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The role of sodium in activation of uterine smooth muscle Singh, Harcharan


Extracellular action potentials and isometric contractile tension have been recorded simultaneously in vitro from uterine longitudinal smooth muscle of the pregnant cat, pregnant rabbit, estrogen-treated rabbit, and estrogen-treated rat. Action potentials were recorded from the surface of the muscle strips with glass electrodes having a large tip diameter. Tension was recorded with an RCA transducer. Spontaneous contractions are associated with a series of action potentials. During relaxation no electrical activity is observed. Electrical and mechanical activities were first recorded in Kreb's Ringer medium and then in sodium-poor media (replacement of sodium chloride with choline chloride or sucrose). Sufficient reduction in the external sodium concentration resulted in increased amplitude (peak to peak) of the biphasic action potential spikes. The duration of the peak to peak deflection and the maximum rate of potential, change remained unchanged. However, decrease in the external sodium concentration reduced the frequency of the action potentials, considerably in the cat, and less so in the rabbit and rat. The external sodium concentration was reduced in stepwise ; fashion to ½, ¼ , ⅛ its initial value. Each successive decrease in the external sodium concentration was accompanied by a prompt initial contraction, followed by very slow relaxation and subsequent resumption of spontaneous contractions accompanied by action potentials. With cat uteri reduction of the sodium concentration of the medium to a level of 15-20 mEq/1 resulted in a greatly prolonged contraction with eventual relaxation when tissues failed to contract. This paralysis was associated with cessation of action potentials. The electrical responses of uteri of the other two species (rabbit and rat) during exposure to sodium-poor media were similar to those observed with the cat uterus. However, the mechanical activity of rat and rabbit uteri in sodium poor media was different from that of the cat uterus. Decrease in the external sodium concentration below 25-30 mEq/1 usually resulted in prolonged contractions, and finally to complete failure of the tissue to relax (even after 2-2½ hours). Outbursts of action potentials at irregular intervals were seen in the initial stages of this persistent contraction but eventually action potentials also disappeared. It was difficult to reconcile these facts with the "Sodium Hypothesis". A selective inward flow of sodium ions probably cannot account for the initiation of action potentials in uterine smooth muscle since considerable reduction of the external sodium concentration (down to 15-20 mEq/1 in cat and 25-30 mEq/1 in the other two species) did not affect the characteristics of the action potentials in the expected manner. However, further reduction in sodium did result in electrical and mechanical inactivity. The view that an outward flow of intracellular anions might be responsible for depolarization (14) receives further support from the present studies. In addition to many differences from other types of excitable tissue (nerve, cardiac and skeletal muscle), uterine smooth muscle also shows considerable intra- and inter-species variation.

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