Voltage-clamp analysis of membrane currents and excitation-contraction coupling in a crustacean muscle

J Muscle Res Cell Motil. 2001;22(4):329-44. doi: 10.1023/a:1013154612985.

Abstract

A single fibre preparation from the extensor muscle of a marine isopod crustacean is described which allows the analysis of membrane currents and simultaneously recorded contractions under two-electrode voltage-clamp conditions. We show that there are three main depolarisation-gated currents, two are outward and carried by K+, the third is an inward Ca2+ current, I(Ca). Normally, the K+ currents which can be isolated by using K+ channel blockers, mask I(Ca). I(Ca) activates at potentials more positive than -40 mV, is maximal around 0 mV, and shows strong inactivation at higher depolarisation. Inactivation depends on current rather than voltage. Ba2+, Sr2+ and Mg2+ can substitute for Ca2+. Ba2+ currents are about 80% larger than Ca2+ currents and inactivate little. The properties of I(Ca) characterise it as a high threshold L-type current. The outward current consists primarily of a fast, transient A current, I(K(A)) and a maintained, delayed rectifier current, I(K(V)). In some fibres, a small Ca2+-dependent K+ current is also present. I(K(A)) activates fast at depolarisation above -45 mV, shows pronounced inactivation and is almost completely inactivated at holding potentials more positive than -40 mV. I(K(A)) is half-maximally blocked by 70 microM 4-aminopyridine (4-AP), and 70 mM tetraethylammonium (TEA). I(K(V)) activates more slowly, at about -30 mV, and shows no inactivation. It is half-maximally blocked by 2 mM TEA but rather insensitive to 4-AP. Physiologically, the two K+ currents prevent all-or-nothing action potentials and determine the graded amplitude of active electrical responses and associated contractions. Tension development depends on and is correlated with depolarisation-induced Ca2+ influx mediated by I(Ca). The voltage dependence of peak tension corresponds directly to the voltage dependence of the integrated I(Ca). The threshold potential for contraction is at about -38 mV. Peak tension increases with increasing voltage steps, reaches maximum at around 0 mV, and declines with further depolarisation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Calcium Channel Blockers / pharmacology
  • Calcium Channels / physiology
  • Cations, Divalent / pharmacology
  • Cations, Monovalent / pharmacology
  • Cobalt / pharmacology
  • Crustacea / physiology*
  • Electric Conductivity
  • Male
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Muscle Contraction / drug effects
  • Muscle Contraction / physiology*
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / physiology*
  • Patch-Clamp Techniques / methods
  • Patch-Clamp Techniques / statistics & numerical data
  • Potassium / physiology

Substances

  • Calcium Channel Blockers
  • Calcium Channels
  • Cations, Divalent
  • Cations, Monovalent
  • Cobalt
  • Potassium