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J Neurophysiol (March 3, 2004). doi:10.1152/jn.01004.2003
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Submitted on October 20, 2003
Accepted on February 24, 2004

Distinct Intracellular Calcium Profiles Following Influx Through N- vs L-Type Calcium Channels: Role of Ca2+-induced Ca2+ release

Keith Tully and Treistman N. Steven*

* To whom correspondence should be addressed. E-mail: steven.treistman{at}umassmed.edu.

Selective activation of neuronal functions by Ca2+ is determined by the kinetic profile of the intracellular calcium ([Ca2+]i) signal, in addition to its amplitude. Concurrent electrophysiology and ratiometric calcium imaging were used to measure transmembrane Ca2+ current and the resulting rise and decay of [Ca2+]i in differentiated pheochromocytoma (PC12) cells. We show that equal amounts of Ca2+ entering through N-type and L-type voltage gated Ca2+ channels result in significantly different [Ca2+]i temporal profiles. When the contribution of N-type channels was reduced by {omega}-conotoxin MVIIA treatment, a faster [Ca2+]i decay was observed. Conversely, when the contribution of L-type channels was reduced by nifedipine treatment, [Ca2+]i decay was slower. Potentiating L-type current with BayK8644, or inactivating N-type channels by shifting the holding potential to -40 mV, both resulted in a more rapid decay of [Ca2+]i. Channel-specific differences in [Ca2+]i decay rates were abolished by depleting intracellular Ca2+ stores with thapsigargin or by blocking ryanodine receptors with ryanodine, suggesting the involvement of Ca2+-induced Ca2+ release (CICR). Further support for involvement of CICR is provided by the demonstration that caffeine slowed [Ca2+]i decay while ryanodine at high concentrations increased the rate of [Ca2+]i decay. We conclude that Ca2+ entering through N-type channels is amplified by ryanodine receptor mediated CICR. Channel-specific activation of CICR provides a mechanism whereby the kinetics of intracellular Ca2+ leaves a fingerprint of the route of entry, potentially encoding the selective activation of a subset of Ca2+-sensitive processes within the neuron.




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