Low sodium intake induces an increase in renal monoamine oxidase activity in the rat. Involvement of an angiotensin II dependent mechanism.

AIMS: The interplay between natriuretic dopamine and antinatriuretic angiotensin II represents an important mechanism for the regulation of renal sodium and water excretion. Monoamine oxidase is the main metabolizing pathway for dopamine in the renal cortex. In this study, we have analysed the effect of low sodium feeding and AT1 receptor blockade on renal dopamine metabolism by monoamine oxidase. METHODS: Four groups of rats were studied: 1, normal salt diet (NS); 2, low salt diet (LS); 3, NS receiving Losartan (Los, specific AT1 receptor antagonist, 20 mg kg(-1) bwt day(-1), NS + Los); 4, LS receiving Los (LS + Los). RESULTS: Urinary dopamine excretion was lower in LS than in NS rats (543 +/- 32 vs. 680 +/- 34 ng day(-1) 100 g(-1) bwt, P < 0.05). When treated with Los, DOPAC excretion and urinary DOPAC/dopamine ratio fell significantly in the LS + Los group as compared with the LS group (1199 +/- 328 vs. 3081 +/- 681 ng day(-1) 100 g(-1) bwt and 1.90 +/- 0.5 vs. 5.7 +/- 1.2, respectively, both P < 0.02). Losartan increased hydroelectrolyte excretion in the LS group. No changes were found in the NS + Los group. Aromatic L-amino acid decarboxylase activity in cortex was similar in NS and LS rats. Instead, monoamine oxidase activity was higher in cortical homogenates from LS rats (in nmol mg tissue(-1) h(-1): NS 7.66 +/- 0.52; LS 9.82 +/- 0.59, P < 0.05) and this difference was abolished in LS + Los rats (7.34 +/- 0.49 nmol mg tissue(-1) h(-1), P < 0.01, vs. LS). CONCLUSIONS: We have concluded that low levels of dopamine in the urine of LS rats are because of an increase in the activity of renal monoamine oxidase and that angiotensin II mediates this increase through stimulation of AT1 receptors.

Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+-ATPase.

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on alpha-adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na+,K+-adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na+,K+-ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the alpha-adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM. The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na+,K+-ATPase on the Ser23 residue. The level of PKC induced Na+,K+-ATPase phosphorylation was determined using an antibody that only recognizes Na+,K+-ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na+,K+-ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na+,K+-ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na+,K+-ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium-dependent protein phosphatase, calcineurin, increased the state of Na+,K+-ATPase phosphorylation. The results imply that phosphorylation of renal Na+,K+-ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.

Effects of thyroid hormone on the renal dopaminergic system.

This study determined the effects of thyroid hormone on the renal dopaminergic system. Surgical thyroidectomy (Tx) and treatment with 2-thiouracil (Thio) decreased renal cortex Na+/K+ ATPase activity and urinary volume. Tx also decreased urinary Na+ and urinary L-DOPA without changing urinary excretion of Dopamine (DA). Thio treatment decreased slightly urinary L-DOPA and Na+, but increased urinary excretion of DA. In both models of thyroid hormone deficiency, the ratio urinary DA/DOPA increased. Changes after Thio treatment were reversed after one month of drug withdrawal. Treatment with T3 via osmotic minipump increased Na+/K+ ATPase activity and urinary L-DOPA, did not change urinary DA, and increased the ratio DA/DOPA. To further analyze the effects of thyroid hormone deficiency, we administered selective DA1 (SCH-23390), DA2 (Sulpiride), and a non selective (Haloperidol) DA receptor antagonists to Thio treated and control animals. The DA1 antagonist decreased diuresis, natriuresis and urinary L-DOPA in control, but had no effect in Thio treated rats. Sulpiride had no effect in either group. The combination of SCH-23390 plus Sulpiride decreased urinary L-DOPA and urinary volume only in Thio treated animals. Haloperidol decreased urinary volume in Thio treated animals, but had no effect in controls. Our findings suggest that renal DA synthesis is to some extent dependent on thyroid hormone levels, and that the response of DA receptors is altered by thyroid hormone deficiency, indicating a role of this hormone in the regulation of the renal dopaminergic system.

Effect of glucocorticoids on renal dopamine production.

This study assess the effects of glucocorticoids on dopamine excretion and evaluates the participation of renal dopamine in the effects of glucocorticoids on renal function and Na+ excretion. Dexamethasone (i.m.; 0.5 mg/kg) was administered to male Wistar rats on day 2 or on days 2 and 5. Daily urinary excretions of Na+, dihydroxyphenylalanine (DOPA), dopamine and dihydroxyphenylacetic acid were determined from day 1 to day 7. Renal function was evaluated 8 h after dexamethasone administration in a separate group. The first dose of dexamethasone increased about 100% diuresis and natriuresis, increased urinary DOPA and renal plasma flow, and did not affect urinary dopamine or the other parameters evaluated. These effects were not affected by previous administration of haloperidol. The second dexamethasone dose increased about 200% diuresis and natriuresis, increased urinary dopamine, DOPA, dihydroxyphenylacetic acid, Uosm x V and both glomerular filtration rate and renal plasma flow. Carbidopa administered before the second dexamethasone dose blunted both the diuretic and the natriuretic response whereas haloperidol abolished or blunted all the effects of the second dexamethasone dose. These results show that modifications in renal dopamine production produced by corticoids may contribute to the effects of these hormones on Na+ balance and diuresis and suggest that regardless the factor that promotes an increase in renal perfusion and glomerular filtration rate during long term administration of glucocorticoids, a dopaminergic mechanism is actively involved in the maintenance of these hemodynamic changes.

Regulation of cell cyclic AMP in medullary thick ascending limb of Henle in a rat model of chronic renal failure.

Chronic renal failure (CRF) is accompanied by adaptive changes in electrolyte reabsorption in the thick ascending limb of Henle of surviving nephrons. To study the cellular mechanism of this adaptation, we measured intracellular cAMP in micro-dissected medullary thick ascending limb (mTAL) segments in rats with CRF. mTAL exhibited in CRF an increase of basal cAMP from 25.6 +/- 10.0 in controls to 65.8 +/- 11.3 fmol mm-1 tubule in CRF (P < 0.05). Vasopressin and calcitonin stimulated mTAL adenylate-cyclase in a dose-dependent manner in controls but failed to stimulate in CRF. Likewise, maximal stimulation with 10(-3) M 3-isobutyl-1-methylxanthine (IBMX) plus 10(-5) M forskolin increased cAMP in controls to 63.0 +/- 16.0 but not in CRF, where maximal stimulated values remained at 63.1 +/- 18.8 fmol mm-1 tubule (P NS). Alpha2-adrenoreceptor activation with clonidine at concentrations ranging from 10(-8) to 10(-6) M diminished cAMP production by 37% in CRF (P < 0.05), whereas no differences were found in controls. Thus, the basal intracellular cAMP is increased in rat mTAL in CRF. The finding that neither forskolin nor vasopressin were able to further augment intracellular cAMP would suggest that stimulatory pathways of the adenylate-cyclase system are activated in the basal state. However, mTAL cells in CRF seem to retain the response of normal epithelium to inhibitory pathways such as the one mediated by alpha2-adrenoreceptors.

Changes in glomerular filtration rate and renal plasma flow in cirrhotic rats during converting enzyme inhibition.

During the development of cirrhosis ascites-edema, peripheral vasodilatation, hypotension and an increase of the plasma concentration of several neurohormones are frequently observed. Such complex changes in the hormonal profile hinders the assessment of the relative role of each in the pathophysiology of this disease. The purpose of this work was to evaluate in a rat model of experimental cirrhosis (phenobarbital/CCl4) the role of the renin-angiotensin system in the pre-ascitic stage of the disease using the converting enzyme inhibitor captopril. Cirrhotic rats showed diminished renal and hepatic perfusion. Compared to normal rats, glomerular filtration rate in cirrhotic rats was reduced from 0.75 +/- 0.11 to 0.42 +/- 0.06 mL/min/100 g BW, and renal plasma flow was reduced from 2.37 +/- 0.28 to 1.58 +/- 0.16 mL/min/100 g BW; the indocyanine green slope changed from -0.095 +/- 0.028 to -0.057 +/- 0.01; the plasma sodium concentration fell from 144 +/- 1.5 to 131 +/- 5.40 mEq/L (all < .05). The mean arterial pressure was not reduced in the cirrhotic rats. There was no ascites. Both the acute (25 mg i.v.) and chronic (25 mg i.p. daily plus 25 mg/L in drinking water) administration of captopril to cirrhotic rats induced an increase in glomerular filtration rate and renal plasma flow along with a steeper slope in indocyanine green decay (p < .05 for all three parameters) when compared to non-treated cirrhotic animals. No changes were observed in controls. In the balance studies, an increase in urinary volume along with a decrease in urinary osmolality was recorded in cirrhotic rats on chronic captopril treatment. In conclusion, our results show an activation of the renin-angiotensin system in these rats, as disclosed by the inhibition of the converting enzyme, as well as a possible interaction with ADH.

Demethylation of 3-O-methyldopa in the kidney: a possible source for dopamine in urine.

The possibility that demethylation of 3-O-methyldopa (OM-dopa) in the kidney could provide a source for dopamine in the urine was explored in male Wistar rats aged 60-90 days, using in vivo and in vitro approaches. The results showed that endogenous OM-dopa is filtered, reabsorbed and extensively metabolized in the kidney. Infusion of OM-dopa into anesthetized rats increased significantly urinary excretion of Na+, dopa, dopamine, and 3,4 dihydroxyphenylacetic acid. Whole kidney homogenates, slices from renal cortex, and microdissected proximal tubules produced significant amounts of both dopa and dopamine when incubated with OM-dopa. Renal cortex slices produced dose-dependent amounts of dopa and dopa-mine when incubated with 1-100 microM OM-dopa. Incubation of microdissected proximal tubule segments with 1 microM OM-dopa produced a fourfold (P < 0.025) increment in dopa and a twofold (P < 0.05) increment in dopamine (an effect similar to that observed with 1 microM L-dopa). One micromolar OM-dopa or 1 microM L-dopa decreased (P < 0.05) Na(+)-K(+)-adenosinetriphosphatase activity measured at maximal velocity condition in proximal tubules. In conclusion, these experiments show that in vitro the kidney is able to produce dopamine by demethylation of OM-dopa, while the results of the OM-dopa infusion suggest that this conversion may also occur in vivo.

Cellular adaptation of the rat medullary thick ascending limb to chronic renal failure.

Chronic renal failure (CRF) is accompanied by adaptive changes in renal and extrarrenal epithelial ionic transport. Fluid reabsorption in the thick ascending limb of Henle is increased and the capacity to lower the urine osmolality in water diuresis is preserved. To study the cellular mechanism of this adaptation, we measured intracellular cAMP in microdissected medullary thick ascending limb (mTAL) segments in rats with CRF. mTAL exhibited in CRF nephrons an increase of basal cAMP from 6.0 +/- 1.5 in controls to 47.0 +/- 10.3 fmol. mm-1 tubule in CRF (P < 0.05). Maximally stimulated cAMP levels (10(-3) M IBMX plus 10(-5) M Forskolin) were different from basal levels in controls (6.0 +/- 1.5 vs 63.1 +/- 18.8, P < 0.05) but not from basal levels in CRF (47.0 +/- 10.3 vs 63.0 +/- 16.0, P = N.S.). Preincubation with the adenylate cyclase inhibitor 2'5'-dideoxyadenosine (DDA) 10(-4) M produced no changes in cAMP in controls (93.7 +/- 10.3% of DDA untreated samples) whereas it decreased to 76.2 +/- 8.8% (24% inhibition) in CRF (P < 0.05). No differences between controls and CRF groups were found in basal and stimulated cAMP in red blood cells and distal colon. The data would suggest that the cAMP pathway is an intracellular signal for mTAL adaptation in epithelial transport and that the adenylate-cyclase system is specifically activated in CRF.

Cellular adaptation of the rat medullary thick ascending limb to chronic renal failure.

Chronic renal failure (CRF) is accompanied by adaptive changes in renal and extrarrenal epithelial ionic transport. Fluid reabsorption in the thick ascending limb of Henle is increased and the capacity to lower the urine osmolality in water diuresis is preserved. To study the cellular mechanism of this adaptation, we measured intracellular cAMP in microdissected medullary thick ascending limb (mTAL) segments in rats with CRF. mTAL exhibited in CRF nephrons an increase of basal cAMP from 6.0 +/- 1.5 in controls to 47.0 +/- 10.3 fmol. mm-1 tubule in CRF (P < 0.05). Maximally stimulated cAMP levels (10(-3) M IBMX plus 10(-5) M Forskolin) were different from basal levels in controls (6.0 +/- 1.5 vs 63.1 +/- 18.8, P < 0.05) but not from basal levels in CRF (47.0 +/- 10.3 vs 63.0 +/- 16.0, P = N.S.). Preincubation with the adenylate cyclase inhibitor 25-dideoxyadenosine (DDA) 10(-4) M produced no changes in cAMP in controls (93.7 +/- 10.3

of DDA untreated samples) whereas it decreased to 76.2 +/- 8.8

(24

inhibition) in CRF (P < 0.05). No differences between controls and CRF groups were found in basal and stimulated cAMP in red blood cells and distal colon. The data would suggest that the cAMP pathway is an intracellular signal for mTAL adaptation in epithelial transport and that the adenylate-cyclase system is specifically activated in CRF.

[The role of vascular radiology in the diagnosis of "suspicious" images of the upper urinary tract]. / Papel de la radiología vascular en el diagnóstico de las imágenes "sospechosas" del tracto urinario superior.

Accurate diagnosis of "suspect" lesions from the Upper Urinary Tract (U.U.T.) is of major significance for implementing an appropriate treatment. There are many diseases and situations which may condition the occurrence of a suspect image, i.e., of a repletion defect of the U.U.T., in the U.I.V., the vascular imprints being just one of them. This paper contributes two cases where the suspect lesion was due to this cause and vascular radiology was decisive to reach a diagnosis.

Portal pressure, renal function and hormonal profile after acute and chronic captopril treatment in cirrhosis.

The acute effects of captopril in cirrhosis are well known but there are few descriptions of the pattern of response to chronic administration of captopril in this disease. Nine nonuraemic cirrhotic patients with ascites and portal hypertension were studied after 1 week on fixed sodium and water intake (balance diet) and following acute and chronic treatment with captopril (three doses of 25 mg every 30 min and 75 mg.day-1 for three weeks, respectively). Whilst on the balance diet, 7/9 patients were unable to excrete the amount of sodium ingested. After the acute administration of captopril, a significant reduction was seen in arterial blood pressure (86.9 vs 77 mm Hg), with no change in the intra-hepatic pressures (free suprahepatic pressure, FSHP: 15.0 vs 12.1 mm Hg and wedged suprahepatic pressure, WSHP: 22.9 vs 20.7 mm Hg). After chronic captopril treatment, a drop was observed in portal pressure (FSHP: 9.4 mm Hg and WSHP 18.8 mm Hg, NS) and the arterial pressure returned to its basal level. The plasma aldosterone concentration decreased, whilst noradrenaline and dopamine increased significantly, the latter more than the former, leading to a reduction in the noradrenaline/dopamine ratio (14.5 vs 5.0). Seven out of nine patients showed enhanced natriuresis and the remaining two, who previously had had a positive sodium balance failed to do so. These haemodynamic, hormonal and renal changes were interpreted as evidence of blockade of angiotensin II generation by captopril, and also as a homoeostatic response by the sympathetic nervous system.