Mechanisms in the return of vascular tone following sympathectomy in man

The response of the hand blood vessels has been determined to the intra-arterial administration of ephedrine and reserpine and to the intravenous administration of norepinephrine before and at intervals after sympatheetomy. The constrictor response to ephedrine slowly disappeared over a period of six weeks, during which time the vessels became increasingly sensitive to norepinephrine. The dilator effect of reserpine was abolished early after sympatheetomy but became slowly re-established over the subsequent three months. The findings suggest that residual vasoconstrictor fibers cannot account for the return of tone following sympatheetomy but that changes in the catecholamine content of the tissue and increasing sensitivity to both these and circulating amines may be closely related to this phenomenon.

The cholinergic blocking action of adrenergic blocking agents in the pharmacological analysis of autonomic innervation

The adrenergic blocking agents tolazoline, phentolamine, piperoxan, yohimbine, phenoxybenzamine, bretylium and guanethidine block the excitatory actions both of cholinergic nerves and of added acetylcholine on a variety of vertebrate smooth muscle preparations. These cholinergic blocking actions often occurred with concentrations lower than those required to block the response of the guinea-pig vas deferens to stimulation of the adrenergic hypogastric nerve. The anti-acetylcholine activities of these drugs have been studied in detail, using the guinea-pig rectum and the toad bladder as test organs. In preparations sensitive to eserine, the anticholinesterase actions of the drugs competed with their anti-acetylcholine actions, so that either potentiation or block of responses to acetylcholine and to cholinergic nerve stimulation occurred with different concentrations. The responses of the toad bladder to acetylcholine were not potentiated by eserine. This enabled the antagonism of acetylcholine by the anti-adrenergic drugs to be estimated without interference from their anticholinesterase activity. When blocking activity was assessed on guinea-pig rectum previously treated with dyflos, the results were qualitatively similar to those on the toad bladder. Phenoxybenzamine often completely blocks responses both to added acetylcholine and to cholinergic nerve stimulation in concentrations less than those required to block adrenergic nerves. Guanethidine and piperoxan also show strong cholinergic blocking activity. Bretylium, yohimbine, tolazoline and phentolamine were less potent. However, in concentrations required to block the effect on the vas deferens of hypogastric nerve stimulation, these drugs at least halved the effects of acetylcholine and often of cholinergic nerve stimulation. It is concluded that these adrenergic blocking agents cannot be used to distinguish conclusively between adrenergic and cholinergic nerves. For reliable analysis of autonomic innervation, the substances released upon nerve stimulation must be identified by specific biochemical techniques or bioassay.

CENTRAL CARDIOVASCULAR ACTIONS OF GAMMA-AMINOBUTYRIC ACID

The vasomotor and cardiac effects of gamma-aminobutyric acid (GABA) administered by different routes were studied in cats and dogs. The cat was resistant to the action of GABA administered intravenously or into a vertebral artery. Intrathecal injection of GABA into the cat depressed the vasomotor reponse to spinal compression. In dogs, intravenous, intrathecal or intraventricular injection, or topical application of GABA to the floor of 4th ventricle consistently produced hypotension and depressed the reflex and direct excitability of the vasomotor neurones, located at the supraspinal and spinal levels. Bradycardia observed after intravenous and intraventricular injections of GABA into dogs was abolished by stellate ganglionectomy but not by vagotomy. It has been attributed to depression of the central sympathetic neurones.

A FUNCTIONAL STUDY OF DISTRIBUTION OF CARDIAC SYMPATHETIC NERVES

In dogs with unilaterally sympathectomized hearts, the remaining intact stellate ganglion was electrically stimulated 14–58 days following sympathectomy. Pressures were simultaneously recorded from all four chambers of the heart and the resulting responses compared with those observed in similarly stimulated but nonsympathectomized animals. Augmentor responses were markedly decreased in some animals but relatively unchanged in others, and it is evident that considerable variation exists in the distribution of sympathetic fibers in different animals. A common finding was markedly delayed augmentation in which responses were practically absent during the initial periods of stimulation. Chronotropic responses appeared to be largely dependent upon intact connections via the right sympathetic nerves. It may be concluded that the sympathetic cardiac nerves from the right side go primarily to the atria and nodal tissue, with a variable supply to the ventricles. In some animals the latter supply is very large while in others it is minimal. The left sympathetic cardiac nerves furnish the major supply to the ventricles, with variable but generally lesser distribution to the atria.