Extrinsic innervation of the canine abdominal vena cava and the origin of cholinergic vasoconstrictor nerves

Abstract

The effects of electrical stimulation of the greater splanchnic nerve and the vagus nerve on the middle segment of the inferior vena cava (i.v.c.) were studied in anesthetized and unilaterally or bilaterally aderenalectomized dogs. The vascular response of the segment was measured as the change in pressure of an intravascular cuff inserted into the lumen of the segment. Electrical stimulation of the greater splanchnic nerve of either side caused an increase in the tension of the vein with a delay of < 3 s. The tension increased with frequency of stimulation until it reached a maximum at the frequency of 32 Hz. The maximum tension development was consistently larger, by a factor of .apprx. 2, when the right greater splanchnic nerve was stimulated than when the left was. It seems that the right greater splanchnic nerve supplies more sympathetic nerve fibers to the middle segment of the i.v.c. than does the left. The response to electrical stimulation of the right or left greater splanchnic nerve was markedly reduced by phentolamine or hexamethonium. The response remaining after treatment with phentolamine or hexamethonium was augmented by neostigmine and diminished by atropine. The electrical stimulation of the vagus nerve elicited no response of the vein before and after treatment with neostigmine. The effects of electrical stimulation of the right greater splanchnic nerve were further confirmed by a preparation in which the middle segment of the i.v.c. was perfused with Krebs solution in situ. The relation between the frequency of stimulation and the magnitude of the response was almost the same as in the in vivo experiments. Apparently, the response to electrical stimulation of the right greater splanchnic nerve was reduced by phentolamine and the remaining response was augmented by neostigmine and abolished by atropine. The middle segment of the canine i.v.c. receives both adrenergic and cholinergic excitatory innervation, preferentially via the right greater splanchnic nerve. The infrarenal segment of the i.v.c. was also examined in the in vivo experiment, in which it was shown that this segment received much clearer dominant innervation of the right greater splanchnic nerve. The infrarenal segment of the i.v.c. is also supplied with excitatory adrenergic fibers and, in some cases, with the cholinergic ones from the right greater splanchnic nerve.