A dopaminergic receptor modulates catecholamine release from the cat adrenal gland

Nicotine tolerance to PC12 cell line: acute and chronic exposures modulate dopamine D2 receptor and tyrosine hydroxylase expression

PC12 is a clonal cell line from chromaffin tumor of rat adrenal pheochromocytoma that releases catecholamine including dopamine, which via interaction with its receptor (D(1) and D(2) receptor), is known to be involved in reward and reinforcement properties of many addictive drugs like nicotine. Nicotine tolerance is the key aspect of nicotine addiction. However, nicotine tolerance on dopamine receptors in PC12 cell line is poorly understood. In this paper, we have demonstrated the tolerance to acute and chronic nicotine administrations on PC12 cell line on the basis of the expressions of dopamine receptors and tyrosine hydroxylase, the rate-limiting enzyme of dopamine biosynthesis, by Western blot, immunohistochemistry and in situ hybridization. In vitro treatment of nicotine resulted in similar expressional changes of dopamine D(2) receptor and tyrosine hydroxylase at protein and mRNA levels in dose- and time-dependent manner, whereas dopamine D(1) receptor did not reveal any positive output. Moreover, moderate to strong signals were obtained from 0.1 to 10 microM of nicotine concentrations and the signals were gradually decreased at 100 and 1000 microM nicotine concentrations relative to the untreated control cell line. Therefore, this study implied a new approach towards nicotine tolerance which is likely to be related to the modulation of dopamine D(2) receptor and tyrosine hydroxylase expressions by chronic and acute nicotine exposures in PC12 cell line.

Neurotransmitters in carotid body function: the case of dopamine--invited article

The carotid body (CB) is the main peripheral chemoreceptor. The present model of CB chemoreception states that glomus (type I) cells are the primary receptors, which are synaptically connected to the nerve terminals of the petrosal ganglion neurons. In response to hypoxia, hypercapnia and acidosis, glomus cells release one (or more) transmitter(s) which, acting on the nerve terminals of chemosensory neurons, increases the afferent discharge. Among several molecules present in glomus cells, dopamine, acetylcholine and 5'-adenosine-triphosphate have been proposed to be the excitatory transmitters in the CB. Beside these putative excitatory transmitters, other molecules modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. In this review, we focus on the role played by dopamine in the CB chemoreception, with emphasis on the open question if the reported differences on its actions on the generation of afferent chemosensory activity reflect true species differences. The available data suggest that dopamine may play a modulatory role within the cat CB, while in the rabbit CB, dopamine is an excitatory transmitter. Therefore, the reported differences on the actions of exogenously applied dopamine and its participation on the generation of afferent chemosensory activity appear to reflect true species differences.

Roles of dopaminergic d(1) and d(2) receptors in catecholamine release from the rat adrenal medulla

The aim of the present study was designed to establish comparatively the inhibitory effects of D(1)-like and D(2)-like dopaminergic receptor agonists, SKF81297 and R(-)-TNPA on the release of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused model of the rat adrenal medulla. SKF81297 (30microM) and R-(-)-TNPA (30microM) perfused into an adrenal vein for 60 min, produced great inhibition in the CA secretory responses evoked by ACh (5.32x10(- 3) M), DMPP (10(-4) M), McN-A-343 (10(-4) M), high K(+) (5.6x10(-2) M), Bay-K-8644 (10microM), and cyclopiazonic acid (10microM), respectively. For the release of CA evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid, the following rank order of inhibitory potency was obtained: SKF81297>R-(-)-TNPA. However, R(+)-SCH23390, a selectve D(1)-like dopaminergic receptor antagonist, and S(-)-raclopride, a selectve D(2)-like dopaminergic receptor antagonist, enhanced the CA secretory responses evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid only for 0~4 min. The rank order for the enhancement of CA release evoked by high K(+), McN-A-343 and cyclopiazonic acid was R(+)-SCH23390>S(-)-raclopride. Also, the rank order for ACh, DMPP and Bay-K-8644 was S(-)-raclopride > R(+)-SCH23390. Taken together, these results demonstrate that both SKF81297 and R-(-)-TNPA inhibit the CA release evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors and the membrane depolarization from the isolated perfused rat adrenal gland without affecting the basal release, respectively, but both R(+)-SCH23390 and S(-)-raclopride facilitate the CA release evoked by them. It seems likely that the inhibitory effects of SKF81297 and R-(-)-TNPA are mediated by the activation of D(1)-like and D(2)-like dopaminergic receptors located on the rat adrenomedullary chromaffin cells, respectively, whereas the facilitatory effects of R(+)-SCH23390 and S(-)-raclopride are mediated by the blockade of D(1)-like and D(2)-like dopaminergic receptors, respectively: this action is possibly associated with extra- and intracellular calcium mobilization. Based on these results, it is thought that the presence of dopaminergic D(1) receptors may play an important role in regulation of the rat adrenomedullary CA secretion, in addition to well-known dopaminergic D(2) receptors.

Facilitation of quantal release induced by a D1-like receptor on bovine chromaffin cells

Dopaminergic receptors are found on bovine adrenal chromaffin cells and have been implicated in the facilitation of an inward calcium current [Artalejo et al., (1990) Nature 348, 239-242] that could enhance release. However, previous studies using incubations of long duration (minutes) with dopaminergic receptor antagonists have found instead an inhibition of catecholamine release. In this work we used brief (subsecond) chemical depolarizing stimuli to reexamine the role of dopaminergic receptors on exocytosis from bovine adrenal chromaffin cells. Responses to consecutive depolarizing stimuli were compared using amperometry to monitor vesicular release events and intracellular fura-2 to examine Ca2+ dynamics within individual cells. Restoration of intracellular Ca2+ levels to their initial values following exposure to 60 mM K+ was found to be prolonged unless the exposure was brief (0.5 s) and the cells were maintained at 37 degrees C. However, with these optimum conditions, a second stimulation evoked more exocytotic events than the first. This effect was blocked by SCH-23390, a D1 antagonist, in a dose dependent fashion, but not by raclopride, a D2 antagonist. The D1 agonist, SKF-38393, enhanced the number of exocytotic events as did prior exposure of the cell to epinephrine. Taken together, the data indicate that released catecholamines can enhance their own release by interaction with a D1-like receptor on bovine adrenal chromaffin cells.

Effects of increasing infusion rates of dopamine, dobutamine, epinephrine, and phenylephrine in healthy anesthetized cats

OBJECTIVE

To determine the cardiopulmonary effects of increasing doses of dopamine, dobutamine, epinephrine, and phenylephrine and measure plasma concentrations of norepinephrine, epinephrine, and dopamine in cats anesthetized with isoflurane.

ANIMALS

6 healthy adult cats.

PROCEDURES

Each cat was anesthetized with isoflurane (1.5 X minimum alveolar concentration) on 4 occasions. Cardiopulmonary measurements were obtained after a 30-minute stabilization period; 20 minutes after the start of each infusion dose; and 30, 60, and 90 minutes after the infusion was discontinued. Cats received 5 progressively increasing infusions of epinephrine or phenylephrine (0.125, 0.25, 0.5, 1, and 2 microg/kg/min) or dobutamine or dopamine (2.5, 5, 10, 15, and 20 microg/kg/min). The order of treatment was randomly allocated. Results-All 4 treatments increased oxygen delivery. Heart rate (HR) increased during administration of all drugs except phenylephrine, and mean arterial pressure increased during administration of all drugs except dobutamine. A progressive metabolic acidosis was detected, but whole-blood lactate concentration only increased during administration of epinephrine and dobutamine. Systemic vascular resistance index increased during administration of phenylephrine, decreased during administration of dobutamine, and remained unchanged during administration of dopamine and epinephrine. A positive inotropic effect was detected with all treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

During anesthesia in cats, administration of dopamine, dobutamine, and epinephrine may be useful for increasing cardiac output, with dopamine having the most useful effects. Administration of phenylephrine increased cardiac and systemic vascular resistance indexes with minimal effect on HR and may be useful for increasing mean arterial pressure without increasing HR.

Neurochemical, neuroautonomic and neuropharmacological acute effects of sibutramine in healthy subjects

Sibutramine is a neuropharmacological drug that exerts central (CNS) and peripheral effects including noradrenaline (NA), and serotonin (5-HT) uptake inhibition. In addition, the drug is able to induce release from DA axons. We measured levels of circulating neurotransmitters in 20 healthy subjects during supine-resting (fasting) state before and after 15 mg of oral sibutramine. Systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) were also monitored. Sibutramine triggered sustained and progressive increase of NA, NA/Ad ratio and DBP. Slight increases of DA were also registered between the 60 and 240 min periods. The rise in DA tended to fade progressively, reaching basal level at 360 min period. Diastolic blood pressure, but neither SBP nor HR, showed significant increases that correlated positively with NA/Ad ratios. Slight but significant negative correlation was also found between DBP and DA. This correlation tended to fade throughout the trial to show no significance at the 360 min period. Although neither plasma serotonin (f-5HT) nor platelet serotonin (p-5HT) values showed significant variation throughout the trial, the f-5HT/p-5HT ratio showed significant decrease throughout. Significant negative correlation was found between f-5HT/p-5HT ratio and NA/Ad ratio. Our results indicate that sibutramine stimulates neural sympathetic activity but not adrenal sympathetic activity in healthy individuals. Further, sibutramine lowers parasympathetic activity. The moderate rise in diastolic blood pressure triggered by sibutramine would be associated with CNS-NA enhancement plus parasympathetic inhibition.

Effects of dopamine on the adrenal gland of Podarcis sicula (Reptilia, Lacertidae)

The effects of dopamine administration on the adrenal gland of a lizard, Podarcis sicula, are described. Dopamine (0.7mg/100g body wt/day for 4 consecutive days) raised plasma ACTH and corticosterone levels (ACTH: from the basal level of 4.40+/-0.05-7.30+/-0.08pg/ml 24h after the fourth dopamine injection; corticosterone: from 3.59+/-0.03ng/ml in untreated lizards to 7.40+/-0.05ng/ml 24h after the fourth dopamine injection), showing a stimulatory effect on the pituitary-interrenal axis activity. In the chromaffin tissue dopamine apparently enhanced the activity of PNMT enzyme; in fact a strong raise in the number of adrenaline cells and a decrease in the number of noradrenaline cells were observed, decreasing the numeric NA/A cell ratio, from 1.4/1 of control specimens to 0.5/1 24h after the fourth dopamine injection. At EM level, chromaffin cells contained both NA and A granules, as well as very clear granules (CG); CG granules showed granular elements ranging between 340 and 347A in diameter. These cells might be the morphological expression of a process of catecholamine resynthesis, due to a possible increase in catecholamine release, following exposure to dopamine.

Functional cooperation between neurosteroids and D2 dopamine antagonists on KCl-evoked [3H]noradrenaline release: modulation by calcium channel blockers

It has recently been proposed that neurosteroids, such as dehydroepiandrosterone sulphate and pregnenolone sulphate, interfere with the dopamine system in the central nervous system. According to our previous report showing that the butyrophenone, spiperone, slightly enhances the evoked release of [3H]-noradrenaline ([3H]NA) in the presence of these sulphated steroids, the present study was carried out to document the putative interplay between steroids and spiperone, which is known to be a prototypic D2 dopamine antagonist and also a 5-HT2 serotonin antagonist. For this purpose, the paradigm of KCl-evoked [3H]NA release from preloaded rat hippocampal slices was used to investigate the interactions between neurosteroids, spiperone and the voltage-sensitive calcium channels (VSCCs). The selective 5-HT2 serotonin antagonist ritanserine was ineffective, whereas sulpiride, a selective D2 dopamine antagonist mimicked the action of spiperone, thus suggesting that the blockade of D2 dopamine receptors accounted for the modulatory effect of spiperone on neurosteroid-induced modulation of evoked [3H]NA release. In addition, this facilitation of KCl-evoked [3H]NA release by the combination of a steroid and a D2 dopamine antagonist was partially inhibited by the L- and N-type VSCC blockers nifedipine and omega-conotoxin GVIA, respectively. The present results provide in-vitro functional evidence for the putative role of VSCCs in the interplay between steroids and D2 dopamine receptors.

Inhibitory mechanism of bromocriptine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal medulla

The purpose of this study was to determine whether bromocriptine affects the catecholamines (CA) secretion evoked in isolated perfused rat adrenal glands, by cholinergic stimulation, membrane depolarization and calcium mobilization, and to establish the mechanism of its action. The perfusion of bromocriptine (1-10 microM) into an adrenal vein, for 60 min, produced relatively dose-dependent inhibition in the secretion of catecholamines (CA) evoked by acetylcholine (ACh, 5.32 mM), DMPP (100 microM for 2 min), McN-A-343 (100 microM for 2 min), cyclopiazonic acid (CPA, 10 microM for 4 min) and Bay-K-8644 (10 microM for 4 min). High K+ (56 mM)-evoked CA release was also inhibited, although not in a dose-dependent fashion. Also, in the presence of apomorphine (100 microM), which is also known to be a selective D2-agonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly depressed. However, in adrenal glands preloaded with bromocriptine (3 microM) in the presence of metoclopramide (15 microM), a selective D2-antagonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid considerably recovered as compared to that of bromocriptine only. Taken together, these results suggest that bromocriptine can inhibit the CA secretion evoked by stimulation of cholinergic receptors, as well as by membrane depolarization, in the perfused rat adrenal medulla. It is thought this inhibitory effect of bromocriptine may be mediated by inhibiting the influx of extracellular calcium and the release from intracellular calcium stores, through the activation of dopaminergic D2-receptors located in the rat adrenomedullary chromaffin cells. Furthermore, these findings also suggest that the dopaminergic D2-receptors may play an important role in regulating adrenomedullary CA secretion.

Block by 5-hydroxytryptamine and apomorphine of recombinant human neuronal nicotinic receptors

The effects of 5-hydroxytryptamine and apomorphine on human neuronal nicotinic acetylcholine receptor/channels were examined by expressing these channels in Xenopus oocytes. Functional channels were expressed by combining one type of alpha subunits (alpha3 or alpha4) and one type of beta subunits (beta2 or beta4). 5-Hydroxytryptamine (100 microM to 1 mM) and apomorphine (10 to 100 microM) inhibited an inward current activated by acetylcholine in the oocytes expressing the channels. The sensitivity to 5-hydroxytryptamine or apomorphine depended on subunit combinations. When concentration-response relationship was obtained for the acetylcholine-activated current, the maximal response was reduced by these compounds. The inhibition by these compounds exhibited voltage-dependence: the inhibition was augmented at negative potentials. The results suggest that 5-hydroxytryptamine and apomorphine noncompetitively inhibits human recombinant nicotinic acetylcholine receptor/channels, presumably by acting on channel pores.

Dopamine and sympathoadrenal activity in man

The sympathetic adrenal (SA) activity can be modulated by dopamine (DA) through D2 receptors. In man, using D2 antagonists, it has been demonstrated that endogenous DA plays an inhibitory modulation of the SA system during high degree of SA activation. D2 agonists are able to induce a decrease in norepinephrine (NE) release either in vitro or in vivo. This effect leads, in vivo, to a decrease in blood pressure (BP) and to an activation of arterial baroreceptors. Therefore, in vivo, the D2 mediated inhibition of epinephrine (E) release, which is clearly demonstrated in vitro, is overwhelmed by the baroreceptor-mediated activation of the splachnic nerve. As a consequence, the in vivo administration of D2 agonists can induce a different effect on the net peripheral sympathetic tone of an organ, depending on the balance between the degree of the baroreceptor-mediated sympathetic activation and the inhibitory D2-mediated inhibition of NE release at the tissue level. In the present paper we investigated the in vivo effect of placebo (PL) or acute oral bromocriptine (BC) administration on plasma CA and on the cardiac sympatho-vagal balance of 7 normal volunteers, as assessed by power spectral analysis of heart rate (HR) variability (autoregressive method), either in resting or sitting position. Low frequency (LF) and high frequency (HF) components, both expressed in normalized units (nU), and LF/HF ratio were calculated. BC caused a decrease in BP, plasma NE and no change in HR in resting and sitting position. Plasma E increased in sitting position. At the heart level, after BC, we observed, during rest, an increase in LF and LF/HF ratio and a decrease in HF while in sitting position LF did not increase further. These data show that BC, while reducing BP through a decrease of plasma NE, increases LF/HF ratio (sympathetic tone) without any change in heart rate. These data seem to confirm that BC causes an inhibitory modulation of the SA system acting predominantly at the periphery through D2 presynaptic receptors.

Influence of 17- alpha-estradiol on catecholamine secretion from the perfused rat adrenal gland

OBJECTIVES

It has been known that adrenal corticosteroids influence the expression of adrenomedullary catecholamine-synthetizing enzymes and also suppress the emission of axonal-like processes in cultured chromaffin cells. In the present study, it was attempted to investigate the effect of 17-alpha-estradiol on catecholamine (CA) secretion evoked by acetylcholine (ACh). DMPP. McN-A-343, excess K+ and Bay-K-8644 from the isolated perfused rat adrenal gland.

METHODS

Mature male Sprague-Dawley rats were anesthetized with ether. The adrenal gland was isolated by the method of WaKade. A cannula used for perfusion of the adrenal gland was inserted into the distal end of the renal vein. The adrenal gland, along with ligated blood vessels and the cannula, was carefully removed from the animal and placed on a platform of a leucite chamber.

RESULTS

The perfusion of 17-alpha-estradiol (1-100 uM) into an adrenal vein for 20 min produced relatively dose-dependent inhibition in CA secretion evoked by ACh (5.32 mM). DMPP (100 uM for 2 min). McN-A-343 (100 uM for 2 min) and Bay-K-8644 (10 uM for 4 min), while it did not affect the CA secretory effect of high K+ (56 mM). Also, in the presence of 17-beta-estradiol. CA secretion of ACh. DMPP and McN-A-343, without any effect on excess K(+)-evoked CA sectretion was depressed. However, in adrenal glands pre-loaded with 17-alpha-estradiol (10 uM) plus tamoxifen (2 uM), which is known to be a selective antagonist of estrogen receptors (for 20 min). CA secretory responses evoked by ACh. DMPP and McN-A-343 were condiderably recovered as compared to that of 17-alpha-estradiol only, but excess K(+)-induced CA secretion was not affected. However, pre-treatment with 17-alpha-estradiol in the presence of meclopramide (dopaminergic antagonist) did not affect the secretory effect of CA evoked by ACh. DMPP, McN-A-343 and high potassium.

CONCLUSIONS

These results suggest that 17-alpha-estradiol causes the marked inhibition of CA secretion evoked by cholinergic receptor stimulation, but not that by excess K+, indicating strongly that this effect may be mediated by inhibiting the influx of extracellular calcium into the rat adrenomedullary chromaffin cells through the activation of inhibitory estrogen receptors, and it also plays a modulatory role in regulating CA secretion.

Plasma neurotransmitters, blood pressure, and heart rate during supine-resting, orthostasis, and moderate exercise conditions in major depressed patients

Major depressed patients showed greater heart rate, noradrenaline, and free-serotonin values than normal. Conversely, platelet-serotonin values in major depressed patients were significantly lower than normal. Patients registered the normal differential blood pressure reduction during orthostasis. They also revealed progressive and significantly higher heart rate rises during orthostasis and exercise periods, when compared to normals. Whereas noradrenaline showed maximal rises during the two last periods, adrenaline only showed small but significant increase during exercise. The analysis of correlations, together with the above data, suggests that major depressed patients register maximal neural sympathetic activity as well as adrenal glands sympathetic hypoactivity. In addition, these patients show hyperparasympathetic activity, as reflected by the free-serotonin profile. Finally, the fact that both the Hamilton Depression Rating Scale and the self-rating Beck Depression Inventory correlated positively with noradrenaline/adrenaline ratio and free-serotonin values strongly suggests that both neural sympathetic and cholinergic mechanisms are involved in major depression.

Decreased carotid body hypoxic sensitivity in chronic hypoxia: role of dopamine

Previously we showed that prolonged exposure to severe hypoxia produces decreased peripheral chemoreceptor responsiveness to hypoxia and attenuates central nervous system (CNS) chemosensory translation, which together may contribute to the decreased hypoxic ventilatory response (HVR) in chronic hypoxia. In this study, we sought to determine whether the central or peripheral activity of endogenous dopamine modulates this decreased HVR. We examined the effects of peripheral and central dopamine receptor blockade on HVR and carotid sinus nerve (CNS) response to hypoxia in controls and in cats exposed to a simulated altitude of 5500 m for 3 weeks. Domperidone increased CSN response to hypoxia in hypoxic cats to levels similar to those observed in controls. HVR was also augmented by domperidone in hypoxic cats, but remained below that of controls. As a result, the CNS chemosensory translation remained reduced in hypoxic animals. We further treated animals with haloperidol. However, this combined treatment with domperidone and haloperidol led to no further increase in CSN or ventilatory responses to hypoxia, or in CNS chemosensory translation in hypoxic cats. Thus, decreased HVR in hypoxic cats is mediated both by depression of hypoxic sensitivity of the carotid body, which is largely dopaminergic, and by decreased CNS chemosensory translation which must involve non-dopaminergic mechanisms.

Acute dopaminergic influence on plasma adrenaline levels in the rat

This study was aimed at in vivo characterisation of the possible role of dopamine receptors in the modulation of adrenaline release from the adrenal medulla in rats. Quinpirole (0.3, 1 and 3 mg/kg s.c., 30 min), an agonist at dopamine D2-like receptors induced a statistically significant increase not only in adrenal dopamine but also in plasma and heart adrenaline levels. The effects of the lowest dose of quinpirole were blocked by domperidone (5 mg/kg s.c., 150 min). Implantation of catheters followed by blood sampling appeared to be a stressful procedure, inducing itself an elevation of adrenal dopamine and of heart adrenaline by 100 and 250%, respectively. To explore the possibility of determining the plasma levels of adrenaline without blood sampling, regression modelling was performed by means of partial least squares regression (PLS) using treatment and levels of heart adrenaline and adrenal dopamine as predictor variables. The selected variables were found to be good predictors of plasma adrenaline levels. Accordingly, the increase in adrenal dopamine and heart adrenaline levels following administration of the dopamine autoreceptor agonist, talipexole, and the classical non-selective dopamine receptor agonist, apomorphine, were interpreted as indicators of the increased adrenomedullary adrenaline release. Neither of the dopamine D2 receptor antagonists used, i.e. domperidone, supposed to have only peripheral effects, nor raclopride, had significant effects on adrenal dopamine and heart adrenaline. Our results support the presence of peripherally located dopamine D2-like receptors, capable of acutely stimulating not only the synthesis of catecholamines, but also the release of adrenaline from adrenals in the conscious rat.

The biology and behaviour of intracerebral adrenal transplants in animals and man

The catecholamine containing chromaffin cells of the adrenal medulla have recently been employed as intracerebral grafts in man and animals with lesions of the nigrostriatal dopaminergic system. This review outlines the basic biology of the chromaffin cell with reference to its efficacy as a source of dopamine in the grafted state. This is followed by an evaluation of the use of these grafts in experimentally lesioned animals and in patients with Parkinson's disease.

Plasma neurotransmitters throughout an oral glucose tolerance test in non-depressed essential hypertension patients

The oral glucose tolerance test (OGTT) with plasma neurotransmitter assays and blood pressure measurements were performed on 68 hypertensive (A and B) and 68 paired normal controls (group C). Those patients who failed to show significant or persistent blood pressure reductions throughout OGTT constitute group A (37 subjects); and those who did show significant and persistent reductions constitute group B (31 subjects). The purpose of this study was to assess if there were any significant differences between those patients whose blood pressure levels normalized throughout OGTT and those who didn't and, further, compare them to their controls. In group A, noradrenaline (NA) was high at the 0' (fasting) period, increasing further at 60' and 90'; however, circulating serotonin (p5HT) did not vary throughout OGTT. Group B, although showing high NA at 0', did not show rises afterwards; whereas, significant and sustained p5HT rises registered throughout postprandial periods. In group C, both p5HT and plasma NA showed significant and sustained increases. Therefore, the NA/p5HT ratio is higher in A, than in B and C. Group A patients were awake and alert throughout. Group B patients were mostly drowsy and many slept light and intermittently. Group C subjects slept throughout, dreaming and showing rapid eye movements. Our findings suggest that the hypertensive syndrome is most severe in those patients who do not show a rise in postprandial circulating serotonin (parasympathetic activity), group A, than those who do exhibit such a rise, group B.

Expression of D2 dopamine receptor mRNA in the arterial chemoreceptor afferent pathway

Dopamine is a major neurotransmitter in the arterial chemoreceptor pathway. In the present study we wished to determine if messenger RNAs for dopamine D1 and D2 receptor are expressed in carotid body (type I cells), in sensory neurons of the petrosal ganglion which innervate the carotid body and in sympathetic neurons of the superior cervical ganglion. We failed to detect D1 receptor mRNA in any of these tissues. However, we found that D2 receptor mRNA was expressed by dopaminergic carotid body type I cells. D2 receptor mRNA was also found in petrosal ganglion neurons that innervated the carotid sinus and carotid body. In addition, a large number of sympathetic postganglionic neurons in the superior cervical ganglion expressed D2 receptor mRNA.

Cross-talk between M2 muscarinic and D1 dopamine receptors in the cat adrenal medulla

In the study reported here we have reached two conclusions. First, the cat adrenal medulla chromaffin cell possesses a dopamine D1 receptor that seems to be coupled to an adenylyl cyclase. Second, this receptor regulates the muscarinic-mediated catecholamine release response through a negative feed-back loop which uses cyclic AMP as a second messenger. These conclusions are supported by the following findings: (i) SKF38393 (a selective D1 receptor agonist), but not quinpirole (a selective D2 agonist), inhibits the methacholine-mediated catecholamine release responses in a concentration-dependent manner (IC50 of around 1-2 microM). (ii) SCH23390 (a selective D1 antagonist), but not sulpiride (a selective D2 antagonist), reversed by 70% the inhibitory effects of SKF38393. (iii) Dibutyril cyclic AMP (500 microM) inhibited by 80% the secretory effects of methacholine.

Three types of bovine chromaffin cell Ca2+ channels: facilitation increases the opening probability of a 27 pS channel

1. Cell-attached patch recordings from bovine chromaffin cells were performed with 90 mM-Ba2+ in the patch pipette and with isotonic potassium aspartate in the bathing solution to zero the membrane potential. Three different types of unitary Ca2+ channel activity could be distinguished in these recordings. 2. A 27 pS Ca2+ channel was distinguished by constructing amplitude histograms and measuring slope conductance. This channel activated over a broad range of potentials (depolarizations greater than -10 mV). 3. A second Ca2+ channel with a slope conductance of 14 pS could also be detected with amplitude histograms. This channel activated with depolarizations greater than -20 mV. 4. An 18 pS Ca2+ channel was observed infrequently indicating that this channel may carry only a small amount of the whole-cell current. This 18 pS channel was sensitive to changes in holding potential. Depolarizing the patch to +10 mV from a holding potential of -80 mV elicited robust unitary activity. Changing the patch holding potential to -40 mV while maintaining test depolarizations to +10 mV completely inactivated the 18 pS channel. Neither the 25 pS nor the 14 pS Ca2+ channels were affected by changes in holding potential in the range from -80 mV to -40 mV, indicating the 18 pS channel was a different type of channel. As the 18 pS channel was observed so infrequently, no detailed studies of it were possible. 5. Chromaffin cell Ca2+ currents exhibited facilitation. Large pre-depolarizations greatly augmented whole-cell currents observed in these cells. Whole-cell currents could double or triple after recruiting facilitation. The application of large pre-depolarizations altered the gating behaviour of the 27 pS Ca2+ channel manifested as dramatically increased channel opening probabilities measured during subsequent test pulses. Large pre-depolarizations induced unitary activity in the 27 pS Ca2+ channel similar to the long-lived openings exhibited by L-type Ca2+ channels in the presence of Bay K 8644. Large pre-depolarizations did not change the gating behaviour of the 14 pS Ca2+ channel. 6. Repetitive depolarizations in the physiological range could also induce facilitation. At the single-channel level facilitation was manifested as a striking increase in opening probability of the 27 pS Ca2+ channel. No effect of repetitive activity was observed on 14 pS channel gating. At the whole-cell level, repetitive depolarizations dramatically increased the current observed. 7. Facilitation of 27 pS Ca2+ channel activity could be induced by changing the holding potential to a depolarized level (greater than or equal to -10 mV).(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine- and adenosine-3',5'-monophosphate (cAMP)-regulated phosphoprotein of 32 kDa (DARPP-32) in the adrenal gland: immunohistochemical localization

The cellular localization of a dopamine- and adenosine-3',5'-monophosphate (cAMP)-regulated phosphoprotein of an apparent molecular weight of 32,000 (DARPP-32) was investigated in mouse, rat, rabbit, guinea pig, cat, monkey (Macaca fascicularis and Marmoset) and human adrenal gland by means of indirect immunofluorescence histochemistry. DARPP-32-like immunoreactivity (-LI) was demonstrated in chromaffin cells in the adrenal medulla of rabbit, guinea-pig, cat, monkey and human, but not in mouse or rat. In the Marmoset monkey, DARPP-32-LI was also observed in the zona glomerulosa of the adrenal cortex. It has been shown that dopamine and dopaminergic agonists inhibit catecholamine release from chromaffin cells and aldosterone secretion from cells in the adrenal cortex. The present results suggest that DARPP-32, an intracellular third messenger for dopamine, may be part of the signal transduction mechanism for dopamine acting on the adrenal gland.

Electrically-evoked catecholamine release from cat adrenals. Role of cholinergic receptors

Catecholamine (CA) release from perfused cat adrenal glands was continuously monitored using an on-line system coupled to an electrochemical detector. This highly sensitive procedure allowed the detection of small changes in the rate of secretion, even using short trains of electrical stimulation or brief acetylcholine (ACh) pulses. CA release was linear with increasing strength of ACh, transmural or splanchnic nerve stimulation. By using specific blockers, the contribution of nicotinic or muscarinic receptors to the overall secretory response to various stimuli could be established. That nicotinic receptors play a major role in mediating the secretory response to all stimuli is shown by the clear inhibition of the response with mecamylamine (10 microM). In contrast, atropine (1 microM) halved secretion evoked by ACh or nerve stimulation but had little effect on the response to trains of transmural electrical stimulation. When transmural electrical stimulation was applied continuously (instead of in trains), increasing the frequency in a step-wise manner, a bell-shaped curve was obtained; secretion reached a peak at 8 Hz and then declined sharply at 16 and 32 Hz. With this stimulation pattern, atropine decreased by 50% the secretion response at the higher frequencies (4-32 Hz). Very few studies are available which define the role of receptors and ionic channels in mediating electrically-evoked CA release. These stimulation patterns have not been used previously and are likely to mimic more closely than those used in earlier studies the physiologic firing pattern of splanchnic nerves innervating adrenomedullary cells.

Dopaminergic regulation of dopamine release from PC12 cells via a pertussis toxin-sensitive G protein

Regulation of dopamine (DA) release from PC12 cells was investigated. Apomorphine and quinpirole, a selective D2 agonist, significantly reduced K(+)-evoked DA release, and this reduction was reversed by haloperidol. Furthermore, spiroperidol, a selective D2 antagonist, and haloperidol, a nonselective DA antagonist, enhanced the K(+)-evoked DA release. Pertussis toxin treatment of the cells abolished the quinpirole-induced reduction of K(+)-evoked DA release. Also, the haloperidol-induced enhancement of K(+)-evoked DA release was not seen in pertussis toxin treated cells. These results, therefore, suggest the presence of D2 receptors on PC12 cells which result in the modulation of K(+)-evoked DA release via a pertussis toxin-sensitive G protein.

Dopaminergic modulation of human adrenal medulla: indirect evidence for the involvement of DA-2 receptors located on the chromaffin cells

1. Pharmacological blockade of DA-2 receptors causes a significantly higher epinephrine (E) response to physical exercise in normal man. 2. The present study was undertaken to evaluate whether dopaminergic modulation of E secretion is present also for a stimulus such as glucagon which acts directly on chromaffin cells. 3. Seven normal males were studied in a single blind randomized manner on two separate occasions, after pre-treatment with placebo or domperidone (DMP; 20 mg orally). Plasma E and norepinephrine, plasma prolactin, heart rate and blood pressure were measured on two separate occasions (placebo vs DMP) before and after a glucagon (1 mg i.v.) stimulation test. 4. Glucagon caused an increase in plasma E either after placebo (P = 0.043) or after domperidone (P = 0.012). DMP administration caused a significant increase in plasma E (P = 0.0008). Absolute increase in plasma E after glucagon was significantly higher after DMP than after placebo (P = 0.030). 5. As the dopaminergic modulation of human adrenal medulla is confirmed also for a stimulus acting directly on chromaffin cells, the results of the present study suggest that the DA-2 receptors responsible for the modulation are located on medullary cells.

Activation of facilitation calcium channels in chromaffin cells by D1 dopamine receptors through a cAMP/protein kinase A-dependent mechanism

Facilitation calcium channels in unstimulated bovine chromaffin cells are normally quiescent but are activated by large pre-depolarizations or by repetitive depolarization in the physiological range. The activation of these 27-pS dihydropyridine-sensitive channels by repetitive stimulation, such as by increased splanchnic nerve activity, can lead to an almost twofold increase in Ca2+ current in these cells. This increase in Ca2+ current is of probable physiological importance in stimulating rapid catecholamine secretion in response to danger or stress. We have identified D1 dopaminergic receptors on bovine chromaffin cells by fluorescence microscopy. Here we show that stimulation of the D1 receptors activates the facilitation Ca2+ currents in the absence of pre-depolarizations or repetitive activity, and that activation by D1 agonists is mediated by cyclic AMP and protein kinase A. The recruitment of facilitation Ca2+ channels by dopamine may form the basis of a positive feedback loop mechanism for catecholamine secretion.

Modulation of secretion by dopamine involves decreases in calcium and nicotinic currents in bovine chromaffin cells

1. Catecholamine secretion from cultured bovine adrenal chromaffin cells was decreased in a dose-dependent manner by the D2 dopamine agonists apomorphine and LY 17 1555. 2. 45Ca2+ uptake was similarly inhibited and whole-cell Ca2+ currents were reduced by apomorphine. 3. These inhibitory effects of D2 agonists depended on the secretagogue used, being much more pronounced for nicotine-evoked responses compared to high K+ stimulation, indicating another possible site of action of apomorphine up-stream of Ca2+ entry. 4. Inhibition by apomorphine of nicotine-evoked responses could not be explained by competitive antagonism against nicotine or DMPP (1,1-dimethyl-4-phenyl-piperazinium iodide). 5. Apomorphine caused reductions of inward whole-cell nicotinic current evoked by ACh and nicotine. 6. Inhibition of nicotine-evoked secretion and 22Na+ influx by apomorphine were not affected by tetrodotoxin, and voltage-dependent, whole-cell Na+ currents were unaltered by apomorphine. 7. No evidence was obtained for increases in K+ conductance by apomorphine. 8. Action potentials recorded in whole-cell current clamp were blocked by apomorphine when they were triggered by nicotinic depolarization but not when they were elicited by direct electrical stimulation. 9. Inclusion of GDP-beta-S in the pipette internal solution did not affect apomorphine-dependent inhibition of nicotinic-evoked responses, while the decrease in whole-cell Ca2+ current induced by apomorphine was completely inhibited in the presence of GDP-beta-S. 10. Increases in cyclic AMP caused by cholera toxin and forskolin did not change the apomorphine-dependent inhibitory effects on nicotine-evoked secretion, indicating that changes in cyclic AMP levels caused by dopamine receptor stimulation are probably not involved.

Dopamine release from bovine adrenal medullary cells in culture

The present study tested whether release of dopamine from isolated bovine adrenal medullary cells in culture could be stimulated or inhibited by secretagogues and modulators known to affect noradrenaline and adrenaline release from adrenal medullary chromaffin cells. K+ depolarization or activation of voltage-sensitive Na+ channels by veratridine both stimulated dopamine release. Ca2+-dependent dopamine release was also stimulated by the mixed nicotinic-muscarinic agonist, carbachol. Carbachol-induced dopamine release was inhibited by a nicotinic but not by a muscarinic antagonist and dopamine release was also stimulated by a selective nicotinic agonist, 1,1-dimethyl-4-phenyl-piperazinium. Carbachol-induced dopamine release was inhibited by substance P and by neuropeptide Y. Histamine also stimulated dopamine release, while angiotensin II and glutamate produced no significant stimulation of dopamine release. Noradrenaline and adrenaline were released in response to the above agents with a profile almost identical to that of dopamine. The results indicate that dopamine can be directly released from adrenal medullary cells in response to stimulation of those cells and suggest that the dopamine release originates from chromaffin cells similar or identical to those storing noradrenaline and adrenaline. A possible role for dopamine, released from adrenal chromaffin cells, in modulating catecholamine release from the chromaffin cells and/or contributing to circulating plasma dopamine is discussed.

Separation and culture of living adrenaline- and noradrenaline-containing cells from bovine adrenal medullae

Separation of viable adrenaline-containing from noradrenaline-containing chromaffin cells in large amounts has been achieved. The procedure involves collagenase digestion of bovine adrenomedullary tissue, isolation of cells through gentle filtration, separation of chromaffin from nonchromaffin cells on discontinuous gradients of the radiopaque contrast Renografin, and separation of adrenaline-enriched from noradrenaline-enriched fractions after centrifugation on self-generated Percoll gradients. Collection of 1-ml Percoll fractions gave two clear-cut catecholamine peaks. The denser peak was enriched in adrenaline and phenylethanolamine-N-methyltransferase (PNMT), suggesting that over 90% of cells were adrenergic. The lighter peak was preferentially enriched in noradrenaline but not in PNMT. With this information, we could collect by gentle aspiration two main fraction layers of larger volumes; one at the bottom of the Percoll gradient, which contained essentially adrenaline-storing cells and the other at the top of the gradient, enriched in noradrenaline cells. Those cells could be maintained viable for at least 1 week in primary monolayer cultures, as shown by neutral red staining and trypan blue exclusion. This method will allow the identification of chemical components, receptors, or ionic channels present in one specific type of cell, to determine their relevance to the regulation of the differential secretion of specific materials present in one but not in the other cell type and to ascertain whether the released materials from one cell type affect the functions of the other.

Acute changes in dopamine levels in rat adrenal glands after administration of dopamine receptor agonists and antagonists

The study was aimed at in vivo pharmacological identification of the possible dopamine (DA) receptor(s) involved in changes of the DA level in rat adrenal glands. Previous work in this laboratory has shown that the DA level is largely controlled by the rate of catecholamine synthesis. The rats were killed by decapitation after various periods of drug administration and the catecholamine content of adrenal glands and forebrain was measured by high-performance liquid chromatography with electrochemical detection. Administration of the DA D-1 + D-2 receptor agonist, apomorphine, induced a statistically significant increase in DA levels in the adrenal glands. The same effect was noted after administration of the DA D-2 receptor agonist, quinpirole. The DA D-2 receptor antagonist, raclopride, blocked the apomorphine-induced increase in adrenal DA levels but had no effect per se on these levels. The DA D-1 receptor agonist, SKF 38393, and the DA D-1 receptor antagonist, SCH 23390, did not have any effect on apomorphine-induced changes in DA content in the adrenals. The DA elevating effect of the DA D-2 receptor agonist, quinpirole, in the adrenals was completely blocked by the DA D-2 receptor antagonist, domperidone. This compound does not cross the blood-brain barrier readily and is thus supposed to act mainly on peripheral tissues. In support of this, the dose of domperidone used did not affect brain DOPAC levels. Our data, together with observations reported in the literature, indicate that the adrenal medulla contains DA receptors of the D-2 subtype, which are capable of controlling the DA level in rat adrenal glands.

Effect of apomorphine on adrenal medullary catecholamine levels

The effects of the dopamine receptor agonist, apomorphine, on the total catecholamine content of the adrenal medulla were studied in normotensive rats. Apomorphine (3, 15, 30 mg/kg SC) induced a dose-dependent decrease in catecholamine content of the adrenal gland. The action of apomorphine was suppressed by previous treatment with the non specific dopamine receptor antagonist, haloperidol (9 mg/kg IP), or the D2 antagonist domperidone (2 mg/kg IP), but not by the D1 antagonist SCH 23390 (1 mg/kg IP). The apomorphine-induced decrease in adrenal catecholamine concentration was suppressed by denervation of the adrenal medulla, i.e. unilateral section of splanchnic fibers performed 5 days before. These results show that, under our experimental conditions, the effect of apomorphine is due to the activation of D2 dopamine receptors probably located on splanchnic nerve endings and suggest the existence of a peripheral D2 dopaminergic system which modulates adrenal medullary catecholamine content.

gamma-Aminobutyric acid uptake and localization in bovine chromaffin cells in primary culture

gamma-Aminobutyric acid (GABA) uptake was studied in bovine chromaffin cells maintained in primary culture. Uptake was found to be dependent on Na+, but not on K+ and Ca2+ ions; it was found that 2 Na+ ions were necessary for each molecule of GABA transported. 2,4-Dinitrophenol, ouabain and vanadate inhibited GABA uptake showing the energy dependency of the system. Two affinity sites were demonstrated, a high affinity site and a low affinity site with Km values of 10 microM and 170 microM, respectively. While the low affinity site did not show large variations with culture age, the Km of the high affinity site increased from 1 microM in freshly isolated cells to 10 microM in 3-9 day-old cells. GABA uptake was unaffected by glutamic acid, aspartic acid, glycine and catecholamines, while taurine, beta-alanine, nipecotic acid and L-2,4 diaminobutyric acid inhibited GABA uptake. Nipecotic acid and L-2,4 diaminobutyric acid acted as competitive inhibitors modifying Km values of the high affinity site. Subcellular studies performed on [3H]GABA-loaded chromaffin cells showed that GABA was not in secretory granules but was recovered in the 100,000 g soluble fraction. The GABA uptake process associated with chromaffin cells may be an important mechanism for regulating the modulation of catecholamine secretion. In addition, the presence of GABA in the cytosol indicates that this molecule may be an effector of chromaffin cell activity in addition to modulating catecholamine secretion.

Decreased catecholamine content in parkinsonian adrenal medullae

Autopsy specimens of adrenal medullae from parkinsonian and nonparkinsonian patients were analyzed for free catecholamines by high-performance liquid chromatography with electrochemical detection. The total free catecholamine content (nanomoles free catecholamine per milligram protein) was significantly lower in the parkinsonian patients than in the control population when the values were corrected for age and time from death to organ harvest. It is not established whether this decreased catecholamine content in the adrenals of parkinsonian patients is a concomitant of the disease itself or whether it is secondary to drug therapies used to treat the symptoms of Parkinson's disease.

Dopamine receptors on adrenal chromaffin cells modulate calcium uptake and catecholamine release

The presence of dopamine-containing cells in sympathetic ganglia, i.e., small, intensely fluorescent cells, has been known for some time. However, the role of dopamine as a peripheral neurotransmitter and its mechanism of action are not well understood. Previous studies have demonstrated the presence of D2 dopamine receptors on the surface of bovine adrenal chromaffin cells using radioligand binding methods and dopamine receptor inhibition of catecholamine release from perfused adrenal glands. In the present study, we provide evidence confirming a role of dopamine receptors as inhibitory modulators of adrenal catecholamine release from bovine chromaffin cell cultures and further show that the mechanism of modulation involves inhibition of stimulated calcium uptake. Apomorphine gave a dose-dependent inhibition (IC50 = 1 microM) of 45Ca2+ uptake stimulated by either nicotine (10 microM) or membrane depolarization with an elevated K+ level (60 mM). This inhibition was reversed by a series of specific (including stereospecific) dopamine receptor antagonists: haloperidol, spiperone, sulpiride, and (+)-butaclamol, but not (-)-butaclamol. In addition, the calcium channel agonist Bay K 8644 was used to stimulate uptake of 45Ca2+ into chromaffin cells, and this uptake was also inhibited by the dopamine receptor agonist apomorphine. The combined results suggest that dopamine receptors on adrenal chromaffin cells alter Ca2+ channel conductance, which, in turn, modulates catecholamine release.

Modulation of adrenal catecholamine release by DA2 dopamine receptors in the anaesthetized dog

1. The effects of DA2 agonist, quinpirole (50 micrograms/kg, i.v.) and a DA2 antagonist, domperidone (50 micrograms/kg, i.v.) on the release of adrenal catecholamines were evaluated in the anaesthetized and vagotomized dog. 2. Stimulations (5 V pulses of 2 ms duration for 3 min) of the splanchnic nerve at frequencies of 1, 3 and 5 Hz were applied randomly before and after injection of the drug. 3. The results show that quinpirole reduces significantly the release of adrenaline at 1 and 3 Hz but not at 5 Hz, while the release of noradrenaline is reduced at 1 Hz but not at 3 and 5 Hz. Inversely, domperidone potentiates significantly the release of both catecholamines at 3 and 5 Hz, but not at 1 Hz. 4. There was no change in basal release of adrenal catecholamines, adrenal blood flow or heart rate after both drug treatments. 5. The mean arterial pressure was not affected by domperidone treatment but there was a significant reduction in basal mean arterial pressure after the injection of quinpirole. 6. There was no change in any of these parameters during electrical stimulation. 7. Therefore, these results strongly suggest that DA2 dopamine receptors are present at the level of the adrenal medulla and that their activation could mediate an inhibitory modulation on the adrenal catecholamines release within a certain range of electrical stimulation.

Effects of alpha- and beta-adrenoceptor agonists and antagonists on ATP and catecholamine release and desensitization of the nicotinic response in bovine adrenal chromaffin cells

The effects of a number of alpha- and beta-adrenoceptor agonists and antagonists on the modulation of secretion from bovine adrenal chromaffin cells were investigated. Secretion was induced by nicotine, 56 mM K+, histamine or Ba2+ and was detected by the ATP luciferin-luciferase bioluminescence technique or by the measurement of endogenous catecholamines (CA) by HPLC coupled with electrochemical detection. ATP release from freshly isolated cells by 5 microM nicotine was only weakly inhibited by adrenaline and noradrenaline and even then required high concentrations (greater than 500 microM), while dopamine (1 microM-1 mM) and isoproterenol (100 microM) had no effect. Clonidine (100 microM), oxymetazoline (100 microM), yohimbine (100 microM), and propranolol (5 microM) all produced inhibition of nicotine-induced ATP release with the order of potency:propranolol greater than oxymetazoline greater than clonidine = yohimbine. The inhibitory effect by propranolol could not be reversed by high concentrations of adrenaline or isoproterenol. In chromaffin cell monolayer cultures, all alpha 2-adrenoceptor agents tested (clonidine, oxymetazoline and yohimbine), produced a dose-dependent, Na+-sensitive, non-competitive inhibition of nicotine-induced catecholamine release with little effect on the catecholamine release induced by K+ (56 mM), histamine (10 microM) or Ba2+ (2.2 mM). (+/-)Propranolol caused a similar pattern of inhibition, however, this inhibition was also observed by (+)propranolol, an isomer with little beta-adrenoceptor antagonist activity. The effects of clonidine and propranolol on desensitization of nicotine-induced CA secretion were also investigated. The degree of desensitization of the nicotinic response was dependent on the concentration of nicotine to which the cells were pre-exposed. Desensitization was detected as the decrease in response to a near EC50 concentration of nicotine (5 microM) following pre-incubation of cells to nicotine in the range of 0.3-300 microM. The desensitization had a threshold of 1 microM nicotine and was maximal at 3 microM nicotine in the pre-incubation. Both clonidine (50 microM) and (+/-)propranolol (5 microM) inhibited CA secretion induced by nicotine (0.3 microM-300 microM) during the pre-incubation period. However, regardless of this inhibition of secretion, neither clonidine nor propranolol had an effect on either the onset, or the rate of nicotine-evoked desensitization subsequently observed. These data suggest that inhibition of the nicotinic response and desensitization of the nicotinic response are regulated independently.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine D2 receptor binding in adrenal medulla: characterization using [3H]spiperone

The possibility that dopamine may function as a neuromodulator or neurotransmitter in the adrenal gland, and not merely serve as a precursor to the catecholamines, has been suggested. If this hypothesis is correct, receptors for dopamine should be identifiable in the adrenal. The present work demonstrates the existence of a high-affinity receptor in adrenal medulla using [3H]spiperone as the radioligand to label the receptors. [3H]Spiperone bound rapidly, reversibly, and with high affinity to bovine adrenal medullary membranes. Scatchard analysis yielded a Kd of 0.09 nM and a Bmax of 51 fmol/mg protein. In competition binding experiments, dopaminergic antagonists were at least 100 times more potent in displacing [3H]spiperone from its binding sites than adrenergic or serotonergic receptor antagonists. Similarly, agonists at the dopamine receptor more readily competed for [3H]spiperone binding than other receptor agonist drugs tested. Furthermore, D2 selective antagonists and agonists were much more potent than D1 receptor ligands. These results suggest that [3H]spiperone may bind to a high-affinity D2 dopamine receptor in adrenal medulla.

D2 dopamine receptors on bovine chromaffin cell membranes: identification and characterization by [3H]N-methylspiperone binding

Although dopamine-containing cells are known to be present in sympathetic ganglia, the site of action and the role of dopamine in ganglion function remain obscure. In the present work, we evaluated the interaction of dopamine receptor ligands with particulate membrane fractions from bovine chromaffin cells and adrenal medullary homogenates using the D2 dopamine receptor radioligand [3H]N-methylspiperone ([3H]NMSP). Scatchard analysis of [3H]NMSP saturation experiments revealed a Bmax of 24.1 +/- 1.6 fmol/mg of protein and a KD of 0.23 +/- 0.03 nM in the particulate fraction from adrenal medulla homogenates and a Bmax of 26.5 +/- 2.7 fmol/mg of membrane protein and a KD of 0.25 +/- 0.02 nM in the particulate fraction prepared from isolated adrenal chromaffin cells. There were approximately 1,000 receptors/cell. There were no detectable levels of specific [3H]NMSP binding in the particulates prepared from adrenal cortical or capsular homogenates. Competition studies with the nonradioactive D2 receptor antagonists spiperone, chlorpromazine, and (-)-sulpiride revealed KI values of 0.28, 21, and 196 nM, respectively. The (+) isomer of butaclamol displayed a 604-fold higher affinity than the (-) isomer. Competition studies with the dopamine receptor agonists dopamine and apomorphine revealed affinities of 3,960 and 417 nM, respectively. A correlation coefficient of 0.96 was obtained in studies comparing the potencies of drugs in inhibiting specific [3H]NMSP binding in bovine adrenal medullary homogenates and in inhibiting specific [3H]NMSP binding to brain D2 dopamine receptors. In summary, radiolabeling studies using [3H]NMSP have revealed the presence of D2 dopamine receptors on bovine adrenal chromaffin cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of decreased dopamine synthesis on the development of hypertension induced by salt loading in spontaneously hypertensive rats

To clarify role of dopamine in the development of hypertension, the effect of a dopamine synthesis inhibitor on blood pressure and urinary output of catecholamines was investigated in spontaneously hypertensive rats (SHR) fed with high sodium diet. Rats were orally given carbidopa, an inhibitor of peripheral DOPA decarboxylase, or the vehicle for 4 weeks. Carbidopa administration accelerated significantly the development of hypertension as compared to the control SHRs with the vehicle. Carbidopa administration resulted in a significant decrease of urinary excreted sodium, urinary dopamine and renal content of dopamine. Conversely, carbidopa administration resulted in a significant increase of urinary excreted norepinephrine, urinary epinephrine and renal content of norepinephrine as compared with control SHRs. These results suggest that decreased dopamine synthesis in kidneys and probably other peripheral tissue accelerates the development of hypertension, mediated by a decrease of natriuresis and an enhancement of sympatho-adrenomedullary activity. Dopamine plays an important role in its protective action against the development of hypertension enhanced by salt loading, and decreased dopaminergic mechanisms accelerated hypertension in SHR.

Characterization of a dopaminergic receptor that modulates adrenomedullary catecholamine release

Nicotine evokes the release of catecholamines from bovine adrenal glands perfused with oxygenated Krebs-bicarbonate solution. Two 2-min pulses of 5 microM nicotine, at 40-min intervals (S1 and S2), gave net catecholamine outputs of 45.2 +/- 3.6 and 29.1 +/- 3.5 micrograms/8 min, respectively. Apomorphine (1 or 10 microM) markedly inhibited catecholamine release during S2 to 9.1 +/- 2.2 and 0.5 micrograms/8 min, respectively. Haloperidol (0.5 microM) reversed the inhibitory effects of apomorphine. Haloperidol alone enhanced catecholamine release induced by nicotine to 67.9 +/- 7.9 micrograms/8 min. [3H]Spiperone binds to adrenomedullary membranes with a KD of 0.24 nM and a Bmax of 117 fmol/mg of protein. Whereas spiperone and haloperidol potently displaced such binding, 3,4-dihydroxyphenylethylamine (dopamine) and sulpiride were poorer displacers, and SCH23390, prazosin, phenoxybenzamine, propranolol, BAY-K-8644, and nitrendipine did not displace [3H]spiperone bound. These data strongly suggest that, as in the cat, the bovine adrenal medulla chromaffin cell contains a dopaminergic receptor that modulates the catecholamine secretory process triggered by stimulation of the nicotinic cholinoceptor. Such a receptor seems to be of the D2 type and might be involved in a sympatho-adrenal cooperative mechanism contributing to the maintenance of cardiovascular homeostasis during stressful situations as well as to the pathogenesis of hypertension. If so, selective dopaminergic agonists might prove clinically useful in the treatment of hypertension.

Continuous monitoring of catecholamine release from perfused cat adrenals

Catecholamine release from perfused cat adrenal glands has been continuously monitored by on-line connection of the perfusion fluid emanating from the gland to an electrochemical detector. This method allowed: the recording of basal levels and fluctuations of catecholamine release with good reliability; the study of the release of catecholamines in response to even submicromolar concentrations of nicotine or acetylcholine and 3 mM increments of the K+ concentration, perfused in pulses of a few seconds, and to determine the effects of drugs and ionic manipulations on such secretory responses; the analysis of the 'physiological' secretory response evoked by electrical stimulation of the splanchnic nerves, allowing the study of presynaptic as well as postsynaptic components of the effects of drugs and ions on chemical neurotransmission at the splanchnic--chromaffin cell synapse; the continuous monitoring of the kinetics of the secretory process during sustained depolarization with several secretagogues and its correlation with the kinetics of activation and inactivation of potential sensitive Ca channels. In addition, this method avoids the time-consuming procedures of collecting samples and assaying them individually through tedious fluorimetric or radioenzymatic techniques.

Potentiation of hemorrhage-evoked catecholamine release by prior blood loss in cats

The effect of prior blood loss on the plasma catecholamine response to acute hemorrhage (H) was assessed in alpha-chloralose-urethane anesthetized cats. Animals sustained an initial H period of 0 (samples only), 10, or 20% H total blood volume. Ninety minutes after reinfusion of the shed blood, all animals sustained a rapid 20% H. The catecholamine and arterial pressure responses to this second 20% H were assessed every 2 min for 20-min duration. Plasma norepinephrine increased modestly in the 0/20% group (+0.63 +/- 0.13 ng/ml) and 10/20% group (+0.66 +/- 0.07 ng/ml), whereas the 20/20% group showed a much larger (P less than 0.01) mean increase of 3.58 +/- 1.16 ng/ml. Plasma epinephrine did not increase after 0/20% (+0.05 +/- 0.02 ng/ml), increased slightly after 10/20% H (+0.10 +/- 0.05 ng/ml), and demonstrated a large significant increase after 20/20% H (+0.48 +/- 0.18 ng/ml). The differential effect of 20% H on catecholamine release, depending on the magnitude of prior blood loss, was not the result of altered mean arterial or pulse pressure responses to H. Correlation analyses revealed that the mean increases in epinephrine and norepinephrine during the post-H sampling period were well correlated in each animal (r = 0.864, P less than 0.001). The data indicate that the priming effect of prior blood loss on H evoked catecholamine release: requires an initial blood loss of 20% of total blood volume; occurs rapidly, as it is seen by 90 min after the initial H period; and affects epinephrine and norepinephrine similarly. We conclude that the immediate past secretory history of the sympathoadrenal system affects its responsiveness to subsequent blood loss.