THE ACTIVATION OF VARIOUS TYPES OF UTERINE MUSCLE DURING STRETCH-INDUCED CONTRACTION

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Abstract
The electrical and mechanical activity of uterine strips from human beings, cats, rats, and rabbits was studied using glass pore surface electrodes and isometric recording of tension. Uterine strips from animals treated with estrogen, or with estrogen and progesterone, were compared with those from pregnant animals. Marked differences in both electrical and mechanical activity were found among the various strips studied. Strips from estrogen or pregnant cats were activated primarily by propagated "ail-or-none" action potentials. Propagated action potentials were recorded from rat uterine strips of all types, but they originated from several sites during the same burst, and did not spread over the entire uterus or even a few millimeters in some strips. Mechanical strain appeared to play a role in activation in rat uterus. In strips from uteri of nonpregnant rabbits, action potentials were small, and propagation was slow and limited in extent. Action potential conduction played a minor role in activation in these tissues. Uterine strips from pregnant rabbits resembled those from cats and rats. Action potentials similar to those of animal uteri were recorded from strips of pregnant human uterus. Progesterone pretreatment and pregnancy increased the number of action potentials and duration of contraction. Progesterone pretreatment reduced the sensitivity of cat uterus to stretch. The effect of reduction of external sodium concentration on propagation of uterine action potentials was studied. The results are discussed in relation to the mechanisms underlying electrical activity and to the question of whether electrical activity is a determinant of mechanical activity in the uterus.

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