Calcium-induced calcium release in smooth muscle - Loose coupling between the action potential and calcium release

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.