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

Selective activation of neuronal functions by Ca is determined by the kinetic profile of the intracellular calcium ([Ca]i) signal, in addition to its amplitude. Concurrent electrophysiology and ratiometric calcium imaging were used to measure transmembrane Ca current and the resulting rise and decay of [Ca]i in differentiated pheochromocytoma (PC12) cells. We show that equal amounts of Ca entering through N-type and L-type voltage gated Ca channels result in significantly different [Ca]i temporal profiles. When the contribution of N-type channels was reduced by ω-conotoxin MVIIA treatment, a faster [Ca]i decay was observed. Conversely, when the contribution of L-type channels was reduced by nifedipine treatment, [Ca]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 [Ca]i. Channel-specific differences in [Ca]i decay rates were abolished by depleting intracellular Ca 2+ stores with thapsigargin or by blocking ryanodine receptors with ryanodine, suggesting the involvement of Ca-induced Ca release (CICR). Further support for involvement of CICR is provided by the demonstration that caffeine slowed [Ca]i decay while ryanodine at high concentrations increased the rate of [Ca]i decay. We conclude that Ca 2+ 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 Ca leaves a fingerprint of the route of entry, potentially encoding the selective activation of a subset of Ca-sensitive processes within the neuron.