The influence of physostigmine on sympathetic outflow and haemodynamics by an action upon the pontomedullary region of the cat.

Administration of physostigmine into the left vertebral artery of the anaesthetized cat elicited a simultaneous decrease of various haemodynamic parameters. This action seems to be mediated by the sympathetic nervous system since interruption of spinal sympathetic pathways by means of thoracicolumbalectomy prevented both the depressor effect and the negative chronotropic action of physostigmine. Moreover, the simultaneous decrease in blood pressure and splanchnic nerve activity produced additional evidence in support of the involvement of the sympathetic nervous system with respect to the observed changes in haemodynamics. The influence of midcollicular transection and pithing upon the cardiovascular effects of physostigmine confirms the earlier established central mechanism which is located within the lower brain stem. It is concluded that stimulation of structures within the pontomedullary region gives rise to the observed changes in haemodynamic parameters. The sympathetic part of the autonomic nervous system plays a dominant role in the depressor effect of physostigmine.