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Department of Autonomic Physiology, Tokyo Medical and Dental University, Japan.
1. Neurones were acutely dissociated from the rat nucleus basalis. Whole-cell patch clamp recordings of calcium currents (ICa) and fura-2 microfluorimetric recordings of intracellular free Ca2+ concentration ([Ca2+]i) were made simultaneously. 2. Depolarization from -60 to 0 mV elicited ICa and a gradual increase in [Ca2+]i. After repolarization, ICa terminated in 0.7 ms, and [Ca2+]i recovered to control exponentially (1-5 s). 3. Both ICa and the transient [Ca2+]i increase in response to step depolarizations, were abolished in Ca2+ free extracellular solution and in Cd(2+)-containing solution. 4. Depolarizations from -90 mV to membrane potentials less negative than -40 mV induced ICa and an increase in [Ca2+]i. Depolarization to 0 mV elicited the maximum ICa, and produced the largest increase in [Ca2+]i. There was a parallel relationship between the [Ca2+]i increase and the magnitude of the ICa. 5. The [Ca2+]i increase was associated with an increase in total Ca2+ influx when the duration of the step depolarization was varied. The relationship between the total Ca2+ influx and the peak of [Ca2+]i transient reached an asymptote as total Ca2+ influx exceeded 200 pC. A similar finding was made when more than thirty action potentials were used in increasing [Ca2+]i. 6. The process of the [Ca2+]i recovery was slowed down by lowering the temperature, by an intracellular dialysis with vanadate, by extracellular application of a mitochondrial inhibitor, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), and by Na(+)-free external solution. It was unaffected by membrane potential (-50 to -130 mV). 7. When pipette solution contained a high concentration of fura-2 (200 microM), the [Ca2+]i increase per 1 pC of Ca2+ influx decreased, and the [Ca2+]i recovery was slowed. 8. The results indicate that the ICa through voltage-dependent Ca2+ channels elevates [Ca2+]i. The neurones possess a large capacity for Ca2+ buffering, and the recovery of [Ca2+]i requires both the Ca2+ pump and membrane Na(+)-Ca2+ exchange.
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