The amino-reactive reagent, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS),1 considerably reduces the uptake of the sulfhydryl agent, parachloromercuriphenylsulfonic acid (PCMBS), but does not reduce its effects on cation permeability and on cation transport. These data indicate that PCMBS enters the membrane by at least two channels, one sensitive and the other insensitive to SITS, with only the latter leading to the cation-controlling sulfhydryl groups. Substitution of phosphate or sulfate for chloride results in an inhibition of PCMBS uptake via the SITS-insensitive pathway. These and other data lead to the conclusion that the SITS-sensitive pathway is the predominant one for anion permeation, and the insensitive one for cation permeation. Parachloromercuribenzoate (PCMB), an agent that is more lipid-soluble than PCMBS, penetrates faster but has a smaller effect on cation permeability. Its uptake is less sensitive to SITS. These and other observations suggest that the cation permeation path involves an aqueous channel in the membrane.

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