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      Characteristics of the Norepinephrine-Sensitive Ca 2+ Store in Vascular Smooth Muscle


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          A comparison was made between the properties of the norepinephrine- and caffeine-sensitive Ca<sup>2+</sup> store in both intact and skinned smooth muscle of the rabbit mesenteric artery. After a first application of 10<sup>-5</sup> M norepinephrine, reapplication of norepinephrine did not induce a second contraction in Ca<sup>2+</sup>-free medium. However, following this sequence 25 m M caffeine still induced a large contraction. The rates of Ca<sup>2+</sup> leakage and Ca<sup>2+</sup> filling of the norepinephrine-sensitive store were much faster than those of the caffeine-sensitive one. The amplitude of the norepinephrine-induced contraction in Ca<sup>2+</sup>-free medium also depended on the amount of Ca<sup>2+</sup> present in the caffeine-sensitive store. In the saponin-treated skinned muscle caffeine induced a Ca<sup>2+</sup> release only after loading with Ca<sup>2+</sup>, whereas norepinephrine was unable to induce Ca<sup>2+</sup> release in the skinned preparation even after loading with Ca<sup>2+</sup>. The release of Ca<sup>2+</sup> from the caffeine-sensitive store could be activated by Ca<sup>2+</sup> itself when the skinned muscle was loaded with Ca<sup>2+</sup> above 10<sup>-6</sup> M. These results suggest that the norepinephrine-sensitive Ca<sup>2+</sup> 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, Ca<sup>2+</sup> released from the norepinephrine-sensitive store by norepinephrine may induce Ca<sup>2+</sup> release from the caffeine-sensitive Ca<sup>2+</sup> store which may be comprised of the sarcoplasmic reticulum.

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          Author and article information

          J Vasc Res
          Journal of Vascular Research
          S. Karger AG
          23 September 2008
          : 21
          : 1
          : 43-52
          Department of Pharmacology, University of Miami School of Medicine, Miami, Fla., USA
          158493 Blood Vessels 1984;21:43–52
          © 1984 S. Karger AG, Basel

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          Pages: 10
          Research Paper


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