Monovalent cation permeabilities of the potassium systems in the crab giant axon

Permeability ratios for pairs of monovalent cations permeating the two potassium systems proposed for the giant axon of the crab Carcinus maenas (M.E. Quinta-Ferreira, E. Rojas & N. Arispe, J. Membrane Biol. 66:171-181, 1982b) were estimated from measurements of the reversal potential of the currents under voltage-clamp conditions. With K+ inside the axon, permeability ratios from the reversal potential of the currents through the late channel are: PRb/PK = 0.9, PNH4/PK less than 0.2 and PCS/PK = 0.18. With CS+ inside the ratios are: PK/PCS = 8.7, PRb/PCS = 7.1 and PNH4/PCS = 2.4. The analysis of the inward currents carried by Rb+ or NH+4 showed similar reversal potentials for the early transient component and the late sustained component. Whence, the sequence of permeabilities for the two types of potassium channels is: PK greater than PRb greater than PNH4 greater than PNa = PCS. The time constants for the activation of the two components recorded either in K-, Rb-, or NH4-artificial seawater are twice as large as the corresponding time constants measured in Na-artificial seawater.