Summary
The aim of the study was to elucidate peripheral effects of ouabain on the parasympathetic innervation of the heart, effects that could contribute to the experimentally and clinically well established “vagal effect of cardiac glycosides”. The experiments were carried out with ouabain concentrations of 3×10−7 and 10−6 mol/l, which were considered “therapeutic”, as they increased force of contraction and did not elicit arrhythmias in incubated chicken atria.
In atrial preparations of chickens and guinea-pigs the negative chronotropic and inotropic effects of acetylcholine (ACh) were not altered by 3×10−7 mol/l ouabain. Resting efflux of ACh from perfused chicken hearts was increased by ouabain from 10 to a maximum of 30 pmol/g min, whereas release of ACh evoked by bilateral vagal stimulation at 3 or 20 Hz for 1 min was unchanged (resting release subtracted). In contrast, release of ACh caused by unilateral vagal stimulation was augmented by ouabain up to 200% of the control. Release by unilateral stimulation (80 pmol/g; 20 Hz) was calculated for each experiment by averaging the releases evoked by consecutive stimulation of the right and left nerves. Ouabain infused for 90 min did not alter the tissue content of ACh (5.5 nmol/g).
Within 2 days after unilateral (left) vagal transsection (denervation of cardiac ganglia) the release of ACh evoked by stimulation of the intact nerve (20 Hz) increased from about 80 to 200 pmol/g, whereas the release from the lesioned nerve markedly declined. One day after denervation, ouabain had lost the ability to facilitate the release of ACh evoked by stimulation of the intact nerve, whereas the release by stimulation of the lesioned nerve was still increased.
It is concluded that ouabain at “therapeutic” concentrations increased resting release of ACh but did not influence the mechanism of action potential-evoked release of ACh. The effect of exogenous ACh on sinus node activity was not enhanced by ouabain. The observation that ouabain increased release of ACh caused by unilateral, but not by bilateral vagal stimulation was explained by an increase in the number of activated postganglionic neurons arising from those (contralateral) ganglia that received a subthreshold input from the stimulated vagus nerve.
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Supported by the Deutsche Forschungsgemeinschaft
Some of the results are part of the M.D. theses of M. Feinauer and B. Ullrich
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Feinauer, M., Lindmar, R., Löffelholz, K. et al. Ouabain enhances release of acetylcholine in the heart evoked by unilateral vagal stimulation. Naunyn-Schmiedeberg's Arch. Pharmacol. 333, 7–12 (1986). https://doi.org/10.1007/BF00569652
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DOI: https://doi.org/10.1007/BF00569652