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A prejunctional mechanism involving an alpha 1-adrenergic receptor may exert control on the release of acetylcholine from parasympathetic nerve endings in the heart. To test this hypothesis in vivo, rats were prepared for electrical stimulation of the vagus nerves. Blood pressures and heart rates were monitored, and the animals were treated with alpha-agonists and alpha-antagonists. The alpha 1-selective agonist phenylephrine significantly attenuated vagally induced bradycardia in a dose-dependent fashion (ED50 = 19 micrograms/kg). This is consistent with the hypothesis that there is alpha-adrenergic inhibition of ACh release. In contrast, the alpha 2-selective agonist, BHT-920, caused no change in heart rate during vagal stimulation. The effects of phenylephrine to raise heart rate and blood pressure during vagal stimulation were blocked by the alpha 1-selective antagonist prazosin (ID50 approximately 1 microgram/kg) but not by the alpha 2-selective antagonists yohimbine and rauwolscine. This further supports an alpha 1 assignment to the prejunctional adrenergic receptor mechanism, which can regulate the release of acetylcholine from cardiac parasympathetic neurons.

Parasympathetic effects on in vivo rat heart can be regulated through an alpha 1-adrenergic receptor.