Calcium and potassium currents in muscle fibres of an insect (Carausius morosus).

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Abstract
A 3 electrode voltage-clamp was used to investigate membrane currents in the skeletal muscle fibers of the stick insect, C. morosus. Contraction was blocked by hypertonic solutions. Membrane currents elicited by step depolarizations consisted of an inward current, an early outward current and a delayed outward current. The reversal potential of the delayed outward current did not change when SO42- was substituted for Cl-, but shifted by 14.1 mV when [K]0 was increased from 20 to 40 mM in SO42- solution, suggesting that the delayed current is carried by K+. Both early and delayed outward currents were substantially reduced by 129 mM tetraethylammonium (TEA) ions. The small size of the shift in the reversal potential of the delayed outward current with increased pulse duration suggests that the delayed current measured flows mainly across the surface membrane. Increasing [Ca]O made the apparent reversal potential for the inward current (120 mM TEA Ringer) more positive and increased the size of the maximum inward current. However, Ca-currents showed saturation with increasing [Ca]O, indicating that there is a site to which Ca2+ bind during their passage through the membrane. The Kd of this site was 7.3 mM at 0 mV and was voltage-dependent. Inward currents were blocked by 1 mM La3+ or Cd2+, or by substitution of Co2+ or Ni2+ for Mg2+. Sr and Ba were able to permeate the channel but Na+ and Mg2+ appear impermeant. As expected from the low intracellular Ca concentration, the instantaneous current-voltage relation of the Ca current rectified strongly in the inward direction. Both constant field theory and the simplest, single site, Eyring rate theory model predict the rectification of the instantaneous current-voltage relation. The rate theory model also predicts saturation of the Ca current with [Ca]O.