Ontogeny of local sarcoplasmic reticulum Ca2+ signals in cerebral arteries -: Ca2+ sparks as elementary physiological events

Ca2+ release through ryanodine receptors (RyRs) in the sarcoplasmic reticulum is a key element of excitation-contraction coupling in muscle. In arterial smooth muscle, Ca2+ release through RyRs activates Ca2+-sensitive K+ (K-Ca) channels to oppose vasoconstriction. Local Ca2+ transients ("Ca2+ sparks"), apparently caused by opening of clustered RyRs, have been observed in smooth and striated muscle. We explored the fundamental issue of whether RyRs generate Ca2+ sparks to regulate arterial smooth muscle tone by examining the function of RyRs during ontogeny of arteries in the brain. In the present study, Ca2+ sparks were measured using the fluorescent Ca2+ indicator fluo-3 combined with laser scanning confocal microscopy. Diameter and arterial wall [Ca2+] measurements obtained from isolated pressurized arteries were also used in this study to provide functional insights. Neonatal arteries (<1 day postnatal), although still proliferative, have the molecular components for excitation-contraction coupling, including functional voltage-dependent Ca2+ channels, RyRs, and K-Ca channels and also constrict to elevations in intravascular pressure. Despite having functional RyRs, Ca2+ spark frequency in intact neonatal arteries was approximate to 1/100 of adult arteries. In marked contrast to adult arteries, neonatal arteries did not respond to inhibitors of RyRs and K-Ca channels. These results support the hypothesis that RyRs organize during postnatal development to cause Ca2+ sparks, and RyRs must generate Ca2+ sparks to regulate the function of the intact tissue.