Fenoldopam Reverses Cyclosporine-Induced Renal Vasoconstriction in Kidney Transplant Recipients

Cyclosporine causes renal vasoconstriction and reduced renal blood flow that may contribute to chronic nephrotoxicity. This effect has not been consistently reversed by available pharmacologic agents. The efficacy of orally administered fenoldopam, a dopamine-1 (DA-1) agonist with renal vasodilator properties, was evaluated in six patients whose condition was stable 3 to 6 months following renal transplantation. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by inulin and p-aminohippurate (PAH) clearances, respectively, at baseline, after the acute oral administration of 100 mg of fenoldopam, and following 3 weeks of chronic oral fenoldopam therapy (100 mg thrice daily). Mean ERPF increased from 3.15 +/- 0.17 mL/s/1.73 m2 (189 +/- 10 mL/min/1.73 m2) at baseline to 3.48 +/- 0.17 mL/s/1.73 m2 (209 +/- 10 mL/min/1.73 m2) 4 hours after acute administration of fenoldopam (P = 0.04). Urine flow rate and fractional excretion of sodium also increased after acute administration, but not significantly. Mean systolic (SBP) and diastolic blood pressure (DBP) decreased maximally by 18 and 6 mm Hg, respectively, and mean pulse rate increased maximally by 8 bpm between 75 and 90 minutes after both acute and chronic administration. GFR was unchanged following both acute and chronic administration. The increase in ERPF was not maintained to the end of the dosing interval during chronic administration, probably due to the short half-life of fenoldopam. However, the renal vasodilatory response was still observed 3 to 4 hours after readministration of the drug following 3 weeks of oral dosing. Thus, fenoldopam significantly reverses the renal vasoconstriction caused by cyclosporine in renal transplant recipients.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular signaling in the regulation of renal Na-K-ATPase. I. Role of cyclic AMP and phospholipase A2

We have reported that dopamine (DA) inhibits Na-K-ATPase activity in the cortical collecting duct (CCD) by stimulating the DA1 receptor, and the present study was designed to evaluate the mechanism of this effect. Short-term exposure (15-30 min) of microdissected rat CCD to DA, a DA1 agonist (fenoldopam), vasopressin (AVP), forskolin, or dibutyryl cAMP (dBcAMP), which increase cAMP content by different mechanisms, strongly (approximately 60%) inhibited Na-K-ATPase activity. 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase, completely blocked Na-K-ATPase inhibition by DA or fenoldopam, and IP20, an inhibitor peptide of cAMP-dependent protein kinase A (PKA), abolished the Na:K pump effect of all the cAMP agonists listed above. To verify whether the mechanism of pump inhibition by agents that increase cell cAMP involves phospholipase A2 (PLA2), we used mepacrine, a PLA2 inhibitor, which also abolished Na-K-ATPase inhibition by DA or fenoldopam, as well as by AVP, forskolin, or dBcAMP. Arachidonic acid (10(-7) - 10(-4) M) inhibited Na-K-ATPase activity in dose-dependent fashion. Corticosterone, which induces lipomodulin, a PLA2 inhibitor protein inactivated by PKA, equally abolished the pump effects of DA, fenoldopam, forskolin, and dBcAMP, suggesting that lipomodulin might act between PKA and PLA2 in cAMP-dependent pump regulation. We conclude that dopamine inhibits Na-K-ATPase activity in the CCD through a DA1 receptor-mediated cAMP-PKA pathway that involves the stimulation of PLA2 and arachidonic acid release, possibly mediated by inactivation of lipomodulin. This pathway is shared by other agonists that increase cell cAMP and thus stimulate PKA activity.

Tolerance to hypoactivity and sensitization to hyperactivity after chronic treatment with a presynaptic dose of lisuride in rats

We studied the adaptive changes of the locomotor effects of lisuride, a selective agonist for dopamine (DA) D2 receptors, and the functional state of D1 and D2 receptors after repeated administration of lisuride at a dose supposed to act preferentially on DA autoreceptors. Rats were treated daily with saline or lisuride, at a dose that causes a significant reduction in locomotor activity when given to naive rats (25 micrograms/kg i.p.), for 33 days and the effect of different challenging doses of the drug on locomotor activity was measured at different times during and after the treatment. The functional state of D1 and D2 DA receptors was evaluated by measuring SKF 82526-stimulated and LY 171555-inhibited adenylate cyclase (AC) activity in the caudatus/putamen, nucleus accumbens and substantia nigra and naive and chronically treated rats. There was a progressive decline in the ability of lisuride to decrease locomotor activity in rats given daily injections of lisuride, and there was a marked reduction in the threshold dose of lisuride for causing hypermotility. The functional state of DA receptors, positively or negatively linked to AC activity, was not modified by the treatment. The most suitable explanation of the reported adaptive behavioral changes is a down-regulation of DA autoreceptors after chronic treatment with presynaptic doses of lisuride.

Comparative effects of fenoldopam mesylate and nitroprusside on left ventricular performance in severe systemic hypertension

To compare the effects of fenoldopam (n = 15), a selective dopamine-1 agonist, and nitroprusside (n = 14) on left ventricular (LV) function in severely hypertensive subjects (diastolic blood pressure (BP) greater than 120 mm Hg), both agents were infused to reduce diastolic BP by 40 mm Hg (or less than 110 mm Hg). Indexes of LV systolic and diastolic functions were obtained using gated radionuclide angiography before the initiation of treatment and after targeted BP was achieved. Both fenoldopam and nitroprusside effectively reduced systolic and diastolic BP to target levels. Changes in heart rate, peak filling rate and relative end-diastolic volume were similar with both agents. Baseline ejection fraction increased after infusion of both drugs. The magnitude of the increase in ejection fraction was far greater with fenoldopam than with nitroprusside (+22% vs +8%; p = 0.04), despite a lesser reduction in systolic BP (-12 vs -22%, p = 0.002). Furthermore, the reduction in relative end-systolic volume (-35 vs -20%; p = 0.04), and increase in the ratio of peak systolic pressure to relative end-systolic volume (+43 vs +6%; p = 0.007) were greater after fenoldopam than after nitroprusside. The greater increment in parameters of LV systolic function produced by fenoldopam than by nitroprusside suggests an effect on LV performance that is independent of afterload reduction.

5-Hydroxytryptamine receptor activity of the dopamine receptor agonist fenoldopam in canine tracheal smooth muscle

Fenoldopam is a new vasodilator undergoing clinical trials for the treatment of hypertensive emergencies. Its pharmacologic effects result from activation of vascular dopamine-1 receptors. In canine tracheal smooth muscle strips, fenoldopam caused a concentration- and calcium-dependent increase in tension, which was not antagonized by atropine, indomethacin or the dopamine-1 receptor antagonist, SCH 23390. The EC50 (1.89 x 10(-6) M) exceeded that of serotonin or acetylcholine (8.38 x 10(-8) and 8.25 x 10(-8) M, respectively). Maximum tension was similar for fenoldopam and serotonin (11.6 +/- 1.5 g, n = 7 and 13.8 +/- 0.8 g, n = 24; P greater than .2) and considerably greater for acetylcholine (20.5 +/- 1.3 g, n = 14; P less than .005). The serotonin antagonists ketanserin and methysergide reversed completely the effect of fenoldopam (5 x 10(-7) M) with IC50 values of 2.5 x 10(-9) and 2.7 x 10(-9) M, respectively. Phentolamine, rauwolscine and chlorpheniramine were also effective, but they were less potent (IC50 values 6.6 x 10(-8), 1.0 x 10(-7) and 1.7 x 10(-7) M, respectively). By contrast, only very high concentrations (IC50, 5.3 x 10(-5) M) of terazosin produced an inhibition of fenoldopam-induced tension increases. The effect of antagonists could be overcome by increasing the fenoldopam concentration. Experiments performed on strips precontracted with serotonin (5 x 10(-8) M) revealed a very similar order of potency for the five antagonists. The addition of serotonin did not increase the tension produced by supramaximal concentrations of fenoldopam (and vice-versa), whereas acetylcholine increased tension further.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the enantiomers of fenoldopam in human plasma by reversed-phase high-performance liquid chromatography after chiral derivatization

Fenoldopam, a selective agonist at peripheral dopaminergic (DA-1) receptors, is administered as a racemic mixture and, consequently, an indirect stereospecific high-performance liquid chromatographic assay was developed to study the disposition of the individual enantiomers in human subjects. Fenoldopam enantiomers were extracted from alkalinized plasma into ethyl acetate prior to precolumn derivatization with the chiral reagent 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (GITC). The resulting diastereomers were separated on a reversed-phase butylsilica column and determined using triple-electrode coulometric detection. The limits of determination and detection for the S- and R-enantiomers of fenoldopam were 0.5 and 0.25 ng/ml, respectively. A linear response was observed for (S)- and (R)-fenoldopam concentrations ranging from 0.5 to 50 ng/ml in plasma. The intra-day relative standard deviations (R.S.D.s) for the plasma assay at nominal concentrations of 0.5, 5 and 50 ng/ml were 17.4, 5.2 and 6.9%, respectively, for (S)-fenoldopam and 9.9, 6.2 and 7.4%, respectively, for (R)-fenoldopam. The inter-day R.S.D.s of the method at these concentrations were 9.3, 7.7 and 7.4%, respectively, for (S)-fenoldopam and 9.5, 1.9 and 7.3%, respectively, for (R)-fenoldopam. The mean accuracy of the method at concentrations of 0.5, 5 and 50 ng/ml in plasma was found to be 106.4, 111.8 and 108.9%, respectively, for (S)-fenoldopam and 116.2, 104.2 and 111.2%, respectively, for (R)-fenoldopam. The assay developed was sufficiently sensitive, accurate and precise to support pharmacokinetic studies in human subjects.

Manipulation of dopamine receptors alters hypoxic pulmonary vasoconstriction in isolated perfused rat lungs

Using an isolated, perfused rat lung model, we examined the hypoxic pulmonary vasoconstriction (HPV). We studied the alterations in HPV induced by the selective DA1 receptor agonist, fenoldopam, the selective DA1 antagonist, SCH 23390, as well as a combination of these agents. Fenoldopam significantly attenuated HPV. SCH 23390 had no effect on HPV, but was ableto block the effect of fenoldopam. These data confirm the presence of vasodilatory DA1 receptors in the pulmonary vascular bed. The data further suggest that ongoing DA1 activity may be important in counterbalancing some pathologic pulmonary hypertensive states.

Effect of fenoldopam on the acute and subacute nephrotoxicity produced by amphotericin B in the dog

The effect of the selective dopamine DA1 receptor agonist fenoldopam (1 microgram/kg/min i.v.) on the acute nephrotoxic response to amphotericin B (2 mg/kg i.v.) has been studied in the anesthetized dog. Animals were prepared for the measurement of blood pressure, renal blood flow, urine flow, glomerular filtration rate and sodium and potassium excretion. Amphotericin B was given over 20 min and the animals were followed for an additional 160 min. The fenoldopam infusion was started 20 min before amphotericin B and was continued for the duration of the experiment. In control animals, amphotericin B markedly increased renal vascular resistance without affecting blood pressure and thus produced a significant reduction in renal blood flow. The renal vasoconstrictor response to amphotericin B was not attenuated by fenoldopam. Concomitant with the renal vasoconstriction produced by amphotericin B was a marked reduction in glomerular filtration rate, sodium excretion and urine flow rate, which lasted for at least 160 min after amphotericin B treatment. Fenoldopam did not have any effect on the initial reductions in glomerular filtration rate, sodium excretion and urine flow rate but did produce a significant return of these parameters toward control levels by 160 min, despite the continued renal vasoconstriction. The effect of fenoldopam (0.5 microgram/kg/min) given continuously by i.v. infusion on the subacute nephrotoxic response produced by amphotericin B given every other day for 8 days was also investigated. One day after the start of the fenoldopam infusion, venous samples were drawn for the analysis of serum creatinine and blood urea nitrogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the dopamine receptor expressed by rat glomerular mesangial cells in culture

Incubation of cultured rat glomerular mesangial cells with dopamine caused an increase in cyclic AMP formation in a concentration-dependent manner (Ka apparent 2.2 microM). The selective dopamine D1 receptor agonists, fenoldopam, SKF 38393 and (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (6,7-ADTN) also produced concentration-dependent increases in cyclic AMP with mean Ka apparent values of 0.04 microM, 0.02 microM and 1.02 microM, respectively. Although fenoldopam and SKF 38393 were more potent than dopamine, they were partial agonists with efficacies, relative to dopamine, of approximately 60 and 35%, respectively. The dopamine analogue, 6,7-ADTN, in contrast, behaved as a full agonist. Dopamine-stimulated cAMP formation was inhibited in a concentration-dependent manner by the D1-selective antagonist, SCH 23390, with a Ki of 0.06 nM. In contrast, the D2-selective antagonist, domperidone, was four orders of magnitude less potent than SCH 23390, having a Ki of 2072 nM. In addition, SCH 23388, the stereoisomer of SCH 23390, was observed to be two orders of magnitude less potent than SCH 23390, indicating the stereoselective nature of the receptor. The potency series for the selective agonists and antagonists is the same as that described, using identical experimental conditions, for the D1 receptor expressed by a cell line of central origin confirming that the peripheral DA1 and the central D1 dopamine receptor are pharmacologically similar.

Cardiovascular Function During Induced Hypotension by Fenoldopam or Sodium Nitroprusside in Anesthetized Dogs

Fenoldopam, a selective dopamine1 receptor agonist, has been recommended for induced hypotension because it effectively lowers arterial blood pressure and improves renal perfusion. We examined cardiovascular functions during hypotension induced by fenoldopam or sodium nitroprusside. In eight halothane-anesthetized dogs, the left ventricle (LV) was instrumented with pressure and ultrasonic dimension transducers for the assessment of LV contractility using the analysis of the pressure-diameter relationship. Blood flow distribution was measured by radioactive microspheres. Doses of fenoldopam and nitroprusside were titrated to reduce mean arterial blood pressure to 60 mm Hg. After 40 min of hypotension, fenoldopam and nitroprusside caused similar increases in heart rate (17% +/- 4% vs 19% +/- 10%, respectively) and decreases in systemic vascular resistance (-24% +/- 5% vs -27% +/- 4%). Hypotension induced by fenoldopam was associated with higher LV end-diastolic pressure (4.4 +/- 0.6 vs 2.5 +/- 1.1 mm Hg) and end-systolic meridional wall stress (33.0 +/- 4.3 vs 17.8 +/- 2.1 g/cm2) when compared with nitroprusside. There were no significant changes in cardiac output and cardiac contractility as expressed by the slope (Ees) of the LV end-systolic pressure-diameter relationship, velocity of shortening of the diameter, and percentage of wall thickening of the LV. In contrast to nitroprusside, which decreased renal blood flow from 197 +/- 19 to 163 +/- 15 mL/min, renal blood flow increased during fenoldopam-induced hypotension from 187 +/- 20 to 239 +/- 18 mL/min. The increase in renal perfusion was similar in upper, middle, and lower regions of the kidney; however, it was more in the medulla compared with the cortex (37% +/- 17% vs 25% +/- 7%).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of selective D1 and D2 dopamine agonists on cockroach salivary gland acinar cells in vitro

1. The ability of the selective dopamine receptor agonists, fenoldopam and SKF38393 (D1) and quinpirole and LY163502 (D2), to mimic the effect of dopamine on cockroach salivary gland acinar cells has been investigated. 2. Intracellular recordings of the membrane potential established that all the agonists mimicked dopamine (i.e. they induced a hyperpolarization which was occasionally followed by a depolarization), whether applied by addition to the superfusate or locally by pressure ejection. 3. The rank order of potency of the agonists to induce a hyperpolarization was (equipotent molar ratio relative to dopamine in parentheses): dopamine (1) much much greater than fenoldopam (1000) greater than SKF38393 (3500) greater than LY163502 (13750) greater than quinpirole (35000). 4. The agonists also elicited secretion from the salivary gland when superfused onto the preparation. SKF38393 and quinpirole did not induce the same maximum rate of secretion as dopamine. The rank order of potency of the agonists was (minimum effective concentration in parentheses): dopamine (0.03 microM) much much greater than fenoldopam (4.8 microM) greater than SKF38393 (40.8 microM) greater than quinpirole (132 microM). 5. Both the hyperpolarizing and secretory responses to all the agonists were antagonized by the selective D1 antagonist, (+)-SCH23390, but not the selective D2 antagonist, (+/-)-sulpiride. 6. These results support the idea that the same receptors mediate both the hyperpolarizing and secretory responses, and that they are similar to the mammalian D1 receptor.

Effects of dopamine DA1-receptor blockade and angiotensin converting enzyme inhibition on the renal actions of fenoldopam in the anaesthetized dog

Experiments were performed in anaesthetized dogs to characterize the renal effects of the selective dopamine DA1-receptor agonist, fenoldopam. Intrarenal artery infusion of fenoldopam (0.01-10 micrograms/kg per min) caused dose-related renal vasodilation. At low doses (0.01-0.3 micrograms/kg per min), renal vasodilation occurred without concomitant falls in blood pressure but was accompanied by increased urine output. This diuresis was most probably a result of reduced tubular reabsorption since glomerular filtration rate was not increased. Both fenoldopam-induced renal vasodilation and diuresis were blocked to a similar extent by the selective dopamine DA1-receptor antagonist, SCH 23390 (30 micrograms/kg, intravenously), suggesting that both effects were mediated by dopamine DA1-receptors. In the presence of the angiotensin converting enzyme inhibitor, captopril (1 mg/kg, intravenously, + 20 micrograms/kg per min, intrarenal artery), fenoldopam (0.01-0.3 micrograms/kg per min) significantly increased fractional excretion of sodium, despite reducing blood pressure; neither of these effects were observed in captopril-free dogs. These observations support the view that the inhibitory effect of fenoldopam on tubular function, and its vasodepressor activity, may be opposed by angiotensin II resulting from fenoldopam-induced renin release.

DA1 dopamine receptors in renal cortical collecting duct

Renal dopamine DA1 receptors are linked to the regulation of sodium transport. We have previously reported the presence of DA1 receptors in the proximal convoluted tubule (PCT) but not in the distal convoluted tubule. However, the DA1 receptor in the collecting duct, the final determinant of electrolyte transport, has not been studied. DA1 receptors were studied in the microdissected cortical collecting duct (CCD) of rats by autoradiography with use of the selective DA1 radioligand 125I-Sch 23982 and by measurement of adenylate cyclase (AC) activity. Specific binding of 125I-Sch 23982 to CCD was saturable with radioligand concentration. The dissociation constant (Kd) was 0.46 +/- 0.08 nM (n = 5), and the maximum receptor density (Bmax) was 1.41 +/- 0.43 fmol/mg protein (n = 5). The DA1 antagonist Sch 23390 was more effective than the DA1 agonist fenoldopam in competing for specific 125I-Sch 23982 binding. Fenoldopam stimulated AC activity in CCD in a concentration-dependent (10(-9)-10(-6) M) manner. The ability of fenoldopam to stimulate AC activity was similar in CCD and PCT even though DA1 receptor density was 1,000 times greater in the CCD than in the PCT. In additional studies, fenoldopam stimulation of AC activity did not influence vasopressin-stimulated AC activity. We conclude that the DA1 receptor in rat CCD is tightly coupled to AC stimulation and that there is no interaction between DA1 agonist-stimulated and vasopressin-stimulated AC activity in the CCD.

Hypotensive and bradycardic effects elicited by spinal dopamine receptor stimulation: effects of D1 and D2 receptor agonists and antagonists

In anesthetized rats, intrathecal (i.t.) administration, at the upper thoracic level of the spinal cord of fenoldopam (a selective dopamine D1-receptor agonist) or quinpirole (a selective D2-receptor agonist) decreased blood pressure (BP) and heart rate (HR) in a dose-dependent manner. Apomorphine, a nonselective DA receptor agonist, produced similar effects. Apomorphine-induced hypotension was competitively antagonized by either SCH 23390 or remoxipride, selective D1- and D2-receptor antagonists, respectively, but only remoxipride antagonized the bradycardia. Furthermore, SCH 23390 antagonized the hypotensive effect of fenoldopam but did not change that induced by quinpirole. Remoxipride antagonized the hypotensive effect of quinpirole but did not alter the hypotensive effect of fenoldopam. Quinpirole-induced bradycardia was antagonized only by remoxipride. Bradycardia elicited by fenoldopam did not appear to be generated by dopamine receptor stimulation, as suggested by the lack of blocking effects of SCH 23390 and remoxipride. Data obtained with fenoldopam were corroborated with use of SK&F 38393, another dopamine D1-receptor agonist. We conclude that hypotensive effects of i.t.-administered DA receptor agonists appear to result from activation of spinal D1- and D2-receptors whereas bradycardia is related only to activation of spinal D2-receptors.

Evidence against a role for dopamine D1 receptors in the myocardium of the pig

1. We investigated the presence of dopamine D1 receptors in the myocardium of anesthetized pigs using intravenous infusions of dopamine, alone and after alpha- and beta-adrenoceptor blockade and intracoronary infusions of the selective D1 receptor agonist, fenoldopam. 2. Intravenous infusion of dopamine (2.5, 5 and 10 micrograms kg-1 min-1 for 10 min, n = 6) caused dose-dependent changes in heart rate (from 94 +/- 6 to 132 +/- 10 beats min-1, P less than 0.05), the maximal rate of rise of left ventricular pressure (LVdP/dtmax; from 2280 +/- 170 to 4800 +/- 410 mmHgs-1, P less than 0.05), mean arterial blood pressure (from 87 +/- 5 to 62 +/- 3 mmHg) and systemic vascular resistance (from 40 +/- 4 to 28 +/- 2 mmHgl-1 min, P less than 0.05). The increases in heart rate and LVdP/dtmax were abolished when dopamine was infused after alpha- and beta-adrenoceptor blockade. The vasodilator response was, however, only minimally affected. 3. Intravenous infusions of dopamine decreased coronary vascular resistance from 0.90 +/- 0.06 to 0.53 +/- 0.07 mmHg ml-1 min 100 g (P less than 0.05). This action of dopamine was not observed when dopamine was infused after blockade of the alpha- and beta-adrenoceptors. 4. Pretreatment with alpha- and beta-adrenoceptor blockade had no effect or only slightly attenuated the dopamine-induced decrease in vascular resistance of the brain, kidneys, adrenals and small intestine. 5. In 7 animals, intracoronary doses of 0.04, 0.1, 0.2 and 0.4g kg1- min 1 of fenoldopam had no effect on coronary venous oxygen content, local myocardial oxygen consumption, coronary blood flow or coronary vascular resistance. However, systemic effects were observed at the highest two doses, as manifested by a drop in mean arterial blood pressure from 82 +/- 4 to 72 +/- 4mmHg (P < 0.05) due to peripheral vasodilatation (e.g. cerebral vascular bed). Heart rate, LVdP/dt,.,, regional myocardial segment length shortening and left ventricular end-diastolic pressure were not affected at these doses. In 2 animals the infusion rate was increased to 4jug kg1 min 1, but again there was no evidence for coronary vasodilatation. 6. We conclude that the intravenous infusion of dopamine after alpha- and beta-adrenoceptor blockade and the intracoronary infusion of fenoldopam provided no evidence for a major role of D1 receptors in the coronary circulation of pigs. The absence of any effect of the employed doses of fenoldopam on LVdP/dt.mx and on regional myocardial segment length shortening also indicates that fenoldopam does not exhibit any inotropic action in this species.

Inhibition of tubuloglomerular feedback by the D1 agonist fenoldopam in chronically salt-loaded rats

1. Chronic dietary NaCl loading in rats is paralleled by an increase of the dopamine concentration in the tubular fluid and humorally mediated inhibition of the tubuloglomerular feedback mechanism at the macula densa. Since these two phenomena are causally linked, the alterations in the tubuloglomerular feedback response by the luminal application of dopamine, the D1 agonist fenoldopam, the D2 agonist bromocriptine and the D1 and D2 antagonists SCH 23390 and metoclopramide were further investigated using the micropuncture technique. 2. Very similar, concentration-dependent inhibition of the tubuloglomerular feedback response was observed for dopamine and fenoldopam. Half-maximal inhibition was achieved at 10(-11) M and the slope factors of the sigmoid concentration-response curves were comparable. Bromocriptine was ineffective. 3. The inhibition of TGF by both agonists could be antagonized very similarly and concentration dependently by the D1 antagonist SCH 23390. At equimolar concentrations of 10(-9) M the inhibition was reduced by approximately 50%. Raising the SCH 23390 concentration to 10(-6) M completely abolished the TGF inhibition. In contrast, TGF inhibition by 10(-9) M-fenoldopam or dopamine was not significantly affected by an equimolar concentration of the D2 antagonist metoclopramide. Increasing metoclopramide concentration to 10(-6) M attenuated tubuloglomerular feedback inhibition by approximately 55%. 4. It is concluded that the inhibition of tubuloglomerular feedback seen during chronic dietary salt loading can be ascribed to the binding of endogenous dopamine to luminal D1 receptors on the macula densa cells.

Use of inotropic agents in critical illness

Inotropic agents are useful in increasing oxygen delivery in critically ill patients. The need for inotropic support requires careful assessment of all the available cardiovascular variables. Following the decision to stimulate contractility, the choice of inotrope should take into consideration the adrenergic receptor populations and their effects on the distribution of blood flow. All the inotropes should be administered for a predetermined effect. If this response is not realized, the inotrope should be discontinued. The indiscriminate use of these powerful but dangerous drugs should be discouraged. However, in periods of decreasing oxygen delivery and consumption, these agents improve tissue oxygenation and prevent the development of isolated or multiple organ dysfunction.

Influence of fenoldopam and quinpirole in the guinea-pig stomach

1. The influence of the selective DA1-agonist fenoldopam and the selective DA2-agonist quinpirole was investigated in the guinea-pig intact stomach model and in guinea-pig gastric corpus muscle strips. 2. In the intact stomach model, quinpirole induced a relaxation from 10(-6) M on. The relaxation by quinpirole (3 x 10(-5) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (10(-7) M) and domperidone (10(-6) M). In the presence of tetrodotoxin, quinpirole (3 x 10(-5) M) induced a contraction. 3. In the same model, fenoldopam induced a relaxation but only at 3 x 10(-5) M. The relaxation by fenoldopam (3 x 10(-5) M) was not inhibited by SCH 23390 (10(-6) M). The relaxant effect of dopamine (3 x 10(-6) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (3 x 10(-7) M), haloperidol (10(-6) M) and domperidone (10(-6) M). 4. In circular muscle strips of the gastric corpus, the electrically induced cholinergic contractions were inhibited by dopamine but not consistently influenced by quinpirole or fenoldopam. 5. Dopamine, fenoldopam and quinpirole induced an increase in basal tone of the strips. The contraction by dopamine (10(-5) M) was significantly antagonized by prazosin and methysergide. 6. No evidence was thus found for the presence of DA1-receptors in both guinea-pig stomach models. Equally, no evidence for the presence of DA2-receptors was found when studying quinpirole in the strips. Although the relaxant effect of quinpirole in the intact stomach seems predominantly mediated via alpha 2-adrenoceptors, an involvement of DA2-receptors cannot be excluded.

Effects of dopamine prodrugs and fenoldopam on glomerular hyperfiltration in streptozotocin-induced diabetes in rats

The effects of dopamine (DA) prodrugs (L-dopa and gludopa) and of a D1-selective agonist (fenoldopam) on glomerular hyperfiltration were studied in the early stage of diabetes in rats. Wistar rats received one injection of streptozotocin (STZ) and were treated 1 week later with L-dopa (2 x 10 mg/kg/day, s.c.), gludopa (2 x 3 or 2 x 10 mg/kg/day, s.c.), or fenoldopam (2 x 0.3 or 2 x 1 mg/kg/day, s.c.). Their renal functions were compared with those of untreated diabetic and nondiabetic control rats. STZ injection led to hyperglycemia that was kept moderate (20-25 mmol/L) by daily insulin therapy (2-4 U of NPH insulin). Within 2 weeks, glomerular hyperfiltration (polyfructosan clearance) developed in diabetic rats (30% increase vs. nondiabetic control). A rise in renal plasma flow (PAH clearance) was sometimes observed. One week of treatment with either L-dopa, gludopa, or fenoldopam normalized the glomerular filtration rate and decreased filtration fraction. These corrections occurred despite similar metabolic disturbance and kidney hypertrophy. Gludopa was less well tolerated by diabetic rats than L-dopa. Results with L-dopa showed that the normalization of glomerular hyperfiltration was linked to DA synthesis and stimulation of D1 receptors, since it was reversed by carbidopa, a dopa decarboxylase inhibitor, and by SCH 23390, a D1-selective antagonist. These data show that DA prodrugs and a D1 agonist can suppress diabetic glomerular hyperfiltration in the very early course of the disease in rats.

Fenoldopam is a partial agonist at dopamine-1 (DA1) receptors in LLC-PK1 cells

Fenoldopam [6-chloro-7,8-dihydroxy-1-(4'-hydroxyphenyl)-2, 3,4,5-tetrahydro-(1H)-3-benzazepine] is a selective dopamine-1 (DA1) agonist with natriuretic/diuretic properties. A component of the natriuretic response to fenoldopam may involve direct DA1 receptor-mediated effects on proximal tubule sodium reabsorption, possibly through stimulation of adenylyl cyclase. Here, we compared the effects of fenoldopam and DA in stimulating cyclic AMP (cAMP) synthesis in LLC-PK1 cells, a renal epithelial cell line that has proximal tubule-like properties and expresses a DA1 receptor linked to stimulation of adenylyl cyclase. Fenoldopam stimulated cAMP accumulation in LLC-PK1 cells in a dose-dependent manner, an effect which could be blocked by the DA1-selective antagonist Sch 23390. Although fenoldopam was more potent than DA (EC50 55.5 +/- 7.75 nM vs. 1.65 +/- 0.64 microM) in stimulating cAMP accumulation in LLC-PK1 cells, the maximum stimulation obtained by fenoldopam was only 37% of the maximum stimulation obtained by DA(Emax 13.0 +/- 2.95 pmol/mg of protein vs. 35.6 +/- 10.19 pmol/mg of protein). Simultaneous incubation of DA and fenoldopam resulted in lower cAMP levels than with DA alone. Incubation of DA with increasing concentrations of fenoldopam produced parallel rightward shifts in the DA dose-response curves. Schild analysis further indicated that fenoldopam acted as a competitive antagonist in the presence of DA, with a pA2 value of 7.38 and a slope of unity. These results indicate that fenoldopam is a partial agonist with low efficacy at DA1 receptors linked to cAMP generation in the LLC-PK1 cells.

Low doses of l-sulpiride down-regulate striatal and cortical dopamine receptors and beta-adrenoceptors

There is now clinical evidence that l-sulpiride has antidepressant effects when administered at low, non-neuroleptic doses. Down-regulation of beta-receptor-linked adenylate cyclase is a well-documented adaptive response to chronic administration of antidepressant drugs. In this study, we investigated dopamine receptor and beta-adrenoceptor changes induced by chronic administration of low doses of l-sulpiride. The data indicate that striatal D1 and D2 receptor function was desensitized by the treatment, which suggests that at low doses l-sulpiride preferentially blocks D2 autoreceptors, leading to increased dopamine release. l-Sulpiride also induced a selective down-regulation of beta-receptor-associated adenylate cyclase activity in the frontal cortex, but not in the striatum, which does not receive norepinephrine projections. Taken together these data suggest that cortical noradrenergic terminals may be endowed with dopamine D2 receptors controlling norepinephrine release and that blockade of this dopaminergic inhibitory modulation may be involved in the antidepressant effects of l-sulpiride.

Attenuation of amphotericin B nephrotoxicity in the dog by the fenoldopam prodrug, SK&F R-105058

Amphotericin B administration to 8 dogs (1 mg/kg.d, i.v.) for 3 days resulted in significant (P less than .01) reductions in 24-hr creatinine clearance. SK&F R-105058 is an N-ethyl carbamate ester prodrug of the selective DA1 receptor agonist, fenoldopam, which, on oral administration to dogs, results in sustained plasma levels of the renal vasodilator, fenoldopam. Treatment of 6 dogs with SK&F R-105058 (10 mg/kg p.o. b.i.d.) resulted in a significant attenuation of the amphotericin B-induced reductions in creatinine clearance observed on days 2 and 3 after initiation of amphotericin treatment. However, the increase in urine flow and fractional sodium excretion induced by amphotericin B was not altered by SK&F R-105058 treatment. Subsequent histological analysis of the kidneys demonstrated lesions consisting of multifocal tubular degeneration, necrosis and mineralization of mostly distal tubules. Quantitation of tubular lesions indicated that SK&F R-105058 significantly reduced the morphological changes induced by amphotericin B. The data indicate that administration of a fenoldopam prodrug can delay amphotericin B-induced reductions in glomerular filtration rate in the dog.

Absence of postsynaptic DA2 dopamine receptors in the dog renal vasculature

In experiments designed to look for functional postsynaptic DA2 dopamine receptors in the dog renal vasculature, the vascular effects of two selective DA2 receptor agonists, bromocriptine and quinpirole, were studied in pentobarbital-anesthetized dogs. Intra-arterial (i.a.) injections of bromocriptine reduced renal blood flow before and after blockade of alpha-adrenoceptors. I.a. quinpirole produced dose-related increases in renal blood flow which were not altered by blockade of alpha- or beta-adrenoceptors or DA1 dopamine receptors. Neither of the selective DA2 dopamine receptor antagonists, domperidone or YM 09151-2, altered the renal vascular effects of quinpirole, but these were reduced by combined H1 and H2 histamine receptor blockade. The results of these experiments do not support the existence of postsynaptic DA2 dopamine receptors mediating vasodilation in the canine renal vasculature.

The effect of food on pharmacokinetics and pharmacodynamics of fenoldopam in class III heart failure

Eighteen patients with New York Heart Association class III congestive heart failure were given single 100 mg oral doses of fenoldopam with food or fasting in a random-order single-blind crossover trial. Before and after each fenoldopam dose, thermodilution cardiac output, right atrial pressure, pulmonary artery pressure, and pulmonary capillary wedge pressure (PCWP) were measured with a balloon-tipped pulmonary artery catheter, and heart rates and blood pressures were recorded with an automated sphygmomanometer. Compared with fasting, bioavailability of fenoldopam was decreased significantly when administered with food: mean peak plasma fenoldopam level decreased from 26.5 (+/- 4.1 SEM) ng/ml to 10.9 (+/- 1.7 SEM) ng/ml (p = 0.0004) and mean area under the concentration-time curve was decreased from 44.7 (+/- 5.8 SEM) ng.hr/ml to 26.8 (+/- 4.1 SEM) ng.hr/ml (p = 0.0001). Fenoldopam administration to fasting patients resulted in decreases in mean arterial pressure, systemic vascular resistance, and PCWP and significant increases in cardiac index without change in heart rate. The maximum changes in mean cardiac index, systemic vascular resistance, and PCWP were greatest 1 hour after oral administration and did not persist beyond 3 hours after administration. In fasting patients, changes in cardiac index were correlated with plasma fenoldopam levels, whereas changes in PCWP and mean arterial pressure did not correlate significantly with the observed fenoldopam level.

SK&F 87516, a close analog of fenoldopam, is a partial agonist at dopamine-1 and alpha-2 receptors and produces stimulation of 5-hydroxytryptamine-2 receptors in the cardiovascular system of the rat

In pentobarbital-anesthetized rats, SK&F 87516 (1.25-80 micrograms/kg/min intravenously over 15 min), the fluoro analog of the selective DA-1 dopamine receptor agonist fenoldopam produced dose-related decreases in carotid artery blood pressure that faded during the infusion period. These effects were abolished by SCH 23390, prolonged by ritanserin, but unchanged by bilateral vagotomy, atenolol, ICI 118,551, idazoxan, methylatropine or S-sulpiride. SK&F 87516 also inhibited the hypotensive effects of clonidine and of the DA-1 receptor agonist fenoldopam, but not of acetylcholine. In pithed rats, SK&F 87516 produced a biphasic vasopressor response. The initial phase was enhanced by SCH 23390 and converted to a transient hypotension by idazoxan. The secondary response was inhibited by ritanserin and enalapril. In pithed, but not in intact rats, SK&F 87516 increased plasma renin activity. In intact rats, SK&F 87516 produced dose-related bradycardic effects that were inhibited (50%) by idazoxan, methylatropine or bilateral vagotomy and abolished by chlorisondamine or pithing. In pithed rats pretreated with either saline or idazoxan, SK&F 87516 reduced the tachycardia to electrical stimulation of preganglionic more than that to postganglionic cardioaccelerator nerve fibers. However, it did not modify heart rate increases evoked by intravenous norepinephrine. In conclusion, SK&F 87516 produces hypotension via vascular DA-1 receptor stimulation. The fading of this effect during the infusion of SK&F 87516 may be related to the partial agonist property of this compound at DA-1 receptors and the stimulation of 5-hydroxytryptamine-2 receptors. SK&F 87516 also behaves as a partial agonist at alpha-2 adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraocular pressure increases with fenoldopam, but not nitroprusside, in hypertensive humans

Fenoldopam mesylate stimulates adenyl cyclase in porcine ocular trabecular meshwork and raises intraocular pressure in humans. To clarify whether this results from direct activation of the dopamine-1 receptor or indirectly from baroreflex sympathetic stimulation after blood pressure reduction, intraocular pressure was measured in 14 patients with accelerated/malignant hypertension, randomized between intravenous fenoldopam or sodium nitroprusside. Intraocular pressure was measured with a Perkins tonometer, before and at the twentieth minute of each dose increment. In seven patients with a mean blood pressure of 232/131 mm Hg treated with fenoldopam, intraocular pressure increased in a dose-dependent fashion, from 16 +/- 1 to 20 +/- 2 mm Hg (p less than 0.005). In contrast, seven patients with a mean blood pressure of 225/134 mm Hg treated with sodium nitroprusside exhibited no change in intraocular pressure (15 +/- 1 versus 14 +/- 1 mm Hg) despite similar blood pressure reduction. Increases in heart rate were not significantly different. Rates of urinary excretion of norepinephrine plus epinephrine increased significantly relative to baseline (p less than 0.05) but were not different between groups. These data suggest that the increase in intraocular pressure with fenoldopam results from specific activation of the dopamine-1 receptor and is not caused by baroreflex sympathetic stimulation. Because dopamine-1 receptors may modulate intraocular pressure, dopamine-1 receptor blockers might be useful therapy for glaucoma.

A single dose study of the effects of fenoldopam and enalapril in mild hypertension

To clarify the contribution made by the renin-angiotensin system activation to the short lived hypotensive response to fenoldopam, the haemodynamic effects of a combination of fenoldopam (100 mg) and enalapril (5 mg) were compared with fenoldopam alone, enalapril alone and placebo in a balanced, randomised, double blind, single dose study in eight hypertensive patients. Fenoldopam caused an acute fall in blood pressure which lasted approximately 3 h after dosing and was associated with a reflex tachycardia. Enalapril caused a more gradual fall in blood pressure (onset 2 h) without a reflex tachycardia. The combination of drugs produced greater reductions in blood pressure sustained for a longer period than fenoldopam alone and with a more rapid onset than enalapril alone. In combination the hypotensive effects of fenoldopam and enalapril were clearly additive and not synergistic. Activation of the renin-angiotensin system does not antagonise significantly the hypotensive effect of fenoldopam.

Do renal tubular dopamine receptors mediate dopamine-induced diuresis in the anesthetized cat?

The aim of this study was to investigate whether the pentobarbitone anesthetized cat is a suitable preparation in which to characterize renal tubular dopamine receptors. Intravenous infusion of dopamine (10-100 micrograms/kg/min) resulted in a dose-related increase in mean blood pressure (MBP), urine output, sodium excretion (UNaV), and fractional sodium excretion (FENa). This diuretic effect occurred despite little change in glomerular filtration rate, suggesting that it is a consequence of decreased tubular reabsorption. Dopamine (10-100 micrograms/kg/min, i.v.) also induced marked dose-related falls in renal vascular conductance; however, renal blood flow was not significantly reduced. The increases in MBP, urine output, UNaV, and FENa induced by dopamine (10-100 micrograms/kg/min, i.v.), were unaffected by pretreatment of cats with either the selective dopamine DA1 or DA2 receptor antagonists, SCH 23390 (30 micrograms/kg, i.v.), or domperidone (100 micrograms/kg, i.v.) respectively. In contrast, pretreatment of cats with the nonselective alpha-adrenoceptor antagonist, phentolamine (1 mg/kg, i.v.) prevented dopamine-induced increases in urine output and MBP. Infusion of the selective dopamine DA1 receptor agonist fenoldopam (0.01-10 micrograms/kg/min) into the left renal artery failed to increase left renal vascular conductance, or left kidney urine output, UNaV, or FENa. In conclusion, this study provides no evidence for the involvement of renal tubular dopamine receptors in dopamine-induced diuresis in anesthetized cats. Rather, the diuretic effect appears to be linked to stimulation of alpha-adrenoceptors.

Effects of fenoldopam on renal blood flow and systemic hemodynamics during isoflurane anesthesia

The authors compared the systemic hemodynamic and renal vascular effects of hypotension induced by fenoldopam with those produced by the most commonly used hypotensive agent, sodium nitroprusside, in 10 dogs. Mean arterial pressure decreased 26% +/- 3% from control following infusion with fenoldopam, and 30% +/- 2% following infusion with sodium nitroprusside (these decreases were not significantly different between the groups). Renal blood flow (RBF) was preserved during fenoldopam-induced hypotension (214 +/- 16 mL/min at baseline and 197 +/- 16 mL/min after fenoldopam-induced hypotension). In contrast, RBF decreased from 223 +/- 17 mL/min to 167 +/- 12 mL/min during sodium nitroprusside-induced hypotension (P less than 0.02). The differences in RBF between the two groups occurred in spite of the fact that cardiac output and pulmonary capillary wedge pressure were kept similar between the two groups. The authors conclude that fenoldopam, a selective dopamine1 (DA1) receptor agonist, preserves blood flow to the kidney during induced hypotension. On the other hand, sodium nitroprusside is a nonselective arteriolar and venous vasodilator that redistributes blood flow away from the kidneys during induced hypotension.

Pulsatile gonadotropin secretion and basal prolactin levels during dopamine D-1 receptor stimulation in normal women

The effect of the specific dopamine D-1 receptor agonist Fenoldopam on pulsatile gonadotropin secretion and prolactin (PRL) secretion was investigated in normal women. The gonadotropin response to subsequent gonadotropin-releasing-hormone (GnRH) administration was also studied. Eight women received 8-hour infusions of either Fenoldopam (0.5 microgram/kg per minute) (Smith Kline and French, Harrow, United Kingdom) or placebo. After 7 hours of infusion, GnRH was given intravenously. The luteinizing hormone (LH) response to GnRH was significantly higher during Fenoldopam compared with placebo (LH; 13.1 +/- 9.0 versus 9.4 +/- 4.3 IU/L). Basal LH levels, pulse amplitude, and pulse frequency during Fenoldopam infusion were not different from placebo. Prolactin levels increased significantly during Fenoldopam (24 +/- 2 micrograms/L) compared with placebo (16 +/- 2). The results suggest that D-1 receptor stimulation does not affect pulsatile gonadotropin secretion but increases the pituitary responsiveness to GnRH. Additionally, dopamine and Fenoldopam have opposite effects on PRL secretion, the latter increasing PRL levels.

Intrarenally produced angiotensin II opposes the natriuretic action of the dopamine-1 receptor agonist fenoldopam in rats

There are reports of an increase in renin release after the administration of fenoldopam which probably results from the activation of dopamine (DA)-1 receptors located on juxtaglomerular cells in the kidney. However, the functional significance of this finding in terms of modulating the tubular response to DA-1 receptor stimulation remains to be determined. In this study we have examined the effect of an increase in renin-angiotensin system activity during the administration of fenoldopam on the natriuretic and diuretic action of this compound. Intravenous infusion of fenoldopam (0.5 microgram/kg/min) for 30 min produced significant increases in urine output and urinary sodium excretion without causing any changes in glomerular filtration rate, renal blood flow and mean arterial blood pressure, a phenomenon suggestive of a direct tubular site of action. In animals treated with the angiotensin converting enzyme inhibitor captopril, both the diuretic and natriuretic effects of fenoldopam were potentiated markedly. In comparison with fenoldopam infusion in control animals, urine output, urinary sodium excretion and potassium excretion increased by approximately 4-fold (375 +/- 24 vs. 97 +/- 3 microliters/30 min, P less than .01), 9-fold (50 +/- 5 vs. 6 +/- 1 microEq/30 min, P less than .001) and 2-fold (46 +/- 8 vs. 24 +/- 2 microEq/30 min, P less than .05), respectively, in animals receiving a bolus injection of captopril (1 mg/kg i.v.) 30 min before onset of fenoldopam infusion. Whereas no significant changes in renal blood flow occurred when fenoldopam was given to control rats, in animals treated with captopril, fenoldopam produced a modest but significant increase in renal blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic inhibition of glycoprotein hormone alpha-subunit in normal subjects

The pituitary glycoprotein hormones thyrotropin (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) consist of two noncovalently linked subunits, alpha and beta. In addition to producing intact hormone, the pituitary releases free alpha-subunit, which is stimulated by gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH). However, little is known about the dopaminergic regulation of free alpha-subunit in vivo. The effect of dopamine (DA), metoclopramide (MTC), and the specific DA D-1 receptor agonist, fenoldopam, on circulating alpha-subunit levels was studied in normal men and women. Normal women received 4-hour infusions of either glucose (n = 6) or DA at rates of 0.04 (n = 6), 0.4 (n = 6), and 4.0 micrograms/kg.min (n = 6). After 3 hours, 10 mg MTC was administered intravenously (IV). The high dose of DA significantly lowered alpha-subunit levels (P less than .05). No response to MTC was observed in any of the groups. Six women received glucose or DA infusion (4.0 micrograms/kg.min) for 18 hours. DA significantly reduced basal alpha-subunit levels compared with control infusion (P less than .05). MTC administration after 17 hours induced a significant increase in alpha-subunit levels on the day of DA infusion compared with control (P less than .05). In a third study, nine normal males received fenoldopam (0.5 microgram/kg.min) or placebo infusions for 4 hours. Fenoldopam did not affect basal alpha-subunit levels, but the alpha-subunit response to a GnRH/TRH bolus was significantly increased during fenoldopam compared with control (P less than .05). The results suggest that alpha-subunit release may be modulated by the dopaminergic system in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In vitro responses of equine digital vessels to dopamine and fenoldopam

The in vitro responses of isolated vascular preparations of digital arteries and veins obtained from healthy anaesthetised horses were determined for dopamine and fenoldopam. The digital vessels were harvested, cut into 4 mm vascular segments, suspended in tissue baths and attached to force-displacement transducers. Dose-response studies between 10(-8) and 10(-4)M concentrations were performed for all drugs. The change in tension of each vascular ring was measured in grams of force. The reactivity between palmar and plantar digital vessels and baseline vascular responses were determined for dopamine. The vascular responses of dopamine were compared to in vitro data for other known vasoconstrictor agents. The mechanism of vasoconstriction induced by dopamine was further defined using prazosin, a specific competitive alpha-1 adrenoceptor antagonist. The vasodilating ability of fenoldopam, a dopamine-1 (DA-1) receptor agonist, was also determined using noradrenaline- preconstricted vascular segments from palmar digital vessels. The effective concentration to produce 50 per cent of the maximal response (EC50) and the maximal contraction in grams of force per milligram of the vascular ring (g/mg) were calculated. There were no differences in the reactivity between the palmar and plantar digital vessels. Dopamine produced intense constriction in arteries and veins but only at very high molar concentrations. Prazosin decreased significantly the sensitivity of the veins to dopamine (increased the mean EC50 values) but not the arteries. Prazosin had no effect on the maximal contractions of the vessels. Fenoldopam produced very little relaxation of either the arteries or veins. These results suggest that dopamine produces constriction in equine digital arteries and veins and that the constriction is only partially mediated by alpha-1 adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective role of dopamine in the natriuresis produced by iso-osmotic saline infusion

Endogenous dopamine (DA) selectively contributes to the natriuresis (UNaV) produced by infusion and dietary consumption of sodium chloride. The present study in anesthetized rats determined whether DA has a role in the natriuresis produced by small (2.0 +/- 0.11% increase in body weight) and large (17.9 +/- 0.58% increase in body weight) increments in extracellular fluid volume with iso-osmotic saline. Small volume expansion increased urine flow (V) by 59 +/- 15%, UNaV by 155 +/- 31%, and dopamine excretion by 25 +/- 9%. DA1-receptor blockade with SCH 23390 (SCH, 1.0 microgram/kg/min), attenuated the natriuresis; an increase in UNaV of only 69 +/- 15%. Large volume expansion increased V by 1,026 +/- 215% and UNaV by 2,735 +/- 899%, without affecting dopamine excretion. Increments in V and UNaV were unaffected by increasing doses of SCH (1.0 microgram/kg/min; 10 micrograms/kg/min; and 50 micrograms/kg bolus, followed by 10 micrograms/kg/min). Adequacy of DA1-receptor blockade was demonstrated by the fact that SCH (1.0 microgram/kg/min) attenuated the natriuresis produced by the DA1-receptor agonist, fenoldopam (0.1 micrograms/kg/min ia). We conclude that endogenous DA contributes to the natriuresis produced by small, but not large, increases in extracellular fluid volume with iso-osmotic saline.

Sodium nitroprusside potentiates the depressor response to the phosphodiesterase inhibitor zaprinast in rats

To determine if the presence of an activator of guanylate cyclase alters the depressor response to a selective inhibitor of low Km cyclic GMP (cGMP) phosphodiesterase (PDE), zaprinast (3-30 mg/kg) was given i.v. to conscious, spontaneously hypertensive rats during a steady state of i.v. infusion of sodium nitroprusside (15 micrograms/kg per min). Sodium nitroprusside significantly increased the magnitude of the depressor response to zaprinast. In contrast, fenoldopam (20 micrograms/kg per min), an activator of adenylate cyclase, did not affect the depressor response to zaprinast. Zaprinast (10 mg/kg) significantly decreased mean arterial pressure (MAP) in rats given an infusion of sodium nitroprusside, an activator of soluble guanylate cyclase, at doses of 15 and 25 micrograms/kg per min but not at a dose of 5 micrograms/kg per min. However, in rats given atrial natriuretic peptide (ANP; 0.5, 1 and 2 micrograms/kg per min), an activator of particulate guanylate cyclase, zaprinast (10 mg/kg) did not affect MAP. In contrast to the potentiation of the depressor response to zaprinast, sodium nitroprusside (15 micrograms/kg per min) significantly attenuated the reductions in MAP produced by CI-930, a selective inhibitor of low Km cAMP PDE. It is concluded that sodium nitroprusside, but not ANP or fenoldopam, potentiates the depressor response to zaprinast. Furthermore, the potentiation of the depressor response to zaprinast is dependent upon the dose of sodium nitroprusside and is selective for zaprinast; the depressor response to CI-930 is attenuated by sodium nitroprusside.

Selective peripheral dopamine-1 receptor stimulation. Differential responses to sodium loading and depletion in humans

Dopamine-1 (DA1) receptors in the renal tubules may be involved in the regulation of sodium homeostasis. To test this hypothesis, fenoldopam, a selective DA1 agonist, was infused at 0.05 microgram/kg/min i.v. in 16 normal male subjects in metabolic balance at 300 or 10 meq sodium. Renal function studies were performed by standard p-aminohippurate, inulin, and lithium clearances for three periods: 1) precontrol (2 hours), 2) experimental (3 hours), and 3) postcontrol (2 hours). DA1 receptor stimulation in sodium-loaded individuals increased the following parameters during the experimental period: urine flow rate, from 12.5 +/- 0.4 to 15.5 +/- 0.5 ml/min (p less than 0.05); urinary sodium excretion, from 309 +/- 12 to 489 +/- 18 mu eq/min (p less than 0.001); renal plasma flow, from 631 +/- 19 to 717 +/- 21 ml/min (p less than 0.005); fractional sodium excretion, from 2.2 +/- 0.1% to 3.4 +/- 0.1% (p less than 0.001); fractional lithium excretion, from 26.2 +/- 0.7% to 32.1 +/- 0.8% (p less than 0.005); and distal sodium load, from 10.7 +/- 0.4 to 13.8 +/- 0.5 ml/min (p less than 0.05). The increase in fractional sodium excretion was greater than that of fractional lithium excretion (p less than 0.0001). Distal sodium reabsorption decreased from 78.3 +/- 0.8% to 73.2 +/- 1.1% but the change was not statistically significant. In contrast, sodium-depleted subjects exhibited no significant changes except in renal plasma flow, which rose from 550 +/- 13 to 625 +/- 17 ml/min (p less than 0.0001). Glomerular filtration rate remained unchanged through the entire study. These results indicate that diuretic and natriuretic responses are mediated by DA1 receptors at both proximal and distal tubular sites. Attenuation of the DA1 natriuretic response during sodium depletion suggests a direct inhibition of cellular DA1 mechanisms in the renal tubule or recruitment of nondopaminergic compensatory homeostatic mechanisms within the kidney.

Age-related changes of dopamine sensitive cyclic AMP generation in the rat frontal cortex

The dopamine (DA) D-1 and D-2 receptors coupled to 3',5'-cyclic adenosine monophosphate (cAMP) generation were studied in membrane particles of the frontal cortex in young (3-month-old), adult (12-month-old) and aged (24-month-old) male Sprague-Dawley rats. Activation of D-1 receptors with DA, apomorphine or fenoldopam enhanced accumulation of cAMP in the frontal cortex of young rats. The stimulatory effect elicited by DA on cAMP generation declined by about 20% in adult rats. No further decline in cAMP accumulation was noticeable in aged animals. The response to dopaminergic agonists was blocked by the D-1 receptor antagonist SCH 29390 in the three age groups examined. The presence of D-2 receptors, negatively coupled to cAMP generation, was demonstrated by incubating frontal cortex membrane particles with SCH 23390 and then with DA. This inhibitory response, was also elicited with D-2 receptor agonists quinpirole or bromocriptine in the absence of SCH 23390 in which these compounds produced a decrease in cAMP. The decrease in cAMP caused following D-2 receptor stimulation was shown to be enhanced with age. No difference was observed between the three age groups of animals in the activation of cAMP production by forskolin. The present data suggest a selective decrease in the coupling between the D-1 receptor and cAMP generation in the frontal cortex of adult and aged rats and of an age-dependent increase in the coupling between the D-2 receptor and cAMP inhibition. The functional consequences of these biochemical changes may have important implications in the aging of the rat frontal cortex.

Assessment of intravenous fenoldopam mesylate in the management of severe systemic hypertension

To evaluate the acute BP response to iv fenoldopam mesylate (FNP), 14 patients with severe hypertension (diastolic BP 120 to 170 mm Hg) were studied in an open-label trial. Initial infusion rate of FNP was 0.1 microgram/kg.min. Titration to diastolic BP goal (95 to 110 mm Hg) was followed by a constant infusion phase (greater than or equal to 6 h), a detitration phase (2 h), and a postinfusion phase. FNP reduced BP by 27/29 mm Hg (p less than .001) with no significant effect on heart rate. Maintenance of the BP effect was noted through the 6 h of constant rate infusion. Mild, transient vasodilating-associated adverse effects were noted with FNP. We conclude that FNP is an effective, well-tolerated iv antihypertensive agent for acute BP reduction in a severely hypertensive population.

Dopamine stimulation of cAMP production in cultured opossum kidney cells

Dopamine receptors have been identified in many tissues including the kidney. To establish an in vitro system as a model for dopamine action, we studied the effect of dopamine (DA) receptor agonists and antagonists on adenosine 3',5'-cyclic monophosphate (cAMP) formation in opossum kidney (OK) cells. The stimulation of cAMP production in these cells by dopamine was dose dependent, and markedly higher levels were observed in the presence of dopamine plus a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Half-maximal stimulation was found with 1.15 +/- 0.22 microM dopamine. A DA1-receptor agonist, SKF 82526J, stimulated cAMP production, whereas a DA2-receptor agonist, Ly 171555, did not. The stimulatory effects of dopamine and SKF 82526J were abolished by a specific DA1-receptor antagonist, Sch 23390 with half-maximal inhibition concentrations of 1.24 +/- 0.18 and 4.0 +/- 0.5 nM, respectively. In contrast, the DA2-receptor antagonist, spiperone, had no inhibitory effect on dopamine- and SKF 82526J-stimulated cAMP production. Beta-Adrenergic antagonists failed to attenuate the stimulatory effects of dopamine and SKF 82526J on cAMP production. In addition, the beta-adrenergic receptor agonist, isoproterenol, did not stimulate cAMP production. These results suggest that the action of dopamine was not mediated through beta-adrenergic receptors. Furthermore, our results clearly demonstrated the existence of DA1-receptors linked to adenylate cyclase in OK cells.

Lack of effect of lithium on the renal response to DA1-dopamine receptor stimulation by fenoldopam in normal man

1. The effect of oral lithium (300 mg) on the renal response to the selective DA1-dopamine receptor agonist fenoldopam was investigated in seven normal men. Lithium had no influence on sodium excretion and renal haemodynamics during fenoldopam infusion. The fenoldopam-induced rise in PRA was enhanced in the presence of lithium. We conclude that a previously described interaction between lithium and the dopamine agonist gludopa is not mediated by DA1-dopamine receptors or is confined to higher doses of lithium.

Effect of dopamine and a dopamine D-1 receptor agonist on pulsatile thyrotrophin secretion in normal women

The inhibitory effect of a pharmacological dose of dopamine and the specific dopamine D-1 receptor agonist fenoldopam on basal and pulsatile TSH secretion was investigated in normal women. The TSH response to fenoldopam and subsequent releasing hormone administration was also studied. Six women received placebo or dopamine infusion (4.0 micrograms/kg min) for 17 h. After 9 h, blood samples were collected every 10 min between 0800 and 1600 h for measurement of TSH. Eight women received 8-h (0900-1700 h) infusions of either fenoldopam (0.5 micrograms/kg min) or placebo. After 7 h of infusion 10 micrograms TRH, 5 micrograms GnRH and 25 micrograms CRF was given i.v. Blood samples were collected every 10 min. Dopamine infusion as well as fenoldopam infusion significantly reduced both mean basal TSH secretion and TSH pulse frequency compared with corresponding control infusions (P less than 0.05). However, while the effect on TSH pulsatility was comparable (P greater than 0.05), the percentage decrease in basal TSH levels after 16 h of dopamine infusion was 51 +/- 16% (mean +/- SD) and after 7 h of fenoldopam infusion 19 +/- 12% (P less than 0.05). Neither of the drugs affected TSH pulse amplitude and fenoldopam did not influence TRH-stimulated TSH release (P greater than 0.05). The results suggest that dopamine D-1 receptors are involved in modulation of TSH pulsatility probably at the hypothalamic level. It is argued that dopaminergic inhibition of basal TSH secretion and TSH pulsatility is predominantly regulated through dopamine D-2 receptors at the pituitary level, and through D-1 receptors at the hypothalamic level, respectively.

Preservation of renal blood flow during hypotension induced with fenoldopam in dogs

The introduction of drugs that could induce hypotension with different pharmacological actions would be advantageous because side effects unique to a specific drug could be minimized by selecting appropriate therapy. Specific dopamine-1, (DA1) and dopamine-2 (DA2) receptor agonists are now under clinical investigation. Fenoldopam mesylate is a specific DA1 receptor agonist that lowers blood pressure by vasodilatation. The hypothesis that fenoldopam could be used to induce hypotension and preserve blood flow to the kidney was tested. Systemic aortic blood pressure and renal blood flow were measured continuously with a carotid arterial catheter and an electromagnetic flow probe respectively, in order to compare the cardiovascular and renal vascular effects of fenoldopam and sodium nitroprusside in ten dogs under halothane general anaesthesia. Mean arterial pressure was decreased 30 +/- 8 per cent from control with infusion of fenoldopam (3.4 +/- 2.0 micrograms.kg-1.min-1) and 34 +/- 4 per cent with infusion of sodium nitroprusside (5.9 micrograms.kg-1.min-1) (NS). Renal blood flow (RBF) increased during fenoldopam-induced hypotension 11 +/- 7 per cent and decreased 21 +/- 8 per cent during sodium nitroprusside-induced hypotension (P less than 0.01). Sodium nitroprusside is a non-selective arteriolar and venous vasodilator that can produce redistribution of blood flow away from the kidney during induced hypotension. Fenoldopam is a selective dopamine-1 (DA1) receptor agonist that causes vasodilatation to the kidney and other organs with DA1 receptors and preserves blood flow to the kidney during induced hypotension.

Renal and hemodynamic effects of intravenous fenoldopam versus nitroprusside in severe hypertension

The renal and hemodynamic effects of intravenously administered fenoldopam mesylate, a novel dopamine-1 receptor agonist, were compared with those of sodium nitroprusside in 28 patients (18 male; 26 black, two white; average age, 49 +/- 3 years) with an average blood pressure of 219/137 mm Hg, most of whom presented with acute target organ damage. Fenoldopam and nitroprusside lowered blood pressure safely to an average pressure of 176/105 mm Hg; highly significant dose-response relations were found for the 13 patients receiving fenoldopam and the 15 receiving nitroprusside. Volume and sodium, potassium, and creatinine concentrations were measured in freely voided urine specimens both before and during intravenous therapy. In the fenoldopam-treated patients, there were significant increases in urinary flow (92 +/- 21 to 168 +/- 37 ml/hr, p less than 0.003), sodium excretion (227 +/- 73 to 335 +/- 90 mu eq/min, p less than 0.001), and creatinine clearance (70 +/- 11 to 93 +/- 13 ml/hr, p less than 0.003). In the nitroprusside-treated group, however, all these parameters decreased, but not significantly. For direct comparison of the two agents, the increments in urinary flow rate (+76 +/- 20 vs. -16 +/- 15 ml/hr, fenoldopam vs. nitroprusside), sodium excretion (+109 +/- 28 vs. -39 +/- 28 mu eq/min), and creatinine clearance (+23 +/- 6 vs. -11 +/- 7 ml/min) were significantly greater (p less than 0.001 for each) in the fenoldopam-treated group. Significant differences were also obtained when these parameters were calculated as percentage increase over baseline. Fenoldopam and nitroprusside are effective therapies for severe, accelerated, or malignant hypertension, but fenoldopam had additional salutary renal effects in these patients.

Tubuloglomerular feedback and blood flow autoregulation during DA1-induced renal vasodilation

The effect of renal vasodilation produced by the dopamine DA1-receptor agonist, fenoldopam (SKF-82526), on tubuloglomerular feedback (TGF) activity and the autoregulation of renal blood flow (RBF) was determined in euvolemic rats. Fenoldopam (2.5 micrograms.kg-1.min-1 iv) increased RBF by 17% (electromagnetic flow probe) while glomerular filtration rate (GFR) was unchanged; mean arterial pressure was decreased by 6%. Superficial cortical blood flow was increased by 12% (laser-Doppler flowmetry) while single-nephron GFR (SNGFR) and estimated glomerular capillary pressure (stop-flow pressure, Psf) were stable. SNGFR measured at proximal and distal sites along the same nephron was not affected by fenoldopam. Partial inhibition of TGF was indicated by the constancy of distal SNGFR and the proximal-distal SNGFR difference in the presence of increased distal delivery of native fluid. However, fenoldopam did not affect feedback control of Psf evaluated by perfusing artificial fluid through Henle's loop at 0-62 nl/min. Despite the decrease in renal vascular resistance over an arterial pressure range of 130 to 70 mmHg, RBF was autoregulated efficiently during fenoldopam infusion. These results indicate that DA1-receptor activation dilates the preglomerular and efferent arterioles without affecting GFR or glomerular pressure. However, this vasodilatory mechanism operates independent of autoregulation and TGF-induced changes in glomerular pressure such that preglomerular vessels remain responsive to the appropriate signals from these intrinsic control systems. The ability of fenoldopam to blunt feedback control of SNGFR may depend on changes in the filtration coefficient independent of glomerular pressure and/or a constituent of natural tubular fluid.

A preliminary study of the dopamine DA1 agonist fenoldopam in the treatment of chronic left ventricular failure due to ischaemic heart disease

The effects of chronic oral fenoldopam in the treatment of NYHA grade II-III heart failure secondary to ischaemic heart disease, were studied in a placebo controlled, double blind, randomised, parallel group fashion in 20 patients. Nine patients taking placebo and six taking fenoldopam completed the study. Adverse events were similar in each group. There were no significant changes in exercise capacity, ejection fraction, body weight or symptom questionnaires with either treatment. This preliminary study has not revealed any benefit of fenoldopam in heart failure due to ischaemic heart disease.

Intravenous fenoldopam versus sodium nitroprusside in patients with severe hypertension

In an open-label study, we compared the efficacy and safety of intravenous infusion of fenoldopam mesylate with that of sodium nitroprusside in patients with severe hypertension or in hypertensive crisis. Both antihypertensive medications were infused at a maximal dose increment of 0.2 microgram/kg/min (fenoldopam) and 1 microgram/kg/min (nitroprusside), with a maximal infusion rate of 1.5 micrograms/kg/min fenoldopam mesylate or 8 micrograms/kg/min sodium nitroprusside. Once the desired reduction in diastolic blood pressure was achieved (less than 110 mm Hg if initial diastolic blood pressure was 120-149 mm Hg, or by at least 40 mm Hg if initial diastolic blood pressure was 150-190 mm Hg), the maximal infusion rate used was maintained for at least 1 hour, and then, the infusion was slowed gradually over 2 hours. After the infusion treatment, patients remained in the hospital for 2 days of follow-up. Both antihypertensive agents successfully controlled the blood pressure in all the patients by the end of the maintenance periods. Between the baseline and the end of the maintenance period, analysis of variance showed that the changes in the variables induced by fenoldopam mesylate did not differ significantly from those induced by sodium nitroprusside. The incidence of side effects listed were similar in both groups of patients. The detection of toxic levels of thiocyanate in two patients treated with nitroprusside, however, shows that fenoldopam might be preferable for the control of a hypertensive crisis or severe hypertension in patients with decreased renal function.