Effects of estrogen and progesterone on single uterine muscle fibers in the rat

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Abstract
Ovariectomized white rats were treated as follows: group 1, 6.0 µg estradiol benzoate daily for 5 days; group 2, 6.0 µg estradiol for 3 days, then 1.6 µg estradiol plus 12 mg progesterone for 5 days; group 3, untreated controls. Membrane potentials were recorded from single uterine fibers, tension from the entire uterus. Untreated control fibers were quiescent, having a mean resting potential of 35.2 mv. Estrogen-dominated fibers were rhythmically contractile and had a mean resting potential of 57.6 mv. A train of action potentials accompanied and preceded each contraction of the muscle. In certain areas the fibers showed pacemaker-like characteristics, i.e. slow membrane depolarization between action potentials. Progesterone-dominated fibers had significantly higher resting potentials, mean 63.8 mv, but no localized pacemaker areas. Action potentials did not consistently precede or accompany contractions. In groups 1 and 2, acetylcholine stimulated contractions, lowered the membrane potential and increased the discharge rate of action potentials. Epinephrine diminished contractions, raised the membrane potential and abolished the action potential discharge.