substituted for H20.3. D20 substitution greatly reduces the amplitude of the aequorin signals of intact fibres both in twitches and in tetani. This is partly the result of the influence of D20 on aequorin, but the amplitude of the calcium transients is reduced as well.4. In both H20 and D20 Ringer solutions, the amplitude and time course of the sarcoplasmic calcium transient during a tetanus vary greatly with the stimulus frequency. In H20, frequencies high enough to produce mechanical fusion normally produce cytoplasmic calcium concentrations high enough to saturate the myofibrils, with the result that stimulus frequency has very little influence on the mechanical response. Saturation does not occur in D20, and the amplitude and form of fully fused tetani are greatly influenced by the frequency of stimulation.5. Aequorin was used as a calcium indicator to estimate the influence of D20 substitution on the apparent equilibrium association constant for the calcium-EGTA complex. The constant was reduced by more than one order of magnitude in D20 at pD = pH = 70.6. Experiments on mechanically skinned muscle fibres showed that D20 substitution decreased the apparent sensitivity of the myofibrils to calcium, but increased the force of contraction at saturating calcium concentrations. The latter effect is probably responsible for the potentiation of tetanic tension that is observed in certain D20/H20 mixtures or in H20 solutions immediately after a switch from D20.7. In intact muscle fibres the potentiation oftwitch or tetanic tension after a switch from D20 to H20 declines with a half-time of about 25 s; this probably reflects the time course of exchange of intracellular D20 for H20. D. G. ALLEN, J. R. BLINKS AND R. E. GODT 8. The effects of D20 on the intracellular calcium transient appeared and disappeared with a half-time of less than 1 s; this time is of the same order of magnitude as that calculated for the exchange of H20 and D20 in the T-tubules. We conclude that D20 affects calcium release at a site in the T-tubule which is more accessible to the extracellular space than to the intracellular space.

Mechanical modulation of cardiac microtubules

“…Image (a) courtesy of D. Benoist, and from ref. [65] (b) effects on L-type Ca 2+ channels and myofilament Ca 2+ sensitivity [1,28], so any effects on contractility ascribed to microtubules should be interpreted with caution.…”

Inactivation of excitation-contraction coupling in rat extensor digitorum longus and soleus muscles.

“…The decay of tension is called inactivation (Heistracher and Hunt, 1969) and occurs as a result of changes in one of the steps in excitation-contraction coupling that link T-tubule depolarization with Ca release from the terminal cisternae, rather than a change in the ability of the contractile proteins to generate tension. Ca transients measured with Ca indicators show inactivation kinetics during tetanic stimulation (Allen et al, 1984) and during long depolarizing voltage-clamp pulses (Kovacs et al, 1979;Baylor et al, 1983;Eusebi et al, 1985). In addition, rapid inactivation of Ca release from the sarcoplasmic reticulum has been described (Melzer et al, 1984).…”

Contraction kinetics of intact and skinned frog muscle fibers and degree of activation. Effects of intracellular Ca2+ on unloaded shortening.

“…However, the isotonic shortening properties of the fibers were constant with these agents . Since a reduction in the amount of Ca 2' release was a probable cause of the decrease in force with D20 (Allen et al ., 1984) and dantrolene, the results supported the view that Ca 2' does not affect the shortening response during the twitch .…”