Influence of the sodium gradient on contractile activity in pregnant rat myometrium.
The effects of varying the sodium gradient-either by lowering [Na+]o or by increasing [Na+]i on the electromechanical properties of pregnant rat uterine smooth muscle were studied. In normal tissues, complete removal of external sodium ions (choline, Tris or sucrose as substitutes) induced a strong and maintained contraction which was dependent on the presence of extracellular calcium ions, and was sensitive to Ca2+-antagonist drugs (Nifedipine; D 600, Mn2+). Electrical recordings showed that the membrane was transiently hyperpolarized (-10 +/- 2.4 mV, n = 20); after 1 minute depolarization accompanied by a spontaneous spike discharge occurred. Partial withdrawal of external sodium ions resulted in following changes in twitch contractions evoked by electrical stimulation: a linear relationship was found between the time constant of twitch relaxation and the external Na-concentration. In Na-rich tissues, where the Na/K pump was blocked, or in the presence of monensin, Na-free solutions (whatever the substitute, even K+ ions) again triggered strong contractions entirely dependent on external calcium but rather insensitive to Ca-antagonists. The Na-free (K+) induced contraction was larger than the Na-free (choline or Tris)-induced contraction. It was concluded that the sodium gradient was an important factor for the regulation of contractile activity of uterine smooth muscle. Na-Ca exchange appeared to mediate twitch relaxation in normal tissues and was responsible for Ca-influx in Na-rich tissues.
Laboratoire de Physiologie Cellulaire, Institut de Biochimie Cellulaire et Neurochimie du C.N.R.S., Bordeaux, France.
J P
JP
Savineau
J
J
Mironneau
C
C
Mironneau
D000818:Animals; D002118:Calcium / Q000378:metabolism; D005260:Female; D008839:Microelectrodes; D011188:Potassium / Q000378:metabolism; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D014590:Uterine Contraction
D012964:Sodium; D011188:Potassium; D002118:Calcium