Calcium-stimulated sodium efflux from rabbit vascular smooth muscle

• Published in: The Journal of physiology Vol. 388; no. 1; pp. 245 - 260
• Main Authors: Kaplan, J H, Kennedy, B G, Somlyo, A P
• Format: Journal Article
• Language: English
• Published: Oxford The Physiological Society 01-07-1987; Blackwell
• Subjects: Animals; Biological and medical sciences; Biological Transport - drug effects; blood vessels; Blood vessels and receptors; calcium; Calcium - pharmacology; Ethers - pharmacology; Fundamental and applied biological sciences. Psychology; In Vitro Techniques; Ionomycin; Ionophores - pharmacology; Muscle Contraction - drug effects; Muscle, Smooth, Vascular - drug effects; Ouabain - pharmacology; Rabbits; smooth muscle; sodium; Sodium - metabolism; Time Factors; Vertebrates: cardiovascular system; Circulatory system; Calcium; Sodium; Membrane channel; Inorganic element; Blood vessel
• DOI: 10.1113/jphysiol.1987.sp016613
• EISSN: 1469-7793
• ISSN: 0022-3751

1. The effects of the addition of Ca2+ on ouabain-resistant 22Na+ efflux from Na+-loaded strips of rabbit portal anterior
mesenteric vein in Ca2+-free media have been studied. 2. Na+ efflux into Li+ media containing 5 mM-KCl is rapidly and transiently
stimulated some 4- to 5-fold on the addition of Ca2+ (1.2 mM). No stimulation is observed if the Li+ medium is K+ free or
if Na+ replaces Li+ ions. This Ca2+-activated Na+ efflux is not obligatorily coupled to Na+ influx. 3. The stimulation of
Na+ efflux could also be triggered by the addition of 5 mM-K+ to a Ca2+-containing K+-free medium. The Ca2+-activated increase
in Na+ efflux also occurred when K+ was the sole monovalent extracellular cation. Rb+ could substitute for the K+ requirement.
Thus the Na+ efflux is not mediated by a system which has a specific requirement for counter-transport of Li+ or one in which
Li+ but not K+ are counter-transported such as the familiar Na+-H+ exchange system. Acidification of the external medium reduced
the Ca2+-stimulated Na+ efflux, in keeping with the conclusion that this efflux was not due to Na+-H+ exchange. 4. Progressive
reduction of external [Ca2+] increased the time-lag to peak activation of Na+ efflux, suggesting that the effects of added
Ca2+ were mediated by a rise in intracellular Ca2+. Under experimental conditions which did not result in activation of the
Na+ efflux by the addition of extracellular Ca2+ alone (e.g. in Na+ media), addition of Ca2+ plus the Ca2+ ionophore, ionomycin,
stimulated Na+ efflux. This further confirms that intracellular sites for Ca2+ are critical for the activation of Na+ efflux.
In the absence of ionophore, in Na+ media, intracellular Ca2+ is not sufficiently increased when extracellular Ca2+ is added.
A partial (40%) block of Ca2+-activated Na+ efflux by amiloride (2 X 10(-3) M) could also be overcome by the addition of ionomycin.
5. The lack of effect of a variety of inhibitors suggests that the Ca2+-stimulated Na+ efflux mechanism is not mediated via
a Na+-K+-Cl- co-transport system or a Na+-H+ counter-transport system, or Na+-Ca2+ exchange. 6. The activation of Na+ efflux
in smooth muscle by Ca2+ ions seems to involve Ca2+ entry partially via an extracellular Ca2+-intracellular Na+ exchange and
also through other parallel pathway(s), followed by a rise in intracellular Ca2+ that activates Na+ efflux through a Ca2+-sensitive
Na+ channel or other transport pathway.