The effect of Na, K and Cl ions on the resting membrane potential of sino-atrial node cell of the rabbit.

The resting membrane potential of S-A node cell was investigated by observing the response of the membrane potential to change in [K+]O or [Cl-]O under the presence or absence of Na ion. The slope of the change in membrane potential per decade change in [K+]O increased from 12.3 to 44 mV by removal of Na ion from the external medium, suggesting an extensive contribution of Na ion to the resting membrane potential. To determine the relative conductance between Na and K ions, Cl ion in Tyrode solution was substituted with SCN ion, which is 2.1 times more permeable than Cl ion, in order to eliminate the contribution of Cl concentration cell to the resting membrane potential. The contribution of the Cl ion to the resting membrane potential could be examined only in Na-free medium. Acetate ion has been reported to be 0.5 times less permeable than Cl ion. The replacement of Cl ion by acetate ion in various proportions caused a transient depolarization. The slope of this transient depolarization per decade change of [Cl-]O between 10 to 100 mM was determined to be -6.8mV. Under the assumption that the resting membrane potential could be determined by the parallel concnetration cells for Na, K and Cl, relative conductance ratio between K, Na and Cl was calculated to be 1:0.58:0.15.