T-type Ca2+ channel lowers the threshold of spike generation in the newt olfactory receptor cell.
Mechanisms underlying action potential generation in the newt olfactory receptor cell were investigated by using the whole-cell version of the patch-clamp technique. Isolated olfactory cells had a resting membrane potential of -70 +/- 9 mV. Injection of a depolarizing current step triggered action potentials under current clamp condition. The amplitude of the action potential was reduced by lowering external Na+ concentration. After a complete removal of Na+, however, cells still showed action potentials which was abolished either by Ca2+ removal or by an application of Ca2+ channel blocker (Co2+ or Ni2+), indicating an involvement of Ca2+ current in spike generation of newt olfactory receptor cells. Under the voltage clamp condition, depolarization of the cell to -40 mV from the holding voltage of -100 mV induced a fast transient inward current, which consisted of Na+ (INa) and T-type Ca2+ (ICa.T) currents. The amplitude of ICa,T was about one fourth of that of INa. Depolarization to more positive voltages also induced L-type Ca2+ current (ICa,L). ICa,L was as small as a few pA in normal Ringer solution. The activating voltage of ICa,T was approximately 10 mV more negative than that of INa. Under current clamp, action potentials generated by a least effective depolarization was almost completely blocked by 0.1 mM Ni2+ (a specific T-type Ca2+ channel blocker) even in the presence of Na+. These results suggest that ICa,T contributes to action potential in the newt olfactory receptor cell and lowers the threshold of spike generation.
Kawai
F
F
Department of Information Physiology, National Institute for Physiological Sciences, Okazaki, Japan. kawai@nips.ac.jp
Kurahashi
T
T
Kaneko
A
A
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D004056:Differential Threshold; D004553:Electric Conductivity; D007700:Kinetics; D009833:Olfactory Pathways / Q000166:cytology / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010539:Permeability; D012455:Salamandridae / Q000502:physiology*; D011984:Sensory Receptor Cells / Q000502:physiology*; D015222:Sodium Channels / Q000378:metabolism
D015220:Calcium Channels; D015222:Sodium Channels; D002118:Calcium