Dopamine receptor agonists: 3-allyl-6-chloro-2,3,4,5-tetrahydro- 1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-diol and a series of related 3-benzazepines

The N-allyl derivative (SK&F 85174) of 6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-dio l (SK&F 82526) not only retains the exceptional D-1 agonist potency of its parent but also displays reasonably potent D-2 agonist activity, as measured by a dopamine-sensitive adenylate cyclase test and a rabbit ear artery assay, respectively. Several additional N-substituted compounds were prepared to explore the D-2/D-1 agonist relationship. The N-methyl analogue retained good D-2 agonist potency, but this substitution converted D-1 agonist activity into antagonist activity. Most other N-substituents sharply decreased D-2 agonist potency including the N-n-propyl group. This observation was surprising since the introduction of mono- or di-N-n-propyl substituent(s) is commonly linked with retention or enhancement of D-2 agonist potency in other series of dopamine agonists. The N-(2-hydroxyethyl) analogue retains about one-fourth the D-2 potency of SK&F 85174. Several synthetic methods were used to prepare these compounds. N-Allylation of a trimethoxybenzazepine followed by cleavage of the methyl ethers with boron tribromide was the preferred method. Other methods used were direct alkylation of the trihydroxy secondary amine, i.e., SK&F 82526, and an acylation-amide reduction-cleavage method.

Renal effects of SK&F 82526 in anesthetized rats

Dopamine affects renal hemodynamics, renal tubular functions, and the secretion of renin. We have studied the renal effects of SK&F 82526 (an agonist which is selective for the DA1 subclass of dopamine receptors) in anesthetized rats. Infused intravenously at 0.005 mumol/min/kg, this drug increased renal plasma flow and the clearances of PAH and insulin, effects which are consistent with decreased renovascular resistance. Concomitantly, urine flow and K excretion increased, and Na excretion tended to increase. All these effects of SK&F 82526 were antagonized by intravenous metoclopramide (1 mumol/min/kg). Despite its diuretic effect and despite its lack of effect on arterial blood pressure, SK&F 82526 increased arterial plasma renin concentration, suggesting a stimulatory effect on renin secretory rate. Taken together, our results demonstrate that the renal effects of SK&F 82526 mimic those of dopamine.

Modulation by fenoldopam (SKF 82526) and bromocriptine of the electrically evoked release of vasopressin from the rat neurohypophysis. Effects of dopamine depletion

Single neurointermediate lobes were fixed by their stalks to a platinum wire electrode and incubated in Krebs-bicarbonate solution. Vasopressin release into the medium was determined by a radioimmunoassay. Vasopressin secretion was increased by electrical stimulation (15 Hz, 10 s trains with 10 s intervals for 10 min). Fenoldopam (SKF 82526) had a dual effect on vasopressin release, 30 nM decreasing (by 30%) and 3 microM increasing (by 32%) the evoked vasopressin secretion. The facilitatory effect of fenoldopam was antagonized in a concentration-dependent manner by flupenthixol but not by sulpiride. Sulpiride (1 microM) prevented the inhibitory effect of fenoldopam (30 microM). After pretreatment of the rats with the dopamine depleting agent, Ro4-1284 (2 mg/kg i.p. 1 h before the experiments), the evoked vasopressin release was decreased by 21% and the inhibitory effect of fenoldopam disappeared, but the facilitatory effect of fenoldopam was already seen at 30 nM. Similarly, bromocriptine (1-10 microM) decreased the evoked vasopressin release from untreated neurointermediate lobes by 30-40% but increased the vasopressin release by 30% after pretreatment with Ro4-1284. The present findings further support the concept that vasopressin from the neurohypophysis is modulated by dopaminergic mechanisms. Facilitatory effects are mediated via D 1 and inhibition via D 2 receptors. The presence of endogenous dopamine seems to be necessary for the inhibitory effects to occur.

Selective antagonism of the hypotensive effects of dopamine agonists in spontaneously hypertensive rats

Agonists of dopamine receptors can lower blood pressure by vasodilation through action on dopamine1 receptors, inhibition of sympathetic nerve activity by action on dopamine2 receptors, or actions in the central nervous system. Fenoldopam, a selective dopamine1 agonist, piribedil, a selective dopamine2 agonist, and dipropyl dopamine, a mixed dopamine1 and dopamine2 agonist, were injected intravenously in pentobarbital-anesthetized, spontaneously hypertensive rats (SHR). The mechanism for the antihypertensive effect was evaluated by administration of the selective dopamine1 antagonist SCH 23390 and the selective dopamine2 antagonist domperidone. While SCH 23390 only antagonized the hypotensive effects of fenoldopam, domperidone abolished the fall in blood pressure produced by dipropyl dopamine and piribedil but not by fenoldopam. Increments in heart rate and plasma norepinephrine levels accompanied the hypotensive effects of fenoldopam. The increase in heart rate was abolished by a dose of SCH 23390 sufficient to completely block the hypotensive effects and was significantly attenuated by the ganglionic blocking agent hexamethonium, which suggests that the increase in heart rate was due to a baroreceptor reflex. Fenoldopam does not cross the blood-brain barrier, which suggests that its hypotensive effect was mediated by peripheral dopamine1 receptors. Since domperidone does not cross the blood-brain barrier and significantly antagonized the hypotensive and bradycardic effects of dipropyl dopamine and piribedil, these effects were mediated primarily by peripheral dopamine2 receptors. These results indicate that SCH 23390 and domperidone are useful agents to identify the receptor subtype mediating the action of dopamine agonists in SHR.

Use of a post-column immobilized beta-glucuronidase enzyme reactor for the determination of diastereomeric glucuronides of fenoldopam in plasma and urine by high-performance liquid chromatography with electrochemical detection

A post-column enzyme reactor, containing beta-glucuronidase immobilized on controlled-pore glass beads, was developed for use in the high-performance liquid chromatographic (HPLC) analysis of glucuronide metabolites using electrochemical detection. The reactor performance was evaluated with glucuronide conjugates of the new antihypertensive agent, fenoldopam [6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-di ol]. These conjugates, which are electrochemically inactive at 0.6 V vs. Ag/AgCl, were separated by HPLC and passed directly into the post-column beta-glucuronidase reactor, which converted the glucuronides to their electrochemically active aglycone, fenoldopam. The enzyme reactor converted greater than 80% of the entering glucuronide to fenoldopam and produced a linear response for fenoldopam glucuronide in the range 0.4-200 ng injected on-column. The reactor performance was optimal when the mobile phase (methanol-acetate buffer) contained 0-25% methanol, but the efficiency gradually declined thereafter until, at 50% methanol, the reactor was inactive. The working pH range for the mobile phase was 5.5-8.0, with a performance optimum at pH 6.0. The reactor displayed marked stability during usage (greater than 4 months) and during storage (greater than 6 months). The reactor did not hydrolyze the 8-O-sulfate conjugate of fenoldopam but did convert the 1(R) and 1(S) diastereomers of fenoldopam-7-O-beta-glucuronide and 1(S)-fenoldopam-8-O-beta-glucuronide to fenoldopam. An assay was developed for 1(R)-fenoldopam-7-O-beta-glucuronide in plasma and urine by using the deschloro, des-4'-hydroxy analogue of fenoldopam glucuronide as the internal standard. The assay was linear in the range 4-1600 ng/ml. The within-day and between-day coefficients of variation for the method were less than 7% at three plasma fenoldopam glucuronide concentrations.

Alpha-adrenoceptor blocking activity of fenoldopam (SK&F 82526), a selective DA1 agonist

The alpha-adrenoceptor blocking activity of fenoldopam (SK&F 82526), a selective dopamine vascular receptor (DA1) agonist, was evaluated in isolated segmental preparations of rabbit aorta, dog mesenteric and rabbit splenic arteries. Fenoldopam, in concentrations ranging from 10(-6) to 10(-4) M, produced parallel, dextral shifts of concentration-contractile response curves to noradrenaline. Slopes of the Schild regression lines were not significantly different from unity in the three vessels. pA2 values for fenoldopam in the rabbit aorta, dog mesenteric and rabbit splenic arteries were 5.48 +/- 0.08, 5.78 +/- 0.05, and 5.20 +/- 0.05, respectively. In experiments where the drug was added to the bathing medium before exposing the vascular segments to the irreversible alpha-adrenoceptor antagonist, phenoxybenzamine, fenoldopam provided nearly complete protection against alpha-adrenoceptor blockade. These results demonstrate that fenoldopam possesses alpha-adrenoceptor antagonist activity and competes with alpha-phenoxybenzamine, for occupancy at the same receptor site.

Effect of fenoldopam on the aldosterone response to metoclopramide in man

The effect of fenoldopam, a selective DA1-agonist, on the plasma aldosterone response to metoclopramide was studied in six hypertensive patients included in a multicentre double-blind placebo controlled cross-over study of the antihypertensive effects of fenoldopam. Fenoldopam significantly increased baseline plasma renin activity (PRA); baseline plasma aldosterone levels rose slightly. Baseline PRL and the PRL response to metoclopramide were not altered by fenoldopam. After metoclopramide, a significant increase of plasma aldosterone was observed during treatment with fenoldopam, as well as in the placebo-period. The peak values were not significantly different and occurred at 15 min during both treatment periods. These results indicate that fenoldopam does not reduce metoclopramide-induced aldosterone secretion and therefore suggest that the adrenal dopamine receptor is not identical to the vascular DA1 receptor.

Studies with fenoldopam, a dopamine receptor DA1 agonist, in essential hypertension

A series of studies were undertaken to assess the effect of oral fenoldopam, a specific DA1 dopamine receptor agonist on blood pressure and renal function in patients with mild essential hypertension. Six patients with essential hypertension were entered into a dose-ranging study and received either placebo, 25, 50 or 100 mg fenoldopam. A significant, dose-related reduction in diastolic blood pressure, and increase in heart rate was demonstrated (both P less than 0.05), maximal at 45 min to 1 h. Fenoldopam increased plasma renin activity. In a double-blind study, seven patients received a single dose of fenoldopam 100 mg or placebo. Fenoldopam produced a significant fall in systolic (P less than 0.05) and diastolic (P less than 0.01) blood pressure and renal vascular resistance (P less than 0.01). Urine flow rate (P less than 0.05), sodium excretion (P less than 0.01), plasma renin activity (P less than 0.05) and plasma aldosterone (P less than 0.05) increased. Five patients underwent measurement of the above parameters following a single dose of fenoldopam 100 mg with a repeat of these measurements after they had taken fenoldopam 100 mg four times daily for 1 month. The acute response of blood pressure to the single dose appeared unchanged but tachyphylaxis was evident in the responses of heart rate, plasma renin activity and plasma aldosterone.

Pharmacological characterization of dopamine receptors in the stellate ganglia with selective DA1 and DA2 receptor agonists and antagonists

Experiments were performed with fenoldopam (SKF-82526), a selective DA1 receptor agonist, and quinpirole (LY-171555), a selective DA2 receptor agonist, to determine their actions on ganglionic transmission. Fenoldopam caused significant inhibition of the tachycardia elicited during preganglionic stellate stimulation; however, it did not alter the positive chronotropic responses to postganglionic stellate stimulation, suggesting that the compound exerts its inhibitory action at the ganglia. Electrophysiological experiments in the isolated stellate ganglia showed that fenoldopam produced inhibition of ganglionic transmission as indicated by a significant reduction in the magnitude of the compound postganglionic action potential elicited during preganglionic nerve stimulation. The inhibition of ganglionic transmission produced by fenoldopam both under in vivo and in vitro conditions was antagonized by R-sulpiride and metoclopramide, but not by SCH 23390, S-sulpiride or phentolamine. Quinpirole produced significant inhibition of the tachycardia elicited during both preganglionic as well as postganglionic cardiac sympathetic nerve stimulation. This action of quinpirole was antagonized by RS-sulpiride. In electrophysiological experiments it was discovered that quinpirole caused a significant reduction in the magnitude of the compound action potential elicited during stimulation of preganglionic stellate nerve fibers. This inhibition of ganglionic transmission produced by quinpirole was antagonized by S- but not by R-sulpiride. Although phentolamine antagonized the inhibitory action of quinpirole, it was much less effective than S-sulpiride. Norepinephrine also produced inhibition of ganglionic transmission in the isolated stellate ganglia which was antagonized by phentolamine but not by S-sulpiride. These results demonstrate the presence of two subtypes of specific dopamine receptors in the stellate ganglia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fenoldopam on regional vascular resistance in conscious spontaneously hypertensive rats

The effects of fenoldopam, a selective dopamine-1 agonist, on regional blood flow and vascular resistance were examined in conscious unrestrained spontaneously hypertensive rats (SHR). Rats were instrumented chronically with pulsed Doppler flow probes to allow measurement of renal, mesenteric and hindquarters blood flow. Maximal changes in mean arterial pressure, heart rate and regional blood flow were recorded after i.v. administration of fenoldopam (1-1000 micrograms/kg). Fenoldopam produced a dose-dependent reduction in arterial pressure and increased heart rate in the conscious SHR. Significant increases in mesenteric (maximal = 69 +/- 10%) and renal (maximal = 42 +/- 4%) blood flows were observed at all doses of fenoldopam. In the hindquarters, vascular resistance was increased after low doses of fenoldopam (1-30 micrograms/kg), but decreased with higher doses (100-1000 micrograms/kg). After ganglionic blockade, hindquarter vasodilation was observed with fenoldopam at low (10 micrograms/kg) and high (500 micrograms/kg) doses. Pretreatment with metoclopramide (20 mg/kg) or SCH 23390 (30 micrograms/kg), a new selective dopamine-1 antagonist, significantly attenuated the vasodilator responses to fenoldopam in all three vascular beds. Pretreatment with propranolol failed to alter the vascular effects of fenoldopam, but reduced the tachycardia markedly. This study indicates that fenoldopam decreased regional vascular resistance in the renal, mesenteric and hindquarters vascular beds of the conscious SHR with the mesenteric vascular bed demonstrating the greatest reactivity. The vasodilation induced by fenoldopam in these vascular beds appeared to be due to stimulation of vascular dopamine-1 receptors.

The DA1 receptor agonist fenoldopam (SK & F 82526) is also an alpha 2-adrenoceptor antagonist

The alpha-adrenoceptor activity of fenoldopam was evaluated in three different in vitro tissue preparations. In rabbit isolated aortic rings fenoldopam displayed weak alpha 1-adrenoceptor antagonist activity (-log KB = 5.41) and displayed no alpha 1-adrenoceptor agonist activity. In contrast, fenoldopam demonstrated moderately potent antagonist properties at the alpha 2-adrenoceptor in two other tissue preparations. In in vitro model systems used for the characterization of alpha 2-adrenoceptors, fenoldopam competitively antagonized the effects of the alpha 2-adrenoceptor agonist B-HT 920. In the dog isolated saphenous vein and in the isolated field-stimulated guinea-pig ileum, fenoldopam antagonized the effects of B-HT 920 with -log KB values of 7.78 and 7.60, respectively. These data indicate that in addition to being an agonist at DA1 receptors, fenoldopam is also a relatively selective antagonist at alpha 2-adrenoceptors.

Cardiovascular responses in healthy subjects to a novel oral dopamine agonist, fenoldopam

Two double-blind randomized crossover studies were carried out in healthy male subjects to determine the cardiovascular effects of fenoldopam and to assess whether they could be modified by metoclopramide. Blood pressure (BP) and heart rate (HR) were measured supine and upright, before and for up to 4 h after dosing. Single oral doses of fenoldopam 50, 100 or 150 mg or placebo were administered in one study. In the other study, subjects received 20 mg metoclopramide or placebo intravenously, 20 min before an oral dose of 100 mg fenoldopam or placebo. Fenoldopam produced decreases in diastolic BP and increases in HR indicative of an orally active systemic arteriolar vasodilator. Mean peak responses occurred 30-60 min after dosing and values had returned to pretreatment levels within 3-4 h. The decrease in diastolic BP was partially attenuated by pretreatment with metoclopramide. Thus, at least in part, the fall in BP following fenoldopam could be due to an interaction with dopamine receptors.

Ganglionic dopamine receptors as mediators of the inhibition of neurogenic vasoconstriction produced by fenoldopam

We have performed experiments to study the action of fenoldopam, a DA1 receptor agonist, on neurogenic vasoconstriction elicited during preganglionic sympathetic nerve stimulation. Infusion of fenoldopam to anaesthetized dogs decreased mean blood pressure and caused significant impairment of hindlimb vasoconstriction produced by preganglionic lumbar sympathetic nerve stimulation. The inhibitory action of fenoldopam on neurogenic vasoconstriction was antagonized by R-sulpiride but not by SCH-23390. However, SCH-23390 was more effective than R-sulpiride in antagonizing the hypotensive action of fenoldopam. These results demonstrate that fenoldopam produces inhibition of vascular sympathetic neurotransmission by activating dopamine receptors located at sympathetic ganglia, and this receptor may not be similar to the vascular DA1 receptor.

Characterization of the hypotensive action of dopamine receptor agonists fenoldopam and quinpirole in anaesthetized rats

The present study was designed to examine the cardiovascular actions of fenoldopam, a DA1-receptor agonist, and quinpirole, a DA2 receptor agonist, in pentobarbital-anaesthetized rats. Fenoldopam (20, 40 and 80 micrograms/kg) produced short lasting decreases in blood pressure accompanied by tachycardia. Quinpirole (20 and 80 micrograms/kg) caused hypotension and bradycardia which lasted throughout the one hour observation period. The DA1-receptor antagonist SCH 23390 almost completely abolished the hypotensive action of fenoldopam without affecting the hypotension or bradycardia seen with quinpirole. The mixed dopamine receptor antagonist, RS-sulpiride and the selective DA2-receptor antagonist, S-sulpiride, antagonized the cardiovascular actions of quinpirole. These compounds also blocked the hypotension seen with the lowest dose of fenoldopam, but this antagonism was of lesser magnitude than that produced by SCH 23390. Fenoldopam also produced hypotension when given to pithed rats after their resting blood pressure was restored to control level by noradrenaline infusion. This hypotensive action of fenoldopam was antagonised by SCH 23390. Quinpirole, on the other hand, failed to exert any effect in pithed rats. Our results show that fenoldopam and quinpirole produce their cardiovascular actions via activation of DA1- and DA2-receptors respectively. These selective dopamine receptor agonists and the selective DA1-receptor antagonist, SCH 23390 offer novel pharmacological probes for characterization of dopamine receptor subtypes in the kidney and other peripheral organs.

Suppression of sympathetic ganglionic neurotransmission by the selective dopamine-1 receptor agonist fenoldopam (SK&F 82526) in the anesthetized dog

This study was designed to determine the role of dopamine (DA) receptors in modulation of sympathetic ganglionic neurotransmission utilizing the selective DA-1 agonist fenoldopam. Preganglionic stimulation of cardiac sympathetic nerves (0.5-2.0 Hz), in pentobarbital anesthetized dogs, resulted in frequency-dependent tachycardia. Fenoldopam (10 and 30 micrograms/kg/min i.v.) suppressed the tachycardic response 45% at 0.5 Hz with no significant effect at higher stimulation frequencies. In the presence of SK&F 83566 (10 micrograms/kg/min i.v.), a selective DA-1 receptor antagonist, fenoldopam no longer elicited significant inhibition of the preganglionic response. When postganglionic cardiac nerves were stimulated (0.5 Hz), fenoldopam (100 micrograms/kg/min i.v.) inhibited the response 55% with no significant effect at lower doses. Stimulation of sympathetic preganglionic fibers in the autoperfused hindlimb of the dog induced vasoconstriction. Fenoldopam (3 micrograms/kg/min i.a.) produced marked inhibition of nerve-induced constriction that was partially antagonized by SK&F 83566 (3 micrograms/kg/min i.a.). Complete inhibition of the effect of fenoldopam on sympathetic nerve stimulation in the hindlimb could not be achieved, as a component of this action was apparently due to postjunctional alpha-2 adrenoceptor blockade. This was evidenced by a reduction in the pressor response to the selective alpha-2 adrenoceptor agonist B-HT 920 by fenoldopam. These data indicate that fenoldopam stimulates DA-1 receptors in sympathetic ganglia to inhibit neurotransmission and this effect can be reversed by a selective DA-1 receptor antagonist.

Postsynaptic D1 and D2 dopamine receptors are present in rabbit renal and mesenteric arteries

Measuring adenylate cyclase activity as a biochemical index of dopamine (DA) receptors, it was found that the selective D1 DA receptor agonist, SKF 82526, was able to stimulate the cAMP formation in rabbit renal and mesenteric arteries, an effect blocked by haloperidol and by SCH 23390. The D2 DA receptor agonist, bromocriptine, elicited a concentration-dependent inhibition of adenylate cyclase activity in both arteries of either normal or 6-hydroxydopamine pretreated rabbits, this effect being prevented by (-)-sulpiride but not by (+)-sulpiride. These data indicate that both D1 and D2 postsynaptic DA receptors, associated with stimulation or inhibition of adenylate cyclase activity, are present on the wall of rabbit renal and mesenteric arteries.

Characteristics of D-1 dopamine receptors labelled by [3H]fluphenazine in homogenates of rat neostriatum

The properties of the site labelled by [3H]fluphenazine in membranes prepared from the rat neostriatum were examined using radioligand binding methodology. Binding of [3H]fluphenazine was rapid, saturable and of high affinity (K4 = 0.4 nM). Drug displacement experiments demonstrated that the site labelled by [3H]fluphenazine possessed pharmacological characteristics consistent with those of a D-1 dopamine receptor.

Hemodynamic effects of an oral dopamine receptor agonist (fenoldopam) in patients with congestive heart failure

Dopamine receptor stimulation causes vascular and neurohumoral responses that may be beneficial in patients with heart failure. Oral inactivity, emesis and adrenergic-induced arrhythmias have limited the use of currently available compounds. Fenoldopam (SKF-82526-J) is a new, orally available, selective, dopamine-receptor agonist with potent renal vasodilating properties (six times that of dopamine) without positive inotropic or adrenergic activity. Drug efficacy was clinically evaluated in 10 patients with heart failure after single oral doses of placebo and 50, 100 and 200 mg of medication. Placebo produced no changes. Peak efficacy was noted 30 minutes to 1 hour after the 200 mg dose with mean blood pressure decreasing from 96 +/- 15 (mean +/- SD) to 83 +/- 8 mm Hg (p less than 0.05), pulmonary capillary wedge pressure decreasing from 23 +/- 6 to 20 +/- 8 mm Hg (p less than 0.05) and mean pulmonary artery pressure decreasing from 32 +/- 9 to 29 +/- 8 mm Hg (p less than 0.05). Systemic vascular resistance decreased from 1,987 +/- 887 to 1,191 +/- 559 dynes.s.cm-5 (p less than 0.05) with a subsequent 55% increase in cardiac index from 2.2 +/- 1.1 to 3.1 +/- 1.3 liters/min per m2 (p less than 0.05). Heart rate and right atrial pressure did not change (p greater than 0.05). No emesis or new tachycardia was noted at any dose. Baseline hemodynamics generally returned within 3 to 4 hours. Fenoldopam, therefore, is a short-acting, orally effective drug that decreases systemic vascular resistance and increases cardiac index in patients with heart failure and represents a new class of oral compounds that may be useful in treating such patients.

Pharmacological analysis of the actions of SKF 82526 on cardiovascular dopamine receptors

We have performed experiments with SKF 82526, a selective dopamine (DA1) receptor agonist to determine whether this compound would activate either ganglionic and/or central DA receptors. Bilateral hindlimb perfusion was carried out at controlled flow rates and changes in hindlimb perfusion pressure were recorded to evaluate the action of SKF 82526 on vascular resistance in anesthetized dogs. Intracisternal administration of SKF 82526 (10 and 40 micrograms/kg) did not produce any changes in blood pressure, heart rate or hindlimb vascular resistance. When the same doses were administered i.v., SKF 82526 produced hypotension and a decrease in perfusion pressure in the innervated limb, whereas perfusion pressure in the denervated limb was not altered. Intravenous SKF 82526 did not produce any changes in heart rate. When given into the lower abdominal aorta, SKF 82526 caused a dose-dependent decrease in perfusion pressure only in the innervated hindlimb, no significant changes in perfusion pressure occurred in the denervated limb. The hypotensive and the hindlimb vasodilatory actions of SKF 82526 could be antagonized by RS-sulpiride. It was discovered that SCH 23390, a selective DA1 receptor antagonist was most potent in blocking the hypotensive action of i.v. SKF 82526; however, it did not influence the neurogenic hindlimb vasodilation produced by intra-aortic SKF 82526. On the other hand, R-sulpiride, another selective DA1 receptor antagonist significantly antagonized the hypotensive as well as hindlimb vasodilatory actions of SKF 82526. S-sulpiride, a selective DA2 receptor antagonist, was least effective in blocking hypotension and did not influence the hindlimb vasodilatory action.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of a novel dopaminergic agonist radioligand [3H]-fenoldopam (SKF 82526) to D-1 receptors in rat striatum

Fenoldopam (SKF 82526), a dopamine agonist which exhibits D-1 receptor subtype selectivity, was evaluated as a radioligand for this receptor subtype. In saturation studies in rat striatal membrane preparations, [3H]-fenoldopam appeared to label a single binding site with a KD of 2.3 +/- 0.1 nM and a Bmax of 590 +/- 40 fmoles/mg protein. In competition binding experiments, binding was shown to be stereoselective, and rank ordering of affinities of dopaminergic and non-dopaminergic compounds closely correlated with potencies of these compounds in stimulating or inhibiting dopamine-sensitive adenylate cyclase (D-1) and in binding to D-1 sites labelled with the antagonist [3H]-cis-flupenthixol. The most potent competitors were the recently identified D-1 selective antagonists, SCH 23390 and SKF R-83566. [3H]-Fenoldopam was also used to assess agonist/D-1 receptor interactions. The results suggest that [3H]-fenoldopam is a useful and selective agonist radioligand for the D-1 receptor.

Arterial medial necrosis and hemorrhage induced in rats by intravenous infusion of fenoldopam mesylate, a dopaminergic vasodilator

Fenoldopam mesylate, a selective, postsynaptic, dopaminergic vasodilator, was administered to rats for assessment of its clinical, toxicologic, and pathologic effects. Groups of 8 male and 8 female rats received 5, 25, 50, or 100 micrograms/kg/min by intravenous infusion for 24 hours. Groups of 12 male and 12 female rats received 2, 8, 16, or 20 mg/kg/day by intravenous injection once daily for 12 days. Tissues were examined by light microscopy. Rats infused for 24-hours with 5-100 micrograms/kg/min of fenoldopam had lesions of renal and splanchnic arteries characterized by medial necrosis and hemorrhage. None were seen in control rats or those administered the compound by intravenous injection. Arteries with four to five layers of medial smooth-muscle cells were most severely and frequently affected. Lesions were particularly severe in interlobular pancreatic arteries and subserosal gastric arteries. They occurred first at 4 hours, were present at low incidence at 8 hours, were induced in unrestrained rats, and were not caused by the experimental procedures employed. The nature and disposition of this novel arterial lesion in the rat suggests that its pathogenesis may be related to the pharmacologic activity of fenoldopam mesylate at the dopamine receptor.

Effects of SK&F 82526 and SK&F 83742 on the renal vascular dopamine receptor

The renal vascular effects of benzazepine derivatives were studied on the isolated perfused rat kidney in the presence of phenoxybenzamine and sotalol after contraction of the vascular bed with prostaglandin F2 alpha. SK&F 82526 was a very potent dopaminomimetic drug (ED50 = 7.6 +/- 0.8 X 10(-9) M) on the renal vascular dopamine receptor. It displayed partial agonist activity (similar to SK&F 38393) and was devoid of alpha-adrenomimetic effects. SK&F 83742 was a potent dopaminolytic drug. It antagonized dopamine-induced relaxation of the renal vascular bed in a competitive way, with an apparent pA2 of 7.47 +/- 0.23.

The effect of oral fenoldopam (SKF 82526-J), a peripheral dopamine receptor agonist, on blood pressure and renal function in normal man

The effect of a single oral dose of 100 mg of fenoldopam on renal function and blood pressure was investigated in seven healthy male subjects in a double-blind placebo controlled study. Mean diastolic blood pressure fell by 10 mm Hg, 45 min after oral dosing and then gradually returned to baseline values. There was an increase in pulse rate and a delayed rise in systolic blood pressure. Measured from 30 to 120 min after drug ingestion, mean effective renal plasma flow increased to 158% of the value observed after placebo; mean glomerular filtration rate rose to 109% of the placebo value. Measured from 120 to 210 min after administration of the drug, effective renal plasma flow and glomerular filtration rate had returned to baseline values. Fenoldopam produced a small increase in the mean sodium excretion rate which was not significantly different from the fall after placebo. No change was detected in urine flow or potassium excretion rate. Mean plasma renin activity increased three-fold 1 h after oral dosing. Plasma aldosterone did not show a parallel increase although the plasma concentration at 1 h was significantly higher than after placebo. The results show a pronounced renal vasodilator effect lasting about 2 h. The findings are consistent with marked DA1 receptor agonist activity.

Determination of fenoldopam (SK&F 82526) and its metabolites in human plasma and urine by high-performance liquid chromatography with electrochemical detection

Fenoldopam [6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-di ol] is a potent renal vasodilator that is currently undergoing Phase II clinical trials. Quantitative analytical methods, based on high-performance liquid chromatography with electrochemical detection (HPLC-ED) after ethyl acetate extraction from plasma or urine were developed for the determination of fenoldopam and its identified metabolites in biological media. The lower limit of quantitation for fenoldopam in plasma was 50 pg/ml. In assays for fenoldopam glucuronide(s) and fenoldopam conjugates, urine was treated with beta-glucuronidase and Glusulase, respectively, and the liberated fenoldopam was quantified by HPLC-ED. A novel assay by dual-electrode (in series) HPLC-ED was developed for the 8-sulfate of fenoldopam. In this method, the 8-sulfate was oxidized to the o-quinone at the first electrode and quantitated at the second electrode after reduction to the catechol. A similar dual-electrode HPLC-ED method was used for 7- and 8-O-methyl fenoldopam. Conjugates of the O-methyl metabolites were determined by HPLC-ED after hydrolysis to O-methyl fenoldopam. These methods have been used to study the kinetics and metabolism of fenoldopam in healthy volunteers. The methods are specific, sensitive, reproducible, and linear over a wide range of concentrations. Precision of the analyses, expressed as coefficients of variation, were less than 10% for all analyses.

Selective peripheral dopamine-1 receptor stimulation with fenoldopam in human essential hypertension

SKF 82526-J, or fenoldopam, a benzazepine derivative, is a selective dopamine-1 (DA-1) agonist devoid of activity at dopamine-2, alpha- or beta-adrenergic receptors. We studied SKF 82526-J in 10 patients with essential hypertension and five normal control subjects on constant 150-meq sodium, 60 meq potassium intake. In the hypertensive patients, during a 6-d placebo period supine blood pressure and heart rate were stable at 156 +/- 6/105 +/- 4 mmHg and 76 +/- 5 beats/min, respectively. In response to a single oral dose of 100 mg of SKF 82526-J, supine blood pressure decreased to a nadir of 141 +/- 5/89 +/- 8 mmHg (P less than 0.0001) at 90 min and remained decreased at 145 +/- 6/99 +/- 3 mmHg (P less than 0.0001) at 4 h. Heart rate increased to 91 +/- 5 beats/min (P less than 0.002), but returned to control levels (82 +/- 5 beats/min) at 4 h. Renal blood flow increased from 371 +/- 57 to a peak of 659 +/- 104 ml/min and renal vascular resistance fell from 34 +/- 5 to 19 +/- 2 dyn sec cm-5 X 10(3) (P less than 0.01). Urine volume, sodium and fractional sodium excretion, and plasma renin activity were increased as a result of SKF 82526-J administration. During the ensuing 3 wk of SKF 82526-J, blood pressure remained decreased and returned to control levels after placebo administration. In contrast, in normal subjects SKF 82526-J administration was associated with a small transient reduction in diastolic pressure only. These results suggest that reduced dopaminergic activity expressed at the peripheral DA-1 receptor may contribute to the pathophysiology and/or maintenance of increased blood pressure in essential hypertension. In addition, the results suggest that peripheral DA-1 receptor stimulation with SKF 82526-J may be efficacious in the treatment of human essential hypertension.

Evaluation of the effects of SKF 82526 and LY 171555 on presynaptic (DA2) and postsynaptic (DA1) dopamine receptors in rat kidney

Effects of dopamine receptor agonists, SKF 82526 and LY 171555 on presynaptic (DA2) and postsynaptic (DA1) dopamine receptors were studied in the isolated perfused rat kidney. SKF 82526 failed to alter [3H]NA release elicited during periarterial nerve stimulation in the kidney whereas LY 171555 caused a concentration-dependent inhibition of the stimulus-induced release of [3H]NA. This inhibitory action of LY 171555 could be antagonized by sulpiride but not by SCH 23390. SKF 82526 caused concentration-dependent renal vasodilation which could be antagonized by SCH 23390. LY 171555 did not produce significant changes in renal perfusion pressure over a wide range of concentrations. These results show that SKF 82526 and LY 171555 are selective agonists at DA1 and DA2 receptors respectively. These agents represent an important class of compounds that would be useful for pharmacological characterization of peripheral dopamine receptors.

Dopamine receptors and hypertension. Physiologic and pharmacologic implications

The endogenous catecholamine dopamine lowers blood pressure by acting on two receptor subtypes: dopamine 1 and dopamine 2. Dopamine 1 receptors subserve vasodilation, especially in the renal, coronary, mesenteric, and cerebral vascular beds. Dopamine 2 receptors have been located at the endings of postganglionic sympathetic nerves and, when activated, inhibit norepinephrine release. Inhibition of emesis and inhibition of prolactin release also appear to be dopamine 2-mediated phenomena. The receptor subtypes have been classified by differences in chemical structure of agonists and by specific antagonists. Dopamine also acts on beta 1 receptors to stimulate the heart and alpha 1 and alpha 2 receptors to cause vasoconstriction. Alpha adrenergic activity and lack of oral availability limit the use of dopamine in the treatment of hypertension. However, studies with the selective dopamine 1 agonist, fenoldopam, and dopamine 2 agonists such as LY 141865 and bromocriptine, indicate that agonists of both receptor subtypes can lower blood pressure in experimental animals and in hypertensive patients. Initial use of dopamine agonists in the treatment of hypertension and its possible involvement in the etiology and maintenance of hypertension are discussed.

Effects of dopamine (DA) and SKF 82526, a selective DA1-receptor agonist, on vascular resistances in the canine forelimb

This study was performed to determine the presence of vascular dopamine (DA1) receptors in the canine forelimb. Local i.a. infusions of DA (2, 4 or 8 micrograms base/min) produced cutaneous and skeletal muscle vasoconstriction in the forelimb comparable to that produced by local i.a. infusions of norepinephrine (0.5, 1 or 4 micrograms base/min). In the cutaneous vasculature, DA produced large artery, small vessel and large vein constrictions. The small vessels and large veins constricted proportionately more than the large arteries. The pattern of constriction produced by DA along the cutaneous vascular tree was similar to that produced by norepinephrine. The forelimb vasoconstriction produced by both DA and norepinephrine was abolished completely by treatment with phentolamine, indicating that both agents produce vasoconstriction mediated by stimulation of alpha adrenoceptors. The failure of DA to produce vasodilation after phentolamine suggests that there are no vascular or DA1-subtype DA receptors in the canine forelimb vasculature. The local i.a. infusion of SKF 82526, a selective DA1-receptor agonist, for 3 min produced cutaneous and skeletal muscle dilation which could be antagonized by either sulpiride or phentolamine. This neurogenic dopaminergic vasodilation produced by SKF 82526 (25, 50 or 100 micrograms base/min) was converted into vasoconstriction after ganglionic blockade. Inasmuch as SKF 82526 does not activate presynaptic or DA2-subtype DA receptors, the neurogenic vasodilation caused by SKF 82526 may be due to a dopaminergic inhibition of ganglionic transmission. Whereas the results of our study fail to provide any evidence for the presence of vascular DA (DA1) receptors in the canine forelimb, they show that SKF 82526, a DA1-receptor agonist, produces neurogenic vasodilation probably by activating ganglionic DA receptors.

Immediate hemodynamic effects of a dopamine-receptor agonist (fenoldopam) in patients with essential hypertension

Systemic, splanchnic, and renal hemodynamics, intravascular volume, and reflexive and endocrine changes were measured before and after a single dose of fenoldopam, a novel antihypertensive agent that acts through stimulation of specific dopamine receptors. A 13% reduction in mean arterial pressure was mediated by a fall in total peripheral resistance association with an increase in cardiac index, heart rate, stroke volume, left ventricular ejection rate, and circumferential fiber shortening. Renal blood flow increased, thereby reducing the renal vascular resistance by more than 40%. In contrast, splanchnic hemodynamics failed to change. Likewise, there were no changes in intravascular volume, plasma renin activity, or norepinephrine, serum aldosterone, or prolactin levels.

A comparison of some renal and cardiovascular effects of a prostaglandin analog, SKF 82526 and ibopamine in anesthetized dogs

The effects of 0.01 to 40 micrograms/kg i.v. of a novel prostaglandin (PG) analog, (+)-4-[3-[3-[2-(1-hydroxycyclohexyl)ethyl]-4-oxo-2 -2-thiazolidinyl]-propyl] benzoic acid, on hemodynamic and renal functions were compared to those of the dopamine agonists SKF 82526 (0.01 to 400 micrograms/kg i.v.) and ibopamine (10-1000 micrograms/kg i.v.) in anesthetized dogs. The PG analog was the most potent renal vasodilator, maximally increasing renal blood flow (RBF) by 220 +/- 52 ml/min at 20 micrograms/kg i.v. SKF 82526 (40 micrograms/kg i.v.) and ibopamine (1000 micrograms/kg i.v.) elevated RBF by 125 +/- 18 and 74 +/- 31 ml/min, respectively. Both the PG analog and SKF 82526 significantly reduced renal vascular resistance (RVR) and were hypotensive, especially at the higher doses. In contrast, ibopamine dramatically increased mean arterial pressure (MAP) by 140 +/- 13 mm Hg at 1000 micrograms/kg i.v. Lower aortic blood flow (LAF), glomerular filtration rate (GFR), urine flow and fractional excretion rates of sodium, potassium and chloride were not altered during the 15-min period following administration of each dose of each of the compounds. The RBF and depressor responses to the PG analog and to SKF 82526 were not substantially influenced by the alpha-adrenergic antagonist, phenoxybenzamine (POB), while the pressor effect of ibopamine was reversed and dilation of the renal and hindlimb vascular beds was unmasked. The PG analog, the most potent of the three vasodilators studied, exhibited a high degree of renal specificity.

Studies on vascular dopamine receptors with the dopamine receptor agonist: SK&F 82526

The vascular effects of a new dopamine receptor agonist, SK&F 82526, were evaluated in isolated rabbit splenic arterial ring segments. In this preparation, previously shown to possess dopamine receptors, SK&F 82526 produced a stereoselective relaxation with the R-enantiomer more active (ED50 1 X 10(-6) M) than the S-enantiomer (ED50 7 X 10(-6) M). Because SK&F 82526 lacks alpha agonist adrenoreceptor activity, a unique feature of these studies was the ability to examine the relaxant action of a dopamine receptor agonist using norepinephrine to contract the tissue in the absence of phenoxybenzamine which is required to antagonize the alpha adrenoreceptor agonist activity of other dopamine receptor agonists. Dopamine receptor antagonists inhibited SK&F 82526-mediated vascular relaxation with the following pA2 values: metoclopramide, 5.20; R-sulpiride, 4.96; and bulbocapnine, 4.62. Initial studies on the location of the receptors, and possible biochemical mechanisms, involved in the relaxation were undertaken. The relaxant effect of SK&F 82526 was decreased when the vascular endothelium was removed. However, removal of the endothelium did not produce a generalized inability to inhibit vascular relaxation because nitroglycerin relaxation of this tissue was not reduced. Phosphodiesterase inhibitors potentiated the vascular relaxant effects of SK&F 82526 only when the endothelium was present. This evidence suggests that a cyclic nucleotide-mediated process may be involved. In summary, direct physiologic evidence for the vascular relaxant effects of SK&F 82526 being mediated on postjunctional dopamine receptors is presented. This drug is a useful agent for the study and characterization of dopamine receptors.

Dopamine and dopamine receptors as target sites for cardiovascular drug action

It is controversial whether dopamine (DA) is a peripheral neurotransmitter in the cardiovascular/renal system. The endogenous concentration of DA in the heart and blood vessels is generally only a fraction (5%) of that of norepinephrine (NE). With perhaps the exception of the kidney, the majority of the evidence suggests a precursor role for this amine rather than that of a neurotransmitter. The main weakness of arguments favoring DA as a vascular neurotransmitter is relative lack of data showing selective DA release and lack of effects of selective DA antagonists on neural stimulation. However, DA receptors have been characterized in cardiovascular tissues and are of two types: DA1 receptors located on vascular smooth muscle (postjunctional), which appear to mediate relaxation of the muscle, and DA2 receptors located on sympathetic nerves (pejunctional), which inhibit NE release. These receptors are interesting and potential target sites for novel cardiovascular drug action for the treatment of hypertension and renal ischemia. Moreover, selective DA receptor agonists will be important tools in understanding the role of DA receptors in normal and disease states.

A new oral renal vasodilator, fenoldopam

Fenoldopam, a dopamine agonist, was evaluated in renal clearance studies during water diuresis after oral doses of 25, 50, and 100 mg. After the 100-mg dose there was an increase in urine flow rate, paraaminohippurate clearance, free water clearance, and an increase in the fractional excretion of sodium, calcium, and uric acid. These effects were evident within the first hour, peaked during the second hour, and lasted about 3 hr. Doses of 50 and 25 mg induced smaller increases. There was no significant change in inulin clearance at any dose. To elucidate the mechanism of action, the studies were repeated after treatment with a dopamine-receptor antagonist (metoclopramide). Metoclopramide greatly diminished the renal effects of fenoldopam. These findings indicate that fenoldopam is an active renal vasodilator in man and increases urine volume, free water clearance, and fractional excretion of sodium by stimulation of renal dopamine receptors.

Potential usefulness of renal vasodilators in hypertension and renal disease: SK&F 82526

SK&F 82526 and its enantiomers have been shown to increase renal blood flow and decrease renal vascular resistance in the anesthetized dog. The effect of the racemate on lowering systemic blood pressure in the anesthetized dog and the spontaneously hypertensive rat has been shown to be caused by the R-enantiomer with the S-enantiomer being devoid of significant activity on blood pressure. The mechanism by which the R-enantiomer decreases blood pressure is not systemic vasodilatation or prejunctional inhibition of norepinephrine release but appears to result from a unique stimulation of the postjunctional dopamine receptor. Racemic SK&F 82526 also has been shown to increase renal blood flow in an ischemic model of acute renal failure.

Analysis of SK&F 82526 in plasma by high-performance liquid chromatography with electrochemical detection

SK&F 82526, a benzazepine with a catechol moiety, is a potent, specific renal vasodilator. The method described here for its determination in plasma uses an ethyl acetate extraction and an ether wash, followed by high-performance liquid chromatographic analysis with electrochemical detection. The detection limit is 160 pM. The method was proven accurate, precise and linear over three orders of magnitude (at plasma concentrations ranging from 50 pg/ml to 50 ng/ml). It has been successfully used for plasma determinations of SK&F 82526 in human volunteers following an oral dose of 25-100 mg.

Pulmonary and systemic vascular effects of SKF-82526-J, a new specific peripheral dopamine receptor agonist, in unanesthetized neonatal lambs

We examined the circulatory effects of SKF-82526-J, a new specific dopamine receptor agonist, using chronically instrumented neonatal lambs. Serial infusion of doses ranging from 1.4 to 337 micrograms/kg/min did not change cardiac output, heart rate, left atrial pressure, systemic arterial pressure (SAP) or vascular resistance (SVR). Pulmonary pressure (PAP) increased from 22.6 +/- 1.1 to 35.8 +/- 4.0 mm Hg at 337 micrograms/kg/min (p less than 0.005) and pulmonary resistance (PVR) changed from 0.099 +/- 0.01 to 0.15 +/- 0.03 mm Hg/ml/kg/min (p less than 0.025). After 3-12 min of recovery, SAP increased in all lambs (range 6-57 mm Hg), while PAP fell, without change in cardiac output. Thus, the ratio of PVR/SVR returned more quickly to baseline (4 min) than either PVR or SVR alone. We conclude, based on physiologic evidence, that the newborn lamb pulmonary circulation has no vasodilating dopamine receptors.

Characterization of the peripheral and central effects of SK&F 82526, a novel dopamine receptor agonist

SK&F 82526 (6-chloro-7,8-dihydroxy-1-(p-hydroxyphenyl)-2,3,4,5-tetrahydro-(1H)-3-benzazepine) was evaluated for its peripheral cardiovascular activity, effect on renal and central dopamine receptors and mechanism of action. Comparisons were made with dopamine. In anesthetized dogs, SK&F 82526 i.v., produced dose-related dilation of the renal vasculature, apparently through stimulation of postsynaptic dopamine receptors. Unlike dopamine, the increased renal perfusion was not accompanied by increases in arterial blood pressure or heart rate, but rather a decrease in arterial blood pressure at higher doses. The relative lack of efficacy of SK&F 82526 on alpha and beta adrenergic receptors was confirmed in pithed rats and isolated electrically paced papillary muscles of guinea pigs. In conscious dogs, oral administration of SK&F 82526 produced dose-dependent and sustained decreases in renal vascular resistance. The renal hemodynamic activity of SK&F 82526 in anesthetized dogs was not attenuated by phenoxybenzamine, propranolol, atropine, mepyramine plus metiamide, meclofenamic acid or reserpine but was inhibited by metoclopramide and bulbocapnine in a manner suggesting competitive antagonism. In the central nervous system studies, SK&F 82526 stimulated dopamine-sensitive adenylate cyclase of rat caudate (which was inhibited in a concentration-dependent manner by haloperidol and bulbocapnine), induced contralateral rotation in caudate-lesioned rats after intracaudal injection, but did not alter serum prolactin levels after systemic administration. Peripheral administration, however, did not result in significant activation of central dopamine receptors, indicating that SK&F 82526 does not readily cross the blood-brain barrier. These findings demonstrate that SK&F 82526 is a potent, selective, orally active renal vasodilator which acts, at least in part, through stimulation of renal vascular dopamine receptors and is without significant effect on central dopamine receptors after systemic administration.

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.