Behavior of uterine muscle in Na-deficient solutions; effects of oxytocin

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Abstract
Myometrial strips, from parturient or 21-day pregnant white rats, were exposed to Na-deficient solutions, and membrane potentials were recorded from single muscle fibers, isometric tension from the entire muscle strip. Reduction of [Na+]0 to 50% of normal had no observable effect on muscle activity. At [Na+]0 below this level, the amplitude and rising velocity of the action potentials declined and, below 10% of normal, muscle activity was abolished. The stimulatory action of oxytocin on the myometrium was unaffected by reductions in [Na+]0 down to 50% of the normal level. Oxytocin (50 µU/ml) stimulated the production of action potentials in muscles previously depressed by exposure to solutions containing 25% and 10% of normal [Na+]0. The data indicate that although the presence of sodium ions in the external solution is necessary for the production and maintenance of activity in uterine smooth muscle, certain quantitative differences exist between uterine muscle and skeletal or cardiac muscle regarding the relationship of membrane activity to [Na+]0.