The effects of the replacement of K+ by Tl+, Rb+, and NH+4 on the muscle membrane potential

The resting membrane potential (RMP) of mouse diaphragm muscle was measured in solutions containing several concentrations of K+ (0.4 to 5 mmol/l) or one of the following cations: Tl+ (0.4, 1 or 2 mmol/l), Rb+ (1, 2 or 5 mmol/l), or NH+4 (4, 8 or 16 mmol/l). In terms of controlling the RMP, the ratios of the efficacies were Tl+:K+:Rb+:NH+4 = 2.5:1.0:1.0:0.12. These ratios are similar to those of the selectivities of the voltage dependent K+ channel (delayed rectifier) in frog nerve and muscle, and this similarity suggests that the resting membrane potential may be controlled by this channel.