Transmembrane Potentials and Contractility in the Pregnant Rat Uterus

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Abstract
Surviving segments from the uteri of pregnant Long-Evans rats were studied. The rats were within 12 hours prepartum. Spontaneous contractility was measured by use of a strain gauge transducer. By means of the microelectrode technique of intracellular recording, transmembrane potentials were observed simultaneously. It was observed that trains of transmembrane action potentials appeared during rhythmic contraction. The frequency was low at the onset, increased to a maximum averaging greater than one per second and then diminished before cessation of firing. The average duration of impulse trains was 35 seconds at 30°C, less than the duration of the total contraction. Slow depolarization between action potentials often appeared after the first several cycles in a train of impulses. This phenomenon resembled the cardiac pacemaker prepotential. In light of similar experimental observations on stretch receptors by other investigators, it was considered possible that interfiber spread of excitation in uterus might be mediated by stretch.