Factors of importance for regulation of uterine contractile activity

The myometrium of mammals consists of smooth muscle, having differing origin, structure and function. Isolated strips from human uterus show different patterns of contraction and different reactions to drugs if the strips are taken from different regions of the uterus. In the rat the myometrium is clearly separated in two layers - one outer longitudinal and one inner circular muscle. These two muscles show differing patterns of contraction. In the longitudinal layer there is a gradual increase in frequency and coordination of contraction towards the end of the gestation. The circular muscle contracts with high frequency and low amplitude until 3--4 days before delivery, when the contractions become more regular, less frequent and greater in amplitude. On day 21, immediately before parturition, spontaneous activity diminishes appreciably, but when delivery begins, strong, regular contractions are again evident. The electrical activity too differs in longitudinal vis-à-vis circular muscle; in the latter there are, as a pregnancy proceeds, characteristic changes in the action potentials. The changes in spontaneous contractions are not explained by changes in the overall ability of the muscle to contract, or of the calcium sensitivity of the muscles, but may be due to variations in the resting membrance potential. D-600, an agent known to reduce calcium influx, was found to reduce the contraction of circular muscle more than that of longitudinal muscle, whereas isoprenaline had a stronger inhibitory effect on the longitudinal than on the circular muscle.