Voltage-dependent activation and inactivation of calcium channels in PC12 cells. Correlation with neurotransmitter release.
The existence and mechanisms of inactivation of voltage-gated Ca2+ channels are important, but still debatable, physiological problems. By using the Ca2+ indicators quin2 and fura-2, we demonstrate that in PC12 cells voltage-gated Ca2+ channels undergo inactivation dependent on both voltage and [Ca2+]i. Inactivation, however, is never complete and a small number of channels remains open during prolonged depolarization, explaining the steady state elevation of [Ca2+]i observed in cells depolarized with high KCl. A close parallel exists between Ca2+ channel inactivation and the transient nature of neurotransmitter release: secretion is rapidly stimulated during the first 30 s of depolarization, when a transient overshoot in [Ca2+]i can be demonstrated, while it is negligible during the following period, despite the persistence of an elevated [Ca2+]i; predepolarization in Ca2+-free medium and subsequent addition of Ca2+ (a condition which allows the development of the voltage inactivation) abolishes the fast phase of secretion, while not modifying the steady state [Ca2+]i eventually attained; and increases in the intracellular Ca2+ buffering decreases the amplitude of the fast secretion phase induced by KCl without altering the steady state [Ca2+]i. We suggest that localized [Ca2+]i gradients form close to the plasma membrane shortly after depolarization and that the [Ca2+]i reached in these regions is the relevant parameter in the regulation of secretion.
D000310:Adrenal Gland Neoplasms / Q000378:metabolism*; D000634:Aminoquinolines; D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism; D016257:Fura-2; D007473:Ion Channels / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D018377:Neurotransmitter Agents / Q000378:metabolism*; D010673:Pheochromocytoma / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats
D000634:Aminoquinolines; D001572:Benzofurans; D007473:Ion Channels; D018377:Neurotransmitter Agents; D011189:Potassium Chloride; C035739:Quin2; D002118:Calcium; D016257:Fura-2; D004298:Dopamine