Faculty Collaboration Database

The voltage sensor of excitation-contraction coupling in skeletal muscle. Ion dependence and selectivity. J Gen Physiol 1989 Sep;94(3):405-28

Date

09/01/1989

Pubmed ID

2481710

Pubmed Central ID

PMC2228956

DOI

10.1085/jgp.94.3.405

Scopus ID

2-s2.0-0024359120 (requires institutional sign-in at Scopus site)   46 Citations

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 Ca2+ 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 greater than Sr greater than Mg greater than Ba for group IIa cations and Li greater than Na greater than K greater than Rb greater than Cs for group Ia. Ca2+, 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.

Author List

Pizarro G, Fitts R, Uribe I, Ríos E

Author

Robert Fitts PhD Professor in the Biological Sciences department at Marquette University

MESH terms used to index this publication - Major topics in bold

Animals
Binding Sites
Calcium Channels
Electric Stimulation
In Vitro Techniques
Ion Channels
Membrane Potentials
Metals, Alkali
Metals, Alkaline Earth
Muscle Contraction
Muscle Relaxation
Muscles
Rana pipiens