THE EFFECT OF SODIUM BASED HYPO-OSMOLALITY ON ARTERIAL SMOOTH-MUSCLE REACTIVITY IN-VITRO

The study tested the hypothesis that the reduced [Na+](e) and hypoosmo lality of normal pregnancy are causally linked to the attenuation of v ascular smooth muscle reactivity in vitro. Aortic rings from nonpregna nt female rats were incubated in physiological medium containing 114 m M NaCl/l and the contractile responses to phenylephrine, KCl and CaCl2 as well as the relaxations to acetylcholine and KCl were compared wit h those of rings incubated in normal medium containing 119 mM NaCl/l, There was no solute substituted for the lowered [Na+]. Experiments wit h phenylephrine were repeated using de-endothelialized rings and intac t rings pretreated with indomethacin. Contractile responses of intact rings (n=11) in hypo-osmolar solution to phenylephrine were significan tly (P<0.001) lower than of those in normal medium (n=11). Responses w ere partially restored by endothelial denudation but not in the presen ce of indomethacin. Relaxations to acetylcholine (n=7 for hypoosmolar; n=6 for normal solution) and KCl (n=7 for each of hypo-and normal osm olar) were significantly enhanced (P<0.05) in rings incubated in hypoo smolar solution. There was no significant difference between the respo nses of the rings to KCl, and CaCl2 in either solution. These effects are similar to some of those previously described for vascular smooth muscle in normal pregnancy suggesting that the reduced [Na+](e) and hy po-osmolarity of normal pregnancy may be contributing to the diminishe d vascular reactivity.