Abstract

The ability of the cations Cs⁺ and Rb⁺ to substitute for K⁺ as agents for causing contraction has been studied in vitro in segments of human uterine muscle. The development of tension was measured isometrically. The tension produced by Cs⁺was identical with that produced by K⁺ but Rb⁺ was less effective than K⁺. This quantitative difference in response to Rb⁺ could not have been due to its bulk relative to the size of the membrane channels since the hydrated Crystal is intermediate between Cs⁺ and K⁺. Possible dependence on a "carrier" mechanism, specific for a single cation, could not be proved by use of inhibitors. Ouabain, though an inhibitor of the contractile response to all 3 cations, did not selectively inhibit any one cation, nor was the blocking effect of phenoxybenzamine cation-specific. Inhibition with 2,4-dinitrophenol was only evident for the response to Cs⁺ and K⁺ and this too was not specific.