The voltage sensor of excitation-contraction coupling in skeletal muscle. Ion dependence and selectivity.

Abstract

Manifestations of excitation-contraction (EC) coupling of skeletal muscle were studied in the presence of metal ions of the alkaline and alkaline-earth groups in the extracellular medium. Single cut fibers of frog skeletal muscle were voltage clamped in a double Vaseline gap apparatus, and intramembrane charge movement and myoplasmic Ca r+ transients were simultaneously measured. In metal-free extracellular media both charge movement of the charge 1 type and Ca transients were suppressed. Under metal-free conditions the nonlinear charge distribution was the same in depolarized (holding potential of 0 mV) and normally polarized fibers (holding potentials between -80 and -90 mV). The manifestations of EC coupling recovered when ions of groups Ia and IIa of the periodic table were included in the extracellular solution; the extent of recovery depended on the ion species. These results are consistent with the idea that the voltage sensor of EC coupling has a binding site for metal cations--the "priming" site--that is essential for function. A state model of the voltage sensor in which metal ligands bind preferentially to the priming site when the sensor is in noninactivated states accounts for the results. This theory was used to derive the relative affinities of the various ions for the priming site from the magnitude of the EC coupling response. The selectivity sequence thus constructed is: Ca > Sr > Mg > Ba for group IIa cations and Li > Na > K > Rb > Cs for group Ia. Ca 2 § the most effective of all ions tested, was 1,500-fold more effective than Na § This selectivity sequence is qualitatively and quantitatively similar to that of the intrapore binding sites of the L-type cardiac Ca channel. This provides further evidence of molecular similarity between the voltage sensor and Ca channels.Address reprint requests to Dr.

The effects of dihydropyridine derivatives on force and Ca2+ current in frog skeletal muscle fibres.

“…This effect, however, became detectable only after Ca2+ had been replaced by other metal ions. This dependence on the cation species in the external solution is to our mind related to recent observations by Pizarro et al (1989). These authors showed that metallic ions in the external solution are needed for the normal functioning of the voltage sensor with an efficiency sequence of the order Ca2+ > Sr2+ > Mg2+ > Ba2+ > Na+ > K+ > Cs+.…”

Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions.

“…Taken together, these findings make it unlikely that the q~ charge movement reflects a necessary step in calcium current activation, whether by itself or as part of some process that also involves excitation--contraction coupling. Any involvement of intramembrane charge in gating the Ca 2+ channel (Pizarro et al, 1989;Rios et al, 1991) can then only implicate q~ and not q~ charge. Finally, they suggest that the influence of the calcium channel blocker nifedipine on q~ charge (Huang, 1990) reflects effects on excitation--contraction coupling rather than calcium channel activation.…”

“…It has been suggested on a number of occasions that the voltage sensor for excitation-contraction coupling may also be the Ca 2+ channel (Pizarro, Fitts, Uribe, and Rios, 1989; for reviews, see Huang, 1988;Rios, Ma, and Gonzalez, 1991). The experiments here applied changes in extracellular osmolarity through a range earlier reported specifically to abolish Ca 2÷ release in intact fibers (Parker and Zhu, 1987).…”

The mechanical hypothesis of excitation—contraction (EC) coupling in skeletal muscle

“…This is clear since the work of Armstrong and colleagues (1972). Long-standing observations, es-9-2 pecially by Frank (1958Frank ( , 1980 on the inhibitory effects of Ca 2+ withdrawal have now been rationalized as consequences of the need of metal cations bound fo the voltage sensor fo permit ifs function (Brum et al, 1988a;Pizarro et al, 1989). Thus, there does not seem fo be a need for actual translocation of Ca 2+ between the outside and the inside of the skeletal muscle cell fo elicit Ca ~+ re]ease.…”