Some effects of trinitrocresolate and valinomycin on Na and K transport across thin lipid bilayer membranes: A steady-state analysis with simultaneous tracer and electrical measurements

Summary

This paper describes the effect of trinitrocresolate anions (TNC⁻) on the electrical conductance (G _m ), and tracer-measured unidirectional Na and K fluxes (M _Na andM _K) across bilayers formed from sheep red cell lipids dissolved in decane. In the absence of TNC⁻, typical low conductances were observed, while the cation fluxes were too low to measure by our techniques (<10⁻¹² moles cm⁻² sec⁻¹). In the presence of TNC⁻ (10⁻² m),G _m increased and TNC⁻ was the main charge carrier in the system. The cationic fluxes were also much increased, but the membranes showed no significant selectivity between K and Na. Furthermore, the Na and K fluxes were at least two orders of magnitude larger than the ionic fluxes calculated fromG _m . Thus, almost all of the K and Na transport across the membrane in the presence of TNC⁻ is electrically silent and is probably carried out as KTNC and NaTNC ion pairs.

In the presence of valinomycin (10⁻⁶ m) and no TNC⁻, both the ion fluxes andG _m were 10³ times larger in KCl than in NaCl, thus exhibiting the characteristic high selectivity of valinomycin for K over Na. In the presence of both valinomycin (10⁻⁶ m) and TNC⁻ (10⁻² m), this selectivity disappeared in that bothG _m andM _Na in the NaCl system were similar to the respective values in the KCl system. Even under these conditions, most of the Na is still transported by a process which does not carry charge.

BothG _m andM _x increased alike and monotonically with increasing temperature over the range 7 to 30°C. In the absence of TNC⁻ the enthalpies of activation were invariably higher in KCl than in NaCl. Addition of TNC⁻ produced equal enthalpies of activation for both Na and K containing systems suggesting a common, temperature-dependent, ratedetermining step in charge transfer and the electrically silent cation fluxes.