Electrical Activity of Single Muscle Cells of Pregnant Uteri Studied With Intracellular Ultramicroelectrodes

Abstract

Membrane potentials of uterine muscle cells were measured with intracellular ultramicroelectrodes in pregnant human, rabbit, guinea pig and cat uteri. In different expts. the mean steady membrane potentials ranged from 21-31 mv in human, 32-52 mv in rabbit, 27-66 mv in guinea pig and was 29 mv in cat. These values are low compared with those of striated muscle and other smooth muscles. Large doses of oxytocin reversed the steady membrane potentials in some of the impaled human, rabbit and cat cells but not guinea pig cells. This "oxytocin reversal" effect resulted in means of -23, -22 and -17 mv in human, rabbit and cat uterus, respectively. Action potentials were rarely seen. A series of action potentials from an injured cell of rabbit uterus were 2 mv high, 0.2 seconds long and had 2 distinct maxima. Action potentials similar to those of frog ventricle were occasionally recorded from guinea pig uterus. The available evidence indicates that the low values of the MP are probably the result of a maintained, moderate increase in the permeability of the membrane to Na rather than of impalement damage or a low K concn. gradient. It is tentatively concluded that the reversed MP''s produced by oxytocin arise from a marked increase in the permeability of the membrane to Na since they are close to the -14 mv predicted on this assumption.