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Comparable sensitivity of flow contraction and relaxation to Na reduction may reflect flow-sensor characteristics.

Physiological salt solution infused through the lumen of a resistance branch of the rabbit central ear artery mounted in an isometric myograph causes both contraction and relaxation. The effect of reductions in extracellular Na up to 26 mM (20% of the NaCl content of the physiological saline solution) on these flow-induced changes in wall force and on the contraction to norepinephrine (10(-6) M) and relaxation to acetylcholine (10(-8) to 3 x 10(-6) M) and papaverine (10(-6) to 3 x 10(-5) M) has been studied. Na in the physiological saline solution was reduced by substitution of NaCl with sucrose or N-methyl-D-glucamine. The effect of either of these substitutions was to reduce both responses, contraction and relaxation, to the same extent. This sensitivity is such that physiological changes in blood Na concentration would be expected to influence flow-induced changes in wall tone. Responses to norepinephrine, acetylcholine, and papaverine were not significantly changed by these alterations in Na. It is argued that since reduction of extracellular Na diminished both contraction and relaxation to a similar extent, leading to relaxation and contraction respectively, that this effect probably occurs at a site common to both flow responses. Because both flow effects occur after endothelium removal and the extracellular matrix binds a large proportion of the Na in the blood vessel wall, this may represent the location of a flow-sensitive mechanism.