The effect of AVP-V receptor stimulation on local GFR in the rat kidney

AIM: Arginine vasopressin (AVP) might influence urinary concentration ability by altering the intrarenal distribution of glomerular filtration rate (GFR). METHODS: To study this possibility we have measured the intracortical distribution of GFR following acute AVP-V1 receptor stimulation in anaesthetized female Sprague-Dawley (SPD) rats during euvolemia and water diuresis by the aprotinin method, allowing two consecutive measurements of zonal GFR in the same kidney. RESULTS: Acute i.v. bolus injection of 50 ng V1 receptor agonist ([Phe2, Ile3, Orn8]-vasopressin) followed by a continuous infusion of 5 ng min(-1) in euvolemic rats reduced GFR by 25% in outer cortex (OC), 20% in middle cortex (MC) and 19% in inner cortex (IC) relative to vehicle infusion (all P < 0.05). In water diuretic rats V1 receptor agonist reduced GFR by 22% in OC, 10% in MC and 11% in IC relative to vehicle infusion (P < 0.05). GFR decreased slightly more in OC than in MC and IC in both euvolemic and water diuretic rats (P < 0.05) indicating a distribution of GFR towards MC and IC. Acute infusion of the selective non-peptide V1 receptor antagonist OPC-21268 in euvolemic rats reduced GFR by 14% in OC, 13% in MC and 11% in IC relative to vehicle infusion (P < 0.05), with no significant difference between the layers. CONCLUSIONS: The change in distribution of GFR not only between OC and IC, but also between OC and MC suggests that the afferent/efferent arterioles and not the medullary vasa recta is the main site of resistance change. We conclude that acute i.v. infusion of V1 receptor agonist in high doses reduces GFR more in superficial than in deep cortex in both euvolemic and water diuretic rats and that this may be of some importance for water conservation, adding to the V2- receptor effect on water permeability of the collecting ducts.     

The effect of AVP-V2 receptor stimulation on local GFR in the rat kidney

The effect of AVP-V2 receptor agonist desmopressin, dDAVP, its non-peptide antagonist OPC-31260 and vehicle infusion on glomerular filtration rate (GFR) in the outer, middle and inner cortex was studied in both hydropenic and water diuretic Inactin anaesthetized female Sprague-Dawley rats using the aprotinin method. Two subsequent GFR measurements were carried out in the same kidney by injection of 125I- and 131I-labelled aprotinin before and after i.v. infusion of dDAVP, OPC-31260 or the vehicle. Acute infusion of dDAVP in hydropenic rats increased total GFR by 14% relative to vehicle infusion, whereas in water diuretic rats it had no effect relative to vehicle. No significant changes in arterial pressure (Pa) or renal blood flow (RBF) were recorded. Infusion of OPC-31260 reduced total GFR by 11% compared with vehicle. These results are consistent with the findings that a presensitization of the vasculature by high plasma levels of AVP is necessary for the renal vascular effects mediated by the V2 or V2-like receptors to occur. The ratio between inner and outer cortex GFR remained unchanged from control to experimental condition as follows: dDAVP infusion in hydropenic rats, 0.504 vs. 0.494 in control; vehicle infusion in hydropenic rats, 0. 393 vs. 0.392; OPC-31260 infusion in hydropenic rats, 0.517 vs. 0. 523; dDAVP in water diuretic rats, 0.547 vs. 0.543; vehicle in water diuretic rats, 0.413 vs. 0.417. Thus no significant difference in the GFR response was observed between superficial and deep cortical layers of the rat kidney.     

Effect of furosemide on local and zonal glomerular filtration rate in the rat kidney

Furosemide has been reported to produce disproportional changes in blood flow in cortical zones and to inhibit tubuloglomerular feedback (TGF), suggesting that furosemide might alter the intracortical distribution of glomerular filtrate. We have tested this hypothesis by a new method for measuring local and total glomerular filtration rate (GFR) based on proximal tubular accumulation of the basic polypeptide aprotinin (mol wt 6513). Local GFR was calculated in tissue samples dissected from outer cortex (OC), inner cortex (IC) and the corticomedullary border zone (CM) from the plasma clearances of two aprotinin tracers injected i. v. before and after a 3 min i. v. infusion of 25 mg kg^-1 furosemide. The mean of five samples from each region was used to determine zonal GFR. Isotonic saline was infused at a rate corresponding to urine flow. Furosemide reduced whole kidney GFR from 1.17 to 1.00 mL min^-1 and gave a similar reduction of renal artery blood flow. Urine flow increased from 0.6 to 17% of GFR. Haematocrit (? 0.48) and plasma protein concentration (? 55 mg mL^-1) were maintained while the arterial blood pressure tended to decline (118pL5 mmHg to 108pL6 mmHg, P < 0.05). GFR in OC, IC and CM (1.58, 1.18, 0.42 mL min^-1 g^-1) fell to 87, 88 and 88% of control after furosemide infusion respectively. The furosemide/control ratio for each sample showed a coefficient of variation of about 3%. We conclude that furosemide produced a modest GFR reduction that was uniform throughout the renal cortex. The homogenous GFR response suggests a similar TGF constriction tone in preglomerular vessels of deep and superficial nephrons.     

Transmural distribution of biochemical markers of total protein and collagen synthesis,myocardial contraction speed and capillary density in the rat left ventricle in angiotensin ll-induced hypertension

The effect of angiotensin II-induced hypertension on selected biochemical parameters was studied in Sprague-Dawley rats. Angiotensin II infusion at rates of 41.7 μg h^-1 kg^-1 and 12.5 μg h^-1 kg^-1 for 2, 5, 10 and 15 days elevated the systolic blood pressure from 143 ± 7 mmHg to 215–230 mmHg (P < 0.001) and 185–195 mmHg (P < 0.001), respectively. The left ventricular weight/body weight ratio increased 10–14% (P < 0.05) and 23–32% (P < 0.001) after 2–15 days in rats treated at the lower and higher infusion rates, respectively. Prolyl 4-hydroxylase (PH) activity, a marker of collagen synthesis, was evenly distributed in the left ventricle. PH activity increased by about 100% in both subendocardial and subepicardial layers of the left ventricular wall after angiotensin II infusion for 10 days at 41.7 γ h^-1 kg^-1, but remained unaltered at 12.5 μg h^-1 kg^-1. No change was observed in hydroxyproline concentration. Myosin isoenzymes (V₁-V₃), which reflect myocardial contractility, were unevenly distributed in the left ventricular wall: the proportion of the fast-turnover isoenzyme (V₁) was smaller in the subendocardial layer than in the subepicardial layer. The proportion of V_l decreased after treatment in both layers. Alkaline phosphatase activity, a marker of capillary density, was evenly distributed transmurally in the left ventricular wall. Angiotensin II caused a slight decrease in this activity in both myocardial layers. The results suggest that the elevation of blood pressure leads to transmurally evenly distributed changes in biochemical parameters reflecting collagen synthesis, capillary density and contractile properties of the myocardium.     

Effects of receptor blockade on metabolism and renal actions of vasopressin in conscious dogs

Vasopressin – but not the V₂ receptor agonist [deamino-cis1,D-Arg8]-vasopressin (dDAVP) – may mediate natriuresis in dogs. The present study investigated this phenomenon by use of nonpeptide antagonists to V_1a and V₂ receptors 1-{1-[4- (3-acetylaminopropoxy)benzoyl]-4-piperidyl}-3,4-dihydro-2 (1H)-quinolinone (OPC-21268) and 5-dimethylamino-1-{4- (2-methylbenzoylamino)-benzoyl}-2,3,4,5-tetrahydro-1H-benzazepine (OPC-31260), respectively) hypothesising that only V_1a inhibition would reduce the natriuresis. In conscious dogs vasopressin secretion was suppressed by water loading (2% body weight) and replaced by infusion of vasopressin (50 pg min^?1 kg^?1) resulting in physiological plasma concentrations (plasma levels of AVP (pAVP) = 2.0 ± 0.1 pg mL^?1). In this setting, OPC-21268 did not change the rate of sodium excretion. OPC-31260 increased water excretion 12-fold without significant changes in sodium excretion. Heart rate, mean arterial blood pressure, glomerular filtration rate, and clearance of endogenous Li⁺ were unchanged. During vasopressin infusion, both antagonists increased pAVP, OPC-21268 by 20% and OPC-31260 by 100% (2.0 ± 0.2–4.0 ± 0.3 pg mL^?1). In the absence of vasopressin infusion, OPC-31260 did not increase pAVP. Thus, the increase in pAVP appeared to be due to a decrease in metabolic clearance rate. The results indicate that the present dose of V_1a receptor inhibitor OPC-21268 does not reduce sodium excretion and that both vasopressin antagonists inhibit vasopressin metabolism.     

Intrarenal Distribution of Blood Flow and Glomerular Filtration during Chronic Unilateral Ureteral Obstruction

Paired hydronephrotic (HN) and hypertrophic (HT) rat kidneys were studied after 6 days with complete unilateral ureteral obstruction without exposing the kidneys. Total HN renal blood flow (RBF), estimated by total microsphere (MS) uptake and from local ¹²⁵I-antipyrine (Ap) uptake, averaged about 3/sec (n=147) for the extensor digitorum communis muscle and 3.39±0.68 m/sec (n=142) for the femoral quadriceps muscle. 4 of control. HN kidney GFR was reduced to about 1/2 of control level as estimated from inulin clearance of HT kidney times the HN to HT ratio for mean single nephron filtration rate, determined by ¹⁴C-ferrocyanide. Whereas blood flow (Ap) was proportionately reduced in outer and inner cortex (OC and 1C), fractional flow to the outer medulla (OM) was doubled as compared to controls (p < 0.01). Filtration was well preserved in deep as compared to superficial glomeruli with a smaller deep nonfiltering fraction (p > 0.02). Thus the results oppose the current concept that HN is characterized by disproportionate circulatory damage to IC and OM with little or no filtration in deep nephrons. In HT kidneys average RBF (MS) and GFR rose by about 1/2. Whereas total blood flow (Ap) rose proportionately in OC and IC, it remained at control level in OM, indicating dissociation between the total RBF and GFR and the effective blood flow to the OM zone.     

Central AT1 and AT2 receptors mediate chronic intracerebroventricular angiotensin II‐induced drinking in rats fed high sodium chloride diet from weaning

Intracerebroventricular (ICV) angiotensin (AIl) administration stimulates central AII receptors to induce water consumption in rats. The aim of this study was to determine the role of brain AT₁ and AT₂ receptors in mediating chronic ICV AII‐induced drinking in rats raised on normal or high sodium chloride diets from weaning. Rats were weaned at 21 days of age and placed on normal or high sodium chloride diet for 10–12 weeks. At adulthood, the animals were instrumented with brain lateral ventricular cannulas and femoral arterial catheters. Low dose chronic central AII infusion (20 ng min^?1) significantly (P < 0.05) increased water intake in both groups of rats when compared with their respective controls of 24 h artificial cerebrospinal fluid infusions. In a separate group of high sodium fed rats, coinfusion of AII with the AT₁ receptor antagonist, losartan (0.25 μg min^?1) or the AT₂ receptor blocker, PD 123319 (0.50 μg min^?1) blocked chronic ICV AII‐induced drinking. Upon reinfusion of AII water intake increased above control. Following the cessation of AII infusions, water intake returned to values not significantly different from control (P > 0.05). In contrast, in the normal sodium fed rats losartan, but not PD 123319, blocked the AII‐mediated water intake. The data demonstrate that in high sodium chloride fed rats AII stimulates both central AT₁ and AT₂ receptors to induce drinking, while in the normal sodium chloride fed rats the peptide activates the drinking response primarily by stimulation of central AT₁ receptors.     

Impaired glomerular permselectivity for albumin in chemically medullectomized WKY rats

Chemical renal medullectomy with 2-bromo-ethylamine hydrobromide (BEA) has been used to study the importance of the renal medulla in blood pressure regulation. However, conclusive evidence as to whether BEA treatment affects the glomerular barrier is lacking. In the present study, the effects of BEA upon glomerular permselectivity for albumin were studied using isolated kidneys (IPK) perfused at a low temperature (8 °C) to inhibit tubular reabsorption of proteins. Sixteen WKY rats (W_B) received an i.v. injection of BEA (150 mg kg^-1) while 10 rats served as controls (W_C). Volume balance, urinary osmolality and creatinine clearance (GFR) were measured in metabolic cages. Acute paired experiments (n=9) were performed 5–7 weeks after BEA. The rats were anaesthetized and the total in vivo albumin excretion was recorded. The kidneys were then isolated and perfused for measurements of inulin clearance (GFR) and fractional albumin clearance without tubular reabsorption of protein. The nine BEA treated rats showed polyuria and hypoosmotic urine. In vivo GFR was lower in the BEA treated groups when measured with creatinine clearance (459±22 vs. 213±41 μL min^-1 100 g^-1 body wt, P<0.001), while GFR was not significantly changed in the IPK (W_C=135±27, W_B=92±14 μL min^-1 100 g^-1 body wt, n.s.) when perfused at identical pressures. The fractional albumin clearance was increased three times in the BEA group (W_B=9.6±3.4J, P<0.05). Moreover, albumin excretion in vivo was similar in the two groups despite low GFR in the BEA group. We conclude that BEA treatment affects glomerular permselectivity for albumin.     

Number of nephrons in hypertrophic kidneys after unilateral nephrectomy in young and adult rats

Summary The number of functional nephrons in hypertrophied kidneys was determined in adult non diuretic rats which had been uni-nephrectomized either before 45 days (NY) or after 90 days (NO). This number was calculated by dividing the whole kidney GFR by the single nephron GFR, measured either by micropuncture or the¹⁴C-ferrocyanide infusion technique. Kidney weight was greater in NY rats (1.48±0.08 g SE) than in NO rats (1.23±0.10). In micropuncture experiments on NO rats the number of nephrones (34300±1600) was similar to that of normal adult rats. In NY rats this number was significantly greater (44100±1800,P<0.001).The ferrocyanide technique gave similar results (NO rats: 34800±2100; NY rats: 43900±3600) and indicated that the ratio of superficial to juxtamedullary SNGFR was not altered in the hypertrophied kidneys.The length of microdissected proximal tubules was increased in hypertrophied kidneys of both NY and NO rats and this increase affected thepars recta andpars convoluta equally.Single nephron GFR and F/P Inulin in late proximal and early distal tubules were similar in NO and NY rats.In conclusion, our results demonstrate that when uni-nephrectomy is performed on young rats, the compensatory hypertrophy of the remaining kidney is the result of an increase in both the size and the number of nephrons.     

The role of antidiuretic hormone in cold-induced diuresis in the anaesthetized rat

The aim of this study was to investigate whether the increased diuresis in consequence of hypothermia is due to a depression of the hypothalamic release of antidiuretic hormone (ADH). The plasma concentration of antidiuretic hormone and the effect of intravenous (i.v.) administration of 65 ng kg^?1 desmopressin (selective V₂-receptor agonist) were determined in the anaesthetized rat. In spite of a 50% (P < 0.001) decrease in glomerular filtration rate, urine flow increased sixfold (P < 0.01) and urine sodium excretion increased sevenfold (P < 0.05), whereas urine osmolality decreased (P < 0.001). At the same time plasma antidiuretic hormone decreased from 7.5 ± 1.1 to 3.8 ± 0.4 pg mL^?1 (P = 0.01). After injection of desmopressin urine flow was completely restored, whereas urine osmolality and sodium excretion were only partially normalized. Since tubular conservation of water and fractional water reabsorption decreased during hypothermia, the diuresis must have resulted from an augmented loss of water. This is further supported by the fact that osmolal excretion was not influenced either by hypothermia or by desmopressin. It is concluded that the diuresis in consequence to hypothermia is due both to a decrease in the release of ADH and to a reduction of renal medullary hypertonicity.     

Arginine vasopressin excretion in response to volume expansion in the healthy human,and in patients with glomerulonephritis

The urinary excretion of arginine vasopressin (AVP) was studied during volume expansion (VE) in nine healthy normotensive individuals and 14 patients with active IgA glomerulonephritis (GN). The studies were started after 17–18 h of food and fluid deprivation (hydropenia, HP) and VE was induced by a continuous infusion of Ringer solution up to an amount corresponding to 3% of the body weight. The clearance of inulin and PAH, urine osmolality and urinary excretion of sodium and AVP were determined. The AVP excretion decreased in response to VE in the healthy individuals, both when related to GFR (from 129+ 17 pg min ‘ 100 ml ’ GFR during HP to 65 ± 9 after 3% VE, P < 0.01)andtobody surface area (BSA) (from 134 ± 22 pg min“¹ 1.73 m^-2 BSA to 75 ±11, P < 0.05). In the patients with IgA GN, who had normal blood pressure and normal GFR, the AVP excretion tended to decrease, but the change was not significant (0.05 < P < 0. 1). The patients with hypertension but essentially normal GFR, and those with hypertension and markedly decreased GFR did not change their renal excretion of AVP in response to VE. If related to the GFR, the latter patients had a markedly increased AVP excretion.     

Modulation of vascular contractile responses to α1-and α2-adrenergic and neuropeptide Y receptor stimulation in rats with ischaemic heart failure

In order to evaluate adaptational changes in vascular function in congestive heart failure (CHF), we studied the contractile responses of isolated arterial and venous blood vessels from rats suffering from CHF induced by coronary artery ligature, resulting in a myocardial infarction. The contractile responses of the basilar, femoral and renal arteries and of the iliac vein were examined in relation to adrenergic and neuropeptide Y (NPY) receptor function by the action of the α₁ agonist phenylephrine, the α₂ agonist clonidine and NPY. The contractile force was measured (in mN) and in% of K⁺-induced contraction as well as pD₂ to each agonist. When stimulated by a 60 mM K⁺-buffer solution, the femoral and renal arteries from CHF rats responded with a stronger contraction (E_max; 9.4 ± 0.6 and 9.8 ± 0.6mN) than the corresponding Sham vessels (E_max; 6.2 ± 0.7 and 5.6 ± 0.4 mN respectively, P < 0.001). On the contrary, the iliac vein of CHF responded less to K⁺ than the Sham iliac vein (E_mas 2.5 ± 0.2 and 3.7 ± 0.5 mN, P < 0.01). The CHF iliac vein responded with a weaker contraction when stimulated with phenylephrine (E_max 1.9 ± 0.4 mN) and showed a lower sensitivity (pD₂ 5.6 ± 0.1) than the corresponding sham vessel (E_max 5.7 ± 2.3mN and pD₂ 6.3 ± 0.5, P < 0.05). The CHF renal artery was less sensitive to clonidine (pD₂ 6.4 ± 0.6) than the Sham renal artery (pD₂ 7.2 ± 0.1, P < 0.05). The results indicate differences between CHF and Sham vessel segments according to both contractile capacity induced by K⁺-depolarization and to agonist induced contractile capacity and sensitivity. The differences are not of general nature but vary according to the vascular bed examined.     

Effects of inhibition of neuronal nitric oxide synthase on NMDA-induced changes in cerebral blood flow and oxygen consumption

This study was performed to determine whether neuronal nitric oxide synthase (nNOS) is involved in altering regional cerebral blood flow (rCBF) and oxygen consumption during N-methyl-D-aspartate (NMDA) receptor stimulation. A craniotomy was performed in rats, under isoflurane anesthesia, to expose the cerebral cortex. For the control group (n=7), an NMDA patch (10^–3 M) was applied to the exposed cortex (ipsilateral cortex, IC) for 10 min before determining rCBF and O₂ consumption. The patch was changed every 5 min. To block nNOS, 7-nitroindazole (7-NI, 25 mg/kg i.p.) was administered 30 min before NMDA application (7-NI group, n=7). The autoradiographic technique was used to determine rCBF and regional O₂ consumption was measured using cryomicrospectrophotometry. Blood pressure, heart rate, blood gases, and hemoglobin were similar between the two groups. In the control group, rCBF (108±32 ml/100 g per min) and O₂ consumption (4.8±0.8 ml O₂/100 g per min) of the IC where NMDA was applied were higher than those of the contralateral cortex (CC) (78±16 ml/100 g per min and 3.1±0.4 ml O₂/100 g per min, respectively). Neither rCBF nor O₂ consumption of the IC of the 7-NI group was statistically different from that of the CC. However, O₂ consumption of the IC of the 7-NI group was lower (3.9±1.0 ml O₂/100 g per min) than that of the IC of the control group. Our data demonstrated that a direct cortical application of NMDA increased O₂ consumption and rCBF, and that pretreatment with 7-NI not only attenuated the effects of NMDA on rCBF but also decreased the O₂ consumption during NMDA receptor stimulation. Electronic Publication     

5-HT1A, but not 5-HT2 and 5-HT7, receptors in the nucleus raphe magnus modulate hypoxia-induced hyperpnoea

Aim: In the present study, we assessed the role of 5‐hydroxytryptamine (5‐HT) receptors (5‐HT_1A, 5‐HT₂ and 5‐HT₇) in the nucleus raphe magnus (NRM) on the ventilatory and thermoregulatory responses to hypoxia. Methods: To this end, pulmonary ventilation (V_E) and body temperature (T_b) of male Wistar rats were measured in conscious rats, before and after a 0.1 μL microinjection of WAY‐100635 (5‐HT_1A receptor antagonist, 3 μg 0.1μL^?1, 56 mm ), ketanserin (5‐HT₂ receptor antagonist, 2 μg 0.1μL^?1, 36 mm ) and SB269970 (5‐HT₇ receptor antagonist, 4 μg 0.1 μL^?1, 103 mm ) into the NRM, followed by 60 min of severe hypoxia exposure (7% O₂). Results: Intra‐NMR microinjection of vehicle (control rats) or 5‐HT antagonists did not affect V_E or T_b during normoxic conditions. Exposure of rats to 7% O₂ evoked a typical hypoxia‐induced anapyrexia after vehicle microinjections, which was not affected by microinjection of WAY‐100635, SB269970 or ketanserin. The hypoxia‐induced hyperpnoea was not affected by SB269970 and ketanserin intra‐NMR. However, the treatment with WAY‐100635 intra‐NRM attenuated the hypoxia‐induced hyperpnoea. Conclusion: These data suggest that 5‐HT acting on 5‐HT_1A receptors in the NRM increases the hypoxic ventilatory response.     

Renal endothelin system and excretory function in Wistar-Kyoto and Long-Evans rats

Aim: The role of the kidney endothelin system in the renal regulation of fluid and electrolyte excretion was investigated in Wistar–Kyoto (WKY) and Long–Evans (LE) rats in which we found previously marked differences in the renal excretory responses to endothelin A receptor blockade. Methods: The selective endothelin A and B receptor antagonists BQ‐123 (16.4 nmol kg⁻¹ min⁻¹) and BQ‐788 (25 nmol kg⁻¹ min⁻¹) were infused i.v. for 50 min in conscious chronically instrumented WKY and LE rats and their renal function and renal endothelin system were studied. Results: Without effects on glomerular filtration rate or renal blood flow, BQ‐123 and BQ‐788 decreased by more than 50% (P < 0.01) both urine flow rate and electrolyte excretion in WKY rats but only urine flow rate (P < 0.05) in LE rats. Endothelin‐1 content, preproET‐1/GPDH mRNA ratio, B_max and K_d of total endothelin receptors in renal cortex did not differ between the two strains. In contrast, plasma endothelin‐1 concentration (0.58 ± 0.04 vs. 1.05 ± 0.01 femtomol mL⁻¹; P < 0.01), renal papillary ET‐1 concentration (68 ± 5 vs. 478 ± 62 fmol mg⁻¹ protein; P < 0.01) and preproET‐1/GPDH mRNA ratio (0.65 ± 0.09 vs. 0.88 ± 0.05; P < 0.05) as well as total endothelin receptor number in renal papilla (B_max 5.3 ± 0.4 vs. and 9.0 ± 1.2 pmol mg⁻¹ protein; P < 0.05) were markedly lower in LE than in WKY rats. In vitro studies showed that in both strains ET_B receptors on renal cortical membranes amounted between 65% and 67% and on papillary membranes between 85% and 88%. Conclusion: The present data show that the selective ET_A or ET_B receptor blockade differentially affects tubular water and salt handling, which becomes apparent in conditions of low renal papillary endothelin receptor number and tissue endothelin‐1 concentration.     

Type A and B monoamine oxidase activities in the human and rat kidney

The present work has examined the distribution of the two isoforms of monoamine oxidase (MAO), type MAO-A and MAO-B, in the cortex and medulla of the human and rat kidney. Homogenates of renal cortex and renal medulla were prepared in 67 mmoles 1^-1phosphate buffer (pH = 7.2) and MAO activity was determined with [³H]5-hydroxytryptamine ([³H]5HT) and [¹⁴C]β-phenylethylamine ([¹⁴C]β-PEA) as preferential substrates of type A and type B MAO, respectively. K_m and V_max values for the two substrates were also calculated. Both MAO-A and MAO-B are present in the cortex and the medulla of the human and rat kidneys. In the human kidney, MAO-A activity was found to be similar in the cortex (V_max= 142.70±45.05 nmoles mg^-1protein h^-1) and medulla (V_max= 133.91±35.51 nmoles mg^-1protein h^-1); MAO-B activity was found to be higher in the cortex (V_max= 166.19±19.75 nmoles mg¹protein h^-1) than in the medulla (V_max= 92.91±13.22 nmoles mg^-1protein h^-1). In the rat kidney, MAO-A was also found to be similar in the cortex (V_max= 62.35±1.74 nmoles mg^-1protein h^-1) and the medulla (V_max= 59.42±0.97 nmoles mg^-1protein h^-1) and higher than the activity of MAO-B in the two renal areas (cortex, V_max= 31.06±1.09 nmoles mg^-1protein h^-1; medulla, V_max= 14.93±0.97 nmoles mg^-1protein h^-1). No statistically significant differences were found between the K_m values towards [³H]5HT and [¹⁴C]β-PEA in the cortex and the medulla of the human and rat kidneys. The results show that in both renal areas, activity of the enzyme is higher in the human kidney than in the rat kidney. Furthermore, in the human kidney, in contrast with the rat kidney, MAO-B activity closely follows MAO-A activity.     

Role of glomerular prostanoid in control of glomerular filtration rate in rats

Summary It is generally accepted that the main action of glomerular prostanoids (GPs) on glomerular filtration rate (GFR) is to modulate the activity of different vasoconstrictors, specially in states of renal hypoperfusion. However it was also suggested that GPs may directly affect GFR. The present study was focused on this last hypothesis, in different experimental models, in rats.In adriamycin induced acute renal failure, the transient decrease of GFR is associated with higher levels of thromboxane B₂. Later on, when GFR returns to normal, vasodilator prostaglandins synthesis was also increased.In captopril induced renal failure in Na depleted rats (where GPs synthesis remained normal), stimulation of PGE₂ and PGI₂ production by K and NaCl was associated with a significant improvement of GFR. Furthermore, the increase in GFR induce by NaCl was prevented by inhibition of prostaglandin synthesis.Infusion of atrial natriuretic peptide in euvolemic rats induce a marked elevation both of GFR and PGE₂ synthesis. It was abolished by previous administration of prostaglandin synthesis inhibitor.In conclusion, glomerular prostanoids may influence GFR, either directly, or as mediator or modulator of other vasoactive hormones.Abbreviations GPs glomerular prostanoids - PG prostaglandin - TX thromboxane - ANP atrial natriuretic peptide - GFR glomerular filtration rate - PE polyethylene Nachtrag zu den Hauptreferaten des 19. Kongresses der Gesellschaft für Nephrologie in Göttingen (Klin Wochenschr 66/18)     

The effects of P2X receptor agonists on renal sodium and water excretion in anaesthetized rats

Aim: To investigate in vivo effects of P2X receptor activation on sodium and water excretion in urine. Methods: The clearance experiments were carried out in anaesthetized rats during intravenous infusion (2 μmol kg^?1 + 20 nmol (kg min)^?1, v = 40 μL min^?1) of P2X receptors agonists: α,β‐methylene ATP (α,β‐meATP) and β,γ‐methylene ATP (β,γ‐meATP). Cortical blood flow (CBF) was estimated by laser Doppler flux during intrarenal artery infusion of β,γ‐meATP (20 nmol (kg min)^?1, v = 2 μL min^?1). Influence of α,β‐meATP and β,γ‐meATP on the activity of Na‐K‐ATPase was investigated in isolated proximal tubules. Results: Intravenous infusion of β,γ‐meATP resulted in a marked, progressively increasing diuresis and this effect was accompanied by a progressive increase in the sodium excretion rate. The glomerular filtration rate was unaffected. The effects of β,γ‐meATP were abolished by P2 receptor antagonist PPADS (70 nmol (kg min)^?1). CBF increased by 16 ± 2% during renal artery infusion of β,γ‐meATP. Furthermore, α,β‐meATP and β,γ‐meATP increased 1.5‐fold lithium clearance (C_Li). Sodium excretion, expressed as a fraction of the distal delivery (C_NaC_Li^?1), increased 1.5‐fold during infusion of α,β‐meATP or β,γ‐meATP. Both agonists at 10^?6 m produced a statistical significant decrement in the ouabain‐sensitive ATPase activity about 16–20% and these effects were blocked in the presence of PPADS. Conclusion: Activation of P2X receptors increased renal sodium and water excretion. Mechanistically, P2X agonists increased renal perfusion and inhibited sodium reabsorption via an Na‐K‐ATPase‐dependent mechanism.     

Involvement of arginine vasopressin in endogenous central histamine-induced reversal of critical haemorrhagic hypotension in rats

Objective and design:Histamine is a potent stimulator of arginine vasopressin (AVP) release and therefore, the role of AVP was studied in the reversal of critical haemorrhagic hypotension induced by endogenous central histamine after inhibition of histamine N-methyltransferase (HNMT) activity in rats. Material:In 48 ethylurethane-anaesthetised male Wistar rats cardiovascular parameters and plasma hormone concentrations were measured. Treatment:Haemorrhage-shocked rats with mean arterial pressure (MAP) 20–25 mmHg were injected intracerebroventricularly (icv) with HNMT inhibitor metoprine (20 g) after pre-treatment with V_1a, V_1b and V₂ receptor antagonists – [-mercapto-,cyclopentamethylenepropionyl¹, O-me-Tyr²,Arg⁸]AVP (10 g/kg; iv), SSR149415 (10 mg/kg; ip) and [adamantaneacetyl¹,O-Et-D-Tyr²,Val⁴, aminobutyryl⁶,Arg^8,9]AVP (10 g/kg; iv), respectively, or saline. Methods:MAP, heart rate (HR) and regional haemodynamics were monitored within 2 h after treatment or to death if it occurred earlier. Plasma hormone concentrations were measured using enzyme immunoassays. ANOVA followed by Neuman-Keules test, and Fishers exact test were used to compare the results. Results:Metoprine produced a long-lasting increase in MAP, HR, renal, hindquarters and mesenteric blood flows, and a 100% survival at 2 h (P < 0.05 vs. the control group). The action was associated with increased plasma AVP concentration (587.5 ± 98.9 vs. 387.3 ± 125.2 pg/ml; P < 0.05) in comparison to the control group as measured at 20 min after treatment. V_1a, but not V_1b and V₂, receptor antagonist inhibited metoprine-induced haemodynamic effects, with no influence on survival at 2 h. SSR149415 did not influence ACTH and adrenaline plasma concentrations in the metoprine-treated group. Conclusion:AVP, acting via V_1a receptors, is involved in endogenous central histamine-induced reversal of critical haemorrhagic hypotension in rats.Received 9 October 2003; returned for revision 2 December 2003; accepted by A. Falus 23 December 2003     

The effect of kinin and prostaglandin inhibitors on the renal response to angiotensin-converting enzyme inhibition: a micropuncture study in the dog

It has been suggested that angiotensin-converting enzyme (ACE) inhibition is accompanied by enhanced bradykinin and prostaglandin activities, which may contribute to the renal haemodynamic actions of ACE inhibitors. Therefore we investigated renal function by clearance and micropuncture techniques in dogs maintained either on normal or low-salt diet before and after ACE inhibition with an i. v. bolus of 0.1 mg/kg ramiprilat followed by an infusion of 5 μg kg⁻¹ min⁻¹. Subgroups each comprising six dogs were also treated with either HOE-140, a bradykinin B₂ receptor antagonist, or the cyclooxygenase inhibitor indomethacin. In general, renal effects of ramiprilat were more pronounced in dogs fed on low salt than in those on normal diet. In dogs on low salt, the mean arterial pressure decreased by 20% 20 min after ramiprilat application, whereas the total renal blood flow rose by 71% from 4.71 to 8.06 ml min⁻¹ g kidney weight⁻¹ and the glomerular filtration rate (GFR) by 28% from 0.74 to 0.95 ml min⁻¹ g⁻¹. Single-nephron glomerular blood flow and single-nephron GFR rose by 55% and 23% respectively. The total and the single-nephron filtration fraction decreased by 25% and 23% respectively. There were no substantial changes in glomerular and peritubular capillary and tubular pressures, but a significant increase in the ultrafiltration coefficient, K _f, by 103% from 3.55 nl/ mmHg to 7.19 nl/mmHg (26.7–54.0 nl/kPa) was observed. Afferent and efferent arteriolar resistances decreased in parallel by 55% and 47%. Prior and concomitant intrarenal arterial infusion of HOE-140 at a dose that blocked the vasodilatory effect of 9 ng kg⁻¹ min⁻¹ bradykinin had no significant effects in dogs on low salt but attenuated the relative rise in renal and single-nephron glomerular blood flow and K _f by 21%, 27% and 26% respectively in dogs on low salt (P<0.01). No such effects were observed with indomethacin. We conclude that ACE inhibition in the dog results in a parallel decrease in afferent and efferent resistance and significantly increases K _f. This latter effect is partly mediated by the kinin system under conditions of Na⁺ depletion.