CHANGES IN CYTOSOLIC CALCIUM LEVEL IN VASCULAR SMOOTH-MUSCLE STRIP MEASURED SIMULTANEOUSLY WITH CONTRACTION USING FLUORESCENT CALCIUM INDICATOR FURA-2

Publisher Website
Google Scholar
Abstract
In rat aortic strips, muscle contraction was recorded simultaneously with cytosolic Ca++ level, which was indicated by the 500 nm fluorescence of Ca++ indicator, fura 2, due to excitation at either 340 nm (F340) or 380 nm (F380) and the ratio of F340 to F380 (R340/380). On the addition of 72.7 nM K+ or 1 .mu.M norepinephrine, muscle contraction followed the increase in R340/380 (resulted from the increased F340 and the decreased F380). Cytosolic Ca++ concentrations of resting, 72.7 mM K+-stimulated and 1 .mu.M norepinephrine-stimulated aortas were tentatively calculated as 228 .+-. 25, 1784 .+-. 154 and 1528 .+-. 180 nM, respectively. Cumulative addition of K+ or norepinephrine induced concentration-dependent increase in both muscle tension and R340/380. However, norepinephrine induced greater contraction than K+ when both of these stimulants induced similar increase in R340/380. Addition of 10 mM tetraethylammonium and 1 .mu.M Bay k8644 caused rhythmic contractions which followed the rhythmic changes in R340/380. EGTA decreased the muscle contraction and decreased R340/380. In Ca++-free solution, addition of 10 .mu.M norepinephrine or 20 mM caffeine induced transient increase in both muscle tension and R340/380. Tension changes always were preceded by the fluorescent changes. Verapamil (10 .mu.M) decreased both tension development and R340/380 in high K+- and norepinephrine-stimulated tissues. Sodium nitroprusside (1 .mu.M) also decreased both tension and R340/380 in norepineprine-stimulated tissues, whereas it decreased tension more strongly than R340/380 in high K+-stimulated tissues. These results indicate that vasoconstrictors and vasodilators may modulate smooth muscle contraction by changing the cytosolic Ca++ concentrations and also by changing the sensitivity of contractile elements to Ca++.