Identification of dopamine receptor in Tetrahymena thermophila by fluorescent ligands

Two types dopamine receptor present in the cell membrane of vertebrates. But in this study D1 receptor was identified in the invertebrate ciliates protozoan, Tetrahymena thermophila by use of fluorescent ligands. D1 specific agonist SKF-38393 binds specifically to Tetrahymena. The specific binding of SKF-38393 was encountered by equimolar addition of D1 antagonist thus showed no binding of ligands. In addition, it was also proved that the D1 specific agonist did not cross bind with the D2 type receptor due to the equimolar addition of D2 selective antagonist spiperone. Interestingly this study also showed that the dopamine receptor present in the endoplasmic reticulum and endosomes of Tetrahymena as well as cell membrane which was revealed by laser scanning microscope. Therefore, this evidence supports the existence of a D1 receptor in the ciliate protozoan.

Changes in apical dendritic structure correlate with sustained ERK1/2 phosphorylation in medial prefrontal cortex of a rat model of dopamine D1 receptor agonist sensitization

Rats lesioned with 6-hydroxydopamine (6-OHDA) as neonates exhibit behavioral and neurochemical abnormalities in adulthood that mimic Lesch-Nyhan disease, schizophrenia, and other developmental disorders of frontostriatal circuit dysfunction. In these animals a latent sensitivity to D1 agonists is maximally exposed by repeated administration of dopamine agonists in the postpubertal period (D1 priming). In neonate-lesioned, adult rats primed with SKF-38393, we found selective, persistent alterations in the morphology of pyramidal neuron apical dendrites in the prelimbic area of the medial prefrontal cortex (mPFC). In these animals, dendrite bundling patterns and the typically straight trajectories of primary dendritic shafts were disrupted, whereas the diameter of higher-order oblique branches was increased. Although not present in neonate-lesioned rats treated with saline, these morphological changes persisted at least 21 days after repeated dosing with SKF-38393, and were not accompanied by markers of neurodegenerative change. A sustained increase in phospho-ERK immunoreactivity in wavy dendritic shafts over the same period suggested a relationship between prolonged ERK phosphorylation and dendritic remodeling in D1-primed rats. In support of this hypothesis, pretreatment with the MEK1/2-ERK1/2 pathway inhibitors PD98059 or SL327, prior to each priming dose of SKF-38393, prevented the morphological changes associated with D1 priming. Together, these findings demonstrate that repeated stimulation of D1 receptors in adulthood interacts with the developmental loss of dopamine to profoundly and persistently modify neuronal signaling and dendrite morphology in the mature prefrontal cortex. Furthermore, sustained elevation of ERK activity in mPFC pyramidal neurons may play a role in guiding these morphological changes in vivo.

Activity of protein kinase C is important for 3alpha,5alpha-THP's actions at dopamine type 1-like and/or GABAA receptors in the ventral tegmental area for lordosis of rats

In the ventral tegmental area, progestogens facilitate sexual receptivity of rodents via actions at dopamine type 1-like and/or gamma-aminobutyric acid type A receptors and activation of downstream signal transduction molecules. In the present study, we investigated whether effects of progesterone's metabolite, 3alpha,5alpha-THP, to enhance lordosis via actions at these receptors in the ventral tegmental area requires phospholipase C-dependent protein kinase C. The objective of this study was to test the hypothesis that: if progestogens' actions through dopamine type 1-like and/or gamma-aminobutyric acid type A receptors in the ventral tegmental area for lordosis require protein kinase C, then inhibiting protein kinase C in the ventral tegmental area should reduce 3alpha,5alpha-THP-facilitated lordosis and its enhancement by dopamine type 1-like or gamma-aminobutyric acid type A receptor agonists. Ovariectomized, estradiol (E(2); 10 microg s.c. at h 0)-primed rats were tested for their baseline lordosis responses and then received a series of three infusions to the ventral tegmental area: first, bisindolylmaleimide (75 nM/side) or vehicle; second, SKF38393 (100 ng/side), muscimol (100 ng/side), or vehicle; third, 3alpha,5alpha-THP (100, 200 ng/side) or vehicle. Rats were pre-tested for lordosis and motor behavior and then tested for lordosis after each infusion and 10 and 60 min after the last infusion. Rats were tested for motor behavior following their last lordosis test. As has been previously demonstrated, 3alpha,5alpha-THP infusions to the ventral tegmental area increased lordosis and effects were further enhanced by infusions of SKF38393 and muscimol. Infusions of bisindolylmaleimide to the ventral tegmental area attenuated 3alpha,5alpha-THP-, SKF38393-, and/or muscimol-facilitated lordosis. Effects on lordosis were not solely due to changes in general motor behavior. Thus, 3alpha,5alpha-THP's actions in the ventral tegmental area through membrane receptors may require activity of protein kinase C.

In the ventral tegmental area, the membrane-mediated actions of progestins for lordosis of hormone-primed hamsters involve phospholipase C and protein kinase C

Progestin-facilitated lordosis of rodents is enhanced by activation of dopamine type 1 (D(1)) or GABA(A) receptors, their downstream G-proteins, and/or second messengers in the ventral tegmental area (VTA). We examined whether the ability of progestins to enhance lordosis via actions at D(1) and/or GABA(A) receptors is contingent upon activation of the second messenger phospholipase C (PLC) and its associated kinase, protein kinase C (PKC), in the VTA. If the actions of progestins through D(1) and GABA(A) receptors in the VTA are mediated through PLC and PKC, then inhibiting PLC formation (Experiment 1) or blocking PKC (Experiment 2) should reduce progestin-facilitated lordosis and its enhancement by D(1) (SKF38393) or GABA(A) (muscimol) receptor agonists. In Experiment 1, ovariectomised hamsters, primed with oestradiol (10 microg; h 0) + progesterone (100 microg; h 45), were pretested for lordosis and motor behaviour (h 48) and then infused with the PLC inhibitor, U73122 (400 nM/side), or vehicle. Thirty minutes later, hamsters were retested and then received infusions of SKF38393 (100 ng/side), muscimol (100 ng/side), or vehicle to the VTA. Hamsters were post-tested for lordosis and motor behaviour 30 min later. In Experiment 2, a similar protocol was utilised except that instead of the PLC inhibitor hamsters were infused with the PKC inhibitor, bisindolylmaleimide (75 nM/side). Systemic progesterone, SKF38393-, and muscimol-facilitated lordosis was attenuated by infusion of the PLC inhibitor, U73122, or the PKC inhibitor, bisindolylmaleimide, compared to vehicle to the VTA. Thus, the actions of progestins in the VTA to enhance lordosis through D(1) and/or GABA(A) may include downstream activity of PLC and PKC.

Receptor affinities of dopamine D1 receptor-selective novel phenylbenzazepines

We prepared a series of 18 novel substituted phenylbenzazepine congeners of the dopamine D1/D5 receptor partial-agonist SKF-83959 (R,S-3-methyl-6-chloro-7,8-dihydroxy-1-[3'-methylphenyl]-2,3,4,5-tetrahydro-1H-benzazepine) and characterized their potency and selectivity in assays of dopamine, 5-HT and adrenoceptors in rat brain tissue or membranes of genetically transfected cells. The R-enantiomer of SKF-83959 (MCL-202) and three other novel racemic 1-phenyl-7,8-dihydroxybenzazepines (MCL-204, -203, and -207) showed very high dopamine D5 receptor affinity; MCL-209 displayed the greatest dopamine D5 receptor affinity. These five potent novel ligands also had >100-fold selectivity for dopamine D1 over dopamine D2, D3, serotonin 5-HT-2A receptors and alpha2-adrenoceptors. They require further functional testing to characterize their intrinsic activity, and for potential stimulant-antagonist actions, as observed with SKF-83959 and MCL-202.

Functional analysis of the human D5 dopamine receptor missense and nonsense variants: differences in dopamine binding affinities

The functional analysis of expressed human gene variants is important in the study of genetic susceptibility to diseases, pharmacogenetic traits and for the investigation of the human genetic diversity at the molecular level. We have performed the analysis of sequence polymorphisms in the human D5 dopamine receptor gene (DRD5) predicting missense and nonsense amino acid changes in the receptor protein. The amino acid substitutions in the human D5 dopamine receptor are: Leu88 to Phe in the putative second transmembrane domain, Ala269 to Val in the third intracellular and Pro330 to Gln in the third extracellular loops, Asn351 to Asp in the seventh transmembrane and Ser453 to Cys in the C-terminal domains and Cys335 to Stop in the third extracellular loop. The two amino acid substitutions in the transmembrane domains had an effect on agonist binding to the human D5 dopamine receptor. Asn351 to Asp resulted in an approximately 10-fold decrease in dopamine and threefold decrease in R(+)-SKF-38393 binding affinities. Leu88 to Phe resulted in a small increase in dopamine binding affinity. Antagonist binding affinities were mostly unaffected by the amino acid substitutions with the exception of Leu88 to Phe, which showed small reductions in binding affinities of SCH-23390 and risperidone. The existence of functionally different variants of the human dopamine receptors might have phenotypic consequences given their importance in central nervous system physiology and pharmacology.

Elimination of palmitoylation sites in the human dopamine D1 receptor does not affect receptor-G protein interaction

We have eliminated putative palmitoylation sites in the carboxyl tail of the human dopamine D1 receptor by replacing the two cysteine residues with alanines either separately or together. The wild type and the three mutated dopamine D1 receptors were stably expressed in baby hamster kidney cells and characterized to detect any resulting alterations in receptor-G protein interactions. The three mutant dopamine D1 receptors retained the same proportion of high affinity state for agonists as wild type receptors and also no difference was observed in the stimulation of adenylyl cyclase activity. Our results are in contrast to those observed with the beta 2-adrenoceptor and consistent with similar studies of luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptors, alpha 2-adrenoceptors, muscarinic M2 receptors and thyrotropin releasing hormone (TRH) receptors. Thus, we suggest that palmitoylation appears to play a unique role in the beta 2-adrenoceptors, and appears not to be essential in G protein coupling for the dopamine D1 receptors.

D1-like dopaminergic activation of phosphoinositide hydrolysis is independent of D1A dopamine receptors: evidence from D1A knockout mice

Accumulated evidence suggests that dopamine and dopamine D1 agonists can activate phospholipase C in both brain and peripheral tissue. The receptor that mediates the hydrolysis of phosphoinositides has not been identified. The cloned dopamine D1A receptor that is generally thought to be linked to adenylyl cyclase, has also been proposed to couple to phospholipase C. However, a number of studies have suggested that this signaling pathway is mediated via a distinct D1-like dopamine receptor. We tested whether the D1A site plays a role in stimulating phosphoinositide hydrolysis by using the dopamine D1A-deficient mutant mice as a test model. Results show that although D1 dopamine receptor-mediated product on of cAMP is completely absent in membranes of D1A-deficient mice, D1 receptor-mediated accumulation of inositol phosphate is identical in tissues of mutant and wild-type animals. Furthermore, the coupling of [3H]SCH23390 binding sites in striatal or frontal cortex membranes to G alpha s is markedly reduced, although coupling of [3H]SCH23390 binding sites to G alpha q was unaltered in tissue taken from D1A mutant mice compared with control animals. These results clearly demonstrate that dopaminergic stimulation of inositol phosphate formation is mediated by a D1 dopamine receptor subtype that is distinct from the D1A receptor that activates adenylyl cyclase.

Block by apomorphine of acetylcholine receptor channels expressed in Xenopus oocytes

Effects of apomorphine and other compounds related to dopamine receptors on nicotinic acetylcholine receptor channels were investigated by expressing functional channels in Xenopus oocytes. When channels were expressed with a combination of alpha 3 and beta 4 subunits, acetylcholine activated an inward current, and apomorphine suppressed the current in a concentration-dependent manner with an IC50 value of about 3 microM. SKF38393 (R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol; dopamine D1 receptor agonist; 3 and 30 microM), quinpirole (dopamine D2 receptor agonist; 30 microM), SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zepine; dopamine D1 receptor antagonist; 10 microM) or sulpiride (dopamine D2 receptor antagonist; 10 microM) also inhibited the acetylcholine-activated current whereas dopamine (100 microM) was ineffective. The inhibition by apomorphine of the acetylcholine-activated current was also apparent when alpha 3 subunit was combined with beta 2 subunit instead of beta 4 subunit, or beta 4 subunit was combined with alpha 2 or alpha 4 subunit instead of alpha 3 subunit to express channels. The results suggest that apomorphine blocks acetylcholine receptor channels through a binding site that is similar to, but cannot be included in dopamine receptors. The binding site may not be present in a single specific subunit.

Desensitization, phosphorylation and palmitoylation of the human dopamine D1 receptor

The regulation and post-translational modifications of the human dopamine D1 receptor were studied in the baculovirus-eukaryotic cell expression system. Baculovirus constructs containing either the DNA encoding the dopamine D1 receptor or a DNA encoding a c-myc epitope tagged dopamine D1 receptor (c-myc-dopamine D1 receptor) were used to infect Spodoptera frugiperda (Sf9) insect cells. Expressed dopamine D1 and c-myc-dopamine D1 receptors bound agonists and antagonists with affinities and a rank order of potency characteristic of a classical dopamine D1 receptor pharmacological profile. In membrane preparations from cells expressing c-myc-dopamine D1 receptor, the photoaffinity label [125I](3-methyl-2-[4'-azidophenyl]-2,3,5-tetrahydro-2H-3-benzazepine) ([125I]MAB) bound specifically upon photolysis. A major broad band of approximately 48 kDa was detected. This species was identified in immunoblots by the monoclonal antibody raised against the c-myc epitope of c-myc-dopamine D1 receptor was isolated by immunoprecipitation from whole cells and was shown to be post-translationally modified by phosphorylation and palmitoylation. Exposure of cells expressing c-myc-dopamine D1 receptor to dopamine for 15 min resulted in a reduction in the maximal dopamine stimulated adenylyl cyclase activity, which was accompanied by an increased phosphorylation of the receptor and a rapid redistribution of surface c-myc-dopamine D1 receptor as detected by in situ immunofluorescence. Dopamine exposure also resulted in an increased level of incorporation of [3H]palmitic acid into the receptor. Thus, we provide the first evidence that the human dopamine D1 receptor undergoes agonist-dependent desensitization, phosphorylation and palmitoylation.

Cerebellar and striatal dopamine receptors: effects of reeler and weaver murine mutations

The presence and the binding characteristics of D1 and D2 receptors were investigated in normal-reeler and normal-weaver mutant mice utilizing [3H]spiperone (D2 antagonist), [3H]SKF 38393 (D1 agonist), and [3H]DA as ligands. Analysis of the binding data showed that in the cerebellum there are two binding components for all [3H]ligands. Comparison of the binding constants from cerebellum and striatum showed that in cerebellum the high affinity-low capacity component has similar affinity with that of striatum. The reeler and weaver mutations affected the binding of all ligands: In reeler, total cerebellar specific binding sites for [3H]spiperone and [3H]SKF 38393 decrease significantly (approximately 50% and approximately 70%, respectively), while those for [3H]DA show a small (approximately 10-15%) but not significant decrease. In weaver, total cerebellar specific binding sites for [3H]spiperone, [3H]SKF 38393, and [3H]DA also decrease significantly (approximately 60%, approximately 70%, and approximately 50%, respectively). In reeler striatum [3H]SKF 38393 binding (Bmax) is significantly decreased (approximately 24%), while [3H]spiperone and [3H]DA binding (Bmax) is not affected. In weaver striatum, [3H]SKF 38393 binding is significantly increased (approximately 40%), while [3H]DA binding (Bmax) decreases significantly (approximately 70%). On the basis of the cytoarchitectural aberrations that characterize the cerebellum of these mutants and some well-established information regarding the dopaminergic system of the cerebellum, the above results indicate that in this region a) D1 receptors are mainly localized on granule cells and b) D2 receptors are localized postsynaptically on granule cells and presynaptically on the DA fibers innervating the cerebellum.

Characterization of D1 receptors mediating dopamine-stimulated growth hormone release from pituitary cells of the goldfish, Carassius auratus

Previously, we demonstrated that dopamine (DA) stimulates GH release from the pituitary of goldfish, and this action is mediated by D1-like receptors. In the current study, we have provided evidence for the presence of D1-specific binding sites in the pituitary cells of goldfish. These D1-binding sites were found to be saturable, stereospecific, and selective for D1 ligands. The rank order of binding affinity of these D1-binding sites is (+)SCH23390 > SKF83566 >> (-)SCH23390 > domperidone > LY171555 >> serotonin. The association of these D1-binding sites with [3H]SCH23390, a D1-specific radioligand, was rapid, reversible, and exhibited a high binding affinity in the nanomolar range. The Kd values were estimated to be 33.7 +/- 8.5 nM for mixed populations of pituitary cells and 10.9 +/- 2.5 nM for pituitary cell preparations enriched with somatotrophs. Autoradiographic studies revealed that specific binding of [3H]SCH23390 was predominantly localized in the pars distalis, not in the neurointermediate lobe of the goldfish pituitary. Furthermore, these D1-binding sites in the goldfish pituitary cells could be functionally correlated with the GH-releasing actions of DA. Since these D1-binding sites exhibited the expected pharmacological properties of mammalian D1 receptors, we conclude that DA D1 receptors are present in the goldfish pituitary and are responsible for the mediation of DA D1-stimulated GH release. The apparent similarities of the D1 receptor pharmacology between goldfish and mammals also suggests that DA D1 receptors are highly conserved during vertebrate evolution.

Chronic treatment with MK-801 decreases D2 dopamine receptor function in rat striatum

Present results show that a single treatment with dizocilpine (MK-801, 0.25 mg/kg IP) failed to modify the specific binding to D1 or D2 DA receptors. In contrast, repeated administrations for 3 weeks resulted in a statistically significant decrease of [3H]Spiroperidol binding to cortical or striatal membranes but did not change the number or the apparent affinity of [3H]MK-801 binding in well-washed cortical membranes. Consistent reduction in specific D2 receptor mediated behavior was obtained. The data suggest that the changes in DAergic function following repeated administrations with MK-801 could be suggestive of potential therapeutic uses of negative allosteric drugs in some DA related dysfunctions.

Analysis of the agonist activity of fenoldopam (SKF 82526) at the vascular 5-HT2 receptor

1. The 5-HT2 receptor agonist activity of fenoldopam (SKF 82526) was characterized in the rabbit isolated aorta preparation. 2. Fenoldopam was an agonist at the vascular 5-HT2 receptor with lower affinity and efficacy than the naturally occurring agonist 5-hydroxytryptamine (5-HT). Fenoldopam had an affinity (pKA) of 5.84 +/- 0.04 and efficacy (tau) of 0.57 +/- 0.04, whereas 5-HT had a pKA of 6.65 +/- 0.12 and tau of 2.66 +/- 0.41. 3. The constrictor effects of fenoldopam and 5-HT were competitively antagonized by the 5-HT2 antagonist, ketanserin, with pKB values of 8.81 +/- 0.11 and 8.83 +/- 0.10 respectively. 4. Prior incubation with fenoldopam produced a concentration-related rightward shift of a subsequent 5-HT concentration-response curve. This inhibition was specific for 5-HT since constrictor responses to angiotensin II were unaffected. 5. This study indicates that the D1 receptor agonist, fenoldopam, acts as an agonist at the vascular 5-HT2 receptor, but with an affinity and efficacy less than that of the naturally occurring agonist, 5-HT.

Stereospecific effects of ascorbic acid and analogues on D1 and D2 agonist binding

Ascorbic acid inhibited the specific binding of both the D1 agonist, [3H] SKF 38393, and the D2 agonist, [3H] N-0437 at physiologically relevant concentrations. This inhibition was both stereospecific and receptor selective. Using ligand concentrations approximating their KD's, the IC50's for ascorbate and two structural analogues, isoascorbate and D-glucoascorbate, were determined. The rank order of IC50's at both D1 and D2 were D-glucoascorbate greater than isoascorbate greater than ascorbate. However, the IC50 for each compound was greater at D1 than D2. Evaluation of the relationship between the IC50 for ascorbate and the ligand concentration using both the D1 and the D2 ligand yielded data inconsistent with competitive inhibition models. Preliminary experiments were conducted to evaluate the site and type of inhibition with results consistent with an allostearic effect at the level of the receptor.

Postnatal development of striatal dopamine function. II. Effects of neonatal 6-hydroxydopamine treatments on D1 and D2 receptors, adenylate cyclase activity and presynaptic dopamine function

To evaluate the influence of patch and matrix ingrowth of DA terminals upon striatal DA (dopamine) receptor function, we performed bilateral intrastriatal (i.s.) or single intracisternal (i.c.) injections of 6-hydroxydopamine (6-OHDA) into rat pups at various postnatal ages and determined D1 and D2 receptor binding, adenylate cyclase activities and markers for presynaptic DA terminal density and turnover as the animals matured. All injection schedules yielded: (a) variable and partial loss of DA, (b) increased DA turnover, (c) small (15-40%) increases in D1 receptor number but no change in affinity for antagonist ([3H]SCH 23390), (d) 2-3-fold increases in affinity of D1 receptors for agonist (SKF 38393) with preserved regulation of agonist affinity by guanine nucleotide, (e) no significant changes in DA-, guanine-nucleotide-, manganese- and forskolin-stimulated AC (adenylate cyclase) activity. D2 receptor binding was evaluated between 1 and 7 weeks of age in animals with i.s. treatment and 7 and 10 weeks of age in animals with i.c. treatment and was reduced by 40-50% with both treatment regimens. [3H]mazindol binding, a marker for presynaptic terminal DA transport sites, was reduced 30-40% by multiple i.s. or i.c. treatment regimens. In animals treated with one i.s. injection, [3H]mazindol binding was reduced 70% at 1 week of age, equal to control by 2 weeks and 14-46% greater than control between 3 and 7 weeks. We conclude that striatal D1 receptor sites maintain their density and second messenger function independently of postsynaptic DA terminal ingrowth, whereas the development of D2 receptor sites is sensitive to disruptions of DA terminal ingrowth.

Pharmacological evaluation of SCH-12679: evidence for an in vivo antagonism of D1-dopamine receptors

The benzazepine compound SCH-12679 has been shown to have clinical efficacy against aggressive behavior in mentally deficient patients. The purpose of the present investigation was to evaluate the potential mechanism of action of SCH-12679. Because of the structural similarity of SCH-12679 to compounds influencing D1-dopamine receptors, even though in vitro studies indicated no direct action on this receptor, investigations focused on the possibility that in vivo SCH-12679 antagonizes the function of this dopamine receptor subtype. After i.p. administration to neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats, SCH-12679 reduced, dose-dependently, the locomotor activity induced by SKF-38393, a D1-dopamine agonist. A dose of SCH-12679 that antagonized the activity induced by SKF-38393 in neonatally lesioned rats also blocked various behaviors observed after administration of this D1-dopamine agonist. SCH-12679 did not alter the activity or behavioral responses induced by quinpirole, a D2-dopamine agonist, when administered to 6-OHDA-lesioned rats. SCH-12679 antagonized the self-mutilation behavior and behavioral responses induced by L-dihydroxyphenylalanine in neonatal-6-OHDA lesioned rats in a manner similar to the prototypic D1-dopamine antagonist SCH-23390 and, like SCH-23390, produced a deficit in avoidance responding in unlesioned rats. SCH-12679 produced a small, transient activation of locomotor activity immediately after administration to neonatal-6-OHDA-lesioned rats that was not observed in unlesioned or adult-6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence from functional and autoradiographic studies for the presence of tubular dopamine-1 receptors and their involvement in the renal effects of fenoldopam

Evidence from receptor-ligand binding and biochemical studies seems to suggest the possible existence of tubular dopamine DA-1 receptors in the rat kidney. However, it is not yet clear whether these putative tubular DA-1 receptors are involved in the functional renal responses elicited during the administration of DA and DA receptor agonists. In the present study we have examined the renal effects of several doses of selective DA-1 receptor agonist fenoldopam in pentobarbital-anesthetized rats in an attempt to unmask a direct tubular DA-1 receptor-mediated diuresis and natriuresis. Additionally, we have performed receptor-ligand binding and autoradiographic studies to examine the presence and localization of DA-1 receptors in various regions of the rat kidney. At the highest dose studied (2 micrograms/kg/min), fenoldopam produced diuresis and natriuresis, which was accompanied by a significant decrease in blood pressure and also a significant increase in glomerular filtration rate. At 1 micrograms/kg/min, the diuretic and natriuretic effects of fenoldopam were observed in the absence of any changes in blood pressure and glomerular filtration rate, but there was a significant increase in renal blood flow. However, at 0.5 micrograms/kg/min, fenoldopam-induced natriuresis and diuresis was not accompanied by any changes in blood pressure, renal blood flow or glomerular filtration rate, implying a direct tubular effect. These effects of fenoldopam appear to be mediated via activation of DA-1 receptors, because they were antagonized by the selective DA-1 antagonist SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)

A retrospective analysis of fenoldopam renal excretion in 65 subjects: evidence for possible intrarenal formation of fenoldopam from its metabolites

Clinical studies have suggested that the dopamine DA1 agonist, fenoldopam, may exhibit nonlinear renal excretion in humans. A retrospective population pharmacokinetic analysis of the renal excretion of fenoldopam and one of its major metabolites, fenoldopam-8-sulfate, was conducted in 65 healthy volunteers to examine this phenomenon. Fenoldopam-8-sulfate exhibited a mean (+/- SE) renal plasma clearance of 129 +/- 4 ml/min, which was independent of its AUC. In contrast, fenoldopam renal plasma clearance ranged from 2220 to 150 ml/min and decreased nonlinearily with increasing fenoldopam AUC. Fenoldopam renal clearance was characterized as a function of fenoldopam AUC using a nonlinear saturation model. The analysis predicted an initial maximal renal clearance of 2852 ml/min, which decreased to 78 ml/min at maximal inhibition. The fenoldopam AUC required to half-saturate fenoldopam renal clearance was 5.2 ng x hr/ml. The elevated clearance values for fenoldopam, beyond normal physiologic limits for renal blood flow in man, suggest that intrarenal formation of fenoldopam from one or more of its circulating metabolites may be contributing to the observed nonlinear decreases in fenoldopam renal excretion. Preliminary data from our laboratory suggest that in vivo desulfation of fenoldopam-8-sulfate to fenoldopam does occur in the dog.