Effect of inhibition of the electrogenic Na⁺/K⁺ pump on the mechanical activity in the rat uterus

Abstract

The effects of ouabain and K(+)-free solution were studied in estrogen-primed rat uterine strips under resting tone or repeatedly stimulated with KCl, acetylcholine or oxytocin applied for 20 minutes at 60 minute intervals. These effects were compared with those of the K+ channel opener cromakalim. In preparations under resting tone, ouabain (0.1 mM and 0.3 mM) induced rhythmic contractions which disappeared after 20-30 minutes whereas at a higher concentration (1 mM) it evoked a rapid, phasic response followed by a small tonic contraction. Exposure of the strip to a K(+)-free solution induced either rhythmic waves, which ceased after 8-10 minutes, or a single phasic contraction which was followed by a small and slow increase in the resting tone (54 +/- 10 mg after 180 min exposure). Nifedipine (0.3 microM) abolished the rhythmic or phasic component of these responses but failed to modify the late small tonic contraction induced by ouabain 1 mM or by K(+)-free solution. Ouabain (0.1-1 mM) or K(+)-free-evoked responses disappeared after short (4 min) or prolonged (60 min) exposure to a Ca(2+)-free, 3 mM EGTA-containing solution. Cromakalim (10 nM-0.1 mM) did not induce any variation in the resting tone either in the presence or in the absence of Ca2+ in the medium. In strips repeatedly stimulated with acetylcholine (0.1 mM) or oxytocin (1 microM), ouabain (0.3 mM), K(+)-free-solution and cromakalim (10 microM) reduced the amplitude of the initial, phasic response and progressively decreased the oscillatory component of the response to these agonists. Conversely, the successive responses evoked by KCl 60 mM in similar experimental conditions were not affected by ouabain or cromakalim. Ouabain (0.3 mM), K(+)-free solution and cromakalim (10 microM) decreased the Ca(2+)-independent, maintained contractions induced by acetylcholine or oxytocin after prolonged exposure to a Ca(2+)-free, EGTA-containing medium. These inhibitory effects were partially or completely reversed in the presence of the non-selective potassium channel blocker tetraethylammonium (10 mM) or in a Ca(2+)-free solution containing 60 mM K+. In conclusion, these results suggest that the response induced by ouabain or K(+)-free solution in estrogen-primed rat myometrium involves Ca2+ influx through potential-operated calcium channels but not Ca2+ release from intracellular stores. In addition, our results show that prolonged exposure to ouabain or K(+)-free medium decreases membrane receptor-mediated responses in rat uterus. This inhibitory effect seems to be the result, at least in part, of a decrease in the cytosolic level of K+, due to the inhibition of the electrogenic Na+ pump.

Inhibitory effects of okadaic acid on rat uterine contractile responses to different spasmogens

“…Ouabain, K+-free and low Na+ (25 mM) solutions induced biphasic responses composed of an early rapid (ouabain and K+-free solution) or slower developed (low Na+ solution) transient contraction followed by a small sustained contraction. Ouabain and K+-free solution have been shown to inhibit the electrogenic Na+ pump and to induce contractions in various types of smooth muscles (Lamb et al, 1988;Tsuchiya et al, 1990;Ausina et al, 1996). A decrease in the [Na+],,/[Na+Ii ratio by reduction of [Na+I0 also results in tension development and the relative rate of contraction is inversely related to "a+], (Savineau et al, 1987;Sat0 and Aoki, 1991).…”

Gestation changes sodium pump isoform expression, leading to changes in ouabain sensitivity, contractility, and intracellular calcium in rat uterus

“…Data from a previous study on rat myometrial preparations showed that 100–300 μ mol/L ouabain induces a significant increase in rhythmic contractions and no increase in resting force (Ausina et al. ). These findings are in broad agreement with ours.…”

“…Different concentrations of ouabain can therefore be used as a way to interrogate and functionally separate the a isoforms in a tissue (Monteith and Blaustein 1998). Ouabain at different concentrations has been reported indirectly to increase Ca 2+ signaling in myometrium but no measurements of intracellular Ca 2+ were made, and the very high concentrations of ouabain used were not designed to allow the effects on the different isoforms to be tested (Ausina et al, 1996).…”

Influence of calixarene C-99 on contractile activity of rat large intestine smooth muscles

“…Відомо, що уабаїн пригнічує ацетилхолінові скорочення вісцеральних гладень-ких м'язів, причому спостерігається переважне інгібування фазного компонента скоротливих відповідей [1,34]. Дослідженнями S. Usune і колег встановлено, що така дія уабаїну залежить від внутрішньоклітинної концентрації іонів Na + і усува-ється за наявності блокатора Na 2+ і супроводжується зниженням напруження м'язів за умови тонічного ско-рочення, отримане нами відносне збільшення тонічного компонента ацетилхоліно-вих скорочень за попереднього блокування натрієвої помпи уабаїном може бути пов'язане з пригніченням витоку іонів K + через ВК Са -канали.…”