HORMONAL AND IONIC INFLUENCES ON THE MEMBRANE ACTIVITY OF UTERINE SMOOTH MUSCLE CELLS1

Abstract

Membrane resting and action potentials were recorded from strips of uterine muscles selected from rats and rabbits at known stages of pregnancy. The measurements were made with the “sucrose gap” methods which permits continuous recording of membrane activity for several hours. Strips of muscle from a parturient, i.e. estrogen-dominated, uterus exhibited the following; (a) Cyclic periods of quiescence and activity, which were characterized by alternate depolarization and repolarization of the membrane. Action potentials were discharged during the depolarization phase, (b) Oxytocin, applied during a quiescent period, immediately depolarized the membrane and potentiated the action potential discharge, (c) Pcrfusion with Cafree Krebs solution depolarized the membrane to a level where all action potentials were abolished and oxytocin was ineffective, (d) Addition of 0.25 mM Ca (10% of normal concentration) to the Ca-free solution repolarizcd the membrane somewhat and under these conditions oxytocin further increased the membrane potential to a level where action potentials were generated. Strips of muscles from late pregnant animals, i.e. progesterone-dominated, showed no cyclic membrane activity and were unresponsive to oxytocin. Isotonic potassium chloride depolarized both estrogen and progesteronedominated strips about 28 mV. Istonic potassium sulfate produced 46 mV of depolarization in the estrogen muscles and 54 mV in the progesterone muscles.