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Original Article

Loose Coupling between the Action Potential and Calcium Release

M.L. Collier^a, G. Ji^a, Y.-X. Wang^a, and M.I. Kotlikoff^a

^a Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6046
Department of Animal Biology, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104-6046.215-573-6810

mik{at}vet.upenn.edu

Calcium-induced calcium release (CICR) has been observed in cardiac myocytes as elementary calcium release events (calcium sparks) associated with the opening of L-type Ca²⁺ channels. In heart cells, a tight coupling between the gating of single L-type Ca²⁺ channels and ryanodine receptors (RYRs) underlies calcium release. Here we demonstrate that L-type Ca²⁺ channels activate RYRs to produce CICR in smooth muscle cells in the form of Ca²⁺ sparks and propagated Ca²⁺ waves. However, unlike CICR in cardiac muscle, RYR channel opening is not tightly linked to the gating of L-type Ca²⁺ channels. L-type Ca²⁺ channels can open without triggering Ca²⁺ sparks and triggered Ca²⁺ sparks are often observed after channel closure. CICR is a function of the net flux of Ca²⁺ ions into the cytosol, rather than the single channel amplitude of L-type Ca²⁺ channels. Moreover, unlike CICR in striated muscle, calcium release is completely eliminated by cytosolic calcium buffering. Thus, L-type Ca²⁺ channels are loosely coupled to RYR through an increase in global [Ca²⁺] due to an increase in the effective distance between L-type Ca²⁺ channels and RYR, resulting in an uncoupling of the obligate relationship that exists in striated muscle between the action potential and calcium release.

Key Words: calcium-induced calcium release • smooth muscle • Ca²⁺ sparks • excitation–contraction coupling • action potential signaling

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This article has been cited by other articles:

				R. ZhuGe, K. E. Fogarty, R. A. Tuft, L. M. Lifshitz, K. Sayar, and J. V. Walsh Jr. Dynamics of Signaling between Ca2+ Sparks and Ca2+- Activated K+ Channels Studied with a Novel Image-Based Method for Direct Intracellular Measurement of Ryanodine Receptor Ca2+ Current J. Gen. Physiol., December 1, 2000; 116(6): 845 - 864. [Abstract] [Full Text] [PDF]

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