DA Agonists Research Articles

Comparative evaluation of lisuride and terguride, ergot alkaloid derivatives, on the field-stimulated vas deferens of mouse

The ergot alkaloid derivatives, lisuride (LIS) and terguride (TDHL), known to interact with central dopamine receptors as agonist and partial agonist, respectively, were studied on the field-stimulated mouse vas deferens, where recently the existence of presynaptic dopamine receptors has been evidenced. LIS was a competitive antagonist at prejunctional alpha 2 and DA1 receptors situated on the sympathetic nerve terminals of the mouse, with a pA2 value of 9.2 and 9.1, respectively. LIS was also able to antagonize the effects of LY 171555, selective DA2 agonist, but the type of interaction cannot be conceptualized in terms of competitive antagonism. Likewise, the type of interaction of TDHL with dopaminergic and adrenergic agonist-activated sites is not suggestive of a competitive antagonism. Based on these results, it seems that central and peripheral pharmacologic profiles of LIS and TDHL cannot be overlapped, LIS being a potent DA1- and alpha 2-antagonist with a high degree of specifity for these receptors.

The therapeutic outcome in schizophrenia of new DA receptor antagonists and agonists in comparison with classical neuroleptic drugs

The therapeutic outcome in schizophrenia of new DA receptor antagonists and agonists in comparison with classical neuroleptic drugs

Effects of specific dopaminergic agonists and antagonists in the open-field test

It has been found that dopaminergic transmission could be involved in some aspects of anxiety. The present study aims to explore this hypothesis further, using specific DA1 (SKF 38393) and DA2 (bromocriptine) agonists or DA1 (SCH 23390), and DA2 (zetidoline) antagonists in the open-field test. The results confirm previous studies indicating that DA1 and DA2 agonists predominantly increasee locomotor activity, while DA1 and DA2 antagonists predominantly decrease it. However, at low doses, the four drugs increase the peripheral ambulation score significantly and, with the exception of zetidoline, also increase the central ambulation score. The observations made with zetidoline confirm the hypothesis that a specific presynaptic DA2 antagonism could be a determinant for the disinhibitory effects of low doses of neuroleptics. A collateral action on 5HT transmission also suggested to explain an hypothetic anxiolytic action of DA agonists and SCH 23390 at lower doses.

Receptor selectivity of cholecystokinin effects on mesoaccumbens dopamine neurons

Extracellular recording techniques were combined with antidromic stimulation to examine the effects of C-terminal cholecystokinin (CCK) fragments and CCK antagonists on the activity of identified mesoaccumbens dopamine (MADA) neurons in chloral hydrate-anesthetized rats. These experiments were designed to determine the receptor selectivity of sulfated CCK octapeptide (CCK-8S) effects on MADA cells. Neither CCK tetrapeptide (CCK-4) nor unsulfated CCK octapeptide (CCK-8U) significantly altered MADA cell basal firing rate or responsiveness to the inhibitory effects of the D2 DA agonist quinpirole. As reported previously for ventral tegmental area DA cells, CCK-8S produced increases or decreases in the firing rate of most MADA cells sampled. CCK-8S also enhanced the sensitivity of MADA neurons to quinpirole-induced inhibition. This increase in sensitivity to quinpirole was blocked by pretreatment with the nonselective CCK receptor antagonist proglumide and the preferential CCK-A receptor antagonist CR 1409 but not by the preferential CCK-B receptor antagonist L-365,260. The inactivity of CCK-4 and CCK-8U in these tests and the results with the antagonists suggest that the effects of CCK-8S on MADA neuronal activity are mediated by CCK-A receptors.

D 2 dopamine receptor-mediated mechanisms in the medial preoptic-anterior hypothalamus regulate affective defense behavior in the cat

The role of the dopaminergic innervation of the medial preoptic-anterior hypothalamus (mPO-AH) in regulating the expression of affective defense behavior in the cat has been investigated in the present study. Feline affective defense behavior, characterized mainly by autonomic arousal, ear retraction, growling, hissing and paw striking, was elicited by electrical stimulation of the ventromedial hypothalamic nucleus (VMH). Following the establishment of a stable threshold current for eliciting the hissing response of the behavior, the effect of injecting various DAergic agonists and antagonists into the mPO-AH on the hissing threshold was determined. The microinjection of the non-selective DA agonist apomorphine (0.03, 0.16, 0.33, 0.66, 1.56 and 3.3 nmol) into the mPO-AH facilitated hissing in a time- and dose-dependent manner. This effect was mimicked by the D2-selective agonist LY 171555 (0.2 and 1.0 nmol) but not by the D1-selective agonist SKF 38393 (1.7 and 17 nmol), and was blocked by the non-selective and the D2-selective antagonists haloperidol (1.3 nmol) and sulpiride (14.5 nmol), respectively. The injection of the D1-selective antagonist SCH 23390 (0.3 nmol), however, did not inhibit apomorphine-induced facilitation of hissing. In addition, the injection of haloperidol (1.3 nmol) and sulpiride (14.5 nmol), but not SCH 23390 (0.3 nmol), alone inhibited the behavior. It was therefore concluded that dopaminergic stimulation of the mPO-AH may facilitate the expression of affective defense behavior in the cat via a D2 receptor-mediated mechanism. The physiological significance of this effect and the interaction between dopaminergic, noradrenergic and serotonergic innervation of the mPO-AH in modulating the expression of affective defense behavior in response to threatening stimuli are discussed.

Selective dopamine DA1 stimulation with fenoldopam in cirrhotic patients with ascites: A systemic, splanchnic and renal hemodynamic study

We studied the effects of fenoldopam, a selective dopamine DA1 agonist on systemic and splanchnic hemodynamics, renal blood flow and sodium excretion in 12 patients with alcoholic cirrhosis and ascites. Hepatic, azygos and renal veins were catheterized before and after intravenous administration of fenoldopam, 0.05 μg/kg/min for 1 hr and increased to 0.1 μg/kg/min for another hour. Mean arterial pressure progressively decreased (from 83 ± 7 to a minimum of 77 ± 8 mm Hg 100 min after starting the infusion) but returned to baseline level at 120 min. Plasma norepinephrine and renin activity increased (respectively from 567 ± 297 to 919 ± 375 pg/ml, p < 0.05, and from 17 ± 14 to 23 ± 15 ng/ml/hr, p < 0.05). Renal blood flow, urine output or sodium excretion did not change. Sodium output decreased at 1 hr from 6.9 μmol/min to 4.0 μmol/min, p < 0.05. Both hepatic venous pressure gradient and azygos blood flow significantly increased by 21%. We conclude that the acute administration of fenoldopam did not improve renal hemodynamics or function in patients with cirrhosis and ascites. In addition, dopamine DA1 agonism caused further increases in norepinephrine concentration and plasma renin activity. Portal pressure also increased, probably because of an increase in mesenteric blood flow. These results question the renal benefit and raise concern about the use of dopamine agonists in patients with cirrhosis and ascites. (HEPATOLOGY 1991;13:111–116).

Selective dopamine DA1 stimulation with fenoldopam in cirrhotic patients with ascites: A systemic, splanchnic and renal hemodynamic study

We studied the effects of fenoldopam, a selective dopamine DA1 agonist on systemic and splanchnic hemodynamics, renal blood flow and sodium excretion in 12 patients with alcoholic cirrhosis and ascites. Hepatic, azygos and renal veins were catheterized before and after intravenous administration of fenoldopam, 0.05 micrograms/kg/min for 1 hr and increased to 0.1 micrograms/kg/min for another hour. Mean arterial pressure progressively decreased (from 83 +/- 7 to a minimum of 77 +/- 8 mm Hg 100 min after starting the infusion) but returned to baseline level at 120 min. Plasma norepinephrine and renin activity increased (respectively from 567 +/- 297 to 919 +/- 375 pg/ml, p less than 0.05, and from 17 +/- 14 to 23 +/- 15 ng/ml/hr, p less than 0.05). Renal blood flow, urine output or sodium excretion did not change. Sodium output decreased at 1 hr from 6.9 mumol/min to 4.0 mumol/min, p less than 0.05. Both hepatic venous pressure gradient and azygos blood flow significantly increased by 21%. We conclude that the acute administration of fenoldopam did not improve renal hemodynamics or function in patients with cirrhosis and ascites. In addition, dopamine DA1 agonism caused further increases in norepinephrine concentration and plasma renin activity. Portal pressure also increased, probably because of an increase in mesenteric blood flow. These results question the renal benefit and raise concern about the use of dopamine agonists in patients with cirrhosis and ascites.

Inhibition of proximal tubule Na(+)-K(+)-ATPase activity requires simultaneous activation of DA1 and DA2 receptors

This study examines the receptor mechanisms by which dopamine (DA) inhibits Na(+)-K(+)-adenosinetriphosphatase (ATPase) activity in single permeabilized proximal tubule (PCT). Na(+)-K(+)-ATPase activity was inhibited in the presence of both DA1- and DA2-specific agonists but not by either agonist alone. The inhibition induced by DA (10(-6) M) was attenuated in the presence of either of the two DA2-specific antagonists S-sulpiride and YM 09151 at 10(-5) M and in the presence of the DA1 antagonist SCH 23390 (10(-5) M). PCT adenosine 3',5'-cyclic monophosphate (cAMP) levels were significantly increased in the presence of DA and DA1 agonist, but DA2 agonist had no effect on cell cAMP levels. Na(+)-K(+)-ATPase activity was significantly inhibited in PCT incubated with DA2 agonist (10(-5) M) and dibutyryl (DB)-cAMP (10(-6) M) but not with DA2 agonist (10(-5) M) only. PCT Na(+)-K(+)-ATPase activity was also significantly inhibited in the presence of both DA2 agonist (10(-5) M) and forskolin (10(-6) M). Neither DBcAMP (10(-6) M) nor forskolin (10(-6) M) alone inhibited Na(+)-K(+)-ATPase activity. In tubules incubated with DA (10(-8) to 10(-9) M), the presence of DBcAMP (10(-6) M) enhanced the sensitivity by which DA inhibited Na(+)-K(+)-ATPase activity. We conclude that PCT Na(+)-K(+)-ATPase activity is inhibited by a synergistic action of the DA1 and DA2 receptors, with the DA1 receptor acting to increase cell cAMP levels.

Effect of pramipexole, a dopamine‐1/dopamine‐2 receptor agonist, on sodium excretion and blood pressure in spontaneously hypertensive rats

1. Abnormalities of the renal dopaminergic system have been implicated in the pathogenesis of hypertension in the spontaneously hypertensive rat (SHR). 2. Both DA-1 and DA-2 receptors are present in renal tubules and blood vessels. DA-1 receptors mediate the renal vasodilatory and natriuretic effects of DA but the contribution of DA-2 receptors to these effects is not known. 3. We therefore studied the effect of a novel and selective DA-1 and DA-2 agonist, pramipexole, on MAP, glomerular filtration rate (GFR), urine flow (V), absolute (UNaV) and fractional sodium (FeNa) excretion in 9-18-week-old SHR. Wistar-Kyoto rats (WKY) served as control. 4. Pramipexole given intravenously (1, 10, 100 micrograms kg body wt-1 min-1) decreased MAP in a dose-related manner to a greater extent in SHR (n = 5) than WKY (n = 6) such that at the highest dose of pramipexole, MAP was similar in both groups. Pramipexole did not alter GFR in either WKY or SHR. Pramipexole increased V in a dose-related manner in both WKY and SHR. At 100 micrograms pramipexole kg body wt-1 min-1, V increased eightfold in both SHR and WKY. In contrast, pramipexole increased UNaV to a greater extent in WKY (5.1-fold) than SHR (3.7-fold). 5. These studies show a differential effect of pramipexole on renal function and MAP in SHR and WKY. Pramipexole has a more potent blood pressure lowering effect in SHR than in WKY. However, the natriuretic effect of pramipexole was greater in the WKY than in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of DA1 receptor-adenylate cyclase coupling in proximal convoluted tubules

The natriuretic effect of dopamine (DA) is less in younger than in older animals. The natriuretic (DA) is less in younger than in older animals. The natriuretic effect of DA is due in part to occupation of renal tubular DA1 receptors, which are most abundant in the proximal convoluted tubule (PCT). However, it has not been determined whether ontogenic differences in DA1-receptor density, affinity, or coupling to intracellular second messengers are involved in the reduced natriuretic effect of DA in the young animal. We therefore studied the DA1 receptor by radioligand binding with the DA1 antagonist 125I-SCH 23982 and the effect of DA1 agonists and guanine nucleotides on adenylate cyclase (AC) activity in microdissected PCT during development in Wistar-Kyoto rats (WKY). The dissociation constant (Kd) and maximum receptor density (Bmax) were similar in all groups (Kd, in nM: 11.9 +/- 0.7 at 3 wk, 10.6 +/- 0.4 at 8 wk, and 12.2 +/- 1.2 at 20 wk; Bmax, in fmol/mm PCT: 0.24 +/- 0.02 at 3 wk, 0.23 +/- 0.01 at 8 wk, and 0.24 +/- 0.01 at 20 wk). Basal AC activities were similar, and forskolin (10(-5) M), which directly stimulates AC, increased AC activity to a similar extent in all age groups. However, the DA1 agonists, fenoldopam (10(-5) M) and SND-919-CL2 (10(-6) M), increased AC activity in PCT to a greater extent in 20-wk-old (55 +/- 7%) than in 3-wk-old rats (27 +/- 1%).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine inhibits Na(+)-H+ exchanger activity in renal BBMV by stimulation of adenylate cyclase

To determine a renal tubular mechanism for the natriuretic effect of dopamine (DA) and DA-1 agonists, we measured Na(+)-H+ exchange activity (amiloride sensitive) in rat renal cortical brush-border membrane vesicles (BBMV). Renal cortical tissues were preincubated with ligands before BBMV preparation to study Na(+)-H+ exchange activity in the absence of the added ligands that may compete for ion binding sites of the exchanger. DA and DA-1 agonist-inhibited Na(+)-H+ exchange activity was concentration and time dependent. The inhibitory effect was not due to increased permeability, collapse of the proton gradient, or change in vesicle size and did not extend to Na(+)-glucose symport. DA-2 agonists had no effect, whereas alpha-adrenergic agonists increased Na(+)-H+ exchange activity. Kinetic analysis revealed that the DA-1 agonist inhibited Na(+)-H+ exchange activity by a noncompetitive process. 2',5'-Dideoxyadenosine inhibited adenylate cyclase activity and reversed the inhibitory effect of DA-1 agonist on the exchanger. H4, an isoquinoline sulfonamide, which inhibits protein kinase A, also reversed the inhibitory effect of DA-1 agonist on the exchanger. Thus the DA-1 agonist-mediated inhibition of Na(+)-H+ exchange activity in BBMV is a receptor-mediated adenylate cyclase-linked process.

Is the Release of Dopamine from Medial Prefrontal Cortex Modulated by Presynaptic Receptors? Comparison with Nigrostriatal and Mesolimbic Terminals

Results obtained from our in vitro studies employing superfused slices obtained from three functionally different brain regions rich in DA axon terminals were discussed. Striking qualitative and quantitative similarities were found for the modulation of DA release from the nucleus caudate and the OT of the rabbit. However, the PFC DA terminals showed important differences from the nigrostriatal and mesolimbic DA terminals. Although release modulatory D2 DA autoreceptors could also be demonstrated in superfused slices of the PFC, our results suggest that the cortical nerve terminals may have a lower number of functional autoreceptors or a reduced efficiency of coupling between receptors and inhibition of release. Either possibility could explain (a) the poor inhibitory efficacy of the agonists, (b) the small facilitatory effect of the antagonists, (c) the disproportionate increase in transmitter overflow produced by neuronal uptake inhibitors, and (d) the lack of synergism between uptake inhibitors and DA antagonists. When the efficacy of the autoreceptor mechanisms was evaluated at stimulation frequencies comparable to the in vivo firing rates reported for each of the three neuronal groups, it was found that DA release from the striatum and the OT was tightly modulated by presynaptic D2 DA receptors; whereas release from PFC was not. We propose that the autoreceptor-mediated control of DA release from PFC may not function in vivo, even though modulation of release by presynaptic D2 DA receptors from PFC terminals could be demonstrated under specific experimental conditions in vitro. However, it is envisaged that if in vivo firing rate of the PFC DA neurons is reduced, the inhibitory actions of DA agonists on DA release may be regained. From these and other studies it is apparent that drug effects on autoreceptors are highly dependent on the rate and duration of stimulation applied to a specific neuronal group. We propose that the basal status of activity of a specific neuronal target could determine the type and magnitude of the effect produced by a therapeutic agent acting at release modulatory receptors. The neuronal activity (firing rate and pattern) may be affected by physiological status, disease, and by current or previous drug treatments. The mechanisms by which PFC DA terminals release a larger proportion of their storage pool compared to other mesotelencephalic DA terminals is unknown and may represent a compensatory mechanism to the continuous rapid firing rates at which these neurons are exposed in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral Dopamine Receptors

Since 1979 when two subtypes of peripheral dopamine receptors (DA1 and DA2) were first proposed, much progress has been made to confirm such a view. Cumulative experience with the dog in vivo models suggests that the potency order: fenoldopam greater than dopamine greater than dipropyl dopamine greater than apomorphine characterizes the DA1 receptor while the reverse order: apomorphine greater than dipropyl dopamine greater than or equal to dopamine much greater than fenoldopam distinguishes the DA2 subtype. SCH 23390 for the DA1 and domperidone and (S)-sulpiride for the DA2, as selective antagonists, clearly identify the two subtypes. Increase in cyclic AMP after DA1 activation and decrease after DA2 occupancy have been reported. However, how both increase and decrease in cyclic AMP transduce vascular relaxation remains to be explained. DA1 and DA2 agonist activities of two new structures, CY 208,243 (Sandoz) and dihydrexidine, are reported in this chapter. Addition of benzene to ergoline and benzoquinoline moieties rendered these molecules inactive on the DA2 receptor while bestowing DA1 agonist activity on these otherwise inactive molecules and even obviated the need for two OH groups for DA1 activity. It would appear that the current views on structural requirements for DA1 and DA2 agonist activities will require revision and reexamination.

R(−) and s(+) stereoisomers of 11-hydroxy- and 11 -methoxy-N-n -propylnoraporphine: Central dopaminergic behavioral activity in the rat

R(-)11-Hydroxy-N-n-propylnoraporphine (11-OH-NPa) induced stereotyped behavior in the rat as potently (ED50 = 0.80 mg/kg, i.p.) as R(-)apomorphine (APO) and this effect was blocked by haloperidol; the 11-methoxy congener, R(-)11-MeO-NPa, had a weak effect (ED50 greater than 10 mg/kg) and the S(+) isomers had none. The isomer R(-)11-OH-NPa potentiated locomotion stimulated by apomorphine; S(+)11-OH-NPa inhibited it and the isomers of 11-MeO-NPa were inactive. Catecholaporphines usually are inactive orally, but both R(-) and S(+)11-OH-NPa were similarly potent after oral or parenteral administration. The isomer S(+)11-OH-NPa inhibited spontaneous and apomorphine-induced locomotion (ID50 = 1.8-2.7 mg/kg, p.o. and i.p.) and stereotyped behavior (ID50 = 3 mg/kg, p.o. or i.p.), all without inducing catalepsy. While apomorphine was short-acting (1-2 hr), the effects of R(-)11-OH-NPa persisted up to 6-7 hr and those of the S(+) isomer for at least 2.5 hr; moreover, the efficacy of R(-)11-OH-NPa increased markedly up to 3-4 hr, although its ED50 was independent of time (ED50 = 1.7-1.9 mg/kg, i.p. from 1-3 hr). The total effect of R(-)11-OH-NPa (p.o. or i.p.) over time was more than 10-times greater than that of injected apomorphine. These observations accord with the reported high (nM) affinity of 11-OH-NPa at cerebral DA receptor sites (D2 greater than D1) and weak interactions of the 11-methoxy congener. They support the conclusion that the R(-) and S(+) stereoisomers are neuropharmacologically active, respectively, as DA agonist and apparent antagonist, as was found with the enantiomers of N-n-propylnorapomorphine, perhaps due to the low intrinsic postsynaptic agonist activity of the S(+) isomers. Moreover, 11-OH-NPa was highly bioavailable orally and unusually long-acting; it may be absorbed slowly or have active metabolites. Hydroxy-substitution of aporphines at the 11-position, homologous to the 3-OH of DA, evidently is critical for affinity and activity at the DA receptor. These or other monohydroxyaporphines may represent leads to potentially useful DA agonist or antagonist drugs.

Interaction of central serotonin and dopamine in the regulation of carbaryl-iduced tremor

Carbaryl (50–200 mg/kg, p.o.) produced dose-dependent tremors and inhibition of striatal AChE activity. A dose-dependent elevation of striatal 5-HT and 5-HIAA levels was also observed with carbaryl but at the higher doses (100–200 mg/kg p.o.). L-Trp or 5-HTP or haloperidol potentiated the carbaryl-induced tremors. Further, 5-HTP or haloperidol, when administered (i) alone, reduced the ED 50 value and increased the duration of carbaryl-induced tremors without affecting the maximum tremorogenic response of rats and (ii) together, did not change any of these measures significantly. Atropine (acetylcholine antagonist) completely blocked the tremors produced by carbaryl in the absence or presence of 5-HTP or haloperidol. Methysergide (5-HT antagonist) and bromocriptine (DA agonist) antagonised the potentiating effect of 5-HTP and haloperidol, respectively, on the carbaryl-induced tremors. Furthermore, bromocriptine antagonised the potentiating effect of 5-HTP on the carbaryl-induced tremor but, methysergide failed to achieve this antagonism in presence of haloperidol. These results indicate that carbaryl-induced tremors primarily involve the activation of central cholinoceptors and that the serotonergic potentiation of carbaryl-induced tremors is possibly mediated through the dopaminergic disinhibition of cholinergic neurons.

The Signal Transducer for the Dopamine-1 Regulated Sodium Transport in Renal Cortical Brush Border Membrane Vesicles

We have reported the presence of dopamine-1 (DA-1) and dopamine-2 (DA-2) receptors in renal brush border and basolateral membranes. DA-1 agonists stimulate adenylate cyclase (AC) and phospholipase C (PLC) activity in both membranes. Moreover, the ability of a DA-1 agonist (fenoldopam) to stimulate PLC activity is independent of AC activity. A DA-2 agonist (LY171555) by itself was without effect and did not enhance the ability of the DA-1 agonist to stimulate PLC activity. The DA-1 but not DA-2 agonists inhibit Na+/H+ exchange activity in brush border membrane vesicles (BBMV) and Na+/K(+)-ATPase activity in basolateral membranes. However, cAMP inhibits, while protein kinase C (presumably via PLC activity) stimulates, Na+/H+ exchange activity. We therefore determined the effect of DA-1 agonists on Na+/H+ exchange activity when PLC or AC activity was blocked using neomycin or dideoxyadenosine, respectively. The drugs were incubated with minced renal cortex prior to preparation of BBMV by differential centrifugation and MnCl2 precipitation. Enrichment of BBMV was not affected by drug treatment. The Na+/H+ exchange activity was assessed by measuring amiloride (1 mmol/L) sensitive 22Na+ uptake in BBMV (pHi = 5.5, pHo = 7.5, Nai+ = O, Nao+ = 1 mmol/L). Neomycin inhibited DA and DA-1-stimulated PLC activity in BBMV in a concentration dependent manner (10(-6) to 10(-4) mol/L). Neomycin (10(-4) mol/L) completely blocked the ability of DA and DA-1 agonist to stimulate PLC activity but had no consistent effect on DA-1 inhibited Na+/H+ exchange activity. Dideoxyadenosine inhibited DA and DA-1 simulated AC activity without affecting DA-1 stimulated PLC activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A Renal Dopamine-1 Receptor Defect in Two Genetic Models of Hypertension

Dopamine (DA), via DA-1 receptors, regulates Na+ transport in the kidneys. Dopamine is synthesized from L-DOPA in the proximal tubule and presumably secreted as an autocrine/paracrine substance to stimulate DA-1 receptors localized on proximal tubular cells. We have previously reported the presence of DA-1 receptors in renal cortical homogenates and on the isolated proximal tubule of the rat and rabbit, consistent with the dopamine autocrine/paracrine model. We have localized DA-1 receptors in the proximal straight tubule of the rabbit, and in the cortical collecting duct of the rabbit and rat, but not in the distal collecting tubule or the cortical thick ascending loop of Henle. The presence of functional DA-1 receptors has been substantiated by the coexistence of DA-1 agonist-stimulated adenylate cyclase activity in the same nephron segments in which DA-r receptors have been found. Increased concentrations of intrarenal dopamine induced by dopamine-beta-hydroxylase inhibition with SKF-102698 caused a down regulation of proximal tubular DA-1 receptors and almost complete ablation of DA-1 agonist stimulated adenylate cyclase activity. Thus, dopamine may play a role in the regulation of DA-1 receptors and their linkage with adenylate cyclase. Since alterations in the renal dopaminergic system have been measured in some forms of experimental hypertension, we studied DA-1 receptors and their coupling to adenylate cyclase in the spontaneously hypertensive rat (SHR).(ABSTRACT TRUNCATED AT 250 WORDS)

The Significance of L-Amino Acid Decarboxylase and DARPP-32 in the Kidney

This study examines the role of endogenous dopamine (DA) for the regulation of renal tubular sodium (Na) transport. The enzyme L-amino acid decarboxylase (L-AADC) that converts L-dopa to DA has been localized to the proximal tubule cells with immunocytochemistry. Locally formed DA will inhibit the activity of Na-K-ATPase, the enzyme that yields energy to active Na transport. The effect is of physiological importance during high salt diet. The phosphoprotein DARPP-32, a DA1 receptor associated third messenger is abundant in the medullary thick ascending limb of Henle (mTAL). DARPP-32 is phosphorylated after activation of DA1 receptors. DARPP-32 is in its phosphorylated form a potent phosphatase inhibitor. Activation of the DA1 receptor in mTAL with the DA1 agonist SKF 82526 causes dose-dependent inhibition of Na-K-ATPase activity. The effect involves activation of cAMP protein kinase. It is likely that this effect is potentiated by DARPP-32.

Dopamine-1 receptors in rat proximal convoluted tubule: regulation by intrarenal dopamine

Dopamine (DA) is synthesized in the renal proximal convoluted tubule (PCT) and may act as a paracrine substance at tubular DA-1 receptors to decrease sodium transport. Although DA-1 receptors have been identified in rabbit renal PCT by use of nonselective dopaminergic radioligands, DA-1 receptors have not been localized in specific nephron segments with the use of selective DA-1 radioligands. In these studies we used a novel DA-1 dopaminergic ligand 125I-SCH-23982, which has been shown to have a high affinity for brain and renal DA-1 receptors, to identify DA-1 receptors in the rat renal PCT and distal convoluted tubule (DCT). DA-1 receptors in the microdissected PCT and DCT were studied by a quantitative autoradiographic technique and by measuring adenylate cyclase (AC) activity. No specific 125I-SCH-23982 binding could be measured in the DCT indicating the absence of DA-1 receptors in this segment. Binding of 125I-SCH-23982 to PCT was saturable with time and radioligand concentration and was stereoselective. Saturation isotherm analysis in control rats yielded a dissociation constant (Kd) of 7.5 +/- 0.14 nM (n = 4) and a maximum receptor density (Bmax) of 0.69 +/- 0.04 pmol/mg protein (n = 4). The rank-order potency for agonist and antagonist displacement of 125I-SCH-23982 binding was consistent for DA-1 receptors: SCH-23390 greater than fenoldopam = SKF 38393 greater than SCH-23388. The stimulatory effect of the DA-1 agonist fenoldopam (10 microM) on AC activity was blocked by the DA-1 antagonist SCH-23390 (10 microM) but not by the beta-adrenergic antagonist (-)-propranolol (10 microM), indicating specificity. The DA-beta-hydroxylase blocker, SKF 102698, increased renal DA concentrations threefold, reduced the PCT DA-1 receptor Bmax by 33%, and abolished the stimulatory effect of 10 microM fenoldopam on AC activity in the PCT but had no effect on Kd. It is concluded that DA-1 receptors are present in rat PCT but not DCT and can be regulated by renal DA.

Development of a high affinity and stereoselective photoaffinity label for the D-1 dopamine receptor: synthesis and resolution of 7-[125I]iodo-8-hydroxy-3-methyl-1-(4'-azidophenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine

In an earlier paper, we reported the development of (+-)-7-iodo-8-hydroxy-3-methyl-1-(4'-azidophenyl)-2,3,4,5-tetrahydro- 1H-3-benzazepine (I-MAB) and its 125I analogue ([125I]I-MAB) as selective, high affinity photoaffinity labels for the D-1 dopamine receptor. In this report, we now describe the complete synthesis and resolution of I-MAB and the pharmacological characterization of the stereoisomers in canine striatal membranes. R-(+)-I-MAB showed highly specific dopamine D-1 receptor binding (KD = 0.28 nM) and binds selectively and stereoselectively to the D-1 receptor. These results further confirm the previous suggestion that, in the benzazepine series of DA agonists and antagonists, the activity principally resides in the R-(+) enantiomer, the S-(-) enantiomer being considerably less potent or inactive. Moreover, R-(+)-[125I]I-MAB, upon photolysis, identifies the ligand-binding subunits of the neuronal D-1 receptor, with an apparent Mr of 74,000, 62,000, and 51,000 as assessed by autoradiography following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Photoincorporation of R-(+)-[125I]I-MAB into these polypeptides was stereoselectively blocked by D-1 dopaminergic ligands with an appropriate pharmacologic profile for the receptor. R-(+)-[125I]I-MAB should thus prove to be a useful stereoselective photoaffinity label for the further characterization of the D-1 receptors.