Origin of the cholinergic response of the rabbit intestine to stimulation of its extrinsic sympathetic nerves after exposure to sympathetic blocking agents.

The effect of guanethidine and of bretylium on the response to nerve stimulation has been studied on two types of isolated innervated preparations of rabbit intestine. One preparation was that of the rabbit ileum in which the periarterial (mainly sympathetic) nerves were stimulated: the other was the doubly innervated rabbit colon where either parasympathetic (pelvic) or sympathetic (lumbar colonic) nerves were stimulated. In both preparations guanethidine and bretylium in appropriate dosage specifically blocked the inhibitory effect of sympathetic nerve stimulation while leaving the response to parasympathetic nerve stimulation and to acetylcholine unaltered: the response to noradrenaline was unaltered or potentiated. In the ileum, after the addition of guanethidine or of bretylium, the inhibitory response to periarterial nerve stimulation was replaced in every preparation by a motor response which had the same frequency sensitivity as parasympathetic nerves. In the colon a motor response to sympathetic nerve stimulation was rarely obtained after blocking the inhibitory response. When such a motor response was uncovered it had similar characteristics to the motor response in the ileum. Furthermore, if the parasympathetic nerves were stimulated for prolonged periods both the parasympathetic and sympathetic motor responses were reduced. These results do not support the idea that post-ganglionic sympathetic nerves to the intestine are generally cholinergic and are themselves responsible for the motor responses. The experimental results are more conveniently explained by assuming a mixture of cholinergic and adrenergic fibres in the nerves stimulated.