Miklos Gellai

Reversal of postischemic acute renal failure with a selective endothelinA receptor antagonist in the rat.

Studies were designed to examine the effect of a selective endothelinA (ETA) receptor antagonist, BQ123, on severe postischemic acute renal failure (ARF) in Sprague-Dawley rats. Severe ARF was induced in uninephectomized, chronically instrumented rats by 45-min renal artery occlusion. BQ123 (0.1 mg/kg.min) or vehicle was infused intravenously for 3 h on the day after ischemia. Measurements before infusion (24 h control) showed a 98% decrease in glomerular filtration rate (GFR), increase in fractional excretion of sodium from 0.6 to 39%, and in plasma K+ from 4.3 to 6.5 mEq/liter. All vehicle-treated rats died in 4 d because of continuous deterioration of renal function, resulting in an increase of plasma K+ to fatal levels (> 8 mEq/liter). Infusion of BQ123 significantly improved survival rate (75%) by markedly improving tubular reabsorption of Na+ and moderately increasing GFR and K+ excretion. Plasma K+ returned to basal levels by the 5th d after ischemia. Improved tubular function was followed by gradual recovery in GFR and urinary concentrating mechanism. Additional data from renal clearance studies in rats with moderate ARF (30-min ischemia) and in normal rats with intact kidneys showed that ETA receptor blockade increases Na+ reabsorption and has no effect on renal hemodynamics. These results indicate that in the rat, the ETA receptor subtype mediates tubular epithelial function, and it plays a significant role in the pathogenesis of ischemia-induced ARF. Treatment with the selective ETA receptor antagonist reverses deteriorating tubular function in established ARF, an effect of possible therapeutic significance.

Evidence for Direct Control of Coronary Vascular Tone by Oxygen

The interaction between the effects of oxygen and adenosine on acetylcholine-induced contractile tension was observed in helical coronary arterial strips suspended in physiological salt solution. It was found that (1) steady-state contractile tension was unaffected or depressed 5–20% when oxygen pressure (Po2) was diminished to levels as low as 5–10 mm Hg, (2) contractile tension was markedly depressed at a Po2 of 0 mm Hg, (3) adenosine-induced relaxation of contractile tension was inversely proportional to Po2 in the tissue bath, and (4) in the presence of adenosine at a concentration just adequate to inhibit contractile tension at a Po2 of 10 mm Hg, contractile tension was directly proportional to bath Po2. The latter two observations were usually most apparent at a Po2 between 10 and 40 mm Hg. It is proposed that the vasodilating effect of adenosine on the intact coronary vasculature may be most effective during myocardial hypoxia and that physiological control of coronary vascular tone may be more closely related to variations in local Po2 than to variations in local concentrations of adenosine.

Influence of oxytocin on renal hemodynamics and sodium excretion.

Acute administration of physiological doses of synthetic OT to conscious Long-Evans and Brattleboro homozygous diabetes insipidus rats produced a modest increase in GFR and effective filtration fraction. Chronic administration of OT to DI rats for 9 days in dosages that were antidiuretic (plasma OT ca. 100 pg/ml) increased both GFR and ERPF by 40%. Table 1 summarizes these renal hemodynamic changes and compares them to the renal effects of VP. Further investigation is needed to define the mechanisms responsible for the changes in GFR and/or ERPF produced by acute and chronic administration of OT to conscious rats. Acute administration of physiological doses of synthetic OT to conscious LE and DI rats also produced a brisk natriuresis with a marked increase in the fractional excretion of sodium. A natriuresis was also observed in conscious Sprague-Dawley rats administered physiological amounts of OT by subcutaneous osmotic minipump. The natriuretic effect of the hormone was short lived, however, being observed only during the first 24-hr period of treatment. The nephron site where OT exerts its natriuretic action, either directly or indirectly, is unknown. Renal prostaglandins may contribute to OT-induced natriuresis, but other mechanisms such as increased renal production of nitric oxide and cGMP have not been tested. Although the natriuretic response to OT has also been described for conscious dogs, it probably does not occur in humans and nonhuman primates. Precise localization of specific renal OT receptors has recently been reported for the rat. OT receptors were identified in the macula densa cells of the adult, rat kidney. This location suggests a possible role for OT in the regulation of tubuloglomerular feedback and solute transport. The signal transduction of the renal OT receptor has been recently evaluated in various kidney epithelial cells in culture. OT stimulates phosphoinositide hydrolysis and increases cytosolic calcium concentrations. In fact, VP produces similar cellular responses in renal epithelia, possibly through the OT receptor. Also, OT stimulates soluble guanylate cyclase and increases intracellular cGMP. Whether OT activates soluble guanylate cyclase secondarily through the production of nitric oxide has not been tested. An important role for OT in renal sodium homeostasis under basal conditions is likely, at least for the rat. Moreover, OT possibly mediates dehydration natriuresis in lower animal species. The contribution of OT to renal physiology in humans and in nonhuman primates, if any, remains uncertain.

Antihypertensive activity of the non-peptide angiotensin II receptor antagonist, SK&F 108566, in rats and dogs

SummaryThe antihypertensive activity of the nonpeptide angiotensin II receptor antagonist, SK&F 108566 (E) - α - [[2 - butyl -1 - [(4 - carboxyphenyl)methyl] -1H -imidazol-5-yl]methylene]-2-thiophene propanoic acid, was examined in rats and dogs. SK&F 108566 produced dose-dependent decreases in blood pressure in renin-dependent hypertensive rats. At 10 mg/kg intraduodenally, mean arterial blood pressure fell from between 150–160 mm Hg to approximately 124 mm Hg. A sustained infusion of SK&F 108566 at 25 μg/min intraduodenally normalized blood pressure during 3 days of infusion and for 18 h following cessation of the infusion. Evaluation of the systemic hemodynamic effects of SK&F 108566 in chronically instrumented renin-dependent hypertensive rats demonstrated that the antihypertensive effects of SK&F 108566 were accompanied by a significant increase in cardiac output with little change in stroke volume. In dogs made acutely hypertensive by an intravenous infusion of angiotensin 1, SK&F 108566 resulted in dose-dependent decreases in blood pressure. The antihypertensive activity of SK&F 108566 at 10 mg/kg p.o. was maintained for between 13–15 h, a similar duration of action as observed with enalapril (1 mg/kg, p.o.). Administration of DuP 753 (losartan) intravenously caused a small and short-lived fall in blood pressure in the angiotensin I-infused hypertensive dog. However, the active metabolite of losartan, EXP 3174, resulted in a response of longer duration. In dogs made hypertensive by placement of an ameroid constrictor on the left renal artery, SK&F 108566 (10 mg/kg, p.o.) or enalapril (1 mg/ kg, p.o.) resulted in antihypertensive responses of at least 12 h duration. The data indicate that SK&F 108566 is a long-acting antihypertensive agent in the rat and dog.

BQ-123, a selective endothelin subtype A-receptor antagonist, lowers blood pressure in different rat models of hypertension

Objective To investigate the role of endothelin-1 in the pathogenesis of hypertension directly by using the selective endothelin subtype A-receptor antagonist BQ-123. Methods The antihypertensive and hemodynamic effects of sustained BQ-123 administration were examined in conscious, unrestrained spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto (WKY) rats and renin hypertensive rats. Results Sustained infusions of BQ-123 (0.16–164 nmol/kg per min, intravenously, for 6h) produced dose-dependent reductions in mean arterial pressure in SHR, the maximal reduction being obtained with a dose of 16 nmol/kg per min. This reversible response was evident up to 14 h after the cessation of antagonist infusion. The antihypertensive response to a maximal dose of BQ-123 was associated with bradycardia, but only a minimal reduction in cardiac output (since the stroke volume was elevated) in the SHR. Therefore, the antihypertensive effect of BQ-123 resulted from a decrease in total peripheral resistance. In contrast, in WKY rats the infusion of the high dose (164 nmol/kg per min, intravenously for 6 h) produced a small but significant reduction in mean arterial pressure. BQ-123 did not alter the pressor response or tachycardia observed in pithed SHR following stimulation of the thoracolumbar sympathetic outflow. BQ-123 was also antihypertensive in renin hypertensive rats, lowering the blood pressure to an extent similar to that observed in SHR. Conclusion The data presented indicate a role for endothelin in the pathophysiology of hypertension.

Contribution of Endogenous Endothelin-1 to the Maintenance of Vascular Tone: Role of Nitric Oxide

Studies were designed to compare the effect of the nitric oxide inhibitor, N omega-nitro-L-arginine (L-NNA), and the novel ETB receptor antagonist, RES-701-1, on changes in blood pressure and renal blood flow induced by exogenous endothelin receptor agonists and to determine the effect of L-NNA on basal hemodynamics in conscious, chronically instrumented rats. Infusion of low (nonpressor) doses of L-NNA or RES-701-1 potentiated systemic and renal vasoconstriction induced by bolus injections of endothelin-1 or sarafotoxin 6c. Bolus intravenous injection or sustained infusion of L-NNA alone resulted in dose-dependent increases in blood pressure and decreases in renal blood flow, similar to our recently reported results with RES-701-1. Vasoconstriction induced by inhibition of nitric oxide was attenuated by SB 209670, a mixed ETA/B receptor antagonist, but not by BQ123, an ETA receptor antagonist; neither antagonist altered basal hemodynamics. Collectively, the results indicate that: (1) endothelin plays an important role in the control of basal vascular tone by mediating both vasodilation and vasoconstriction; (2) these effects are mediated by different ETB receptor subtypes in the rat, one located on the endothelium that mediates vasodilation via the nitric oxide pathway, the other located on the vascular smooth muscle that mediates contraction.

Antihypertensive Actions of the Novel Nonpeptide Endothelin Receptor Antagonist SB 209670

Indirect evidence has implicated endothelin-1 in the pathogenesis of hypertension. In the present study we examined such a role directly with SB 209670, a novel nonpeptide endothelin receptor antagonist. The antihypertensive and hemodynamic effects of SB 209670 were examined in conscious, unrestrained spontaneously hypertensive (SHR), normotensive Wistar-Kyoto (WKY), and renin-hypertensive rats. Sustained intravenous infusion of SB 209670 (10 micrograms.kg-1.min-1 for 6 hours) produced a significant, reversible reduction in mean arterial pressure in SHR but not in WKY rats. The antihypertensive response to 10 micrograms.kg-1.min-1 SB 209670 (approximately 25 mm Hg reduction in blood pressure) was associated with bradycardia (16% decrease in heart rate) but only a minimal reduction (3%) in cardiac output, because stroke volume was evaluated (by 15%). Therefore, the antihypertensive effect of SB 209670 resulted from a decrease (13%) in total peripheral resistance. A sustained antihypertensive effect could also be observed after intraduodenal administration of SB 209670 (3 mg/kg) in conscious SHR (reduction of approximately 35 mm Hg 5 hours after administration). SB 209670 (3 mg/kg intravenous bolus) did not alter the pressor response or tachycardia observed in pithed SHR after stimulation of thoracolumbar sympathetic outflow. SB 209670 was also antihypertensive in renin-hypertensive rats, lowering blood pressure to an extent similar to that observed in SHR. Thus, the data presented provide evidence to support a role for endothelin-1 in the pathophysiology of two animal models of hypertension.

The effect of atrial natriuretic factor on blood pressure, heart rate, and renal functions in conscious, spontaneously hypertensive rats.

Atrial natriuretic factors, polypeptides released by atrial myocytes, may play a role in the control of blood pressure and the regulation of renal salt and water excretion. Our studies were designed to assess the role of a synthetic peptide, atriopeptin II, on blood pressure and heart rate, renal hemodynamics, and salt and water excretion in conscious, spontaneously hypertensive rats and in normotensive Wistar-Kyoto rats. Changes in mean arterial pressure and heart rate were recorded following intravenous bolus injections (0.1, 1.0, 10, 100 μg/kg) of atriopeptin II in 5 spontaneously hypertensive and 5 Wistar-Kyoto rats. In a second group of rats the peptide was infused for 90 minutes in two different doses: low dose, 1 μg/kg + 2 μg/kg/hr; and high dose, 10 μg/kg + 20 μg/kg/hr. Bolus injections of atriopeptin II resulted in dose-dependent decreases in mean arterial pressure in the hypertensive, but not in the normotensive, rats; heart rates remained unchanged. Blood pressure decreased gradually during the sustained infusion of both doses of atriopeptin II, with the spontaneously hypertensive strain showing increased sensitivity compared to the Wistar-Kyoto strain. Heart rate decreased in both strains during infusion of the high dose; the decrease was significant only in the hypertensive rats. The low dose of atriopeptin II increased the clearance of free water in both strains of rate; sodium excretion was increased only in the hypertensive rats. The high-dose atriopeptin II was associated with transient natriuresis, unaltered glomerular filtration rate, and decreased effective renal blood flow in both strains. These results 1) show increased sensitivity of spontaneously hypertensive rats to the hypotensive action of atrial natriuretic factor, 2) do not support the hypothesis that an increased glomerular filtration rate is a prerequisite to the natriuretic effect of atrial peptide, and 3) suggest that atriopeptin II may exert separate and dose-dependent effects on renal water and sodium excretion in conscious rats.

Evidence in Support of Hypoxia but Against High Potassium and Hyperosmolarity as Possible Mediators of Sustained Vasodilation in Rabbit Cardiac and Skeletal Muscle

The vasoactive properties of hypoxia, elevated extracellular potassium concentration ([K]o), and hyperosmolarity were studied in helical strip preparations of small coronary and deep femoral arteries (260–700 μm, o.d.) equilibrated in a physiological salt solution with an oxygen tension (Po2) of 100 mm Hg, [K]o of 3.18 mM, and an osmolarity of 304 milliosmoles/liter. Increasing [K]o (2–6 mM) or osmolarity (30–50 milliosmoles/liter) produced relaxation of resting tension in 50% of the coronary strips but had no effect on resting tension in deep femoral strips. Sustained increments in [K]o or osmolarity produced concentration-dependent, transient relaxation of aganist-induced contractile tension in both coronary and deep femoral arterial strips: a potassium increment of 4 mM produced 40% relaxation with 100% recovery within 5–6 minutes, an osmotic increment of 30 milliosmoles/liter caused 20–40% relaxation with 100% recovery within 15–60 minutes, and simultaneous potassium (4 mM) and osmotic (30 milliosmoles/liter) increments produced 85–95% relaxation with 100% recovery within 10–15 minutes. Decreasing Po2 from 100 mm Hg to 10 mm Hg resulted in a sustained 35–40% fall in agonist-induced contractile tension. Although a nonspecific additive interaction was observed during a simultaneous change to high [K]o, hyperosmolarity, and hypoxia, for any given level of vascular tone hypoxia had little or no effect on the degree or the duration of the tension relaxation-recovery sequence produced by elevated [K]o, hyperosmolarity, or both. Therefore, it is proposed that hypoxia is the only one of these three factors that, by a direct interaction with vascular smooth muscle cells, can contribute to sustained vasodilation of small arteries in rabbit cardiac or skeletal muscle.

Identification and function of putative ETB receptor subtypes in the dog kidney.

Summary: Binding studies performed in dog kidney, lung, and spleen using [125I]endothelin (ET) 3 and a series of ETB-selective ligands indicated the presence of two subtypes of ETB receptors, an IRL-1620-sensitive (putative ETB1) and an IRL-1620-insensitive (putative ETB2) receptor. The IRL-1620-sensitive (but not IRL-1620-insensitive) ETB receptors displayed similar pharmacology to the cloned human ETB receptor and a high affinity for the ETB receptor antagonist RES-701. ETB1 receptors predominated in the dog kidney and lung and ETB2 receptors predominated in the dog spleen. Stimulation of ETB receptors in dogs in vivo inhibited sodium reabsorp-tion, as ET-1 infusion in the presence of the ETA antagonist BQ123, but not the mixed ETA/ETB receptor antagonist, (±)SB 209670, resulted in increased sodium excretion. Furthermore, infusion of the ETB-selective agonist Sarafotoxin S6c (S6c) resulted in an increase in sodium excretion, a response that was attenuated by infusion of RES-701. These data indicate that the putative ETB1 receptor may mediate ET-induced inhibition of tubule sodium reabsorption in the dog. In addition, we observed no putative ETB2 receptor-mediated renal vasoconstriction, consistent with the apparent low abundance of this subtype in the dog kidney.