Inactivation of calcium currents in granule cells cultured from mouse cerebellum.
1. Cells dissociated from mouse cerebellum were grown in vitro. Ca2+ channel currents were recorded from granule cells with the patch-clamp technique under conditions which suppressed currents through Na+ and K+ channels and minimized run-down of current through Ca2+ channels. 2. A strong depolarizing voltage step from a hyperpolarized holding potential produced inward Ca2+ channel current that decayed exponentially to a non-zero level. Inward current decayed to approximately 40% of its peak amplitude (range 20-90%). 3. The inward current increased in amplitude when Ca2+ was replaced with Ba2+ or after raising the concentration of extracellular Ba2+, but the rate of decay of current was unaffected. 4. The current-voltage (I-V) relation showed that peak or sustained current increased with voltage pulses more positive than approximately -30 mV, reached a maximum amplitude near +20 mV and became progressively smaller with larger depolarizations. 5. The tail currents produced after rapidly repolarizing the membrane potential to -70 mV from a positive test pulse decayed along a single exponential time course with a time constant of approximately 0.5 ms. The amplitude of tail current measured at a fixed repolarization potential increased as the pre-pulse was made more positive and reached a maximum with pre-pulses more positive than +40 mV. A plot of normalized amplitude of the tail current as a function of the pre-pulse potential was fitted with a Boltzmann relation with V1/2 = approximately + 8 mV and steepness k = 14 mV. 6. Shifting the holding potential to more positive potentials reduced the amplitude of the Ca2+ channel current elicited by the fixed voltage step and abolished the decay of the inward current. The peak current was normalized to the maximum peak current elicited from a very negative holding potential and plotted as a function of holding potential. The points were fitted with a Boltzmann relation for inactivation with V1/2 = approximately -57 mV and steepness k = 14 mV. 7. The onset of inactivation was studied in two-pulse experiments in which the duration of conditioning pre-pulse was varied. Increasing the duration of a pre-pulse to a fixed potential reduced the peak inward current evoked by the second test pulse. Plotting normalized current as a function of pre-pulse duration showed that inactivation developed along a double exponential time course. Both fast and slow time constants decreased as the pre-pulse potential was made more positive.(ABSTRACT TRUNCATED AT 400 WORDS)
Slesinger
P A
PA
Graduate Program in Neuroscience, School of Medicine, University of California, San Francisco 94143-0450.
Lansman
J B
JB
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D002531:Cerebellum / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology; D004594:Electrophysiology; D015640:Ion Channel Gating / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D013997:Time Factors
D015220:Calcium Channels; D004095:Dihydropyridines; D002118:Calcium