Characteristics of the norepinephrine-sensitive Ca2+ store in vascular smooth muscle.
A comparison was made between the properties of the norepinephrine- and caffeine-sensitive Ca2+ store in both intact and skinned smooth muscle of the rabbit mesenteric artery. After a first application of 10(-5) M norepinephrine, reapplication of norepinephrine did not induce a second contraction in Ca2+-free medium. However, following this sequence 25 mM caffeine still induced a large contraction. The rates of Ca2+ leakage and Ca2+ filling of the norepinephrine-sensitive store were much faster than those of the caffeine-sensitive one. The amplitude of the norepinephrine-induced contraction in Ca2+-free medium also depended on the amount of Ca2+ present in the caffeine-sensitive store. In the saponin-treated skinned muscle caffeine induced a Ca2+ release only after loading with Ca2+, whereas norepinephrine was unable to induce Ca2+ release in the skinned preparation even after loading with Ca2+. The release of Ca2+ from the caffeine-sensitive store could be activated by Ca2+ itself when the skinned muscle was loaded with Ca2+ above 10(-6) M. These results suggest that the norepinephrine-sensitive Ca2+ store is distinct from a large fraction of the caffeine-sensitive one, and that the norepinephrine-sensitive store is close to the cell membrane. In vascular smooth muscle, under physiological conditions, Ca2+ released from the norepinephrine-sensitive store by norepinephrine may induce Ca2+ release from the caffeine-sensitive Ca2+ store which may be comprised of the sarcoplasmic reticulum.
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D008638:Mesenteric Arteries / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000032:analysis* / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology*; D011817:Rabbits; D012503:Saponins / Q000494:pharmacology
D012503:Saponins; D002110:Caffeine; D002118:Calcium; D009638:Norepinephrine