Anomalous permeabilities of the egg cell membrane of a starfish in K+-Tl+ mixtures.

Abstract

A B S T R A C T The electrical properties of "inward" rectifying egg cell membranes of the starfish Mediastera aequalis have been studied in the presence of K+-TI + mixtures. When the ratio of the external concentrations of these ions is changed while their sum is kept constant, both the conductance and the zero-current membrane potential go through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (e.g., Goldman's equation). By contrast, when the ratio of the two concentrations is fixed and their sum varied, the potential follows an ideal Nernst slope, consistent with Goldman's equation. The membrane conductance which, according to previous studies on similar membranes, is to be viewed as a function of the displacement of the membrane potential from its resting value AV, shows marked differences between the cases in which K + or T1 ÷ are the predominant ions: when K ÷ is the predominant permeant ion in solution, the addition of small amounts of T1 + inhibits the current, while corresponding blocking effects of K + on the current are not observed when TI + is the predominant permeant ion. Also, the time course of the conductance during voltage clamp is different in the two cases, being much faster in T1 + than in K ÷ solution for comparable values of AV. Most of the above features are accounted for by a model in which it is assumed that the ionic channels have external binding sites for cations and that their permeability properties depend on the species of the cation bound (K + or TI ÷ in the present experiments).

Blocking effects of barium and hydrogen ions on the potassium current during anomalous rectification in the starfish egg.

“…As already described, K(0) for Ba2+ in eqn. (4) (Hagiwara et al 1976), Rb+ (Hagiwara & Takahashi, 1974; unpublished work by S. Hagiwara & S. Miyazaki, 1977) and Tl+ 183 S. HAGIWARA AND OTHERS (Hagiwara et al 1977), show marked blocking effects on the K channel for the anomalous rectification. In contrast, no significant effect is found for Na+ or NH+4 (Hagiwara & Takahashi, 1974).…”

“…Some monovalent cations such as Cs+, Rb+ and Tl+ reduce the K+ currents by blocking the channel when they are introduced into the external solution (Hagiwara & Takahashi, 1974;Hagiwara et al 1976;Hagiwara, Miyazaki, Krasne & Ciani, 1977). Na+, Cs+ and Rb+ have also been shown to block the K channel responsible for the outward rectification when introduced internally in the squid axon (Bezanilla & Armstrong, 1972;French & Adelman, 1975;French & Wells, 1977).…”

Unidirectional K+ fluxes through recombinant Shaker potassium channels expressed in single Xenopus oocytes.

“…Both of these results are inconsistent with single ion pores. Heginbotham and MacKinnon (1993) showed that these channels exhibited another manifestation of multiion occupancy: an anomalous mole-fraction effect (Hagiwara et al, 1977) for current carried by NH ~ and K + ions.…”

Inward rectification in frog skeletal muscle fibres and its dependence on membrane potential and external potassium.

“…Such models are extensions of one first proposed by Armstrong (1975), and have been developed, in various forms, by Hille & Schwarz (1978) Woodbury, 1949;Hille, 1975 (Stanfield, Ashcroft & Plant, 1981) results in the K-dependent activation being replaced by a Tl-dependent inactivation enhanced by hyperpolarization. And in starfish egg (Hagiwara, Miyazaki, Krasne & Ciani, 1977), Tl+, replacing K+, speeds up the activation process.…”