Cp. Toma et al., CHARACTERISTICS AND POSSIBLE MECHANISMS OF LOW-NA+ INDUCED CONTRACTIONS IN RAT AORTA, Naunyn-Schmiedeberg's archives of pharmacology, 352(1), 1995, pp. 88-93
The influence of reducing external Na+ concentration ([Na+](ex)) upon
vascular smooth muscle contractility was investigated using the rat is
olated aorta. NaCl from the physiological saline solution (PSS) was re
placed with either choline-Cl, sucrose, or LiCl to give the following
[Na+](ex) (mM): 115, 85, 55, and 25 (115NaPSS to 25NaPSS). Small reduc
tions in [Na+](ex) (115NaPSS) induced a biphasic contraction, comparab
le in amplitude with the control one induced by phenylephrine 10(-6) M
. Elimination of the endogenous catecholamine participation using eith
er phentolamine 10(-5) M or guanethidine 3.10(-6) M similarly reduces
these contractions to 25% (sucrose replacement). A similar relaxing ef
fect was obtained with D600 10(-5) M, an antagonist of the voltage ope
rated Ca2+ channels (25-30% residual tension for all the substitutes).
Large reductions in [Na+](ex) (25NaPSS) induced contractions comparab
le in amplitude and shape, but less sensitive to phentolamine and guan
ethidine (residual tension 65-75%, sucrose replacement) and insensitiv
e to D600 (all the substitutes). The Na+/K+ ATP-ase inhibitor ouabain
(10(-4) M) elicited slowly developing contractions, the amplitude bein
g 115% of the phenylephrine 10(-6) M control. Phenylephrine further co
ntracted the 115NaPSS precontracted preparations, but was significantl
y less effective in 25NaPSS, although the precontraction levels were s
imilar for the same substitute used. The amplitude of the superimposed
phenylephrine contractions exhibited [Na+](ex) dependence. Phenylephr
ine 10(-6) M failed to further contract the ouabain 10(-4) M precontra
cted rings. We conclude that relatively small reductions in [Na+](ex)
are able to induce contractions of rat aorta primarily through release
of endogenous catecholamines, probably through neural Na+/Ca2+ exchan
ge. Larger reductions in [Na+](ex) appear to cause contraction through
muscular Na+/Ca2+ exchange.