K+ transport across the lamprey erythrocyte membrane: characteristics of a Ba(2+)- and amiloride-sensitive pathway.
The characteristics of K+ transport in erythrocytes from the river lamprey (Lampetra fluviatilis) were investigated using standard radioisotope flux techniques. The cells were shown to have a ouabain-sensitive transport pathway that carried 43K+ and 86Rb+ into the cell at similar rates. Most of the ouabain-resistant 43K+ and 86Rb+ influx was via a pathway that was insensitive to cotransport inhibitors and to the replacement of extracellular Cl- or Na+. This pathway showed a strong selectivity for 43K+ over 86Rb+. It was inhibited fully by Ba2+ (I50 approximately 2.8 mumol l-1), amiloride (I50 approximately 150 mumol l-1) and ethylisopropylamiloride (I50 approximately 3.3 mumol l-1) and less effectively by quinine and by the tetraethylammonium ion. Inhibition by Ba2+ took full effect within a few minutes whereas the full inhibitory effect of amiloride took more than 1 h to develop. Experiments with the membrane potential probe [14C]tetraphenylphosphonium ion gave results consistent with the lamprey erythrocyte membrane having a Ba(2+)-sensitive K+ conductance that was significantly greater than the membrane Na+ conductance and which gave rise to a marked dependence of the membrane potential on the extracellular K+ concentration. The rate constants for Ba(2+)-sensitive 43K+ and 86Rb+ influx decreased (proportionally) with increasing extracellular K+ concentration in a manner that was consistent with the transport being via a conductive pathway. The decrease was attributed to a depolarisation of the membrane (in response to the increasing extracellular K+ concentration) and a consequent decrease in the driving force for the conductive movement of 43K+ and 86Rb+ into the cells. Ba(2+)-sensitive 86Rb+ influx increased significantly with decreasing cell volume and with increasing intracellular pH (at a constant extracellular pH) but increased only slightly with increasing extracellular pH. The pathway operated normally in the complete absence of extracellular Ca2+ but its activity decreased in cells pretreated with ionomycin and EGTA; this suggests a role for intracellular Ca2+ in the operation of the pathway.
University Laboratory of Physiology, Oxford, UK.
K
K
Kirk
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000502:physiology; D004910:Erythrocyte Membrane / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating; D007798:Lampreys / Q000097:blood*; D008564:Membrane Potentials; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000097:blood; D015221:Potassium Channels / Q000502:physiology; D012413:Rubidium / Q000378:metabolism
D015221:Potassium Channels; D001464:Barium; D010042:Ouabain; D000584:Amiloride; D012413:Rubidium; D011188:Potassium; D002118:Calcium