Sodium action potentials induced by calcium chelation in rat uterine smooth muscle

Abstract

Muscular strips of rat myometrium were exposed to Ca-free solutions in the presence of low concentrations of EGTA (0.1–1 mM). A study of electrical activity was carried out using intracellular recording techniques and the double sucrose-gap apparatus. The results were as followed: (1) After 5–15 min the resting membrane potential was depolarized to about −35 mV und a rhythmic activity appeared. The potentials had no overshoot and their duration was noticeably prolonged showing a sustained plateau close to zero potential. Rhythmic activity was observed in a definite membrane potential range. The effects were essentially reversed upon Ca-replacement. (2) The maximal amplitude of the potentials was strongly dependent on the external Na-concentration. At low Na-concentrations (13 mM), the membrane repolarized to its normal resting value (about −50 mV). (3) The potentials were unaffected by tetrodotoxin (2μM). They were inhibited by Mn (0.1 mM) and D600 (0.1–1 μM) and by increasing Mg concentration (1.24 mM) or Ca concentration (10−3 mM) after 1 h in the presence of EGTA. Higher concentrations of Mn (1–2.5 mM) also repolarized the membrane to its normal resting potential. Tetraethylammonium ions (20 mM) increased both amplitude and duration of the responses but did not blocked the repolarization. (4) Inward Na currents characterized by a slow inactivation were recorded on voltage-clamped preparations. (5) Inhibition of Na-pumping with K-free solution or ouabain (0.1–1 mM) blocked the rhythmic activity and maintained the membrane potential at a steady-state depolarized value. Action potentials reappeared by application of inward current pulses which repolarized the membrane to about −45 mV. After 5–15 min the resting membrane potential was depolarized to about −35 mV und a rhythmic activity appeared. The potentials had no overshoot and their duration was noticeably prolonged showing a sustained plateau close to zero potential. Rhythmic activity was observed in a definite membrane potential range. The effects were essentially reversed upon Ca-replacement. The maximal amplitude of the potentials was strongly dependent on the external Na-concentration. At low Na-concentrations (13 mM), the membrane repolarized to its normal resting value (about −50 mV). The potentials were unaffected by tetrodotoxin (2μM). They were inhibited by Mn (0.1 mM) and D600 (0.1–1 μM) and by increasing Mg concentration (1.24 mM) or Ca concentration (10−3 mM) after 1 h in the presence of EGTA. Higher concentrations of Mn (1–2.5 mM) also repolarized the membrane to its normal resting potential. Tetraethylammonium ions (20 mM) increased both amplitude and duration of the responses but did not blocked the repolarization. Inward Na currents characterized by a slow inactivation were recorded on voltage-clamped preparations. Inhibition of Na-pumping with K-free solution or ouabain (0.1–1 mM) blocked the rhythmic activity and maintained the membrane potential at a steady-state depolarized value. Action potentials reappeared by application of inward current pulses which repolarized the membrane to about −45 mV. These results suggest that Ca-chelation induces membrane permeability changes resulting essentially in: 1) an increase in resting Na-conductance, 2) a prolonged Na-inward current entering through channels normally used by Ca ions, and 3) a reduction in K-conductance. However, active Na pumping does indeed participate to the generation of rhythmic activity.

DOI: 10.1007/BF00584815

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@article{Mironneau1982SodiumAP, title={Sodium action potentials induced by calcium chelation in rat uterine smooth muscle}, author={Jean Mironneau and Daniel Eug{\`e}ne and Chantal Mironneau}, journal={Pfl{\"{u}gers Archiv}, year={1982}, volume={395}, pages={232-238} }