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Notes: 1. The effects of the specific angiotensin II receptor type I (AT1) antagonist candesartan on renal proximal tubular sodium transport were studied using lithium clearance. The effects of candesartan on mean arterial blood pressure (MABP), renal plasma flow (RPF), glomerular filtration rate (GFR) and sodium and potassium excretion were also investigated.2. Male Wistar rats were anaesthetized with Inactin (thiobutabarbital sodium; Sigma, St Louis, MO, USA). Clearance markers (8% polyfructosan, 1% para-aminohippuric acid and 4 mmol/l lithium chloride) were given into a jugular vein at the rate of 1.6 mL/h per 100 g bodyweight. Candesartan was given as bolus injection (0.01, 0.1, 0.2, 0.5 and 1.0 mg/kg) followed by 60 min continuous infusion at a rate of 0.5, 5, 10, 25 and 50 µg/min per kg, respectively.3. The non-depressor dose of candesartan (0.01 mg/kg) did not alter RPF or GFR, whereas diuresis, natriuresis and kaliuresis were observed. The higher doses of candesartan reduced MABP, RPF and GFR, although diuresis, natriuresis and kaliuresis were still observed.4. Renal tubular sodium and water reabsorption were inhibited after intravenous administration of candesartan independently of an alteration in arterial pressure. Lithium clearance data indicate that the site of inhibition was in the proximal nephron segment.

Notes: 1. Disturbances in cardiovascular responsiveness to endogenous endothelin-1 (ET-1) may play a significant role in the pathogenesis of essential hypertension. In this study the inotropic responses of cardiomyocytes derived from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains to ET-1 (10-11-10-8 mol/L) were characterized. Isotonic contraction cycles of ventricular cardiomyocytes isolated from age-matched (11 week) WKY and SHR rats were recorded using a rapid digital imaging technique and evaluated by computation of a range of normalized parameters.2. The maximum effect of ET-1, eliciting a 60–70% increase in myocyte shortening after 3 min, was observed at 10-9 mol/L in both strains, and was associated with elevations in the rate of shortening and lengthening, abbreviated latency, contractile cycle prolongation and delayed time to peak shortening.3. No evidence for a significant strain dependent difference in the relative responsiveness to ET-1 was detected. This finding indicates that altered sensitivity to ET-1 is unlikely to be a major factor underlying the development of hypertension in this model.4. The distinct nature of the alterations in contractile parameters produced by ET-1 compared with angiotensin II (AII) suggests that the prevailing cellular mechanisms of action of these peptides are different and that ET-1 is not a paracrine or autocrine inotropic intermediate for AII

Notes: 1. The role of angiotensin as a modulator of proximal glomerulotubular (GT) balance was investigated in anaesthetized rats by examining the relationship between glomerular filtration rate (GFR) and absolute proximal reabsorption (APR) during removal of endogenous angiotensin II (AII) and III (AIII) with enalaprilat (CEI) and then during their subsequent replacement by intravenous infusions.2. Enalaprilat lowered mean arterial blood pressure (MABP) and increased renal blood flow (RBF), GFR, urine flow rate and sodium excretion. Filtration fraction (FF) was not altered. Absolute proximal reabsorption, derived from fractional lithium clearance, increased by only 48% of the change expected for ‘perfect’ GT balance.3. Angiotensin II replacement corrected MABP, GFR and plasma renin level, but reduced RBF and increased FF; APR was decreased and GT balance was restored. Urine flow and sodium excretion remained above control values with AII.4. Replacement with AIII did not correct the hypotension but completely reversed the renal and renin responses to enalaprilat and restored GT balance without affecting FF.5. It was concluded that the relation between proximal reabsorption and GFR is considerably modified by the intrarenal angiotensin concentration. The findings are best explained by a direct stimulation of proximal tubular sodium transport by angiotensin at the concentrations existing in anaesthetized rats.

Notes: 1. A rapid imaging technique with temporal resolution of about 1 ms was employed to describe cell-length changes during the isotonic contraction cycle of adult rat ventricular myocytes at 22°C.2. Parameters of cell contraction and relaxation were defined and values obtained under control conditions and after treatment with 5 μmol/L verapamil.3. Over 15 min, verapamil dramatically reduced the maximum shortening attained. This was associated with delay in both excitation-contraction coupling latency and the time at which maximum rate of shortening occurred. However, peak shortening was recorded earlier in the contraction cycle. Total cell-cycle time was abbreviated under the influence of verapamil and maximum rates of shortening and lengthening were depressed in a similar manner.4. The results highlight the value of improved temporal precision in describing myocyte contractility and validate the use of the parameters defined in the single-cell model for the study of the mechanisms of action of cardiotonic agents.

Notes: 1. The antihypertensive agent rilmenidine has threefold higher affinity for I1 imidazoline receptors compared with α2-adrenoceptors and acts on the central nervous system by reducing sympathetic activity and in the kidney by inhibiting Na+/H+ exchange activity.2. In the present study, we examined: (i) the effects of luminal and peritubular administration of rilmenidine on fluid absorption in superficial proximal tubules; and (ii) the nature of the receptors involved in mediating the action of this drug in the presence of specific antagonists (efaroxan, idazoxan and 2-methoxy-idazoxan). Studies were performed in anaesthetized Sprague-Dawley rats using shrinking split-drop micropuncture.3. Luminal administration of rilmenidine (10−5 and 10−13 mol/L) inhibited proximal tubular fluid absorption. Peritubular rilmenidine at 10−12 and 10−13 mol/L also inhibited fluid uptake, whereas rilmenidine at 10−11 mol/L had a significant stimulatory action.4. In the presence of the I2 〉 I1/α2-adrenoceptor antagonist idazoxan (10−5 mol/L), luminal rilmenidine (10−5 mol/L) stimulated fluid absorption. Stimulation of fluid uptake was also observed when rilmenidine (10−5 mol/L) and the I1 imidazoline receptor antagonist efaroxan (10−5 mol/L) were added together in the luminal fluid. Luminal administration of the selective α2-adrenoceptor antagonist 2-methoxy-idazoxan (10−5 mol/L) resulted in significant attenuation of the inhibitory action of luminal rilmenidine (10−5 mol/L). This indicates that both I1 imidazoline receptors and α2-adrenoceptors are involved in the luminal actions of rilmenidine.5. The effects of luminal and peritubular administration of α-methylnoradrenaline (an α2-adrenoceptor agonist) were compared with those of rilmenidine. Luminal α-methylnoradrenaline, at higher concentrations (10−7 and 10−5 mol/L), inhibited fluid absorption, as was seen with peritubular rilmenidine, but, in contrast with rilmenidine, no stimulatory action was observed. Peritubular α-methylnoradrenaline inhibited fluid uptake at higher concentrations (10−5 and 10−7 mol/L), whereas rilmenidine at these concentrations had no effect. The differences in the concentration-dependent responses for rilmenidine and α-methylnoradrenaline indicate that both imidazoline receptors and α2-adrenoceptors are involved in the actions of these compounds on proximal fluid uptake.

Notes: 〈list xml:id="l1" style="custom"〉1The effects of angiotensin II (AngII) on water and electrolyte transport are biphasic and dose-dependent, such that low concentrations (10−12 to 10−9 mol/L) stimulate reabsorption and high concentrations (10−7 to 10−6 mol/L) inhibit reabsorption. Similar dose-response relationships have been obtained for luminal and peritubular addition of AngII.2The cellular responses to AngII are mediated via AT1 receptors coupled via G-regulatory proteins to several possible signal transduction pathways. These include the inhibition of adenylyl cyclase, activation of phospholipases A2, C or D and Ca2+ release in response to inositol-1,4,5,-triphosphate or following Ca2+ channel opening induced by the arachidonic acid metabolite 5,6,-epoxy-eicosatrienoic acid. In the brush border membrane, transduction of the AngII signal involves phospholipase A2, but does not require second messengers.3Angiotensin II affects transepithelial sodium transport by modulation of Na+/H+ exchange at the luminal membrane and Na+/HCO3 cotransport, Na+/K+-ATPase activity and K+ conductance at the basolateral membrane.4Atrial natriuretic factor (ANF) does not appear to affect proximal tubular sodium transport directly, but acts via specific receptors on the basolateral and brush border membranes to raise intracellular cGMP levels and inhibit AngII-stimulated transport.5It is concluded that there is a receptor-mediated action of ANF on proximal tubule reabsorption acting via elevation of cGMP to inhibit AngII-stimulated sodium transport. This effect is exerted by peptides delivered at both luminal and peritubular sides of the epithelium and provides a basis for the modulation by ANF of proximal glomerulotubular balance. The evidence reviewed supports the concept that in the proximal tubule, AngII and ANF act antagonistically in their roles as regulators of extracellular fluid volume.

Notes: 1. The p-aminophenol (pAP) model of tubular necrosis displays elevated tubular pressures equivalent to ‘stop-flow’, with low glomerular filtration rate (GFR) but maintained blood flow and urine output. Renal function, micropuncture, and morphological studies were performed in anaesthetized rats to examine the causes of filtration failure.2. At the height of pAP-induced renal failure proximal tubular fluid reabsorption (Jv(a) was markedly reduced while proximal and distal free-flow rates measured by tubular fluid collections during venting of the nephron were not significantly different from saline-injected controls. Renal blood flow was maintained over the 4 h observation period despite extensive and selective proximal tubular necrosis. There was no temporal relationship between increased tubular pressure and cast formation.3. Maintained blood and tubular fluid flow rates indicate that activation of tubuloglomerular feedback plays little or no part in pAP-induced renal failure, which is apparently due to high fluid flow resistance in the region of the connecting tubule, late distal convolution or collecting ducts. Morphological appearances were consistent with compression of these segments.

Notes: 1. Renomedullary interstitial cells (RMIC) are characterized by numerous intracellular granules thought to contain renal medullary antihypertensive substances. However, the nature of the trigger for RMIC degranulation remains to be elucidated. The present study examines the effects of acute alterations in mean arterial pressure (MAP) and medullary blood flow (MBF) on RMIC granulation.2. Basal MAP and MBF in anaesthetized Sprague-Dawley rats (n = 4/group) were altered by intravenous infusions of vasoactive agents, including angiotensin II alone or with a nitric oxide (NO) synthase inhibitor (Nω-nitro-l-arginine) or NO donor (sodium nitroprusside), noradrenaline and by carotid artery clamping. Following these treatments, kidneys were examined by electron microscopy and the absolute volume of granules in the renal medulla was calculated using unbiased stereological methods.3. Acute increases in MAP, regardless of the treatment causing the increase, were associated with a reduction in the absolute volume of granules in the range of 42–67%. Regression analysis revealed that only increases in MAP, but not MBF, strongly predict RMIC degranulation.4. Despite previous reports that changes in MBF activate renomedullary antihypertensive activity, we conclude that the change in MAP is an important determinant of the activity of the blood pressure-lowering mechanism of the renal medulla, with the assumption that the medullary lipids mediate the antihypertensive property of the renal medulla.

Notes: 1. Cultured renomedullary interstitial cells (RMIC) isolated from 4-week-old Sprague-Dawley rat kidneys possess ETA receptors, as identified by reverse transcription–polymerase chain reaction (RT-PCR).2. Treatment with endothelin (ET)-1 (10−6 mol/L) increases the intracellular inositol 1,4,5-trisphosphate concentrations within 10 s and intracellular calcium concentrations after 7 s.3. Endothelin-1 (10−7 and 10−10 mol/L) induced increases in intracellular cAMP concentrations, but only in the presence of Nω-nitro-L-arginine, a nitric oxide synthase (NOS) inhibitor. Addition of ET-1 (10−10 mol/L) to the RMIC culture led to increases in intracellular cGMP concentrations through activation of NOS.4. In the presence of ET-1 (10−7 and 10−10 mol/L) and during NOS inhibition, RMIC responded with increased cell proliferation and extracellular matrix (ECM) synthesis. These responses were abolished by BQ-123 (10−6 mol/L), suggesting mediation via the ETA receptor subtype. The proliferative effect of ET-1 was also abolished by atrial natriuretic peptide (10−6 mol/L).5. The present study provides evidence that binding of ET-1 to ETA receptors on RMIC activates several intracellular second messenger systems that mediate cell proliferation and ECM synthesis.6. These results also highlight an important interaction between ET-1 and nitric oxide in the control of RMIC function.