Mechanism-based inactivation of dopamine beta-hydroxylase by p-cresol and related alkylphenols

The mechanism-based inhibition of dopamine beta-hydroxylase (DBH; EC 1.14.17.1) by p-cresol (4-methylphenol) and other simple structural analogues of dopamine, which lack a basic side-chain nitrogen, is reported. p-Cresol binds DBH by a mechanism that is kinetically indistinguishable from normal dopamine substrate binding [DeWolf, W. E., Jr., & Kruse, L. I. (1985) Biochemistry 24, 3379]. Under conditions (pH 6.6) of random oxygen and phenethylamine substrate addition [Ahn, N., & Klinman, J. P. (1983) Biochemistry 22, 3096] p-cresol adds randomly, whereas at pH 4.5 or in the presence of fumarate "activator" addition of p-cresol precedes oxygen binding as is observed with phenethylamine substrate. p-Cresol is shown to be a rapid (kinact = 2.0 min-1, pH 5.0) mechanism-based inactivator of DBH. This inactivation exhibits pseudo-first-order kinetics, is irreversible, is prevented by tyramine substrate or competitive inhibitor, and is dependent upon oxygen and ascorbic acid cosubstrates. Inhibition occurs with partial covalent incorporation of p-cresol into DBH. A plot of -log kinact vs. pH shows maximal inactivation occurs at pH 5.0 with dependence upon enzymatic groups with apparent pK values of 4.51 +/- 0.06 and 5.12 +/- 0.06. p-Cresol and related alkylphenols, unlike other mechanism-based inhibitors of DBH, lack a latent electrophile. These inhibitors are postulated to covalently modify DBH by a direct insertion of an aberrant substrate-derived benzylic radical into an active site residue.

Cardiovascular effects of a new potent dopamine beta-hydroxylase inhibitor in spontaneously hypertensive rats

The cardiovascular effects of a new class of potent inhibitors of dopamine beta-hydroxylase (DBH) were evaluated in spontaneously hypertensive rats (SHR). SK&F 102698 [1-(3,5-difluorobenzyl)imidazole-2-thiol] is the prototype molecule of this class of substituted 1-benzylimidazole-2-thiols and is one of the most potent inhibitors of DBH yet described. After acute p.o. administration in conscious unrestrained SHR, SK&F 102698 elicited a dose-dependent decrease in mean arterial blood pressure. The antihypertensive effect was marked by a gradual onset with long duration of activity. The antihypertensive effect produced by SK&F 102698 was accompanied by bradycardia. SK&F 102698 inhibited DBH in vivo as demonstrated by its ability to increase vascular levels of dopamine (DA) while concomitantly decreasing vascular levels of norepinephrine (NE), thus increasing the overall DA/NE ratio. The chronic cardiovascular effects of SK&F 102698 were evaluated in developing SHR. SHR were administered SK&F 102698 p.o. once daily for 9 weeks beginning when animals were 4 weeks of age. SK&F 102698 (50 mg/kg) significantly attenuated the development of hypertension of these SHR. Tolerance to the chronic effects of DBH inhibition was not observed and blood pressures in drug-treated animals were still reduced significantly 20 hr after drug administration. Vascular catecholamine levels were determined in the mesenteric artery of these chronically treated animals. Vascular DA levels were increased 290%, vascular NE levels were decreased 36% and the DA/NE ratio was increased 520%, as compared to controls. Furthermore, hearts weights of SHR receiving SK&F 102698 were approximately 10% lower than controls. The present study demonstrates that in SHR SK&F 102698 is an effective antihypertensive whose effects are mediated by the novel mechanism of DBH inhibition.

Multisubstrate inhibitors of dopamine beta-hydroxylase. 2. Structure-activity relationships at the phenethylamine binding site

1-Aralkylimidazole-2-thiones have been shown to be potent multisubstrate inhibitors of dopamine beta-hydroxylase (DBH; EC 1.14.17.1). In the present study, a series of 1-benzylimidazole-2-thiones was prepared to explore the effects of substitution in the benzyl ring on the inhibition of DBH. A detailed structure-activity relationship for in vitro activity was discovered and this was shown by a modified Hansch analysis to correlate (r = 0.91) with four key structural features of the benzyl ring: the presence of a hydroxyl at the 4-position, molar refractivity at the 3-, 4-, and 5-positions, inductive effects of the substituents at the 3-, 4-, and 5-positions, and pi-electron density. The affinity (Kis) of eight substituted inhibitors for DBH was shown to correlate (r = 0.75) with the affinity (KD) of comparably substituted tyramines for the ternary DBH-oxygen-tyramine complex. This correlate is used to support the hypothesis that binding of inhibitor to DBH occurs in a fashion that mimics the binding of tyramine substrates. The most potent inhibitors were selected for study in vivo in the spontaneously hypertensive rat model of hypertension. The changes in vascular dopamine and norepinephrine levels that resulted from oral administration of the inhibitors corresponded to the observed reduction in mean arterial blood pressure. A divergence between in vitro potency and in vivo efficacy upon oral dosing was noted and is suggested to result from an in vivo metabolic conjugation of the phenolic group of inhibitor.

Interaction of dopamine beta-mono-oxygenase with substituted imidazoles and pyrazoles. Catalysis and inhibition

The interaction of dopamine beta-mono-oxygenase (DBM) with substrate analogues possessing either imidazole or pyrazole functionalities at the alkyl chain terminus was investigated. 1-(4-Hydroxybenzyl)imidazole (4-HOBI) is an active substrate for DBM, and it exhibits the expected ascorbate- and fumarate-dependencies and normal kinetic behaviour at concentrations up to 10 mM. 4-Hydroxybenzaldehyde was identified as the product formed from 4-HOBI on the basis of h.p.l.c. and g.c.-m.s. analysis, and its formation exhibits the expected 1:1 stoichiometry with O2 consumption. The 4-HOBI/DBM reaction is kinetically comparable with other DBM activities, and 4-HOBI is the first substrate analogue yet reported that exhibits substantial activity though lacking a terminal amino group. Introduction of a methyl substituent at the 2-position of the imidazole ring abolishes substrate activity, probably through a steric effect. 1-(4-Hydroxybenzyl)pyrazole, where imidazole is replaced by the isomeric pyrazole moiety, is a potent DBM inhibitor, and not a substrate. These results represent the first report of an active heterocyclic substrate or inhibitor for this enzyme, and establish the basis for the design of new classes of DBM substrates and inhibitors.

Possible role of glutathione in cofactor regeneration of dopamine beta-hydroxylase

The activity of the enzyme dopamine beta-hydroxylase was determined in rat brain stem by a sensitive coupled radiometric assay. The appropriate copper and dilution parameters have been determined for this tissue. Reduced glutathione has been shown to activate the enzyme homogenate at concentrations of 24-240 microM. This paper shows that glutathione cannot contribute to the inhibitory activity coming from the brain stem. A mechanism is proposed for the role of glutathione in cofactor regeneration of dopamine beta-hydroxylase.

Catecholamine sulfates: end products or metabolic intermediates?

Dopamine-beta-hydroxylase catalyzes the beta-oxidation of dopamine to noradrenaline while phenylethanolamine-N-methyltransferase converts noradrenaline to adrenaline. Since catecholamine sulfates represent the predominant form of catecholamines in human tissues, we have studied the role of dopamine sulfate and noradrenaline sulfate as alternate substrates for dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase, respectively. Dopamine 3-sulfate, dopamine 4-sulfate and noradrenaline 3-sulfate were chemically synthesized and exhaustively purified by ion-exchange chromatography. Dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase were partially purified from human adrenals. Using tyramine as substrate, dopamine-beta-hydroxylase is slightly inhibited by dopamine 3-sulfate according to some irreversible or mixed mechanisms. When dopamine-beta-hydroxylase was incubated with dopamine 3-sulfate or dopamine 4-sulfate, we were not able to find any synthesis of either noradrenaline sulfate or free noradrenaline. Using phenylethanolamine as substrate, the enzymatic activity of phenylethanolamine-N-methyltransferase remains unchanged with addition of dopamine 3-sulfate, dopamine 4-sulfate or noradrenaline 3-sulfate. It was concluded that dopamine sulfate is not an alternate substrate for either dopamine-beta-hydroxylase or phenylethanolamine-N-methyltransferase nor is noradrenaline 3-sulfate an alternate substrate for phenylethanolamine-N-methyltransferase.

Stimulation of vascular dopamine receptors: status and future strategies

The ability to stimulate dopamine receptors in the heart and blood vessels with selective receptor agonists, such as fenoldopam, has been an important advance. Fenoldopam is a selective DA1 receptor agonist and is currently in clinical trials for cardiovascular disease therapy. An alternative approach to stimulating dopamine receptors, while at the same time blunting sympathetic nervous system activity, would be by inhibiting the enzyme dopamine beta-hydroxylase (D beta H), thus increasing the cardiovascular and renal ratio of dopamine to norepinephrine. SK&F 102698, (1-[3',5'-difluorobenzyl]-2-mercaptoimidazole), is a potent inhibitor of D beta H with a Ki of 40 nM against D beta H in vitro. In the spontaneously hypertensive rat SK&F 102698 100 mg/kg orally, increases the dopamine/norepinephrine ratio approximately 5-fold and lowers blood pressure approximately 30 mmHg to normotensive levels. Antihypertensive activity can be achieved with single daily oral administration and neither tolerance nor reflex tachycardia occur. Stimulation of dopamine receptors by inhibition of dopamine beta-hydroxylase is a unique approach toward cardiovascular and renal therapeutics.

Characterization of highly purified dopamine beta-hydroxylase

A modified purification procedure has been developed for dopamine beta-hydroxylase isolated from bovine adrenal medulla. Catalase is included in the homogenization step starting with a suspension of either chromaffin granules or adrenal medulla tissue. With this precaution, the enzyme remains stable in the supernatant solution in preparation for the subsequent purification step involving concanavalin A-Sepharose chromatography. The homogeneous enzyme has a specific activity in the range of 60-70 mumol O2 consumed/min/mg. Using radiolabeled metal ion chelators, it was determined that several of the chelators remained tightly bound to the enzyme after removal of the copper leading to difficulties in establishing stoichiometry of enzyme-bound metal ions.

Bombesin alters the sympathetic nervous system response to cold exposure

The accumulation of dopamine in interscapular brown fat, heart, kidney and mesenteric white fat has been used as an index of sympathetic nervous activity in rats in which dopamine beta-hydroxylase has been blocked. Intracerebroventricular administration of bombesin impaired the cold-induced increase of dopamine accumulation in interscapular brown fat but not in other tissues. These studies provide evidence that bombesin acts within the central nervous system to modify efferent sympathetic nervous system responses to cold exposure in a viscerotopically specific manner. This action of bombesin is consistent with the other central nervous system actions of this peptide, i.e. inhibition of heat production and disruption of thermoregulation.

Mechanism-based inhibition of dopamine beta-monooxygenase by aldehydes and amides

A mechanism for beta-chlorophenethylamine inhibition of dopamine beta-monooxygenase has been postulated in which enzyme-bound alpha-aminoacetophenone is generated, followed by an intramolecular redox reaction to yield a ketone-derived radical cation as the enzyme inhibitory species (Mangold, J. B., and Klinman, J. P. (1984) J. Biol. Chem. 259, 7772-7779). If correct, additional compounds capable of producing enzyme-bound (formula; see text) reductant should inhibit dopamine beta-monooxygenase. Phenylacetaldehyde was chosen to test this model, since beta-hydroxyphenylacetaldehyde is expected to function as a reductant in a manner analogous to alpha-aminoacetophenone. Phenylacetaldehyde exhibits the properties of a mechanism-based inhibitor. Kinetic parameters are comparable to beta-chlorophenethylamine under both initial velocity and inactivation conditions. Since phenylacetaldehyde bears little resemblance to beta-chlorophenethylamine, its analogous inhibitory action provides support for an intramolecular redox reaction (via beta-hydroxyphenylacetaldehyde oxidation to a radical cation) in dopamine beta-monooxygenase inactivation. beta-Hydroxyphenylacetaldehyde was identified as the enzymatic product of phenylacetaldehyde turnover. As predicted, this product behaves both as a time-dependent inhibitor of dopamine beta-monooxygenase and as an electron donor in enzyme-catalyzed hydroxylation of tyramine to octopamine. Phenylacetamide and p-hydroxyphenylacetamide are also found to be mechanism-based inhibitors of dopamine beta-monooxygenase. In this case the product of hydroxylation (beta-hydroxyphenylacetamide) is redox inactive and, therefore, is unable to function as either a reductant or an inhibitor. Thus, mechanism-based inhibitors are divided into two types: type I, which undergoes hydroxylation prior to inactivation, and type II, which only requires hydrogen atom abstraction. A general mechanism for dopamine beta-monooxygenase inactivation is described, in which a common mechanistic radical intermediate is formed from both pathways.

Effects of catecholamine-depleting agents and receptor blockers on basal and angiotensin II- or norepinephrine-stimulated luteinizing hormone release in female rats

Depletion of hypothalamic norepinephrine (NE) and epinephrine by administration of diethyldithiocarbamate abolished the stimulatory effects of intraventricular (IVT) angiotensin II (AII) on LH release in ovariectomized rats pretreated with estradiol and progesterone. The increase in blood LH produced by IVT NE or iv LHRH was unaffected in these drug-treated animals. Selective depletion of hypothalamic epinephrine by the administration of LY134046 (8,9-dichloro-2,3,4,5-tetrahydro-1H-2-benzazepine hydrochloride) potentiated the effect of AII on LH secretion. Blockade of alpha 1-, alpha 2-, or beta-adrenergic receptors resulted in a transient increase in basal LH levels. The stimulation of LH secretion induced by IVT AII or NE was unaffected by alpha 1-receptor blockade with prazosin, but was abolished by alpha 2-receptor blockade with yohimbine. beta-Receptor blockade with propranolol potentiated both NE- and AII-induced LH release. AII receptor blockade with IVT saralasin prevented the LH rise due to AII without modifying that due to NE. Taken together, these data suggest that IVT AII stimulates LH release in ovariectomized rats treated with estradiol and progesterone by releasing endogenous NE, which, in turn, acts on facilitatory alpha 2-receptors to affect LH secretion, presumably by increasing the secretion of LHRH. Exogenous NE also acts at this receptor. beta-Receptors provide inhibitory tone to this facilitatory system, and blockade of this receptor subtype results in potentiated LH responses to both AII and NE.

Multisubstrate inhibitors of dopamine beta-hydroxylase. 1. Some 1-phenyl and 1-phenyl-bridged derivatives of imidazole-2-thione

The synthesis and characterization of some 1-(phenylalkyl)imidazole-2-thiones as a novel class of "multisubstrate" inhibitors of dopamine beta-hydroxylase (DBH) are described. These inhibitors incorporate structural features that resemble both tyramine and oxygen substrates, and as evidenced by steady-state kinetics, they appear to bind both the phenethylamine binding site and the active site copper atom(s) in DBH. A series of structural congeners that incorporate different bridging chain lengths between the phenyl ring (dopamine mimic) and the imidazole-2-thione group (oxygen mimic) define the optimum distance for inhibitory potency and the likely intersite distance in the DBH active site. Additional bridging analogues were prepared to determine the active site bulk tolerance and the effects of heteroatom replacement.

Design and kinetic characterization of multisubstrate inhibitors of dopamine beta-hydroxylase

The synthesis and kinetics characterization of a new class of dopamine beta-hydroxylase (DBH; EC 1.14.17.1) inhibitor, 1-(4-hydroxybenzyl)imidazole-2-thiol, is reported. These inhibitors, which incorporate a phenethylamine substrate mimic and an oxygen mimic into a single molecule, exhibit both the kinetic properties and the potency (Kis approximately 10(-9) M) expected for a multisubstrate inhibitor and are therefore classified as such. Steady-state kinetic experiments with these multisubstrate inhibitors and their substructural analogues support the recently proposed pH-dependent changes in substrate binding order [Ahn, N., & Klinman, J. P. (1983) Biochemistry 22, 3106] and a mechanism whereby the inhibitor binds specifically to the reduced Cu+ form of enzyme at both the phenethylamine substrate site and the active-site copper atom(s). A Yonetani-Theorell double-inhibition experiments indicates mutually exclusive binding of the inhibitor substructures p-cresol and 1-methylimidazole-2-thiol to suggest an extremely short intersite distance between the phenethylamine binding site and the active-site copper atom(s).

[Effect of adrenoblockers on the analgesic effect of GABA-positive preparations]

The experiments on rats have shown that selective alpha 1 and alpha 2 adrenoceptor blockers (prazosin and yohimbine) and an inhibitor of dopamine-beta-hydrolase FD-008 failed to change the antinociceptive effect of baclofen, a direct GABAB receptor agonist. The antinociceptive effect of THIP and depakin, acting predominantly on GABAA receptors, was significantly reduced by prazosin, FD-008 and yohimbine in vocalization test. In tail-flick test the analgetic effect of THIP and depakin was not altered by prazosin and FD-008, but was increased by yohimbine. The role of adrenergic mechanisms in GABAA and GABAB receptor-mediated analgesia is discussed.

Mechanism of action of dopamine on the in vitro release of gonadotropin-releasing hormone

Controversy exists on whether dopamine (DA) stimulates or inhibits GnRH secretion and whether its effects are mediated via alpha-adrenergic receptors or dopaminergic receptors. As a means to examine this conflict, we have utilized an in vitro superfusion system to study the effects of DA, norepinephrine (NE), phentolamine (alpha-antagonist), pimozide (DA antagonist), and two DA agonists (apomorphine and bromocryptine) on GnRH release from isolated mediobasal hypothalami from adult male rats. In this dynamic system, graded concentrations of both NE and DA (2.0 nM to 2.0 microM) led to a dose-dependent increase in GnRH output during the 10 min interval that followed each pulse dose of NE (P less than 0.02) or DA (P less than 0.05). The DA-induced GnRH release was reproducible, consistent, and significant over five successive pulses (20 microM) at 30-min intervals (P less than 0.02). Coinfusion of phentolamine (20 microM) prevented the DA (20 microM) induced release of GnRH (P less than 0.03), but pimozide (20 microM) had no significant effect on DA-induced GnRH release (P greater than 0.3). The two DA agonists, apomorphine and bromocryptine, at doses up to 2.0 microM and 200 nM, respectively, had no significant effect on GnRH release. To determine whether DA was causing a direct stimulation of alpha-adrenergic receptors or being enzymatically converted to NE which could then stimulate alpha-receptors to induce GnRH release, rats were injected with sodium diethyldithiocarbamate (DDC) (550 mg/kg BW) ip, 1 h before death. DDC blocks the enzymatic conversion of DA to NE, and this was reflected by a 37% decrease in hypothalamic NE efflux during the superfusion. However, pulses of DA, even in the presence of DDC, were associated with a marked dose-dependent increase in hypothalamic NE efflux, and DDC failed to prevent the subsequent stimulation of GnRH release. We conclude that the apparent DA-induced release of GnRH is most probably attributable to DA-induced release of hypothalamic NE which, in turn, acts through alpha-adrenergic receptors on peptidergic neurons to stimulate GnRH release.

On the physiological role of neurocuprein: aponeurocuprein is an inhibitor of dopamine beta-monooxygenase

The apoform of neurocuprein, the copper protein from brain and chromaffin granules, was found to be a potent inhibitor of the hydroxylating activity of dopamine beta-monooxygenase, whereas the holoform of neurocuprein has no effect on the activity of the enzyme. The inhibiting capacity of neurocuprein may be due to the property of the apoprotein to chelate copper from the enzyme. A role of neurocuprein as an endogenous protein regulator of dopamine beta-monooxygenase is suggested.

Evidence for a non-precursor dopamine pool in noradrenergic neurones of the dog mesenteric artery

The effects of dopamine beta-hydroxylase inhibition by disulfiram on dopamine and noradrenaline concentrations were investigated at various times in the main trunk and proximal branches of the dog mesenteric artery. The effects of desmethylimipramine and benztropine on the uptake of exogenous dopamine in the proximal branches of the mesenteric artery were also analyzed. The rate constant of dopamine accumulation after dopamine beta-hydroxylase inhibition in the main trunk (0.1137 h-1) was significantly higher than that observed in the proximal branches (0.075 h-1). The rate constants of noradrenaline decline were similar in both segments of the mesenteric artery (about 0.052 h-1). The uptake of exogenous dopamine in the proximal branches of the mesenteric artery was insensitive to benztropine and only blocked by desmethylimipramine. These results suggest the existence of a non-precursor dopamine pool in noradrenergic neurones supplying the proximal branches of the dog mesenteric artery, and also provide evidence in favour of the absence of independent dopaminergic neurones in this vascular area.

The mechanism of inactivation of dopamine beta-hydroxylase by hydrazines

Dopamine beta-hydroxylase is inactivated by phenyl-, phenethyl-, benzyl-, and methylhydrazine, but not by hydrazine itself. With phenyl-, methyl-, and phenethylhydrazine, the rate of inactivation decreases in the presence of ascorbate and increases in the presence of tyramine. Reduction of the enzyme-bound copper occurs with all of the hydrazines tested. In the presence of the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone the carbon-centered radicals generated from each compound are trapped. This is consistent with reduction of the enzyme-bound copper by the hydrazine-containing compounds, resulting in formation of the hydrazine cation radical. Homolytic cleavage of the carbon-nitrogen bond then generates a carbon-centered radical which reacts with the enzyme, resulting in inactivation. Inactivation with [14C]phenylhydrazine results in the incorporation of 0.94 molecule of label per enzyme subunit. Benzylhydrazine behaves as a mechanism-based inhibitor of the enzyme. Both benzyl- and phenethylhydrazine are substrates for dopamine beta-hydroxylase. The second-order rate constant for inactivation of dopamine beta-hydroxylase by benzylhydrazine in the presence of ascorbate is increased about 4-fold when the benzylic hydrogens are replaced with deuterium. The apparent Vmax shows an observed deuterium kinetic isotope effect of 13 +/- 2. The partition ratio for product formation versus inactivation is 11-fold less for alpha,alpha-d2-benzylhydrazine. These results are interpreted in terms of a model where inactivation is due to abstraction of an electron from nitrogen instead of abstraction of a hydrogen atom from the benzylic carbon.

Inhibitory action of hydralazine on catecholamine-synthesizing enzymes prepared from bovine adrenal medulla

The direct effect of hydralazine on catecholamine-synthesizing enzymes was investigated. Hydralazine caused a concentration-dependent inhibition of tyrosine hydroxylase (TH) prepared from bovine adrenal medulla, and a more pronounced effect was obtained by incubating the enzyme with the drug prior to the enzyme assay. Kinetic studies showed that hydralazine increased the apparent Km value of the enzyme for tyrosine and cofactor, 6,7-dimethyl-5,6,7,8-tetrahydropterin (DMPH4), without any change in the Vmax. The inhibitory effect of the drug was irreversible, and an excess amount of FeSO4 failed to restore the enzyme activity inhibited by this drug. Furthermore, hydralazine also inhibited the dopamine beta-hydroxylase (DBH) in chromaffin granule membranes. Hydralazine increased the apparent Km value of DBH for ascorbic acid without any change in the Vmax, and it decreased the Vmax of the enzyme for tyramine with no change in the apparent Km value. The observations described here suggest the possibility that hydralazine presumably causes the inhibition of catecholamine-synthesizing enzymes as a result of allosteric alterations in the molecular structures of these enzymes. It thus seems unlikely that the inhibitory action of hydralazine on these enzymes may totally be based on its metal-chelating activity.

Dopamine and L-dopa: inhibition of thyrotropin-stimulated thyroidal thyroxine release

Previous studies had suggested that norepinephrine (NE) and its precursors dopamine (DA) and L-DOPA acted similarly on iodine metabolism of isolated thyroid cells. Present studies indicate that this similarity extends to the inhibition by catecholamines of TSH-stimulated T4 release by mouse thyroids incubated in vitro. DA (5 X 10(-4) M), like NE, shown previously, inhibits TSH-stimulated T4 release. This inhibition was reversed by the alpha-blockers phentolamine, prazosin, and yohimbine, but not by the beta-blocker L-propranolol. DU-18288 and diethyldithiocarbamate, inhibitors of DA beta-hydroxylase, did not reduce DA inhibition, suggesting that prior conversion to NE was not a condition for DA activity. Apomorphine, a dopaminergic agonist but not a NE precursor, acted like DA, and its inhibition was also reversed by alpha-blockers. Furthermore, sulpiride, a dopaminergic blocker, reversed DA and apomorphine inhibition of TSH stimulation. These results suggest that DA inhibits TSH-stimulated T4 release through both adrenergic and dopaminergic receptors. On the other hand, L-DOPA, exerting an inhibition like that of DA, was also reversed by alpha-blockers, but its activity was greatly diminished by carbidopa, an inhibitor of aromatic L-amino acid decarboxylase, the enzyme converting L-DOPA to DA. This indicated that L-DOPA had to be converted to DA for activity. Both DA and L-DOPA inhibited stimulation of T4 release induced by (Bu)2cAMP, suggesting that their effect was exerted at a locus distal to cAMP generation. Indirect confirmation of a cAMP-independent pathway was obtained when DA inhibited TSH-stimulated cAMP formation, but, contrary to T4 release, this inhibition was not reversed by dopaminergic or adrenergic blockers. Presumably, therefore, DA inhibition of TSH-stimulated cAMP production was not related to T4 release. We conclude that 1) DA inhibits TSH-stimulated T4 release in mouse thyroids via alpha-adrenergic and dopaminergic receptors; 2) L-DOPA has to be converted to DA to produce inhibition; and 3) cAMP is unlikely to be an intermediary in DA inhibition.

Some catecholamine inhibitors do not cause accumulation of nuclear estrogen receptors in rat hypothalamus and anterior pituitary gland

We have recently shown that the dopamine-beta-hydroxylase inhibitor, U-14,624, decreases the concentration of cytosol estrogen receptors in the mediobasal hypothalamus (MBH) and anterior pituitary gland (AP) in ovariectomized rats, but that it also causes cell nuclear accumulation of estrogen receptors. We tried to determine if this is the mechanism by which other catecholaminergic inhibitors decrease the concentration of cytosol estrogen receptors in either the MBH or AP. The previously reported decrease in the concentration of cytosol estrogen receptors in AP by the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine was confirmed. Also, the decrease in the concentration of cytosol estrogen receptors in MBH after treatment with the dopamine-beta-hydroxylase inhibitors, diethyldithiocarbamate and FLA 63 was demonstrated. In no case was an increase in the concentration of nuclear estrogen receptor accumulation detected after treatment with the drugs. Results of assays of norepinephrine and dopamine levels in MBH after the various treatments suggest that, at the dosage used, U-14,624 has a greater effect on norepinephrine and dopamine levels that the other dopamine-beta-hydroxylase inhibitors. The results of these experiments suggest that inhibitors of dopamine-beta-hydroxylase and tyrosine hydroxylase cause decreases in the concentration of cytosol estrogen receptors in either the MBH or AP that are not referable to increased cell nuclear accumulation of estrogen receptors.