High selectivity of the i(f) channel to Na+ and K+ in rabbit isolated sinoatrial node cells.
The ionic selectivity of the hyperpolarization-activated inward current (i(f)) channel to monovalent cations was investigated in single isolated sinoatrial node cells of the rabbit using the whole-cell patch-clamp technique. With a 140 mM K+ pipette, replacement of 90% external Na+ by Li+ caused a -24.5 mV shift of the fully activated current/voltage I/V curve without a significant decrease of the slope conductance. With a 140 mM Cs+ pipette, the i(f) current decreased almost proportionally to the decrease in external [Na+]o as Li+ was substituted. These responses are practically the same as those observed with N-methyl glucamine (NMG+) substitution, suggesting that the relative permeability of Li+ compared with Na+ for the i(f) channel is as low as that of NMG+. When Cs+ or Rb+ was substituted for internal K+, the fully activated I/V relationship for i(f) showed strong inward rectification with a positive reversal potential, indicating low permeability of the i(f) channel for Cs+ and Rb+. These results show that the i(f) channel is highly selective for Na+ and K+ and will not pass the similar ions Li+ and Rb+. Such a high degree of selectivity is unique and may imply that the structure of the i(f) channel differs greatly from that of other Na+ and K+ conducting channels.
Ho
W K
WK
University Laboratory of Physiology, Oxford, UK.
Brown
H F
HF
Noble
D
D
D000818:Animals; D002586:Cesium / Q000378:metabolism; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism; D017136:Ion Transport; D007700:Kinetics; D008094:Lithium / Q000378:metabolism; D008536:Meglumine / Q000378:metabolism; D008564:Membrane Potentials; D010539:Permeability; D015221:Potassium Channels / Q000378:metabolism*; D011817:Rabbits; D012413:Rubidium / Q000378:metabolism; D012849:Sinoatrial Node / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism*
D007473:Ion Channels; D015221:Potassium Channels; D015222:Sodium Channels; D002586:Cesium; D008536:Meglumine; D008094:Lithium; D012413:Rubidium