Extracellular calcium ions are required for muscarine-sensitive potassium current in bullfrog sympathetic neurons.
Cultured bullfrog sympathetic neurons were voltage-clamped in the whole-cell configuration. The extracellular medium contained tetrodotoxin (3 microM) and cesium (1 mM) to block and inward sodium current and a hyperpolarization-activated cation current Attempts were made to separate the M-current from four other potassium currents. Tetraethylammonium (30 mM) was used to block a classical delayed rectifier current (IK) and a fast calcium-activated current (IC). Apamin (30 nM) was used to block a slow calcium-activated current (IAHP). 4-Aminopyridine (1 mM) was used to reduce the amplitude of a transient current (IA). In these conditions, the maximum M-conductance near 0 mV was reduced by as much as 90% when divalent cations such as cobalt (1 mM) were added to the superfusate. The maximum M-conductance was also reduced by as much as 60% when calcium ions were removed from the superfusate. The half-activation voltage in the steady-state activation curve and the reversal potential of the M-current were not significantly changed in the calcium-free solution. It is suggested that the presence of calcium ions in the extracellular space is required for the M-current activation.
Tokimasa
T
T
Department of Physiology, Kurume University School of Medicine, Japan.
Akasu
T
T
D000320:Adrenergic Fibers / Q000502:physiology*; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002478:Cells, Cultured; D004558:Electric Stimulation; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000502:physiology*; D011892:Rana catesbeiana; D011976:Receptors, Muscarinic / Q000378:metabolism / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
D011976:Receptors, Muscarinic; D001030:Apamin; D013779:Tetrodotoxin; D011188:Potassium; D002118:Calcium