Abstract
1. 1. Acetylcholine produces a marked increase in the rate and amplitude of the tarantula heart-beat by its actions on the cardiac ganglion. These effects are greatly potentiated by eserine and are considerably reduced by atropine, hexamethonium, and D-tubocurarine. 2. 2. Methacholine produces a moderate increase in heart-rate and a pronounced increase in the amplitude of the contractions. Nicotine produces a marked increase in heart-rate, but either no change or else a reduction in heart-beat amplitude. 3. 3. The effects of methacoline are blocked completely by atropine, but are unaffected by hexamethonium. The effects of nicotine are reduced considerably by hexamethonium, but not by atropine. 4. 4. It is suggested that both nicotinic and muscarinic receptor sites for acetylcholine may be present in the cardiac ganglion.
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