A comparative investigation of the longitudinal and circular muscles in the pregnant rat uterus (10-15days) was made by means of electrical and mechanical recordings. The response of the circular muscle strip was characterized in the following respects:application of stretch caused acceleration of spontaneous activity that was greater in extent than in the longitudinal muscle strip; tetanic contraction was not produced by repetitive stimuli in the range of 0.1-5Hz; slow potential was dominant in the circular muscle cells. The longitudinal contraction of the uterine segment occurred in synchronization with the change in the intraluminal pressure. Either the longitudinal stretch or the increase in the intraluminal volume caused the acceleration of synchronized activity. Stimulation of the longitudinal muscle caused membrane response in the circular muscle cells and vice versa, suggesting electrical interference between longitudinal and circular muscle cells.

SUMMARY1. In the guinea-pig taenia coil, influences of Na and Ca ions on the recovery process from the K contracture were investigated. In the absence of Na ion (sucrose-Krebs solution), the K contracture did not recover when the external K (143 mM) was returned to the normal concentration (5 9mM), although the membrane was repolarized to normal resting potential.2. After reducing the external K concentration to normal, the addition of Na rapidly terminated the contracture. About 5 mM-Na was enough to produce the relaxation, but the rate of relaxation was slower the lower the Na concentration.3. Lithium could substitute for Na in the relaxation, but Tris-hydroxymethyl aminomethane could not. The possibility of a chloride contribution was excluded.4. Ouabain (2 x 10-g/ml.) and K removal reduced the rate of relaxation by Na ion only slightly. Lowering the temperature also had a small effect, having a Q10 of about 1-4. Therefore, the Na-K pump may not be involved in this process, but a physical process seems responsible.5. The contracture in K-Krebs solution and in sucrose-Krebs solution was dependent on the external Ca concentration suggesting a high Ca permeability of the membrane. When sucrose was isosmotically replaced with Mn, Mg, La or Ca ions the relaxation was produced with a relatively fast speed in the absence of external Na ions.6. These results may be explained by assuming that external Na ions are involved in decreasing the Ca permeability of the membrane and in reducing the intracellular Ca concentration by Na-Ca exchange, energy for which is supplied by Na influx. In the relaxation by polyvalent cations, suppression of the Ca permeability is probably the main factor.

Influences of ionsand temperature on the excitatory action

Summary Effects of placental extract (and perfusate) obtained from humans or rats were studied by recording the electrical and mechanical activities of pregnant rat and mouse myometria. 1) Membrane potential of circular muscle cells of pregnant rat myometrium ranged between 50-60 mV. Slow potentials amounting 20-30 mV dominated the circular muscle cells in contrast to spike potentials in the longitudinal cells.2) The placental extract (as well as rat uterine perfusate) reduced the magnitude of spontaneous contractions of myometrial strips of mouse and rat. The inhibitory effect on contractile response became more pronounced when Mg ions were present in the bathing media. The extracts did not cause a significant change in the membrane potential or action potential discharge, but the compound action potential recorded extracellularly was depressed.3) The extracts potentiated the mechanical response, when the myometrial tissue from pregnant animals was treated with fl-blocking agent (10-7g/ml propranolol) or with bretylium (10-5g/ml). 4) It was argued that placenta and endometrium of the pregnant animal release bioactive substances including a certain inhibitory agent which may cause the depressant effect on the conducting mechanism of excitation, possibly through an indirect action in releasing the endogenous catecholamines.Aside from the primary action of placenta as the locus of exchange of gases, nutrients and wastes between fetal and maternal bodies, it is also known to produce several bioactive substances including hormones and antigens.

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