Surface potential reflected in both gating and permeation mechanisms of sodium and calcium channels of the tunicate egg cell membrane

Abstract

SUMMARY1. Threshold changes of Na and Ca currents due to various polyvalent cations (stabilizing cations) or H+ ions were studied in the egg cell membrane of a tunicate, Halocynthia roretzi, by using the voltage-clamp technique.2. With an increase in [Ca], or a decrease in pH in the external solution, the current-voltage (I-V) relations for the peak of the Na and Ca currents shifted along the voltage axis in the positive direction. These voltage shifts in the I-V relations, measured at a potential of Vi where inward current attains its half-maximum, were shown to be identical to shifts in voltage-dependence of the time courses of Na and Ca currents, and also identical to shifts in the inactivation curves of Na current along the voltage axis.3. The shifts in Vi produced by various polyvalent cations or H+ ions were analysed by the Gouy-Chapman equation for the diffuse double layer, by assuming that a change in Vi directly corresponds to a change in the surface double layer potential.4. The V.-divalent cation concentration relations of Na current were exactly described by the predictions of the theory with a constant value of the surface charge density of le-/(9 A)2.

The effect of extracellular low pH on the plateau current in isolated, single rat ventricular cells - A voltage clamp study.

“…It has been also shown in the tunicate egg cells (OHMORI and YOSHII, 1977) that decrease in external pH shifts the I -V relations for the peak of Ca and Sr currents to a positive direction along the voltage axis. They estimated that the binding constants for H+, Cat, and Sr2+ were 1.2 x 10-5, 0.58, and 0.035 M~', respectively.…”

“…Nevertheless, in order to present the voltage dependent kinetics proposed by OHMORI and YOSHII (1977), the time to peak (tp) and half decay time (th) of the IS were plotted in Fig. 3B.…”

“…Both maximum sodium and potassium conductances are reversibly depressed under acidic conditions. A reduction in external pH also has a marked effect on the voltage dependence of a variety of membrane parameters (HILLE, 1968;DROUIN and THE, 1969;MOZHAYAVA and NAUMOV, 1970;WOODHULL, 1973;DROUIN and NEUMCKE, 1974;SHRAGER, 1974;HILLE et al, 1975;SCHAUF and DAVIS, 1976;OHMORI and YOSHII, 1977;HAGIWARA et a1.,1978;CARBONS et al, 1978).…”

Ca2+‐Dependent K+ Currents Induced by Muscarinic Receptor Activation in Guinea Pig Adrenal Chromaffin Cells

“…2, the removal of external Ca2~caused an inward current accompanied by a slight decrease in resting Ca2~I 1. This phenomenon is explained by the increase in Na~permeability resulting from a loss of the stabilizing action of divalent cations (Ohmori and Yoshii, 1977), because these inward currents were abolished by the replacement of external Na~with a nonpermeable cation, N-methyl-D-glucamine. Five seconds after the exposure of the cells to Ca 2tfree solution, muscarine could still evoke an 'out and an increase in [Ca2~] 1, although these amplitudes were somewhat reduced compared with those in the presence of external Ca 2~.…”

Ionic dependence of Ca 2+ channel modulation by syntaxin 1A

“…The electrostatic actions of the divalent ions affect the membrane field (16)(17)(18); they also interact with sites at the channel pore (19)(20)(21) and induce conformational changes after binding at specific regulatory sites, influencing gating (22,23). Therefore, differences observed on the extent and kinetics of secretion by various divalents cannot be attributed entirely to Ca 2ϩ -channel inactivation or binding to an intracellular Ca 2ϩ sensor but could involve a change in conformation induced by binding of the divalent ion to the channel and passage through the pore.…”