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Comparative effects of Mg, Ca, Sr, and verapamil on the uterine longitudinal muscle of spayed and estrogen-treated rats.

A comparison was made of effects of Mg, Ca, Sr, or verapamil on the mechanical and electrical activities in the uterine longitudinal muscles of spayed and estrogen-treated rats. The muscle strips taken from spayed rat exhibited spontaneous rhythmic activity in the Locke solution which did not contain Mg, whereas spontaneous activity was less frequent in the preparation taken from estrogen-treated rat. The resting potentials were -54 and -61 mV in the spayed and the estrogen-treated preparations, respectively. An initial spike potential followed by plateau potential with abortive spikes on the top was generated in both spayed and estrogen-treated preparations. In the spayed preparation, the frequency of rhythmic contractions was reduced, and the base-line tension was lowered when 0.6 mM Mg was added to the solution. The base-line tension was elevated progressively when the external Ca concentration was raised, and reached a maximal value up to 10 mM. The amplitude of phasic contraction was progressively increased by increasing Ca concentrations in the range from 1.25 to 5 mM, and was reduced by Ca higher than 10 mM. In the estrogen-treated preparation, the amplitude of phasic contraction was increased by increasing Ca concentrations in the range from 1.25 up to 17.5 mM. When the amplitude of phasic contraction was increased, the duration of plateau potential became protracted. Substitution of the external Ca with Sr caused an increase in the spike activity generated on the top of plateau potential. However, the amplitude of phasic contraction was diminished in both the spayed and the estrogen-treated preparations. Verapamil (2 microM) caused a stronger depression of electrical and mechanical activity in the spayed preparation. Results were discussed in relation to the genomic effects of estradiol on the membrane properties so as to change the interaction with divalent cations.