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^-5M 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^-6M. 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.