Membrane calcium activation in excitation-contraction coupling

Calcium-induced calcium release in crayfish skeletal muscle

1. Cut crayfish skeletal muscle fibres were mounted in a triple Vaseline-gap voltage clamp with the Ca(2+)-sensing dye Rhod-2 allowed to diffuse in via the cut ends. Ca2+ currents across the surface/T-tubule membranes (ICa) were recorded simultaneously with changes in myoplasmic Ca2+ concentration (Ca2+ transients). 2. Excitation-contraction coupling in crayfish skeletal muscle fibres is abolished when calcium in the extracellular solution is replaced by Mg2+. 3. The amplitude of the Ca2+ transients elicited by voltage clamp pulses closely followed the amplitude of the peak calcium currents recorded simultaneously across the surface/T-tubule membranes. This included decreases in both parameters as the pulse potential approached ECa (reversal potential for Ca2+), as well as secondary Ca2+ transients accompanying large tail calcium currents occurring upon repolarization from very large depolarizations. 4. A large contribution of sarcoplasmic reticulum (SR) Ca2+ release to the Ca2+ transients was revealed by a large decrease in the transient caused by the calcium-induced calcium release (CICR) blockers procaine and tetracaine. 5. Short pulses which interrupted the calcium current while SR Ca2+ release was in progress at high rates caused the Ca2+ transient to stop rising nearly immediately after the end of the pulse in most fibres. In about 15% of the fibres the Ca2+ transients continued to rise, albeit at a slower rate, for 10-20 ms after the end of the pulse, as if released Ca2+ was able to elicit some further Ca2+ release from the SR for a while. 6. Even with fibres displaying little sign of continued release after termination of short pulses under control conditions, procaine accelerated the decay of Ca2+ transients elicited by short pulses, indicating that continued release was taking place even as the transient was declining. 7. These results suggest that CICR in crayfish fibres is more closely controlled by a small entry of Ca2+ via surface/T-tubule membrane Ca2+ current than by a larger amount of Ca2+ released from the SR. The limited positive feedback of released Ca2+ on further Ca2+ release allows CICR to remain graded (according to ICa) rather than all-or-none.

Excitation-contraction coupling: role of K-activation within the transverse tubular system

Long-duration Ca action potentials induced in crustacean muscle fibers after prolonged exposure to quaternary ammonium ions are accompanied by attenuated tensions with unique time courses. The tensions have three phases. The initial phase, correlated with the upstroke of the spike, is a rapid increase in tension followed by relaxation to or near to resting level (on-tension). In the second phase, tension rises slowly as the spike plateau declines. The final phase is another rapid increase and decay in tension that is correlated with termination of the action potential (off-tension). To observe these tensions, fibers must be exposed to 50-100 mM tetrabutylammonium ion for about 1 hr or to lower concentrations for longer periods (e.g., 5 mM for 20-30 hr). To obtain a similar response in fibers treated with tetraethylammonium ion, higher concentrations or longer soaking periods, or both, are required. Because neither caffeine-induced tensions in intact fibers nor contractile protein and sarcoplasmic reticulum function in skinned fibers were modified by quaternary ammonium ions, their site of action appears to be limited to surface or transverse tubular system membranes, or both. The unique tensions can be explained by considering the mode by which quaternary ammonium ions block K channels in conjunction with a scheme in which activation of K channels within the transverse tubular system controls the driving force for influx of Ca ions.

Dissociation of action potentials from contraction in single crab muscle fibers

In single crab fibers (Callinectes danae) bathed in Ca2+-free media, Ba2+ action potentials did not elicit tension. In contrast, Sr2+ spikes evoked twitches similar in amplitude to those accompanying the control Ca2+ spikes. Tension development in these fibers, therefore, depends on the ionic species carrying the inward current during membrane excitation. The Ca2" or Sr2+ influx appears insufficient to evoke the observed twitch tensions, and it seems necessary to postulate mobilization of an intracellular source of Ca1+. Procaine, which suppresses release of Ca2+ from sarcoplasmic reticulum, depressed twitch tension but did not reduce the overshoot or duration of Ca2+ or Sr2+ spikes. This finding is compatible with the suggestion that the contractions results from release of Ca2+ from the sarcoplasmic reticulum initiated by signals given by the influx of Ca2+ or Sr2+, but not Ba2+.

Effects of verapamil on excitation-contraction coupling in single crab muscle fibers

The effects of verapamil and its optical isomers on the electrical and mechanical characteristics of single muscle fibers of Callinectes danae were studied. Verapamil (10-20 microng/ml) blocked the procaine- and TEA-induced spikes; the blockade was preceded by reduction in the rate of rise of the upstroke and increase in the duration of the action potentials. Inhibition of Ba-spikes required higher concentrations of verapamil (greater than 50 microng/ml). These concentrations reduced the amplitude of the normally occurring graded electrogenic membrane responses and reduced the rate of development of the current-induced tensions. With lower concentrations (10-30 microng/ml) verapamil enhanced the negative afterpotentials and the peak amplitude of the local contractions elicited by depolarizing current pulses, while the graded membrane responses were not markedly modified. Verapamil (1-100 microng/ml) did not affect the resting membrane potential but increased the effective membrane resistance. Determination of the cable characteristics by DC pulses indicated that verapamil (1-10 microng/ml) shortens the membrane length constant, increases the specific resistivity of the sarcoplasm and, in most cases, increases the membrane time constant. Verapamil (10 microng/ml) induced tension in these crab fibers. The contractions were potentiated in Na-deficient media, by increase in [Ca]0, and by membrane depolarization; "Ca-free" salines depressed, and procaine abolished these contractions. The results suggest that verapamil affects both Ca and K conductances and interferes with the Ca-sequestering mechanisms of these fibers. The (-)-isomer of verapamil was more effective than the (+)-isomer with respect to tension development, prolongation and subsequent blockade of procaine-spikes and enhancement of current-induced after-potentials and contractions.

Potassium and calcium conductance in slow muscle fibres of the toad

Slow muscle fibres in isotonic potassium sulphate saline could be easily repolarized to -90 mV. From this membrane potential a regenerative response could be elicited with short depolarizing pulses. 2. This response is blocked by TEA, suggesting that potassium is the main ion involved. 3. In the presence of TEA, a transient depolarization is recorded when the steady hyperpolarization is withdrawn. This anode break response is dependent upon the external calcium and is blocked by cobalt, suggesting that it is due to a calcium conductance. 4. The membrane conductance change was continuously recorded with short pulses at the end of the hyperpolarization. The membrane conductance decayed with at least two components with an average t1/2 of 1-2 and 6-8 sec. TEA blocked the slow component, and the fast one was dependent upon calcium and was blocked by cobalt.

Sites of action of D2O in intact and skinned crayfish muscle fibers

The effect on tension development of replacing 90% of the H2O of the bathing saline with D2O was studied on intact single fibers, and on skinned fibers before and after the latter were treated so as to eliminate Ca-accumulation by the sarcoplasmic reticulum (SR). Excitation-contraction coupling (ECC) of intact fibers is not abolished, but is depressed by D2O so that higher depolarizations are required to elicit a given tension. The reduction in tension at a given level of depolarization is not due to inhibition of the contractile system. The latter showed an enhanced Ca sensitivity; that is, skinned fibers respond to Ca concentrations that are 1-2 orders of magnitude smaller in D2O than in H2O saline. When bathed in D2O saline, intact fibers or skinned fibers with functional SR can still accumulate and release Ca in sufficient quantities to allow repeated induction of maximum tensions. Relaxation is slowed in all three types of preparation, perhaps because of an increased affinity of troponin to Ca in D2O salines.