Epinephrine synthesis by an N-methyltransferase in rat liver

We investigated if liver can synthesize epinephrine in vitro and in vivo. Homogenates of rat liver readily synthesized [3H]epinephrine from [3H]S-adenosylmethionine and norepinephrine. Liver homogenates also N-methylated dopamine at more than twice the rate that they N-methylated norepinephrine. In contrast, adrenal homogenates, which N-methylate norepinephrine to form epinephrine using the enzyme phenylethanolamine-N-methyltransferase (PNMT), methylated dopamine only about 1% as well as norepinephrine. Synthesis of epinephrine by liver homogenates was not significantly inhibited by the PNMT inhibitor SKF 29661 at a concentration that inhibited adrenal homogenate epinephrine synthesis by nearly 90%. These findings indicate that liver can synthesize epinephrine in vitro using an enzyme other than PNMT. Adrenal demedullation of rats reduced plasma epinephrine levels to 7% of control values, but left liver epinephrine and epinephrine-forming enzyme levels unchanged. Treatment of demedullated rats with 6-hydroxydopamine plus reserpine also resulted in dramatically reduced plasma epinephrine levels but no change in hepatic epinephrine and N-methylating enzyme levels. We conclude that the liver synthesizes its own epinephrine.

[Study of catecholamine secretion by the rat adrenal glands using microdialysis in vivo]

Microdialysis technique has been developed to study secretory function of the adrenal gland in anesthesized rats. Concentration of adrenaline and noradrenaline in sequential 20 min microdialysis samples was measured by HPLC with electrochemical detection. The suitability of method was tested by local and central stimulation of catecholamine secretion. In the first case 100 mmol of KCl or 1 mmol of carbachol were added to perfusion medium, in the second one hypovolemic hypotension was applied. All the stimuli used increased catecholamine levels in the adrenal gland dialysates. Institute of Experimental Cardiology of the All-Union.

The determination of the adrenal medullary cell fate during embryogenesis

One subset of neural crest cells, the sympathoadrenal precursors, undergoes a switch in phenotype expression, when they invade the adrenal anlagen and become associated with adrenocortical cells. To investigate the mechanisms responsible for the conversion of noradrenaline synthesizing precursors to adrenaline producing endocrine chromaffin cells we studied the role of glucocorticoids on the initial induction of adrenaline synthesis in embryonic adrenals and cultures of highly purified chromaffin precursor cells. We could show that in vivo differentiation of rat chromaffin precursors commences between 16.3 and 17.3 days of gestation. While adrenaline and the activity of the enzyme phenylethanolamine N-methyltransferase (PNMT), which converts noradrenaline to adrenaline, were present at Embryonic Day 17.3 (E17.3), they were not detectable in E16.3 adrenals. Small amounts of corticosterone were present in E16.3 adrenals and plasma, but in parallel with the initial induction of adrenaline biosynthesis, a sharp rise in organ and plasma glucocorticoid levels occurred until E17.3. Chromaffin precursor cells, isolated at E16.3 and cultured for 4 days, failed to express PNMT activity and adrenaline. However, 0.1 nM dexamethasone was already sufficient for the initial induction of adrenaline and its synthesizing enzyme. Specific glucocorticoid binding of freshly isolated chromaffin (precursor) cells revealed a developmental increase during embryogenesis, yet no glucocorticoid binding sites were detectable in chromaffin precursor cells at E16.3. They appeared at E17.3 in parallel with the initial induction of adrenaline biosynthesis and the enormous rise of adrenal and plasma corticosterone levels. We therefore conclude that glucocorticoids are essential and sufficient to trigger the differentiation of noradrenergic sympathoadrenal precursors to adrenergic chromaffin cells after a functional glucocorticoid receptor system has been established.

Adrenal responses to splanchnic nerve stimulation in conscious calves given naloxone

1. The effects of stimulating the peripheral end of the right splanchnic nerve in the presence of naloxone (2 mg kg-1) have been investigated in conscious 3 to 6-week-old calves. 2. Mean aortic blood pressure rose to significantly higher levels during splanchnic stimulation in bursts at 40 Hz for 1 s at 10 s intervals than it did during stimulation at the corresponding continuous frequency (4 Hz). Furthermore, naloxone significantly reduced the fall in mean vascular resistance in response to both patterns of stimulation. 3. The output of catecholamines from the adrenal gland, together with the proportion of noradrenaline released, was significantly enhanced by stimulating the splanchnic nerves in bursts in animals pre-treated with naloxone and the proportion of noradrenaline released also increased. In both cases the output of adrenaline and noradrenaline was within the same range as that reported previously in normal control animals. 4. Naloxone significantly increased the amounts of enkephalin-like immunoreactivity and corticotrophin-releasing factor (CRF)-like immunoreactivity released from the adrenal gland in response to splanchnic nerve stimulation and raised the proportion of total to free met5-enkephalin that was secreted. 5. Naloxone also inhibited the rise in plasma adrenocorticotrophic hormone (ACTH) concentration during continuous stimulation at 4 Hz, but not during stimulation at 40 Hz in bursts. Under these latter conditions the output of cortisol apparently directly from the adrenal gland was inhibited. The finding that splanchnic nerve stimulation can potentiate the output of cortisol in response to ACTH was confirmed. 6. These results provide evidence that release of enkephalins and of CRF from the adrenal is inhibited by activating opioid receptors within the gland itself.

Rat renal epinephrine synthesis

Rats that underwent adrenal demedullation had a 93% decrease in plasma epinephrine (E) levels, but did not decrease their renal E. Even further treatment with 6-hydroxydopamine and reserpine failed to lower renal E levels. Similarly, urine E levels failed to decrease after adrenal demedullation and renal denervation. There is a renal E-synthesizing enzyme that differs from adrenal phenylethanolamine-N-methyltransferase (PNMT) in that it is only weakly inhibited by SKF 29661 and can synthesize epinine from dopamine, while adrenal PNMT does so poorly. When an adrenalectomized rat received intravenous [3H]methionine, its urine contained radioactivity that appeared to be [3H]E, with small amounts of [3H]epinine. However, after [3H]methionine was infused in the renal artery, the major product in urine appeared to be [3H]epinine, with a small amount of [3H]E. Adrenal demedullation induced renal E synthesis, but denervation returned the rate of renal E synthesis to control values. The combination of adrenal demedullation, 6-hydroxydopamine, and reserpine treatments increased renal E-forming activity to 350% of control. We conclude that appreciable portions of renal and urinary E are synthesized in the kidney by an enzyme distinct from PNMT. The enzyme is induced by some treatments that lower E and NE levels.

Evidence for tonic activation of prejunctional beta-adrenoceptors in guinea-pig pulmonary arteries by adrenaline derived from the adrenal medulla

1. The effects of (+/-)-carteolol 10(-8) M to 10(-6) M, a non-selective beta-antagonist, applied cumulatively, on stimulation-evoked 3H-release at 1 Hz were studied in pulmonary arteries isolated from guinea-pigs. The guinea-pigs were subjected to either bilateral adrenalectomy, adrenalectomy followed by injections of deoxycorticosterone acetate (DOCA) and hydrocortisone, bilateral adrenodemedullation or a sham operation, and then loaded in vitro with [3H]-noradrenaline. 2. Carteolol inhibited 3H-output in arteries from sham-operated animals in a concentration-dependent manner. This inhibitory effect was not found in pulmonary arteries from animals subjected to adrenalectomy or adrenodemedullation. However, DOCA and hydrocortisone pretreatment, did not prevent the disappearance of the carteolol-induced inhibition of 3H-release. 3. Adrenalectomy and adrenodemedullation depleted or markedly reduced the endogenous contents of adrenaline in pulmonary arteries without altering the levels of dopamine and noradrenaline. 4. It is concluded that adrenaline, mainly derived from the adrenal medulla, acts as an endogenous agonist for tonically functioning prejunctional beta-adrenoceptors in guinea-pig pulmonary arteries, probably by being taken up and co-released with noradrenaline.

Extraadrenal adrenaline formation by two separate enzymes

Adrenaline (A) is synthesized in the adrenal medullae by the enzyme phenylethanolamine-N-methyltransferase (PNMT). After surgical removal of the adrenal medullae tissue A levels ranged from 22% of control in the heart to 125% of control in the liver. Use of a novel assay to measure tissue A formation revealed that many tissues can synthesize A using PNMT and another enzyme that N-methylates both noradrenaline and dopamine. These enzymes are non-neuronal, inducible and synthesize a major fraction of tissue and urine A.

Molecular cloning of cDNA and chromosomal assignment of the gene for human phenylethanolamine N-methyltransferase, the enzyme for epinephrine biosynthesis

Phenylethanolamine N-methyltransferase (PNMT; EC 2.1.1.28) catalyzes the synthesis of epinephrine from norepinephrine, the last step of catecholamine biosynthesis. To isolate a cDNA clone for human PNMT, we first isolated a cDNA clone for bovine adrenal medulla PNMT using mixed oligodeoxyribonucleotide probes whose synthesis was based on the partial amino acid sequence of tryptic peptides from the bovine enzyme. By screening a bovine adrenal medulla cDNA library, a cDNA clone with an insert of about 200 base pairs (bp) was isolated. This clone consisted of 84 bp of carboxyl-terminal coding region, which contained amino acid sequences corresponding to two tryptic peptides, and about 100 bp of 3'-untranslated region. Using this cDNA fragment as the probe, we screened a human pheochromocytoma cDNA library and isolated a cDNA clone with an insert of about 1.0 kilobase pairs, which contained the complete coding region of the enzyme. Northern blot analysis of human pheochromocytoma poly(A)+ RNA using this cDNA insert as the probe showed a single RNA species of about 1,000 nucleotides, suggesting that this clone is a full-length cDNA. Determination of the nucleotide sequence revealed that human PNMT consists of 282-amino acid residues with a predicted molecular weight of 30,853, including initial methionine. The amino acid sequence of the human PNMT was highly homologous (88%) to that of the bovine enzyme. Chromosomal assignment of the gene for human PNMT was carried out using mouse-human somatic cell hybrids. The PNMT gene was assigned to chromosome 17.

Synthesis and evaluation of 3-substituted analogues of 1,2,3,4-tetrahydroisoquinoline as inhibitors of phenylethanolamine N-methyltransferase

1,2,3,4-Tetrahydroisoquinoline (THIQ) and aryl-substituted derivatives of THIQ are potent inhibitors of the enzyme that catalyzes the formation of epinephrine--phenylethanolamine N-methyltransferase (PNMT, E.C. 2.1.1.28). In previous studies, we found that substitution of the 3-position of THIQ with a methyl group resulted in enhanced activity as an inhibitor for 3-methyl-THIQ with respect to THIQ itself. To more fully delineate this region of the PNMT active site, we have synthesized and evaluated other 3-substituted THIQ analogues that vary in both steric and electronic character. Extension of the methyl side chain in 8 by a single methylene unit results in diminished potency for 3-ethyl-THIQ, suggesting that this zone of the active site is spatially compact; furthermore, the region of steric intolerance may be located principally on only "one side" of the 3-position of bound THIQs, since the carbonyl containing (bent) analogues 3-(methoxycarbonyl)-THIQ and 3-(aminocarbonyl)-THIQ are much less capable of forming a strong enzyme-inhibitor dissociable complex compared to straight-chain derivatives possessing a similar steric component. The good activity of 3-(hydroxymethyl)-THIQ as a PNMT inhibitor cannot be explained solely by steric tolerance for this side chain. We believe that an active-site amino acid residue capable of specific (i.e., hydrogen bond) interactions is located in close proximity to the 3-position of bound THIQs and that association of the OH functionality with this active-site residue results in the enhanced in vitro potency of this analogue (Ki = 2.4 microM) compared to that of THIQ (Ki = 10.3 microM). Incorporation of a hydroxymethyl substituent onto the 3-position of the potent PNMT inhibitor 7,8-dichloro-THIQ (SKF 64139, Ki = 0.24 microM) did not result in the same enhancement in inhibitor potency for 17 (Ki = 0.38 microM). This result suggests that simultaneous binding in an optimal orientation of the aromatic halogens, secondary amine, and side-chain hydroxyl functionalities to the PNMT active site is not allowed in this analogue.

5-Hydroxytryptamine and catecholamines in developing sympathetic cells of the rat

Appearance of 5-hydroxytryptamine (5-HT) in developing sympathetic cells of prenatal rats was studied using the indirect immunofluorescence method. In consecutive sections, tyrosine hydroxylase (TH) immunoreactivity was considered as a marker for catecholamine-synthesizing cells in general, while phenylethanolamine N-methyltransferase (PNMT) immunoreactivity was used as an indicator of adrenaline synthesis. 5-HT immunoreactivity was observed for the first time in 12.5-day-old embryos in developing sympathetic chain ganglia. On day 13.5, 5-HT-immunoreactive cells were first seen on the preaortic region and on day 14.5 in the developing adrenal gland. Comparison with consecutive sections stained for TH revealed that all TH-immunoreactive cells were also 5-HT-immunoreactive. During later development, however, 5-HT immunoreactivity was retained by some cell types in each sympathetic tissue. In the ganglia, most developing principal nerve cells gradually lost their 5-HT immunoreactivity, while all so-called small intensely fluorescent (SIF) cells remained intensely 5-HT-immunoreactive. In the adrenal medulla, all catecholamine-containing cells showed 5-HT immunoreactivity until day 16.5. The first adrenaline-synthesizing cells appeared at this stage. Occasionally on day 16.5 and constantly on day 17.5 noradrenaline cell islets were distinguished among adrenaline cells. The adrenaline cells retained intense 5-HT immunoreactivity, while the noradrenaline cells were non-reactive to it. In the main retroperitoneal paraganglion, two noradrenaline cell populations were distinguished from day 15.5, one being 5-HT-immunoreactive and the other non-reactive. A third population appeared in this tissue at the time of birth, consisting of adrenaline-synthesizing cells which were also 5-HT-immunoreactive. These results indicate that the 3 sympathetic tissues undergo similar developmental changes: 5-HT immunoreactivity occurs in conjunction with the initiation of catecholamine synthesis and appears first in all catecholamine cells. During maturation it is confined to certain subpopulations in each tissue, i.e. the SIF cells and some principal nerve cells of the ganglion, the adrenaline cells of the adrenal medulla, the adrenaline cells and some noradrenaline cells of the paraganglionic tissue.

Effect of chronic morphine treatment on the adrenaline biosynthesis in adrenals and brain regions of the rat

Phenylethanolamine-N-methyltransferase (PNMT) activity and tissue catecholamine content were examined after 13 days morphine treatment. Prolonged morphine treatment did not alter the PNMT activity in brain regions (A1/C1 and A2/C2 cell groups, medial basal hypothalamus, median eminence). However, the enzyme activity and the adrenaline content were increased in the adrenal medulla of male rats. In parallel, morphine treatment induced adrenal hypertrophy. In female or hypophysectomized male animals the chronic morphine treatment had no effect on adrenal weight but evoked the increase of PNMT activity. It is concluded that the morphine-induced increased adrenaline biosynthesis in the adrenal gland is not fully dependent on the intact pituitary-adrenal axis and may be mediated partly by a neural mechanism (increased splanchnic nerve activity) or by a direct effect of morphine.

A sensitive radioenzymatic assay for epinephrine forming enzymes

Epinephrine (E) is formed in the adrenal medulla by phenylethanolamine-N-methyltransferase (PNMT), and in other tissues. Enzymes other than PNMT may be able to synthesize E, but this has been difficult to investigate because most assays do not have E as their final product. This assay produces 3H-E from norepinephrine (NE) and 3H-S-adenosylmethionine. The 3H-E is isolated on alumina, 3H-S-adenosylmethionine is precipitated and the 3H-E is suspended in diethylhexyl phosphoric acid in toluene for scintillation counting. The assay is sensitive and linear over a wide range. E was formed by most tissues tested. Brain and adrenal contained an enzyme specific for NE, but cardiac ventricle contained an enzyme that methylated both NE and dopamine. Denervated tissues in adrenal medullectomized rats contained very little NE, but still had E and E forming enzyme present. This assay detects a non-neuronal E forming enzyme with activity in vitro and in vivo.

Intracellular pH and catecholamine synthesis in cultured bovine adrenal medullary cells: effect of extracellular Na+ removal

Incubation of cultured bovine adrenal medullary cells in Na+-free sucrose medium or in Na+-free Cs+ medium enhanced the synthesis of 14C-catecholamines from [14C]tyrosine about two- to threefold or sixfold, respectively. The increment of 14C-catecholamine synthesis produced by Na+-free medium was partially dependent on the presence of Ca2+ in the medium. Dibutyryl cyclic AMP also stimulated the synthesis of 14C-catecholamines in adrenal medullary cells, and the effects of Na+ removal and dibutyryl cyclic AMP (5 mM) on the synthesis were almost additive. The intracellular pH measured by using a weak acid 5,5-dimethyloxazolidine-2,4-dione was 7.14 in control cells and when Na+ was replaced by sucrose or Cs+, it shifted down to 6.56 or 5.66, respectively. The fall in intracellular pH and the stimulation of 14C-catecholamine synthesis were similarly dependent on the concentration of Na+ in the medium. The optimal pH of soluble tyrosine hydroxylase was 5.5-6.0 both in control cells and in cells incubated in Na+-free medium. These results suggest that removal of extracellular Na+ increases the synthesis of catecholamines, at least in part, by shifting the intracellular pH toward the optimal pH of tyrosine hydroxylase.

[Effect of stress and the antioxidant ionol on the biosynthesis of catecholamines and the content of dopamine in the heart and adrenal glands]

The effects of a synthetic antioxidant ionol (dibunol) on the biosynthesis and content of catecholamines in the heart and adrenal glands were studied. It was shown that in stress a mobilization of catecholamine reserve is combined with a considerable increase in dopamine concentration. In conditions of physiological rest, ionol did not affect the studied indices of adrenal catecholamine biosynthesis, while in the heart it enhanced the dopamine synthesis and content. With ionol administration, stress did not suppress but, on the contrary, increased the neuronal uptake and noradrenaline biosynthesis, catecholamine concentration remaining practically unchanged. Simultaneously, a manyfold increase in the biosynthesis along with a considerable increase in the concentration of dopamine developed in both organs. The data obtained suggest that ionol realizes its stress-defensive effect to a great extent due to the activation of catecholamine biosynthesis and to a concomitant increase in dopamine accumulation.

[Existence of a peripheral nerve mechanism of interoceptive effects on the function of the adrenal medulla]

In cats with decentralized solar plexus ganglia, discharges in the central stumps of post-ganglionic nerve branches running from the celiac and superior mesenteric ganglia to the adrenal glands, were studied. An obvious peripheral neural connection through C-afferents of peripheral origin was found to exist among the spleen, stomach, small intestine and the adrenals. Electrical stimulation of the central stumps of splenic or superior mesenteric nerves increased adrenaline, noradrenaline and dopamine contents in the adrenals. Simultaneously there was an increase in the catecholamine concentration in the myocardium, medulla oblongata and hypothalamus, indicating the release of hormones into the blood flow and their uptake by the tissues. A peripheral neural mechanism seems to exist which mediates interoceptive influences on the adrenal medullary function.

Immunohistochemical identification of noradrenaline- and adrenaline- synthesizing neurons in the cat ventrolateral medulla

The distribution and morphology of cell bodies containing the catecholamine biosynthetic enzymes dopamine-beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) in the ventrolateral medulla (VLM) of the cat were studied immunohistochemically after intracisternal administration of colchicine. Perikarya immunoreactive to DBH were found throughout the VLM extending from approximately the spinomedullary junction to the level of the superior olivary nucleus. In the caudal VLM DBH neurons were found primarily in the region immediately dorsal to the lateral reticular nucleus (LRN), although a few scattered DBH neurons were also found near the ventral surface of the medulla in and around the parvicellular division of the LRN. These DBH neurons in the caudal VLM were generally fusiform, fusiform-bipolar, or multipolar, with a mean somal area of 422 +/- 32 microns2, and with two to four branching processes. In the rostral VLM DBH neurons formed three distinct groups: one group was found in the nucleus paragigantocellularis lateralis in the region just ventromedial to the retrofacial nucleus (RFN) near the ventrolateral surface of the medulla; the second group was found in the region dorsomedial to the rostral aspects of the nucleus ambiguous and the RFN, and the third group was found in the region along the lateral aspect of the superior olivary nucleus. Perikarya immunoreactive to the adrenaline-synthesizing enzyme PNMT were localized to a more restricted region of the VLM that extended from approximately the rostral aspect of the caudal third of the inferior olivary complex (level of the obex) to the caudal pole of the facial nucleus. These PNMT neurons were fusiform or multipolar with a mean somal area of 273 +/- 21 microns2, and with two to five branching processes. The location, size, shape, and numbers of PNMT-immunoreactive neurons corresponded closely to the rostral groups of DBH neurons, with the exception of the group found along the lateral aspect of the superior olivary nucleus. These data indicate that noradrenaline-synthesizing neurons are primarily found in the caudal VLM and in the region near the superior olivary nucleus, whereas catecholamine neurons in the rostral VLM between these two noradrenergic cell groups synthesize adrenaline. As the VLM has previously been shown to have direct connections with spinal cord, brainstem, and hypothalamic areas implicated in cardiovascular and neuroendocrine regulation, this suggests that DBH- and PNMT-synthesizing neurons are components of neuronal circuits involved in these homeostatic mechanisms.

Adrenaline-synthesizing neurons in the medulla of the cat

In this study, the distribution of neurons containing the adrenaline-synthesizing enzyme phenylethanolamine-N-methyltransferase (PNMT) was mapped in the medulla of the cat. Data from recent studies in the rat suggest that the anatomical structure responsible for cardiorespiratory changes that occur following application of neurotransmitters and drugs to Schlaefke's area on the ventral medullary surface is the nucleus reticularis rostroventrolateralis (RVL), which is distinguished from adjacent regions of the reticular formation, in part, by the presence of adrenaline-synthesizing neurons. To determine whether an equivalent adrenergic population is present in the RVL of the cat, we used antibodies raised against bovine adrenal PNMT to map the distribution of adrenaline-synthesizing neurons in the reticular formation. In the ventrolateral medulla, we found that labeled cells extended from the level of the retrofacial nucleus to the calamus scriptorius. The majority of labeled cells were seen in a nucleus designated RVL at the level of the rostral one-third of the inferior olive. In the dorsomedial medulla, cells were labeled in the caudal aspect of the nucleus tractus solitarii (NTS) and were especially dense in the subnucleus gelatinosus and commissural nucleus of the vagus. A few lightly labeled cells were also present in the rostral pole of the area postrema (AP). In contrast to the rat, few or no immunoreactive cells were found in the rostral NTS, medial longitudinal fasciculus, nucleus paragigantocellularis dorsalis, or periventricular gray. Our results are consistent with the notion that an area of the RVL containing adrenergic perikarya is the anatomical structure responsible for cardiovascular changes that occur when chemicals are applied to Schlaefke's area.

Epinephrine synthesis in the PC12 pheochromocytoma cell line

PC12 is a rat pheochromocytoma cell line which has been increasingly used as a model system for both neural differentiation and chromaffin cell function. PC12 cells have been reported to synthesize dopamine and norepinephrine, but not epinephrine. We have found that PC12 cells do synthesize small amounts of epinephrine and that dexamethasone increases both epinephrine content as well as phenylethanolamine-N-methyltransferase (PNMT) activity. These results suggest that the PC12 cell line may be useful in the investigation of the regulation of PNMT.

Strain-specific differences in levels of the mRNA for the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase

The activity of the epinephrine biosynthetic enzyme phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28) is 3- to 8-fold greater in rats of the Fischer 344 and Buffalo strains. The biochemical basis for the strain differences has been analyzed at the level of PNMT protein and messenger RNA production. Fischer rat adrenals possess approximately 5-fold more PNMT protein than those of the Buffalo rat as established by Western blotting and immunoprecipitation of adrenal gland homogenates. Poly(A)+ RNAs purified from adrenal glands of each strain were translated in a reticulocyte lysate system, immunoprecipitated with antibody to PNMT and fractionated by SDS-PAGE. A 35S-labelled protein of Mr = 34,000 was immunoprecipitated from adrenals of Fischer and Buffalo rats, indicating that the molecular weights of PNMT do not differ in these strains prior to post-translational processing. Hybridization of a 740 base pair (bp) cDNA for PNMT indicated that the mRNAs for PNMT are the same size in the adrenals of both strains. However, the adrenals of Fischer rats contain 2- to 4-fold more PNMT mRNA than Buffalo rats, as established by quantitative dot blot hybridization and Northern blot analysis. The medulla oblongata, the site of cell bodies of central adrenergic neurons, also contains approximately 2-fold more PNMT mRNA in Fischer rats. The strain specificity in the production of PNMT reflects differences in the expression of the gene for PNMT. Thus, an inherited capacity for PNMT expression may in fact provide the intrinsic determinants responsible for neurotransmitter production. These data provide a direct link between regulation of catecholamine enzyme biosynthesis at the genomic level and the availability of specific catecholamines for neurotransmitter and hormonal functions.

Bromocriptine decreases blood pressure of spontaneously hypertensive rats without affecting the adrenomedullary synthesis of catecholamines

Because of controversial data on the role of adrenomedullary catecholamines (CA) in the hypertension of spontaneously hypertensive rats (SHR) and in the hypotensive action of bromocriptine, we studied the effect of chronic bromocriptine treatment on blood pressure (BP), adrenal CA synthesis, tissue CA, and urinary excretions of CA and their metabolites in SHR. We found that the hypertension of 12-week-old SHR (systolic BP, 181 +/- 13 mm Hg) was reduced to 123 +/- 8 mm Hg when they received bromocriptine between 4 and 12 weeks of age (2 X 600 micrograms/kg/day, intraperitoneally). Although the urinary epinephrine (E) and the synthesis of adrenal norepinephrine (NE), E, and dopamine (DA), as well as the tissue content of CA in adrenals, heart, and kidney, remained unchanged after bromocriptine treatment, the urinary NE and DA excretions were lower in bromocriptine-treated SHR than in sham-treated SHR (NE, 2.7 +/- 0.3 versus 4.2 +/- 0.3 nmol/24 h in control SHR; DA, 18.7 +/- 1.6 versus 28.2 +/- 2.2 nmol/24 h in control SHR). In SHR, bromocriptine treatment did not affect the previously observed selectively increased adrenal turnover and synthesis of DA or the urinary excretions of normetanephrine (NM), dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), or homovanillic acid (HVA). The results suggest that the bromocriptine-induced decrease of BP in SHR is not mediated by changes in adrenomedullary CA and that bromocriptine decreases the BP of SHR without affecting the previously observed increased adrenal DA release of SHR. Thus, other central or peripheral dopaminergic agonist actions of bromocriptine bypassing the adrenal medulla must be considered in the hypotensive action of the drug.

Distribution of tyrosine hydroxylase and neuropeptide Y-like immunoreactive neurons in rabbit medulla oblongata, with attention to colocalization studies, presumptive adrenaline-synthesizing perikarya, and vagal preganglionic cells

We studied the distribution, within the rabbit medulla oblongata, of neuronal cell bodies containing either tyrosine hydroxylase or neuropeptide Y-like immunoreactivity. Both avidin-biotin and immunofluorescence procedures were used. Because the two primary antibodies were raised in different species it was possible to perform simultaneous colocalization studies with the immunofluorescence procedure. Tyrosine hydroxylase-containing neurons in the rostral medulla were demonstrated to contain a catecholamine by the colchicine-enhanced FAGLU (formaldehyde-glutaraldehyde) fluorescence histochemical procedure. These neurons are presumably adrenergic, corresponding to the C1 and C2 groups described in the rat. No C3 group was found in the rabbit. The distribution of tyrosine hydroxylase-containing neurons in the caudal medulla was in accordance with previous descriptions of the A1 and A2 groups based on the unenhanced FAGLU procedure. Neuropeptide Y-like immunoreactivity was observed in cell groups corresponding to those already described in the rat, but additional groups were discovered in the rabbit. Some neurons containing neuropeptide Y-like immunoreactivity were observed in nucleus raphe pallidus and these also contained serotonin (5-HT). In the nearby nucleus reticularis gigantocellularis there were occasional neurons that contained neuropeptide Y-like immunoreactivity without any colocalized 5-HT. Neuropeptide Y-like immunoreactivity was also observed in the dorsal motor nucleus of the vagus, rostral to the obex, and these neurons were demonstrated to be true vagal preganglionic cells by colocalization of neuropeptide Y-like immunoreactivity and Fast Blue retrogradely transported from the cervical vagus. We found that neuropeptide Y-like immunoreactivity was colocalized in approximately 75% of the tyrosine hydroxylase-containing neurons in the rostral medulla (C1 and C2 cells). A smaller proportion of the A1 cells also contained this peptide but it was absent from both the most caudal A1 cells and from the A2 cells. Some tyrosine hydroxylase-containing neurons occur in direct apposition to vagal preganglionic cells in both the dorsal motor nucleus of the vagus and the nucleus ambiguous. However, colocalization studies revealed that none of these neurons contained Fast Blue when this dye was retrogradely transported from the cervical vagus. Medullary catecholamine-synthesizing neurons apparently do not contribute axons to the vagus nerve. This finding is consistent with our own studies in the rat but is in contrast to studies in this species published by other workers.

Sympathomedullary activity in one-kidney, one clip hypertensive rats

In order to evaluate more directly the implications of the sympatho-adrenal system in one-kidney, one clip hypertension (1K1C), we studied adrenal catecholamine synthesis after a bolus injection of 3H-tyrosine, tissue norepinephrine and dopamine concentrations, as well as the urinary excretions of norepinephrine, epinephrine, dopamine and those of the major dopamine metabolites, dihydroxphenylacetic acid (DOPAC) and homovanillic acid (HVA) in 1K1C and control uninephrectomized rats. When compared with controls, the hypertensive rats had a markedly enhanced formation of adrenal 3H-norepinephrine and 3H-epinephrine, higher urinary norepinephrine and epinephrine excretions but lower heart and kidney content of norepinephrine and dopamine as well as decreased urinary excretions of the main dopamine metabolites, DOPAC and HVA. These data suggest an increased norepinephrine and epinephrine synthesis in 1K1C hypertensive rats associated with dopamine synthesis, which is normal but probably disproportionally low relative to the synthesis of norepinephrine and epinephrine. This abnormality may be an important pathogenetic factor in this model of experimental hypertension.

Induction of refractoriness of cyclic AMP responses to prostaglandin E1 and epinephrine by prior exposure of guinea pig macrophages to lipopolysaccharide

Prior exposure of guinea pig macrophages to LPS (lipopolysaccharide) resulted in reduced cAMP-generating responses to prostaglandin E1 and epinephrine. LPS-induced refractoriness was diminished when LPS treatment was carried out in the presence of an inhibitor of prostaglandin synthesis, hydrocortisone, or indomethacin, or an inhibitor of protein synthesis, cycloheximide. The release of arachidonic acid and its metabolites, especially prostaglandin E2 and thromboxane B2, increased during incubation of macrophages with LPS. These increases were efficiently antagonized by hydrocortisone, indomethacin, or cycloheximide. Preincubation of macrophages with prostaglandin E1 greatly reduced the subsequent responses of cAMP generation to prostaglandin E1 and unexpectedly also to epinephrine. Thus, increased production of prostaglandins during the LPS treatment is likely to be responsible for decreased cAMP responses to subsequent addition of prostaglandin E1 and epinephrine.