Metabolism and vascular effects of gamma-L-glutamyl-L-dopa on the isolated rat kidney

In silico design, synthesis, characterization and pharmacological evaluation of captopril conjugates in the treatment of renal fibrosis

Application of glutamic acid and taurine conjugates of captopril for kidney targeting.

Novel Derivative of Bardoxolone Methyl Improves Safety for the Treatment of Diabetic Nephropathy

Currently, no effective and safe medicines are available to treat diabetic nephropathy (DN). Bardoxolone methyl (CDDO-Me) has displayed promising anti-DN activity as well as serious side effects in clinical trials, probably because the highly reactive α-cyano-α,β-unsaturated ketone (CUK) in ring A of CDDO-Me can covalently bind to thiol functionalities in many biomacromolecules. In this study, we designed and synthesized a γ-glutamyl transpeptidase (GGT)-based and CUK-modified derivative of CDDO-Me (2) to address this issue. 2 can be specifically cleaved by GGT, which is highly expressed in the kidney, to liberate CDDO-Me in situ. It should be noted that 2 exhibited anti-DN efficacy comparable to that of CDDO-Me with much less toxicity in cells and db/db mice, suggesting that its safety is better than CDDO-Me. Our findings not only reveal the therapeutic potential of 2 but also provide a strategy to optimize other synthetic molecules or natural products bearing a pharmacophore like CUK to achieve safer pharmaceutical drugs.

Development and characterization of glutamyl-protected N-hydroxyguanidines as reno-active nitric oxide donor drugs with therapeutic potential in acute renal failure

Acute renal failure (ARF) has high mortality and no effective treatment. Nitric oxide (NO) delivery represents a credible means of preventing the damaging effects of vasoconstriction, central to ARF, but design of drugs with the necessary renoselectivity is challenging. Here, we developed N-hydroxyguanidine NO donor drugs that were protected against spontaneous NO release by linkage to glutamyl adducts that could be cleaved by γ-glutamyl transpeptidase (γ-GT), found predominantly in renal tissue. Parent NO donor drug activity was optimized in advance of glutamyl adduct prodrug design. A lead compound that was a suitable substrate for γ-GT-mediated deprotection was identified. Metabolism of this prodrug to the active parent compound was confirmed in rat kidney homogenates, and the prodrug was shown to be an active vasodilator in rat isolated perfused kidneys (EC50 ~50 μM). The data confirm that glutamate protection of N-hydroxyguanidines is an approach that might hold promise in ARF.

A new class of NO-donor pro-drugs triggered by γ-glutamyl transpeptidase with potential for reno-selective vasodilatation

This communication describes the synthesis of a new class of N-hydroxyguanidine (NHG) pro-drugs which release nitric oxide (NO), triggered by the action of γ-glutamyl transpeptidase (γ-GT), and have potential for the treatment of acute renal injury/failure (ARI/ARF).

Novel insights into the physiology of renalase and its role in hypertension and heart disease

Renalase is an amine oxidase expressed in kidney, heart, liver, and brain that metabolizes catecholamines. Tissue and plasma levels are decreased in models of hypertension and chronic kidney disease. Its expression is modulated by salt intake, and urinary renalase may regulate catecholamines levels and effect renal sodium and phosphate transport. The renalase knockout mouse is hypertensive in the absence of significant changes in renal function. Sympathetic tone is increased as evidenced by elevated plasma and urine catecholamines. Studies in humans with resistant hypertension indicate that plasma renalase levels are inversely associated with systolic blood pressure. Additionally, a functional mutation in renalase (Glu37Asp), known to be associated with essential hypertension, also predicts more severe cardiac hypertrophy and dysfunction. Lastly, a single dose of recombinant renalase administered subcutaneously to rats with chronic kidney disease or to Spontaneously Hypertensive Stroke Prone rats significantly decreases blood pressure for more than 24 h. Available data suggest that renalase deficiency is associated with increased sympathetic tone and resistant hypertension, and recombinant renalase is a potent antihypertensive agent that may provide a valuable option for treating hypertension in chronic kidney disease.

Role of renalase in the regulation of blood pressure and the renal dopamine system

PURPOSE OF REVIEW

Renalase is a secreted amine oxidase that is synthesized in the kidney, and that metabolizes circulating catecholamines. Tissue and plasma renalase levels are decreased in models of chronic kidney disease. Recent data indicate that renalase deficiency is associated with increased blood pressure and elevated circulating catecholamines. The mechanisms of hypertension in renalase deficiency and the possibility that renalase regulates the renal dopamine system are discussed.

RECENT FINDINGS

Characterization of the renalase knockout mouse model revealed that renalase deficiency increases SBP and DBP. Renal and cardiac functions are unaffected, but there is evidence of sympathetic activation, with elevation of plasma and urine catecholamines. Renalase is continually excreted in urine, and is enzymatically active and could modulate catecholamines levels in tubular fluid. Renalase expression is modulated by salt intake, and recombinant renalase has a potent and prolonged hypotensive effect on blood pressure in Dahl salt-sensitive rats and rats with chronic kidney disease. Plasma renalase levels are inversely associated with SBP in patients with resistant hypertension. A functional mutation in renalase (Glu37Asp) associated with essential hypertension also predicts more severe cardiac hypertrophy, dysfunction, and ischemia in individuals with stable coronary artery disease, comparable blood pressure and normal renal function.

SUMMARY

Urinary renalase metabolizes urinary catecholamines, and perhaps regulates dopamine concentration in luminal fluid, and modulate proximal tubular sodium transport. Renalase deficiency is associated with increased sympathetic tone and resistant hypertension. Recombinant renalase is a potent antihypertensive agent in Dahl salt-sensitive rats and in rats with chronic kidney disease.

Mild acute renal failure potentiates metformin accumulation in the diabetic rat kidney without further impairment of renal function

OBJECTIVES

To analyze, in acute renal failure (ARF) in diabetic rats, how moderate functional ARF would modify metformin (MET) pharmacokinetics and if plasma and renal tissue MET accumulation could aggravate renal insufficiency and/or elicit plasma lactate accumulation.

METHODS

Streptozotocin-induced diabetic rats were allocated to four groups: control, MET, ARF, ARF-MET (6-7 rats per group). MET (100 mg/kg/day) was given per os for two weeks before ARF was induced by drinking restriction and enalapril treatment. The effects of MET and/or ARF were examined in vivo on renal function in conscious rats (metabolic cages) and ex vivo on renal vascular reactivity (isolated kidney).

RESULTS

MET treatment (plasma level: 5.3 +/- 1.4 microg/ml, mean+/-SEM), resulted in biguanide accumulation in cortex and medulla (53 +/- 17 and 80 +/- 40 microg/g respectively). MET was devoid of any effect on creatinine clearance, mean blood pressure or renal vascular resistance, but moderately increased plasma lactate (3.8 +/- 0.5 vs 3.2 +/- 0.2 mM, P<0.05) and decreased angiotensin II-induced renal vasoconstriction. ARF, although mild, decreased renal MET clearance (0.29 +/- 0.05 vs 1.01 +/- 0.31 ml/min/100 g, P<0.05) and increased plasma and renal tissue MET levels (x 2-4). MET however did not worsen the fall in glomerular filtration rate, nor modify renal vascular reactivity. ARF did not change the MET-elicited moderate increase in plasma lactate.

CONCLUSION

Despite the increase in MET plasma and renal tissue levels subsequent to moderate ARF, no harmful metabolic effect on plasma lactate and no further impairment of renal function was observed in MET-treated diabetic rats subjected to ARF.

Renal vascular reactivity to vasopressin in rats with diabetes mellitus

We evaluated how renal vascular reactivity to vasopressin changes when nitric oxide (NO) synthesis varies, as has been reported to occur in the course of insulin-dependent diabetes mellitus. Renal vasoconstrictor responses to vasopressin were obtained in young and older Sprague-Dawley control rats (3 and 10 months old) and in age-matched diabetic rats that had been treated with streptozotocin (60 mg/kg i.v.) at the age of 2 months. In young rats, vasopressin (3-1000 ng/kg/min i.v.) induced in vivo a dose-dependent decrease in renal blood flow, which was diminished in streptozotocin diabetic rats (P<0.05). Similarly, in in vitro perfused kidneys, the concentration-response curve for vasopressin (0.03-10 nM) was shifted 3-fold to the right in kidneys isolated from young diabetic rats (P<0.05). This shift was abolished in the presence of an inhibitor of nitric oxide synthesis, N(G)-nitro-L-arginine (100 microM), in the perfusate. In 10-month-old rats, the in vivo renal vasoconstrictor dose-response curve to vasopressin was shifted 10-fold to the left as compared to that for young rats (P<0.001). This shift was similar in both control and diabetic rats. In conclusion, the present study documented the existence of hyporesponsiveness to vasopressin in the early stage of diabetes, possibly related to nitric oxide overproduction. In contrast, renal vascular hyperreactivity to vasopressin occurs with aging, whether the rats are diabetic or not.

Theodore Cooper Lecture: Renal dopamine system: paracrine regulator of sodium homeostasis and blood pressure

All of the components of a complete dopamine system are present within the kidney, where dopamine acts as a paracrine substance in the control of sodium excretion. Dopamine receptors can be divided into D(1)-like (D(1) and D(5)) receptors that stimulate adenylyl cyclase and D(2)-like (D(2), D(3), and D(4)) receptors that inhibit adenylyl cyclase. All 5 receptor subtypes are expressed in the kidney, albeit in low copy. Dopamine is synthesized extraneuronally in proximal tubule cells, exported from these cells largely into the tubule lumen, and interacts with D(1)-like receptors to inhibit the Na(+)-H(+) exchanger and Na(+),K(+)-ATPase, decreasing tubule sodium reabsorption. During moderate sodium surfeit, dopamine tone at D(1)-like receptors accounts for approximately 50% of sodium excretion. In experimental and human hypertension, 2 renal dopaminergic defects have been described: (1) decreased renal generation of dopamine and (2) a D(1) receptor-G protein coupling defect. Both defects lead to renal sodium retention, and each may play an important role in the pathophysiology of essential hypertension.

Intrarenal serotonin, dopamine, and phosphate handling in remnant kidneys

BACKGROUND

Serotonin (5-HT) and dopamine (DA) are intrarenal autocrine/paracrine substances that regulate phosphate reabsorption. The present studies explored intrarenal serotonin and DA metabolism and the implications for phosphate homeostasis in rats with remnant kidneys, a model for renal failure.

METHODS

The intrarenal productions of serotonin and DA were determined from measurements of renal interstitial fluid (microdialysate) and urine in rats with remnant or intact kidneys. In clearance studies, the effects of infusion of methiothepin, a serotonin receptor antagonist, or gludopa, a renal selective DA precursor, on phosphate and sodium excretion were determined in rats with a remnant or intact kidneys.

RESULTS

Renal interstitial serotonin (5-HT, 3.4 +/- 0.9 pg/min) was fourfold higher than DA (0.6 +/- 0.1 pg/min) in remnant kidneys. Conversely, urinary excretion of serotonin was fourfold less than DA in rats with a remnant kidney (5-HT 0.4 +/- 0.02 vs. DA 1.5 +/- 0.1 ng/min). Infusion of methiothepin or gludopa significantly increased the fractional excretion of phosphate (FE(Pi)) in rats with a remnant kidney from 54 +/- 3 to 67 +/- 7% (P < 0.05) and from 36 +/- 10% to 51 +/- 13% (P < 0.05), respectively.

CONCLUSION

We conclude that serotonin preferentially accumulates in the renal interstitium, whereas DA exits primarily via the tubular lumen. Phosphate excretion is increased by both the acute infusion of the serotonin receptor antagonist and the infusion of gludopa, suggesting that both serotonin and DA modulate phosphate excretion in rats with remnant kidneys.

Effect of gamma-L-glutamyl-L-dopa on phosphate excretion

gamma-L-glutamyl-L-DOPA (gludopa) is a dopamine prodrug that is relatively specific for the kidney. Because dopamine is phosphaturic, the present study compared the phosphaturic effects of the infusion of equimolar doses of gludopa (n = 8), L-DOPA (n = 8), and gamma-L-glutamyl-L-tyrosine (glutyrosine, n = 6). Glutyrosine was used as a control to evaluate the effect of the glutamyl portion of gludopa on phosphate excretion. Sprague-Dawley rats (350 to 400 g) were anesthetized with 5-sec-butylethyl-2-thyobarbituric acid (Inactin; 100 mg/kg, IP) and underwent thyroparathyroidectomy. Clearances were taken during the infusion of normal saline vehicle, followed by the infusion of gludopa, L-DOPA, or glutyrosine, all infused at the rate of 10 nmol/kg bolus and 0.8 nmol/kg/min (iv). To determine the contribution of glutamyl derivative to phosphate excretion, gludopa or L-DOPA was infused in the presence of SCH23390, a DA-1 receptor antagonist. Gludopa infusion significantly increased dopamine excretion (from 1.9+/-0.2 ng/min to 17.0+/-3.9 ng/min, delta15.0+/-3.9 ng/min, P < .008) and fractional excretion of phosphate (from 2.6%+/-0.6% to 34.8%+/-1.8%, delta32.0%+/-1.6%, P < .001). L-DOPA infusion significantly increased dopamine excretion (from 1.4+/- 0.4 ng/min to 9.7+/-1.6 ng/min, delta8.3+/-1.5 ng/min, P < .001) and fractional excretion of phosphate (from 1.7%+/-0.6% to 8.2%+/-2.0%, delta6.4%+/-1.5%, P < .004). Glutyrosine infusion significantly increased fractional excretion of phosphate (from 2.8%+/-0.8% to 17.5%+/-5.2%, delta14.6%+/-4.8%, P < .03) without changing dopamine excretion (delta0.5+/-0.2 ng/min). Infusion of gludopa in the presence of SCH23390 increased fractional excretion of phosphate (from 5.7%+/-2.5% to 12.6%+/-3.5%, delta6.8%+/-2.3%, n = 6, P < .03), whereas SCH23390 completely blocked the phosphaturic effect of L-DOPA. We conclude that gamma-L-glutamyl-L-DOPA is more phosphaturic than L-DOPA in the rat because of the combined effects of dopamine and the glutamyl moiety.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

N-acetyl-L-gamma-glutamyl derivatives of p-nitroaniline, sulphamethoxazole and sulphamethizole for kidney-specific drug delivery in rats

Kidney-specific delivery of p-nitroaniline, sulphamethoxazole and sulphamethizole after either intravenous administration of the L-gamma-glutamyl or N-acetyl-L-gamma-glutamyl derivatives or the parent drugs has been examined in a rat model. All L-gamma-glutamyl derivatives were converted to the corresponding parent drugs within 60 min whereas the N-acetyl-L-gamma-glutamyl derivatives were fairly stable in the systemic circulation after parenteral administration. Concentrations of p-nitroaniline and sulphamethoxazole 20 min after administration of the parent drugs were somewhat higher in the kidney than in the liver and lung. The concentration of sulphamethizole in the kidney was dramatically higher than those in the hepatic and pulmonary tissue. Kidney-specific delivery of the drugs of interest was evaluated by determining the tissue concentrations of the released parent drug and the total drug levels (i.e. drug levels after hydrolysis of all conjugate to the parent drug). For L-gamma-glutamyl-p-nitroaniline released renal levels of p-nitroaniline and total p-nitroaniline concentrations were both higher than those obtained after p-nitroaniline dosing. Use of L-gamma-glutamylsulphamethoxazole resulted in higher total sulphamethoxazole concentrations in the kidney, but did not lead to an increase in released (unconjugated) sulphamethoxazole levels. In contrast, no kidney-selective distribution was observed for L-gamma-glutamylsulphamethizole. Markedly increased kidney distribution was observed for both N-acetyl-L-gamma-glutamyl-p-nitroaniline and N-acetyl-L-gamma-glutamylsulphamethoxazole and the liver and lung concentrations were correspondingly reduced in comparison with parent drug dosing. Use of the N-acetyl-L-gamma-glutamyl-p-nitroaniline conjugate increased the concentration of p-nitroaniline in the kidney to the same extent as did L-gamma-glutamyl-p-nitroaniline. In conclusion, N-acetyl-L-gamma-glutamyl derivatization of certain compounds seems to be useful for kidney-specific drug delivery and preliminary data suggests that lipophilic drugs are better substrates than hydrophilic compounds. Results related to the selectivity of tissue distribution of the derivatives and species differences are discussed.

Intrarenal dopamine production and distribution in the rat. Physiological control of sodium excretion

Dopamine (DA), produced by the renal proximal tubule, has been demonstrated as an intrarenal paracrine hormone mediating diuresis and natriuresis. The precise mechanism by which DA exerts its cell-to-cell action is not fully understood. In the present study, renal interstitial fluid (RIF) DA (by in vivo microdialysis) and urinary DA excretion (UDAV) were compared in anesthetized rats on either normal (0.28% NaCI, NS) or high (4.0% NaCI, HS) sodium balance and in response to acute gamma-L-glutamyl-L-dopa (gludopa) administration. Urine flow (UV) and sodium excretion (UNaV) in HS were greater than in NS rats. UDAV was increased in HS compared with NS rats. RIF DA was significantly lower in HS than NS rats. Gludopa at 3, 5, and 7.5 nmol/kg (IV bolus) produced a larger increase in UDAV than RIF DA. Only the highest dose of gludopa (7.5 nmol/kg), which resulted in a 7.3-fold increase in UDAV and 1.7-fold increase in RIF DA, was associated with significant diuresis and natriuresis. Cortical and medullary blood flow remained unchanged after gludopa (7.5 nmol/kg) administration, while angiotensin II (100 ng.kg-1.min-1) induced significant reduction in cortical and medullary blood flow. Prior bilateral renal denervation did not have a significant effect on basal DA levels (RIF DA and UDAV) or gludopa-induced DA production or natriuresis and diuresis. These data demonstrated that both chronic sodium loading and acute gludopa administration stimulated renal DA production and release predominantly into the tubule lumen, where DA had a direct tubule action in the control of UNaV. Renal DA production and its renal effects were not significantly regulated by renal sympathetic nerve activity.

The renal handling of dopamine originating from L-dopa and gamma-glutamyl-L-dopa

1. The formation and outflow of dopamine and its deaminated metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) was studied in cortical fragments of the rat kidney loaded with L-beta-3,4-dihydroxyphenylalanine (L-dopa) or gamma-glutamyl-L-dopa (GluDOPA). Dopamine and DOPAC in the tissues and in the effluent were assayed by means of h.p.l.c. with electrochemical detection. 2. In rats given 30 mg kg-1 L-dopa, tissue and outflow levels of both dopamine and DOPAC were 3 fold those observed with a lower dose of L-dopa (10 mg kg-1). In rats given GluDOPA (16.7 mg kg-1) levels of dopamine in renal tissues and in perifusate samples were found to be higher than those obtained with an equimolar dose of L-dopa (10 mg kg-1); however, no significant difference was observed for DOPAC. The outflow of both dopamine and DOPAC in kidney slices of rats injected with L-dopa (10 and 30 mg kg-1) or GluDOPA (16.7 mg kg-1) was found to decline monophasically with similar slopes of decline. The rate constants of loss (k, min-1) of DOPAC (10 mg kg-1 L-DOPA, k = 0.0070; 30 mg kg-1 L-DOPA, k = 0.0087; 16.7 mg kg-1 GluDOPA, k = 0.0080) were 2 to 3 fold those of dopamine (10 mg kg-1 L-dopa, k = 0.0027; 30 mg kg-1 L-DOPA, k = 0.0034; 16.7 mg kg-1 GluDOPA, k = 0.0030). With both precursors the DOPAC/dopamine ratio in perifusate samples were 2.0 fold those in the tissues. 3. Tissue and outflow levels of dopamine after incubation of renal tissues with L-DOPA, 50 and 100 MicroM were found to be lower than those observed with GluDOPA (50 and 100 MicroM). DOPAC/dopamine ratios in tissues and perifusate samples of experiments performed with L-DOPA were significantly higher(P<0.01) than those observed with GluDOPA. The outflow of both dopamine and DOPAC in renal slices incubated with L-DOPA (50 and 100 MicroM) were found to decline with time, but presented a biphasic shape. DOPAC/dopamine ratios in perifusate samples were 3 fold that in the tissues with both precursors.4. In conclusion, the present results show that both L-DOPA and GluDOPA give origin to substantial amounts of dopamine and the newly-formed amine undergoes considerable deamination to DOPAC.However, dopamine originating from GluDOPA was less deaminated than that resulting from L-DOPA;it appears that this different behaviour may concern aspects related to the formation of the amine and also those related to its deamination and disposition, namely the processes involved in the access of newly-formed dopamine to MAO.

Regional haemodynamic effects of dopamine and its prodrugs L-dopa and gludopa in the rat and in the glycerol-treated rat as a model for acute renal failure

1. In this study the renal selectivity of dopamine and its prodrugs L-dopa and gludopa, with respect to their effects on regional blood flow, vascular resistance and central haemodynamics was investigated in normal rats and in rats with glycerol-induced acute renal failure (ARF). 2. In normal, anaesthetized rats, dopamine as well as its prodrugs caused a dose-dependent reduction of vascular resistance in the kidney (RR), mesentery (MR) and hindquarters (HQR) (dose range: dopamine: 0.1-5 mumol kg-1 h-1; L-dopa and gludopa: 1-200 mumol kg-1 h-1). Blood pressure and heart rate were affected at the highest dose only. 3. Administration of glycerol induced a preferential renal vasoconstriction; renal blood flow (-60%) and vascular resistance (+190%) were significantly more affected than MR (+40%) and HQR (+60%). This was only ameliorated by a low rate (10 mumol kg-1 h-1) infusion of gludopa: the glycerol-induced reduction of renal flow and increase in RR were significantly attenuated. A high dose of gludopa (100 mumol kg-1 h-1) or any dose of L-dopa or dopamine did not induce this beneficial effect. The glycerol-induced increase in MR and HQR was not attenuated by any of the treatments used. 4. The results indicate that gludopa is not renally selective at a pharmacodynamic level in normal, anaesthetized rats. Contrary to this, a low dose of gludopa does cause a renal selective vasodilatation and reduction of RR in rats with glycerol-induced ARF. This difference could be explained by a difference in renal vascular tone between normal rats and glycerol-induced ARF rats. A high dose ofgludopa does not cause these renal-selective effects: renal resistance and renal flow are at the same level as following glycerol and saline. This is probably due to the systemic effects of the released dopamine.

The dopamine prodrug, gludopa, decreases both renal and extrarenal noradrenaline spillover in conscious rabbits

1. Renal and total noradrenaline (NA) spillover rates were examined under control conditions and during graded infusions of gludopa (gamma-L-glutamyl-L-dopa) in conscious rabbits. 2. Gludopa infusion at 25 and 100 micrograms/kg per min did not alter mean arterial pressure (MAP) and heart rate (HR), but had significant dose-related effects on the renal dopamine (DA) system. At the high dose there were pronounced increases in urinary DA excretion (> 6000-fold) and renal DA content (> 100-fold); renal NA content doubled. 3. Renal venous DA increased after gludopa infusion, but arterial plasma DA concentrations were not significantly changed. Mean arterial plasma gludopa and L-dopa concentrations reached 890, 3190 ng/mL and 3, 10 ng/mL at low and high doses, respectively. 4. Gludopa resulted in a pronounced dose-dependent fall in renal NA spillover, which at 100 micrograms/kg per min accounted for almost half of the reduction in overall NA spillover rate. 5. The significant falls in renal and extrarenal NA spillover rate during gludopa infusion are consistent with suppression of renal and overall sympathetic activity. Gludopa-induced inhibition of renal NA spillover is likely to be due to the actions of DA generated in the kidney on presynaptic DA-2 and alpha-2 receptors. A central sympathoinhibitory mechanism may explain the reduced total NA spillover.

Renal dopamine excretion in healthy volunteers after oral ingestion of L-dopa

L-Dopa is converted to dopamine by aromatic-L-amino acid decarboxylase (AADC). In the kidney, proximal tubular epithelial cells are rich in AADC and urinary free dopamine excretion is a marker for endorenal extraneuronal dopamine synthesis. The urinary free dopamine excretion was analysed in a double-blind cross-over study after oral ingestion of L-Dopa or a placebo in five healthy volunteers. The drug ingestions were separated by one week's wash-out. Since in a preliminary study, two volunteers ingesting a single L-Dopa dose of 500 mg with breakfast experienced nausea, the five volunteers of the present study were given 300 mg L-Dopa (50 mg at 9 am with breakfast, 100 mg before lunch and 150 mg before dinner) without any adverse effects. L-Dopa induced an increase in 24-h urinary dopamine excretion (HPLC with electrochemical detection). Free urinary dopamine (1900 micrograms/24 h) accounted for 0.8% of the daily oral L-Dopa dose and represented 10% of total urinary dopamine excretion. L-Dopa treatment had no significant effect on mean ambulatory arterial blood pressure and heart rate measured from 9 am to 6 pm (Spacelabs) or on 24 h urinary water and sodium excretion.

Renal selective N-acetyl-L-gamma-glutamyl prodrugs: studies on the selectivity of some model prodrugs

1. In this study, a number of structurally different N-acetyl-L-gamma-glutamyl prodrugs were investigated with respect to selective uptake by the kidney in male Wistar rats. 2. All prodrugs were tested in vitro in rat kidney slices and kidney homogenate to study their uptake and conversion. It was found that the prodrugs of para-nitroaniline (agPNA), aminophenyl acetic acid (agAFA), sulphamethoxazole (agSM), sulphadimethoxine (agSDM), propranolol (agPP) and metoprolol (agMP) were accumulated by a probenecid-sensitive carrier. The prodrug of 4'-aminoantipyrine (agAAP) was not accumulated by a probenecid- or buthionine sulphoximine-sensitive carrier. Unlike all other prodrugs, agAAP and agMP were not, or only a very limited extent converted to the parent compound in vitro. 3. agPNA, agAFA and agPP were also investigated in vivo. The tissue distribution of the prodrugs and the parent drugs was established, as was their urinary excretion and pharmacokinetic behaviour. agPNA and agAFA showed selective uptake by the kidney, in contrast to agPP which accumulated in the liver. The distribution of the parent compounds following prodrug administration was as follows: agPNA was found in kidney and plasma: agAFA in kidney only; agPP in liver only. 4. The factors which determine the selectivity of N-acetyl-L-gamma-glutamyl prodrugs are discussed. The main factors are: the transport into the kidney, the conversion rate, the residence time of the prodrug in the kidney and the presence or absence of competition for uptake and conversation by other tissues, e.g. the liver. It is concluded that this prodrug approach offers the possibility of delivering drugs selectively to the kidney, but also that it is not universally applicable.

DA-1 receptors mediate renal effects of the dopamine prodrug, gludopa, in conscious rabbits

1. Eight male rabbits were implanted with Doppler flow probes around the lower abdominal aorta and left renal artery. A 2 week recovery period was allowed prior to the experiment. 2. Normal saline, gludopa at 25 micrograms/kg per min and at 100 micrograms/kg per min were each infused i.v. for 60 min. One week later the same protocol was administered to four of these animals in addition to DA-1 antagonist SCH 23390 (0.3 mg/kg i.v.) before gludopa infusion. 3. Gludopa elicited significant increases in urine flow, urinary sodium excretion and renal blood flow, and decreased renal vascular resistance. These changes were abolished by the DA-1 antagonist. Blood pressure, heart rate and hindlimb blood flow remained unchanged. 4. Urine dopamine excretion was increased 1200-fold and 7800-fold after gludopa administration at 25 micrograms/kg per min and 100 micrograms/kg per min, respectively, while plasma dopamine concentration and plasma renin activity (PRA) were not significantly altered. However, PRA was elevated by gludopa with DA-1 antagonism. 5. The renal vasodilation, natriuresis and diuresis produced by gludopa in conscious rabbits appears to be mediated by locally generated dopamine via DA-1 receptors.