Potassium channels of the lamprey erythrocyte membrane exhibit a high selectivity to K+ over Rb+: a comparative study...

In order to assess the specificity of potassium channels, we examined the K+ transport pathways in the lamprey red blood cells using 86Rb and stable isotope 41K as tracers measured by the mass spectrometric method. Upon replacing 4 mmol/l K+ with 4 mmol/l Rb+ in the incubation medium, the rate coefficient for unidirectional 86Rb influx (Kin) was significantly reduced from 1.48 +/- 0.10 h-1 to 0.82 +/- 0.05 h-1 (P < 0.001). Addition of 1 mmol/l Ba2+ to the incubation medium significantly decreased 86Rb influx in K(+)- (by 53%) and Rb(+)-media (by 26%). Thus, the reduction in the rate coefficient for 86Rb influx in Rb(+)-medium as compared to K(+)-medium was mainly due to the Ba(2+)-sensitive component (0.23 +/- 0.04 vs. 0.78 +/- 0.10 h-1, P < 0.001). The ouabain-sensitive component of 86Rb influx was also higher in K(+)-(Kin = 0.62 +/- 0.05 h-1) than in Rb(+)-medium (Kin = 0.54 +/- 0.05 h-1, P < 0.05). In the presence of 4 mmol/l 41KCl, the average value of the rate coefficient for the total 41K influx was 8.1 +/- 0.5 h-1. The rate coefficient for 41K influx was reduced to 0.82 +/- 0.13 h-1 in the presence of 1 mmol/l Ba2+. Under the conditions of our assays, the rate of 86Rb uptake via potassium channels of the lamprey red cells was only about 11% in K(+)-medium and only about 3% in Rb(+)-medium compared to 41K influx. These data clearly demonstrate a high selectivity of potassium channels for K+ over Rb+, as already reported for some potassium channels in a variety of cells and tissues.