Interactions of oxytocin, potassium, and calcium in the rat uterus

¹ Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland

Ovariectomized white rats were treated as follows: group 1, 70 µg estradiol benzoate every other day for 9 days; group 2, 3.1 mg aqueous progesterone daily for 4 days then 6.2 mg daily for 5 days; group 3, untreated controls. Spontaneous muscle contractions were recorded isometrically from isolated uterine horns. Uteri from the estrogen-treated animals were the most sensitive to the stimulatory action of oxytocin (5 mu/ml). The action of oxytocin remained essentially unchanged in the presence of a tenfold increase in external K concentration in both the estrogen-treated and progesterone-treated muscles. A reduction in the Ca concentration of the surrounding medium markedly reduced the effectiveness of oxytocin. This decline in oxytocin potency was most pronounced in the estrogen-treated uteri.