BIOASSAY OF ACETYLCHOLINE ON THE SINUS VENOSUS OF THE FROG

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Abstract
Electrocardiographic changes induced by the application of acetylcholine (ACh) to the sinus venosus of the frog in situ are described as simple and accurate for the detection of minute amounts of ACh in small sample volumes. The sensitivity and reactivity of the sinus to ACh are increased by pretreatment with eserine, which also prolongs ACh effects. Cardio-inhibitory responses to ACh are either abolished or reversed by atropine, the latter effect revealing the so-called 'sympathomimetic' effect of ACh on the heart. ACh induces a marked increase in the R–P interval of the electrocardiogram which occurs concomitantly with a decrease in the P–R interval. It is suggested that the R–P interval, as opposed to the P–P interval, provides a better estimate for the bioassay of ACh. Changes in temperature of the sinus venosus (induced by application of solutions) must be controlled for meaningful results.