Activity of the Adrenal Medulla and its Regulation

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Abstract
The most recent experimental studies of adrenomedullary activity confirm the fact that adrenal gland represents the equivalent of a ganglion which must be fully integrated in the orthosympathetic system. Its preganglionic innervation is cholinergic; the neuroadrenomedullary junction behaves like any ganglionic synpase in terms of vulnerability, reaction to synaptolytic poisons, or facilitation of transmission. The medullary pheochromocytes have the same origin as the postganglionic neurons and react to the same poisons. In general, any somatic orthosympathetic hypertension results in adrenomedullary intervention. The endocrine secretion parallels and completes the nervous action. The adrenal medulla releases 2 pressor catecholamines into circulation, epinephrine and norepinephrine. The proportions of the latter vary according to the animal species, generally being less than its methylated homologue. Under physiological conditions of activity, hypotension, hypoglycemia, or anoxia are not specifically related to epinephrine or norepinephrine secretion when the experiments are made on the dog. It seems unlikely that in this animal 2 controlling nervous systems separately regulate the activity of adrenomedullary cells containing preferentially epinephrine or norepinephrine. However, in the case of the cat, authors have found results in favor of the double controlling nervous system for epinephrine and norepinephrine. The stimulation of a denervated gland (which exhibits only reduced "paralytic" activity with release of higher norepinephrine proportions) always increases the percentage of epinephrine. In the dog this may amount to 97 to 98% of catecholamines released into circulation. The variations, in the proportions of the 2 pressor amines in different animal species, are difficult to explain; even in the dog, whose adrenal medullae are clearly epinephrine secretory, adrenalinogenic power with methylation of the norepinephrine precursor is fully developed only in the adult animal. At birth the adrenal glands are not only very rich in norepinephrine but they secrete it in abundance - 50% of the mixture. It is only during the 1st months of life that the pheochromocytes progressively acquire the capacity of epinephrine elaboration. These cells have the same origin as the postganglionic orthosympathetic cells which elaborate norepinephrine. It may be supposed that their actual endocrine differentiation is linked with the elaboration of epinephrine. This differentiation is slow and goes on after birth, the rate varying with the animal species. All physiologists accept the intervention of the adrenal medullae in all instances of diffuse orthosympathetic excitation due to stress or the activation of corrective mechanisms. But, there is no agreement on the existence of a constant adrenomedullary secretion or "physiologic adrenalinemia." Only small quantities of intravenously administered epinephrine are necessary to induce facilitatory effects on synaptic transmission, on activity of the cardiac sinal node, and on the calorigenic capacity of tissues. Circulating norepinephrine is largely of adrenergic sympathetic nerve origin. Epinephrine is of adrenomedullary origin.