Diseases of the adrenal medulla

The adrenal glands are vital in the organism's response to environmental stress. The outer cortex releases steroid hormones: glucocorticoids, mineralocorticoids and sex hormones, which are crucial to metabolism, inflammatory reactions and fluid homeostasis. The medulla is different developmentally, functionally and structurally. It co-releases catecholamines (primarily adrenaline and to some extent noradrenaline) as well as peptides by the all-or-none process of exocytosis from chromaffin granules, to aid in blood pressure and blood flow regulation, with regulated increments during the activation of the sympathetic nervous system. The co-released peptides function to regulate catecholamine release, blood vessel contraction and innate immune responses. Pathology within the adrenal medulla and the autonomic nervous system is primarily because of neoplasms. The most common tumour, called phaeochromocytoma when located in the adrenal medulla, originates from chromaffin cells and excretes catecholamines, but may be referred to as secreting paragangliomas when found in extra-adrenal chromaffin cells. Neoplasms, such as neuroblastomas and ganglioneuromas, may also be of neuronal lineage. We will also briefly discuss the catecholamine deficiency state.     

Myelolipoma associated with adrenal ganglioneuroma

Adrenal myelolipomas are rare, benign mesenchymal tumors composed of mature adipose tissue and hematopoietic cells in varying proportions. Although the majority of cases occur as isolated adrenal lesions, myelolipomas have been described in association with various adrenal pathologic conditions. These conditions include enzyme deficiencies and hyperplastic and neoplastic lesions of the adrenal cortex, with perhaps endocrine dysfunction as a common feature. Ganglioneuroma is a benign tumor of the sympathetic nervous system that rarely produces symptoms of endocrine dysfunction. We report an unusual case of myelolipoma associated with ganglioneuroma of the adrenal medulla. The histogenesis of myelolipoma remains speculative. However, the close proximity to adrenal cortical cells within the stroma of ganglioneuroma suggests that the hormonal microenvironment may have played a role in the development of the myelolipoma.     

Hypoxia and hypercapnia increase the sympathoadrenal medullary functions in anesthetized, artificially ventilated rats

Graded hypoxia (FETO2 14-6%) and hypercapnia (FETCO2 6-10%), which were applied for 45s and 2 min, respectively, to urethane anesthetized and artificially ventilated rats produced an increase in adrenal sympathetic efferent nerve activity in parallel with increases in adrenaline and noradrenaline secretion measured in the adrenal venous effluent. Percentage increases in adrenaline and noradrenaline were almost equal. In rats whose carotid sinus nerves (CSN) were bilaterally cut, hypoxia did not produce any effect on adrenal sympathetic nerve activity or catecholamine secretion. In contrast, excitatory adrenal nerve and catecholamine secretory responses to hypercapnia remained unchanged in CSN denervated rats. After severing a splanchnic nerve whose branches innervated the adrenal gland, while maintaining the resting level of catecholamine secretion by low-frequency stimulation of the peripheral end of the splanchnic nerve, hypoxia did not produce any increase in catecholamine secretion. Hypercapnia (FETCO2 8 and 10%), however, induced catecholamine secretion from denervated adrenal medulla, although the magnitude of the response was significantly lower than that in animals with adrenal nerve intact. It is concluded that hypoxia stimulates the adrenal medulla via the carotid chemoreceptor reflex whereas hypercapnia acts mainly via mechanisms besides carotid chemoreceptors such as central chemoreceptors with some direct stimulatory effect on the adrenal medulla. The functional significance of these dual mechanisms of sympathoadrenal excitation during hypoxia and hypercapnia is discussed.     

The distribution of immunoreactive chromogranins, S-100 protein, and vasoactive intestinal peptide in compound tumors of the adrenal medulla

Three adrenal medullary tumors that showed admixtures of pheochromocytomatous elements with ganglioneuroma or ganglioneuroblastoma were studied to determine the distribution of immunoreactive chromogranins, S-100 protein, and vasoactive intestinal peptide (VIP). Two tumors consisted of typical pheochromocytoma cells admixed with mature-appearing ganglioneuroma. The third consisted of pheochromocytoma admixed with ganglioneuroblastoma and contained many immature and cytologically atypical cells. In all cases, chromogranin staining was absent or weak in neuronal perikarya and moderate to intense in varicosities of neuronal processes, a finding consistent with the presumed distribution of secretory granules in neurons. Chromogranin staining was also intense in chromaffin cells. Glial cells that stained for S-100 were randomly scattered among chromaffin cells but accumulated in areas with neuronal processes. Weak staining for VIP was present in neuronal cells in one of the two tumors with ganglioneuromatous features. Intense staining for VIP occurred in the third tumor in both neuronal and apparently nonneuronal cells. We conclude that granule distribution and cell-cell interactions for specific cell types in compound tumors tend to mimic those in normal adrenal medulla and sympathetic ganglia. Although immunoreactive VIP was localized exclusively to neurons in one tumor, as in normal tissues, patterns of staining for VIP across tumors are less predictable.     

Immunohistochemical localization of protein components of catecholamine storage vesicles

1. The distribution of specific proteins in sympathetic neurones has been examined by immunofluorescent histology using antibodies prepared against soluble protein components of the catecholamine storage vesicles of the adrenal medulla.2. Two antigen preparations were separated by ion exchange chromatography of the soluble proteins released on osmotic lysis of catecholamine storage vesicles which had been isolated by centrifugation from homogenates of sheep adrenal medulla. One fraction (AgDH) had high dopamine-beta-hydroxylase activity, while another (AgCB), consisting of the bulk of the protein, had some capacity to bind catecholamines. On disk gel electrophoresis the antigens ran as single bands with very different mobilities.3. Antisera (AsDH) and (AsCB) produced in rabbits to the two antigens were shown to react specifically with their antigens by immunodiffusion and electrophoresis in agarose.4. Indirect immunofluorescent staining of tissue sections was achieved by layering first the rabbit anti-sera, followed by goat anti-rabbit globulin serum which had been conjugated with fluorescein isothiocyanate.5. The adrenal medulla and the cell bodies of sympathetic ganglia showed the most intense green fluorescence with the immune rabbit sera, and hardly stained when pre-immune serum from the same animal was used. The reactivity of the antisera could be abolished by incubation with the corresponding antigen.6. The preterminal and terminal axons of sympathetic nerves also stained specifically but less intensely with both antisera. When the nerves were ligated for up to 24 hr, the portion immediately proximal to the constriction showed an enhanced reaction to the antisera.7. The results provide evidence that sympathetic neurones contain proteins immunologically identical to those involved in the synthesis and storage of noradrenaline in the adrenal medulla, and support the concept that granular vesicles are synthesized in the perikaryon of the neurone and are transported somatofugally in the axon.     

Centrally evoked increase in adrenal sympathetic outflow elicits immediate secretion of adrenaline in anaesthetized rats

To examine whether feedforward control by central command activates preganglionic adrenal sympathetic nerve activity (AdSNA) and releases catecholamines from the adrenal medulla, we investigated the effects of electrical stimulation of the hypothalamic locomotor region on preganglionic AdSNA and secretion rate of adrenal catecholamines in anaesthetized rats. Pre- or postganglionic AdSNA was verified by temporary sympathetic ganglionic blockade with trimethaphan. Adrenal venous blood was collected every 30 s to determine adrenal catecholamine output and blood flow. Hypothalamic stimulation for 30 s (50 Hz, 100–200 μA) induced rapid activation of preganglionic AdSNA by 83–181% depending on current intensity, which was followed by an immediate increase of 123–233% in adrenal adrenaline output. Hypothalamic stimulation also increased postganglionic AdSNA by 42–113% and renal sympathetic nerve activity by 94–171%. Hypothalamic stimulation induced preferential secretion of adrenal adrenaline compared with noradrenaline, because the ratio of adrenaline to noradrenaline increased greatly during hypothalamic stimulation. As soon as the hypothalamic stimulation was terminated, preganglionic AdSNA returned to the prestimulation level in a few seconds, and the elevated catecholamine output decayed within 30–60 s. Adrenal blood flow and vascular resistance were not affected or slightly decreased by hypothalamic stimulation. Thus, it is likely that feedforward control of catecholamine secretion from the adrenal medulla plays a role in conducting rapid hormonal control of the cardiovascular system at the beginning of exercise.     

Nicotinic acetylcholine receptors of adrenal chromaffin cells

In the adrenal medulla, acetylcholine released by the sympathetic splanchnic nerves activates neuronal-type nicotinic acetylcholine receptors (nAChRs) on the membrane of chromaffin cells which liberate catecholamines into the bloodstream in preparation for the fight and flight reactions. On adrenal chromaffin cells the main class of nAChRs is a pentameric assembly of alpha3 and beta4 subunits that forms ion channels which produce membrane depolarization by increasing Na+, K+ and Ca2+ permeability. Homomeric alpha7 nicotinic receptors are expressed in a species-dependent manner and do not contribute to catecholamine secretion. Chromaffin cell nAChRs rapidly activate and desensitize with full recovery on washout. nAChR activity is subjected to various types of dynamic regulation. It is allosterically modulated by the endogenous neuropeptide substance P that stabilizes receptors in their desensitized state, thus depressing their responsiveness. The full-length peptide CGRP acts as a negative allosteric modulator by inhibiting responses without changing desensitization, whereas its N-terminal fragments act as positive allosteric modulators to transiently enhance nAChR function. nAChR expression increases when cells are chronically exposed to either selective antagonists or agonists such as nicotine, a protocol mimicking the condition of chronic heavy smokers. In this case, large upregulation of nAChRs occurs even though most of the extra nAChRs remain inside the cells, creating a mismatch between the increase in total nAChRs and increase in functional nAChRs on the cell surface. These findings highlight the plastic properties of cholinergic neurotransmission in the adrenal medulla to provide robust mechanisms for adapting catecholamine release to acute and chronic changes in sympathetic activity.     

Sympathetic responses of the heart and adrenal medulla in developing Dahl hypertensive rats

Functional development of the sympathetic nervous system was examined in inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats by assessing cardiac and adrenal medullary responses to insulin-induced hypoglycemia at 2, 4, 8, 12, and 16 days of age. Heart ornithine decarboxylase (ODC) activity and adrenal catecholamine content were measured in pups of the two strains 3 hours after administration of either saline or insulin. The centrally mediated increase in sympathetic outflow caused by insulin-induced hypoglycemia was attended by induction of heart ODC activity and depletion of adrenal epinephrine (EPI). No significant differences were found overall between R/JR and S/JR strains with regard to either heart ODC activity or adrenal epinephrine. This was true for basal values obtained from saline-injected pups as well as for measures from insulin-injected pups. Functional innervation of the heart was present in pups of both strains as early as 2 days of age, while in the adrenal medulla a significant response to stimulation was not detected until 8 days of age. While the susceptibility for hypertension in the salt-sensitive animals may well be linked to increased sympathetic tone, the present findings indicate that S/JR rats do not have an accelerated development or a hyperresponsiveness of sympathetic input to either the heart or the adrenal medulla during the pre-weanling period.     

Synaptic activation of rat adrenal medulla examined with a large photodiode array in combination with a voltage-sensitive dye.

The adrenal medulla is innervated by sympathetic preganglionic nerve fibers in the splanchnic nerve. Synaptic activation of the adrenal medulla causes catecholamine secretion which is known to be modified by various neuropeptides and other factors. To understand the neuronal control mechanism of catecholamine secretion, it is necessary to know the transfer function at the synapse and how it is affected by such factors. By using a large photodiode array in combination with a voltage-sensitive dye, membrane potential changes in a slice of the rat adrenal gland were recorded upon brief local electrical stimulation. Electrical signals were recorded only on the portion of the diode array corresponding to the medulla. In a typical record, a spike and an underlying slow potential were observed following a small deflection due to a presynaptic nerve action potential. Both the spike and slow potential were blocked in Ca(2+)-free solution or by hexamethonium, a nicotinic antagonist, but were not affected by atropine, a muscarinic antagonist. The slow potential was interpreted as a nicotinic synaptic potential in the chromaffin cells and the spike as a population action potential. A double pulse experiment revealed that the chromaffin cell action potential began to fail only when the stimulus interval was less than 50 ms (20 Hz). When the stimulus intensity was reduced, the minimal response was found to behave in an all-or-none fashion. This suggested that one nerve fiber is innervating a cluster of chromaffin cells, which may correspond to a previously histologically identified "complex" of cells [Hillarp (1946) Acta. anat. 4, Suppl. 1]. Each complex was innervated by approximately four nerve fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Histamine in the developing sympathoadrenal system

The ontogenetic distribution of histamine in correlation with catecholamines in the developing rat sympathoadrenal system was analyzed by using an indirect immunohistochemical method and a specific rabbit anti-histamine antiserum. Tyrosine hydroxylase (TH) immunoreactivity was used as a marker of catecholamine synthesis. TH immunoreactivity appeared in retroperitoneal sympathetic tissues on embryonic day 12.5 (E 12.5) when it was found in cells of lumbar chain ganglia. In preaortic sympathetic tissue, TH immunoreactivity was observed on day E 13.5 and in adrenal medullae on day E 14.5. Histamine immunoreactivity was expressed in all of these tissues beginning from day E 14.5. First it was found mainly in nerve fibers, but also in some cells. During the embryonic development the number of histamine-immunoreactive cells increased in all sympathetic tissues studied. In newborn rats, histamine immunoreactivity was restricted to a subpopulation of sympathetic cells, i.e. small intensely fluorescent (SIF) cells of sympathetic ganglia, paraganglion-type cells and some adrenaline-synthesizing cells of the adrenal medulla.     

Pheochromocytoma: Rediscovery as a catecholamine-metabolizing tumor

Catecholamine-producing tumors are rare neoplasms derived mainly from chromaffin cells of the adrenal medulla (pheochromocytomas) or, in about 10% of cases, from paraganglia (paragangliomas). Diagnosis of these tumors relies heavily on measurements of urinary or plasma catecholamines or catecholamine metabolites. The metabolites are usually thought to be produced after release of catecholamines into the bloodstream. This, however, ignores observations of over 40 yr ago that catecholamines are metabolized within pheochromocytoma tumor cells. Development of improved methods for measurement of catecholamine metabolites, in particular, plasma concentrations of free normetanephrine and metanephrine, has reestablished the importance of intratumoral catecholamine metabolism. In patients with pheochromocytoma, over 90% of the elevations in plasma free normetanephrine and metanephrine result from metabolism of catecholamines within pheochromocytoma tumor cells. This process occurs continuously and independently of variations in catecholamine release. As a consequence, measurements of plasma concentrations and urinary outputs of normetanephrine and metanephrine provide more reliable methods for diagnosis of pheochromocytoma than measurements of the parent amines. Rediscovery of the importance of intratumoral catecholamine metabolism is leading to a reevaluation of the procedures used to diagnose pheochromocytoma. This review provides an update on the diagnosis of pheochromocytoma, with emphasis on identifying and correcting relevant misconceptions about catecholamine metabolism.     

Reinvestigation of the effect of orexin A on catecholamine release from adrenal chromaffin cells

Orexins have been shown to be implicated in the regulation of adrenal medulla functions. However, there are still inconsistent investigations on the effects of orexins on catecholamine release from chromaffin cells in varying species. In the present study, using the carbon-fiber amperometry, we investigated whether orexin A would stimulate catecholamine release from rat and mouse adrenal chromffin cells. Puff application of orexin A dose-dependently induced amperometric currents in the cultured rat chromaffin cells, which was completely blocked by the selective OX1R antagonist SB-334867 or by the removal of extracellular calcium. Likewise, in the mouse adrenal medulla slices, orexin A also induced catecholamine release mainly through the activation of OX1R. These results gain insight into our understanding of the pharmacological relevance of orexin system in modulating neuroendocrine functions.     

Brief and repeated noise exposure produces different morphological and biochemical effects in noradrenaline and adrenaline cells of adrenal medulla

Exposure to stressful stimuli is known to activate the peripheral sympathetic nervous system and the adrenal gland. In this study, we evaluated the effects of single or repeated bouts of exposure to a readily measurable stressful stimulus (loud noise) on the catecholamine content and ultrastructure of the rat adrenal medulla. In particular, we measured tissue levels of dopamine, noradrenaline, adrenaline and metabolites. In parallel studies, we evaluated the fine ultrastructure of catecholamine cells, including a detailed study of catecholamine granules and a morphometric analysis of adrenaline and noradrenaline medullary cells. Animals were exposed either to a single (6 h) session of loud (100 dBA) noise, or to this noise stimulus repeated every day for 21 consecutive days. There was a marked correlation between biochemical indexes of catecholamine activity and the ultrastructural morphometry of specific catecholamine granules. Exposure to loud noise for 6 h induced a parallel increase in dopamine, noradrenaline, adrenaline and their metabolites, a polarization and an increased numerical density of noradrenaline and adrenaline granules in the cells. After repeated noise exposure, noradrenaline levels were significantly higher than in controls, and adrenaline decreased significantly. In addition, adrenaline cells also exhibited ultrastructural alterations consisting of wide homogeneous cytoplasmic areas and large, pale vesicles.     

Differential diagnosis of pheochromocytomas and paragangliomas

Paragangliomas are of two types, sympathetic and parasympathetic, depending on the type of paraganglion in which they arise. The term pheochromocytoma is reserved for tumors arising in the adrenal medulla. These tumors are usually fairly easy to diagnose. However, several areas are the subject of debate, including the identification of malignant potential, the diagnosis of medullary hyperplasia, and the recognition of composite tumors. Some histologic features can cause problems in differential diagnosis. Paragangliomas may have spindle cell morphology or contain pigment, requiring distinction from mesenchymal tumors and melanoma, respectively. Extensive degenerative change in phenochromocytomas may mimic adrenal cortical tumors. This short review addresses the diagnosis of pheochromocytomas and paragangliomas and discusses useful approaches in the aforementioned problem areas.     

Glucose does not affect catecholamine stimulus-secretion coupling in rat adrenal medulla: relationship to low changes in osmolarity and to insulin

Glucose levels were analyzed to see whether they directly affect the catecholamine release from chromaffin cells. We incubated isolated adrenal medullae of rats in Krebs-Hepes modified solutions with several glucose concentration, in the presence and absence of carbachol or insulin. Transfer of the medulla from a solution with 11.1 mM of glucose to a 0.56 mM one caused an increase in catecholamine secretion. Relative increase in change of glucose levels from 25 to 0.56 mM and from 50 to 0.56 mM enhanced the effect mentioned above. An inhibitory effect was detected after transfer of the medulla from 0.56 mM to 50 mM glucose. However, correction of solution osmolarity with mannitol or NaCl switched back catecholamine secretion to basal levels in all groups, and correction of solution osmolarity with sucrose indicated an impairment of catecholamine release. No difference was observed in stimulated catecholamine secretion (100 microM of carbachol) at all glucose levels. Further, the presence of insulin did not affect catecholamine secretion in all groups. Our results suggest that in isolated adrenal medullae of rats (1) glucose or variations in glucose levels do not affect catecholamine released; (2) isolated adrenal medulla of rat was highly sensitive to hyperosmolarity and extremely sensitive to hyposmolarity; (3) Insulin had no acute direct effect on catecholamine secretion in isolated adrenal medullae of rats.     

Adrenal medullary hyperplasia: A clinicopathologic study of four cases

This study presents the clinicopathologic findings in four new cases of adrenal medullary hyperplasia. The patients presented with episodic hypertension frequently associated with palpitations headache, and diaphoresis. All four had elevated urinary catecholamine levels during attacks, and were thought clinically to have a pheochromocytoma. In each case laparotomy revealed a diffusely enlarged adrenal gland without a discrete tumor. Histologic examination of the adrenals demonstrated a diffuse or diffuse and nodular expansion of the medulla confirmed by morphometric study. Of the four patients, three underwent unilateral and one bilateral adrenalectomy. Two patients who underwent unilateral adrenalectomy have been free of symptoms for three years. Thus, it would appear that adrenal medullary hyperplasia may occur unilaterally or asynchronously in the two glands. The bilaterally adrenalectomized patient has had persistent attacks, suggesting that the stimulus to adrenal medullary hyperplasia may possibly affect other chromaffin tissues. On the basis of our cases and a review of the literature, we propose the following criteria for the diagnosis of adrenal medullary hyperplasia: a clinical history of episodic attacks suggesting pheochromocytoma (generally with associated increased urinary catecholamine levels), an adrenal gland showing diffuse expansion of the medulla into the alae or tail of the gland with or without nodule formation, a medulla composed of enlarged cells with or without pleomorphism, and, most important, an increased medulla/cortex ratio, together with an increased calculated medullary weight asddetermined by morphometric analysis.     

Somatostatin and substance P inhibit catecholamine secretion from isolated cells of guinea-pig adrenal medulla

Chromaffin cells isolated from guinea-pig adrenal glands secrete catecholamines in response to acetylcholine, nicotine, pilocarpine, veratridine, and high [K⁺]. Both substance P and somatostatin inhibit acetylcholine-induced catecholamine secretion. The maximal inhibition of acetylcholine-induced catecholamine secretion produced by substance P and by somatostatin is approximately 60%: the concentrations of the peptides required for half-maximal inhibition of secretion are approximately 0.8 and 2 μM. respectively. The maximal inhibition of catecholamine secretion produced by somatostatin and that caused by substance P are not additive. The effects of the peptides on secretion are readily reversible. Somatostatin and substance P also inhibit nicotine-induced catecholamine secretion, but they do not inhibit catecholamine secretion stimulated by pilocarpine, veratridine, or high [K⁺]. Thus, these peptides specifically inhibit catecholamine secretion linked to stimulation of nicotinic receptors. The inhibition of acetylcholine-induced catecholamine secretion by somatostatin is noncompetitive with respect to acetylcholine, Na⁺ or Ca²⁺.Immunoreactive somatostatin and substance P are present in guinea-pig adrenal glands. It is suggested that these peptides may play a role in the regulation of catecholamine secretion from the adrenal medulla.     

Mediastinal neuroblastoma,ganglioneuroblastoma, and ganglioneuroma: Pathology review and diagnostic approach

Neuroblastic tumors are a group of tumors of the sympathetic ganglia and adrenal medulla that derive from primordial neural crest cells. These tumors include neuroblastoma, intermixed ganglioneuroblastoma, nodular ganglioneuroblastoma, and ganglioneuroma. Neuroblastomas are the most common extracranial solid tumor arising in childhood and may occur in different anatomic sites. Neuroblastic tumors are common mesenchymal tumors of the mediastinum. Herein, we describe advances in our understanding of neuroblastic tumor biology. Pathologists should be aware of diagnostic challenges associated with these tumors to ensure correct histologic diagnosis and appropriate clinical management. We describe updated mediastinal neuroblastic tumor pathology, focusing on morphological, immunohistochemical, and molecular features and differential diagnoses.     

An ultrastructural study of two cases of adrenal ganglioneuroma

Tissue samples of adrenal ganglioneuromas were obtained from two patients: 35- and 47-year-old males. Light microscopic studies showed that these tumors contained Schwann cells and ganglion cells. Electron microscopic examinations revealed numerous unmyelinated and myelinated axons surrounded by Schwann cells. The ganglion cells in the tumors had abundant organelles, such as well-developed cisternae of the rough endoplasmic reticulum, many profiles of the Golgi apparatus, mitochondria, lysosomes, microtubules and neurofilaments. Electron dense cored granules resembling catecholamine granules were present in the ganglion cell bodies and neural processes. These features resmebled those of normal sympathetic ganglion cells. It is concluded that adrenal ganglioneuroma originates from sympathetic ganglion tissue. This study was presented in part at the 26th Annual Meeting of the Clinical Electron Microscopy Society of Japan, Kochi, October 5–7, 1994.     

Catecholamine uptake into isolated adrenal chromaffin cells: inhibition of uptake by acetylcholine

We have investigated the process of catecholamine uptake in guinea-pig chromaffin cells. Isolated guinea-pig chromaffin cells accumulate [3H]norepinephrine and [3H]epinephrine by a saturable transport system. Catecholamine uptake is dependent upon temperature, energy, and extracellular Na+. The apparent KmS for norepinephrine and epinephrine transport are approximately 1 and 3.5 microM, respectively; the transport maximum (Vmax) for both compounds is about 100 pmol/min/mg protein. The uptake of norepinephrine into chromaffin cells is inhibited by imipramine (Ki = 50 nM) and by desmethylimipramine (IC50 = 20 nM). In both its substrate specificity and its sensitivity to pharmacological inhibition, the catecholamine uptake system in chromaffin cells is similar to the catecholamine transport system previously described in sympathetic neurons. Decreasing external Na+ from 130 to 19 mM increases the apparent Km for norepinephrine to 2.8 microM. Decreasing external norepinephrine increases the Na+ concentration required for half-maximal transport. Agents that depolarize chromaffin cells, such as acetylcholine and veratridine, significantly inhibit [3H]norepinephrine uptake. This decrease in uptake is due to an increase in the apparent Km for norepinephrine. The inhibition of [3H]norepinephrine uptake by depolarizing agents cannot be accounted for by the preferential release of newly-accumulated [3H]norepinephrine, or by the competitive inhibition of [3H]norepinephrine uptake by secreted catecholamines. The inhibition of catecholamine uptake by depolarizing agents suggests that the transport system may be regulated by the membrane potential. Norepinephrine and epinephrine that are spontaneously released from the adrenal medulla may be recaptured in vivo. The inhibition of transport by acetylcholine may prevent the re-uptake of catecholamine released during the physiological stimulation of secretion.