SODIUM-DEPENDENT RELAXATION IN ARTERIAL SMOOTH MUSCLE
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1. The effects of isosmotic substitution of choline for sodium on resting tension and on relaxation after noradrenaline-induced contraction was studied in rabbit isolated aortic strips immersed in Hepes-buffered physiologic salt solution (PSS) warmed to 37 degrees C and gassed with 100% O2. 2. Isosmotic substitution of choline for sodium produced a sustained increase in resting tension which effectively prevented any evaluation of the influence of sodium on relaxation. The increase in resting tension was insensitive to 10(-8) mol/l atropine but was abolished by 10 min exposure to calcium-free PSS prior to replacement of sodium. 3. Under sodium-calcium free conditions which eliminated the increase in resting tension observed in sodium-free PSS, stimulation with 10(-5) mol/l noradrenaline initiated contractions that were 55 +/- 7.5% of the control response in normal PSS. Washout of noradrenaline with sodium-calcium-free PSS failed to produce any decrement in tension. However, restoration of the normal sodium resulted in gradual relaxation. 4. These results suggest that sodium is required for relaxation after noradrenaline-induce contraction of arterial smooth muscle.
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