Ion movements in gramicidin pores. An example of single-file transport

Experimental results on ion movement through gramicidin membrane channels are presented and discussed in terms of ion transport in the simplest single-file pore (for review see Urban, B.W. and Hladky, S.B. (1979) Biochim. Biophys. Acta 554, 410-429). Single-channel conductance and bi-ionic potential data for Na+, K+, Cs+, NH4+ and Tl+ are used to assign values to the rate constants of the model. Not all of the rate constants can be determined uniquely and simplifications are introduced to reduce the number of free parameters. The simplified model gives good quantitative fits to the experimental results for Na+, K+, Cs+ and NH4+. For Tl+, although the model accounts qualitatively for the salient features of the results, the quantitative agreement is less satisfactory. Predictions calculated from the model and the fitted rate constants are compared with independent data from blocking and tracer-flux measurements. In agreement with experiment, the model shows that only Tl+ blocks the Na+ conductance significantly. Furthermore, the exponent, n, in the tracer flux ratio rises, as observed, well above unity. The values for the rate constants suggest internal consistency of the model in that entry is always slower to singly occupied pores than to empty pores while exit is always faster from doubly as compared to singly occupied pores. The agreement between model prediction and experimental results suggests that the main features of ion transport in the gramicidin channel arise from cation-cation interaction in a single-file pore.