Potassium channels of the lamprey erythrocyte membrane exhibit a high selectivity to K+ over Rb+: a comparative study of Rb-86 and K-41 transport

In order to assess the specificity of potassium channels, we examined the K+ transport pathways in the lamprey red blood cells using Rb-86 and stable isotope K-41 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 Rb-86 influx (K-in) 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 Rb-86 influx in K+-(by 53%) and Rb+-media (by 26%). Thus, the reduction in the rate coefficient for Rb-86 influx in Rb+-medium as compared to Ks-medium was mainly due to the Ba2+-sensitive component (0.23 +/- 0.04 vs. 0.78 +/- 0.10 h(-1), P < 0.001). The ouabain-sensitive component of Rb-86 influx was also higher in K+-(K-in = 0.62 +/- 0.05 h(-1)) than in Rb+-medium (K-in = 0.54 +/- 0.05 h(-1), P < 0.05). In the presence of 4 mmol/l (KCl)-K-41, the average value of the rate coefficient for the total K-41 influx was 8.1 +/- 0.5 h(-1). The rate coefficient for K-41 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 Rb-86 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 K-41 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.