Blocking effects of barium and hydrogen ions on the potassium current during anomalous rectification in the starfish egg.
1. The blocking effects of Ba+ and H+ on the inward K current during anomalous rectification of the giant egg membrane of the starfish, Mediaster aequalis, were studied using voltage clamp techniques. 2. External Ba2+ at a low concentration (10--100 micron) suppresses the inward K current; the extent of suppression, expressed as the ratio of currents with and without Ba2+, can be described by a conventional bimolecular adsorption isotherm, K/(K + [Ba2+]o), K being an apparent dissociation constant. 3. The dissociation constant, K, decreases as the membrane potential V becomes more negative and can be expressed by K(V) = K(0) exp (zmuFV/RT), where K(0) is the K at V = 0, z is the charge of the blocking ion, and mu is a parameter for the membrane potential dependence of Ba2+ blockage. The value of mu ranges between 0.64 and 0.68. 4. Upon a sudden change in membrane potential the change in the blocking effect of Ba2+ follows first order kinetics; the forward rate constant is membrane-potential-dependent whereas the backward constant is potential-independent. 5. The blocking effect of Ba2+ appears to be independent of the activation of K channels during anomalous rectification. 6. The blocking effect of Ba2+ depends on V alone, in contrast to the activation of the K channel during anomalous rectification which depends on V--VK. 7. In these respects, the effect of Ba2+ is equivalent to the introduction of inactivation into the anomalous rectification. 8. SI2+ and Ca2+ show small but observable blocking effects only at much higher concentrations (about 10--20 mM). 9. The inward K current is suppressed when the external pH is reduced below 6.0. The blocking effect of H+ shows no significant potential dependence. The concentration dependence suggests that three H+ ions simultaneously titrate the acidic groups of each channel (pK = 5.3--5.4). 10. The implications of these results are discussed in terms of molecular models of the potassium channel of anomalous rectification and possible mechanisms of K channel inactivation.
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D005260:Female; D006859:Hydrogen / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D010063:Ovum / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000502:physiology*; D013215:Starfish / Q000502:physiology*; D013324:Strontium / Q000494:pharmacology
D001464:Barium; D006859:Hydrogen; D011188:Potassium; D002118:Calcium; D013324:Strontium