In vivo effects of endothelin-2, endothelin-3 and ETA receptor blockade on arterial,venous and capillary functions in cat skeletal muscle

Ekelund , U. 1994. In vivo effects of endothelin-2, endothelin-3 and ET_A receptor blockade on arterial, venous and capillary functions in cat skeletal muscle. Acta Physiol Scand 150, 47–56. Received 31 March 1993, accepted 25 May 1993. ISSN 0001–6772. Department of Physiology & Biophysics, University of Lund, Sweden. This study describes, in quantitative terms, the effects of endothelin-2 and endothelin-3 on vascular tone (resistance) in large-bore arterial resistance vessels (> 25 /μm), small arterioles (< 25 μm) and the veins, as well as on capillary pressure and fluid exchange in cat gastrocnemius muscle in vivo. Infusion of endothelin-2 or endothelin-3 (200–1600 ng kg^-1 min^-1, i.a.) elicited an initial transient dilation, followed by a dose-dependent, slowly developing constrictor response, being maintained after cessation of the infusion. At the dose of 400 ng kg^-1 min^-1 (n= 9), infused i.a. during 20 min, endothelin-2 caused an average increase in total regional vascular resistance of 80%, and endothelin-3 of 35%, and the site of constrictor action of both peptides was preferentially located to the small arterioles. Endothelin-2 also constricted the veins and, hence, evoked a pronounced capacitance response, whereas endothelin-3 was devoid of any venoconstrictor effect. This difference, via effects on the pre-/post-capillary resistance ratio, led to a more pronounced fall of capillary pressure in response to endothelin-3 than to endothelin-2. The new specific competitive ET_A receptor antagonist, FR 139317, abolished the vasoconstrictor response to both endothelin-2 and endothelin-3 in vivo, whereas the preceding vasodilator responses were unaffected. These results, taken together with those of our previous analogous study of the effects of endothelin-1, indicated that all three endothelins were approximately equally as effective in eliciting the transient dilator response in skeletal muscle in vivo, whereas the order of vasoconstrictor activity was endothelin-1 > endothelin-2 > endothelin-3. Due to an especially pronounced venoconstrictor activity of endothelin-1, this peptide, in contrast to endothelin-2 and -3, evoked a rise in capillary pressure, with a consequent net transcapillary fluid filtration and muscle tissue oedema formation. The results further indicated that the vasoconstrictor responses to all endothelins in skeletal muscle were mediated by the ET_A receptor, whereas the initial transient vasodilator responses seemed to be mediated by the ET_B receptor.     

Effects of glyceryl trinitrate,nitroprusside and nitric oxide on arterial,venous and capillary functions in cat skeletal muscle in vivo

The aim of the present study was to analyse quantitatively, on a cat gastrocnemius preparation in vivo, the effects of i.a. or i.v. administered glyceryl trinitrate (GTN), sodium nitroprusside (SNP) or nitric oxide (NO dissolved in saline) on vascular resistance (tone) in the following consecutive vascular sections: Large-bore arterial resistance vessels (> 25 μm), small arterioles (< 25 μm), and the veins. Effects on hydrostatic capillary pressure (P_cv) and transcapillary fluid exchange were simultaneously recorded. Close-arterially infused GTN (1–4096μg kg tissue^-1 min^-1), SNP (0.5–32 %mUg kg tissue^-1 min^-1) and NO (0.14-0.82 mg kg tissue^-1 min^-1) elicited a generalized dose-dependent dilator response in all three sections, though with a preferential action on the arterial side. Further, these agents caused an increase in P_cv and transcapillary fluid filtration. The sites of action along the vascular bed of these exogenous vasodilators differed from that previously established for endogenous EDNO. Infusion of GTN, SNP and NO during EDNO blockade (L-NAME) could, therefore, not restore the vascular resistance distribution to that prevailing in the initial control state. Myogenic vascular reactivity to standardized transmural pressure stimuli was clearly depressed by GTN and SNP. Intravenously infused GTN (4–512 μg kg body wt^-1 min^-1) and SNP (4–64μg kg body wt^-1 min^-1) decreased arterial pressure and elicited, via reflex sympathetic activation, a dose-dependent vasoconstriction in skeletal muscle, a decrease in P_cv, and net transcapillary fluid absorption. The constrictor response thus overruled the direct dilator effect of the drugs. The plasma volume expansion known to result from long-term systematic administration of nitrovasodilators seems in part to be caused by transcapillary fluid absorption in skeletal muscle.     

Effect of C-peptide administration on whole body glucose utilization in STZ-induced diabetic rats

Recent studies suggest that C-peptide stimulates glucose transport in isolated skeletal muscle. In order to determine the effect of C-peptide on whole body glucose utilization, streptozotocin (60 mg kg^-1) (STZ)-induced diabetic and normal rats were studied using the euglycaemic clamp procedure and continuous infusion of somatostatin (1.0 μg kg^-1 min^-1) in pentobarbital-anaesthetized rats. Plasma insulin levels during the 6.0- and 30.0-mU kg^-1 min^-1 insulin infusions rose to 70–90 μU mL^-1 and 500–700 μU mL^-1, respectively. Blood glucose concentrations were clamped at 7.5–7.9 mmol L^-1 in the diabetic rats and at basal levels or 7.7 mmol L^-1 in the non-diabetic (normal) rats. Biosynthetic human C-peptide (0.5 nmol kg^-1 min^-1) was infused in 12 diabetic and 11 normal rats, resulting in concentrations of 26–41 nmol L^-1. The metabolic clearance rate of glucose (MCR) for the diabetic rats receiving C-peptide (12.0±1.0 mL kg^-1 min^-1) was significantly (P<0.01) higher than that in the diabetic rats given saline (6.3±0.7 mL kg^-1 min^-1) or a randomly scrambled C-peptide (7.8±1.3 mL kg^-1 min^-1) at low-dose insulin infusion but not at the high-dose insulin infusion. In normal rats C-peptide did not significantly increase the MCR for glucose. These results thus demonstrate that C-peptide has the capacity to increase glucose utilization in STZ-induced diabetic rats.     

In-vivo effects of endothelin-1 and ETa receptor blockade on arterial,venous and capillary functions in skeletal muscle

Results from in vitro studies have indicated that endothelin-1 is a main candidate for endothelium-derived contracting factors. The aim of this in vivo study was to describe in quantitative terms the effects of endothelin-1 (ET-1), and of ET_a receptor blockade, on vascular tone (resistance) in large-bore arterial resistance vessels (> 25 μm), small arterioles (< 25 μm) and the veins, as well as on capillary pressure and fluid exchange in cat gastrocnemius muscle. Endothelin-1 (100–1600 ng kg-1 min-1, i.a.) elicited, after an initial transient dilation, a strong dose-dependent constrictor response in all three consecutive vascular sections, yet with a preferential action on the small arterioles and the veins. The vasoconstriction developed very slowly over about 1 h and was also long-lasting after cessation of the infusion. Our main quantitative analysis refers to effects elicited by 20 min long i.a. infusions of ET-1 at a dose of 400 ng kg-1 min-1. At the end of this period, the peptide caused, on average, a three-fold increase in total regional vascular resistance, in turn explained by a 70% increase in large-bore arterial resistance, a 280% increase in arteriolar resistance and a 220% increase in venous resistance. The latter effect was also manifested as a pronounced capacitance response, and as a decrease in the pre- to post-capillary resistance ratio leading regularly to a rise in capillary pressure, net transcapillary fluid filtration and oedema formation which is unusual for a vasoconstrictor. The new specific competitive ET_A receptor antagonist FR 139317 was found to be fully effective in vivo, insofar as it abolished the constrictor response to endothelin-1. ET_A receptor blockade, or administration of phosphoramidon, an inhibitor of ET-1 production, did not influence the level of basal vascular tone, indicating no significant endogenous release of ET-1 under resting conditions. This contrasts to the established pronounced endogenous release of endothelium-derived nitric oxide. Finally, vascular myogenic regulation was found not to be mediated by ET-1. The results, taken together, suggest a possible role of ET-1 in long-term, rather than short-term, regulation of vascular tone in vivo, perhaps especially during pathophysiological conditions.     

Angiotensin II infusion during β-adrenergic stimulation by isoproterenol. Effects on hepatic,splenic and cardiac blood volumes and on the magnitude and distribution of cardiac output in the dog

The cardiac and peripheral vascular adjustments to angiotensin II (0.1–0.2 μg kg^-1 min^-1 i.v.) during high β-adrenergic activity by a continuous isoproterenol infusion (0.2–0.3 μg kg^-1 min^-1 i.v.) were examined in anaesthetized, atropinized dogs. Hepatic, splenic and left ventricular (LV) volume changes were estimated by an ultrasonic-technique, and the blood flow distribution was measured by injecting radioactive microspheres and by electromagnetic flowmetry on the caval veins, the hepatic artery and the portal vein. During isoproterenol infusion, angiotensin II increased the systolic LV pressure by 45 ± 3 mmHg and the stroke volume by 17 ± 6 %. Concomitantly, the hepatic and splenic blood volumes declined by 29 ± 4 and 14 ± 6 ml, respectively, and the LV end-diastolic segment length increased by 3 ± 1 %. The flow through the inferior caval vein increased by 39 ± 9%, whereas the superior vena caval flow remained unchanged. The hepatic arterial flow more than doubled. Thus, at high inotropy by isoproterenol infusion, angiotensin II relocates blood from the liver and the spleen towards the heart. By activating the Frank-Starling mechanism, cardiac output is increased and conducted through the lower body, especially through the hepatic artery, because of the poor autoregulation of flow through this vessel.,     

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.     

Influence of intravenously administered catecholamines on cerebral oxygen consumption and blood flow in the rat

In order to study effects of catecholamines on cerebral oxygen consumption (CMRo₂) and blood flow (CBF), rats maintained on 75 % N₂O and 25 % O₂, were infused i.v. with noradrenaline (2, 5, or 8 μpg. kg^-1. min^-1) or adrenaline (2 or 8, μg. kg^-1.min^-1) for 10 min before CBF and CMR_oz were measured. In about 50% of animals infused with 2–8, μg. kg^-1 min^-1 of noradrenaline, CMRo_z (and CBF) rose. However, there was no dose-dependent response, and CMRo₂, did not exceed 150% of control. The effects of noradrenaline in a dose of 5 μg. kg^-l. min^-1 on CMRo₂, and CBF were blocked by propranolol (2.5μg.kg^-1). In animals infused with adrenaline (8 μg.kg^-1.min^-1) CMRo₂, was doubled and, in many, CBF rose 4- to 6-fold. It is concluded that, when given in sufficient amounts, catecholamines have pronounced effects on cerebral metabolism and blood flow, the effects of adrenaline on CMRo₂, and CBF resembling those observed in status epilepticus.     

Calciuresis elicited by spontaneous rehydration in dehydrated sheep

Cardiovascular and renal responses to a step-up infusion of endothelin-1 (ET-1) (1, 5, and 15 ng kg^-1 min^-1) were investigated in conscious dogs. In addition, the disappearance of ET-l in arterial and central venous plasma after an infusion of 10 ng kg^-1 min^-1 was quantified, and the effects of vasopressin (AVP, 10 ng kg^-1 min^-1) and angiotensin II (AII, 2, 5, and 10 ng kg^-1 min^-1) on plasma ET-1 were investigated. The step-up infusion of ET-1 increased the plasma level from 3.6 ± 0.3 to 243 ± 23 pg ml^-1. Concomitantly, arterial blood pressure increased and heart rate (HR) decreased dose-dependently. Diuresis, sodium, and potassium excretion did not change significantly. However, free water clearance increased during the infusion. Clearance of creatinine and excretion of urea decreased (39 ± 4 to 29 ± 3 ml min^-1 and 87 ± 16 to 71 ± 14 μmol min^-1, respectively). Decay curves for ET-1 in venous and arterial plasma were identical, and initial t_½ was 1.1 ± 0.1 min. Vasopressin increased arterial blood pressure (107 ± 4 to 136 ± 3 mmHg) beyond the infusion period and increased plasma ET-1 (85%). An equipressor dose of AII tended to decrease plasma ET-1. It is concluded that the lung is apparently not important in the removal of ET-1, that the disappearance of ET-1 follows a complex pattern, and vasopressin – in contrast to angiotensin II – is able to increase the plasma concentration of ET-1. The latter may suggest that ET-1 is involved in the prolonged pressor action of AVP observed.     

Effect of vasoactive intestinal polypeptide (VIP) upon myometrial blood flow in non-pregnant rabbit

Vasoactive intestinal polypeptide (VIP) containing nerve fibres have previously been demonstrated in the female genital tract of several mammalian species including the rabbit. These nerve fibres seemingly innervate vascular and non-vascular smooth muscle. For that reason we investigated the dose-relationship between VIP (5, 50, 500 pmol min^-1 kg^-1) and myometrial blood flow (MBF) using Xenon-133 washout technique. VIP increased MBF dose-dependently. VIP was on molar base 100 times more potent than acetylcholine. The action of VIP seems to be direct on vascular smooth muscle rather than mediated by other neurotransmitters, because the MBF increase was not antagonized by atropine, adrenergic blocking agents or naloxone. These findings make it likely that VIP plays a role in the local nervous control of myometrial blood flow.     

Influence of angiotensin II, α- and β-adrenoceptors on peripheral noradrenergic neurotransmission in canine gracilis muscle in vivo

Interactions between angiotensin II and adrenoceptor-mediated effects on peripheral sympathetic neurotransmission were investigated in constant flow blood-perfused canine gracilis muscle in situ, without and with pretreatment by non-competitive α- adrenoceptor blockade. Angiotensin converting enzyme (ACE)-inhibition by benazeprilat increased nerve stimulation (2 Hz, 4 min)-evoked noradrenaline (NA) overflow (+21 ± 5 yo) with α- adrenoceptors intact, but reduced NA overflow (– 18 ± 6%) when α-adrenoceptors were blocked. Vasoconstrictor responses were slightly reduced by benazeprilat. Subsequent infusion of angiotensin II (Ang 11, 20 and 500 ng kg_-1 min_-1 i.v., raising arterial concentrations from 0.6 ± 0.2 PM to 1390 ± 240 and 25 110 ± 3980 PM, respectively) failed to increase NA overflow or to enhance stimulation-evoked vasoconstriction. Adrenaline (0.4 nmol kg_-1 min_-1 i.v.) did not change evoked NA overflow before or after benazeprilat, either with or without α-adrenoceptor blockade, despite high concentrations (± 10 nM) in arterial plasma. Following benazeprilat, propranolol reduced NA overflow (–24 ± 3 yo) only if the α-adrenoceptors were blocked. In conclusion, benazeprilat reduced evoked NA overflow in the presence of α- adrenoceptor blockade to a similar degree as previously shown in the presence of neuronal uptake inhibition in this model. However, contrasting to our previous findings, benazeprilat enhanced NA overflow and reduced the post-junctional response to nerve stimulation in the absence of α-adrenoceptor blockade. This could be related to bradykinin accumulation during ACE-inhibition, in addition to the reduction of Ang II generation. Our data are not compatible with facilitation of NA release by circulating Ang II even at pharmacological dose levels. Although activation of prejunctional β-adrenoceptors may facilitate evoked NA overflow in this model, circulating adrenaline is ineffective under physiological conditions even after α-adrenoceptor blockade. Also, β-adrenoceptor-mediated prejunctional effects do not seem to involve Ang II in canine skeletal muscle in vivo.     

Sympathetic a-adrenergic control of large-bore arterial vessels,arterioles and veins,and of capillary pressure and fluid exchange in whole-organ cat skeletal muscle

The sympathetic nervous control of the vascular bed of cat gastrocnemius muscle was studied with a new whole-organ technique which permits simultaneous, continuous and quantitative measurements of capillary pressure (P_c), capillary fluid exchange and resistance reactions in the whole vascular bed and in its three consecutive sections: large-bore arterial vessels (> 25 μm), arterioles (< 25 μm) and veins. The results demonstrated a distinct neural control of all three consecutive vascular sections, graded in relation to the rate of nerve excitation up to maximum at 16 Hz. Stimulation at high rates, which in the steady state caused an average rise of overall regional resistance from 15.3 to 120 PRU (7.8-fold increase), thus raised large-bore arterial vessel resistance from 8.8 to 64 PRU (7.3-fold increase), arteriolar resistance from 4.5 to 49 PRU (10.9-fold increase) and venous resistance from 2.0 to 7 PRU (3.5-fold increase). The rate of resistance development (PRU s^-1) of the sympathetic constrictor response was much higher in the arteriolar than in the other sections, which indicates that the neural control is especially prompt and efficient in the arterioles. A passive component was shown to contribute to the described responses only on the venous side, but in no case by more than 10% of the total sympathetic venous resistance response, which thus is mainly active. Of special functional importance was that the new technique provided information about the adrenergic control of P_c, in absolute figures. From the control value of 19 mmHg, graded sympathetic stimulation caused a graded decline in P_c, at maximum constriction by about 7 mmHg. This resulted in marked net transcapillary fluid absorption, in turn increasing plasma volume.     

l-Arginine-induced hypotension in the rat: evidence that NO synthesis is not involved

l -Arginine is the biological precursor for nitric oxide (NO). NO is formed continuously in endothelial cells and maintains a certain degree of vasodilator tone under physiological conditions. Although the formation of NO is not primarily controlled by precursor availability, the extent to which extra supplementation with l -arginine may affect endothelial NO formation, and hence, vasodilator tone and systemic blood pressure, is not entirely clear. To address this issue, we infused l -arginine i.v. in anaesthetized normotensive rats pretreated with N^G-nitro-l -arginine methyl ester (l -NAME, 50 or 200 mg^-1) and in untreated controls, under continued recording of mean arterial pressure (MAP). In control animals l -arginine (25 or 100 mg kg^-1 min^-1) had no effect on systemic MAP (111 ± 3 mm Hg), while l -arginine (200 mg kg^-1 min^-1) lowered MAP (to 70 ± 6mmHg). D-Arginine (200 mg kg^-1 min^-1) also induced hypotension; during infusion of D-arginine MAP fell from 106 ± 4 to 64 ± 4 mm Hg. Pretreatment with l -NAME (50 and 200mgkg^-1) elevated MAP to 140 ± 2 and 147 ± 3 mm Hg, respectively, but failed to affect the hypotensive response to l -arginine; during infusion of l -arginine (200 mg kg^-1 min^-1) in rats pretreated with l -NAME (50 and 200 mg kg^-1) MAP fell to 86 ± 9 and 104 ± 6 mm Hg, respectively. Plasma levels of the NO metabolite, nitrate, were 18 ± 4 and 17 ± 3μmol l^-1, respectively, before and after infusion of l -arginine (200 mg kg^-1 min^-1). Trapping of NO to haemoglobin (HbNO) could not be detected, either before or after infusion of l -arginine (200 mg kg^-1 min^-1). We conclude that a high dose of l -arginine may act hypotensive in normotensive rats. This effect does, however, not seem to be based on an augmented formation of NO.     

血管紧张素Ⅱ受体拮抗剂对高血压肾小动脉重建的影响

目的:研究血管紧张素II(AngII)受体拮抗剂对高血压肾小动脉重建的影响。方法:18只4周龄雄性大鼠分为:正常血压大鼠(WKY)组、自发性高血压大鼠(SHR)组、SHR口服losartan组,均饲养至16周。在肾组织切片上分别用光镜和电镜配合计算机图像分析法观测肾组织内小动脉的几何形态学指标和小动脉平滑肌及其间隙,离体肾脏灌流法测定最小肾血管阻力。结果:Losartan组的尾动脉收缩压、肾小动脉壁厚、壁面积、壁厚内径比和中层血管平滑肌细胞宽度以及最小肾血管阻力,均显著低于高血压对照组。结论:AngII受体拮抗剂losartan能预防SHR肾小动脉的重建。     

Effects of angiotensin ll on blood flow in rat submandibular gland

The effect of angiotensin II (Ang II) was studied on blood flow in the submandibular gland and tongue in male rats. Blood flow changes were determined with laser Doppler flowmetry and Ang II was infused into the common carotid artery before and after i.v. doses (18 nmol kg^-l) of the angiotensin II antagonist saralasin. Angiotensin II (10–60 pmol min^-l) dose-dependently increased blood pressure and tongue blood flow, whereas glandular blood flow decreased at all of the doses used. After saralasin administration the angiotensin II effects on blood pressure, tongue and glandular blood flow were significantly diminished (glandular blood flow reduction was diminished from 29%-3%, P < 0.005, n = 9). However, the responsiveness of these 3 parameters to local infusions with noradrenaline (0.75–3.0 pmol min^-1) was unaffected by saralasin. The dose of saralasin used in the present study did not affect any of the parameters on it's own. Our results show that vascular receptors sensitive to angiotensin II operate in the submandibular gland but not in the tongue.     

Effect of atrial natriuretic factor on renal prostaglandin E2 release in the anaesthetized dog

Experiments were undertaken in two groups of barbiturate anaesthetized dogs to examine whether atrial natriuretic factor (ANF) exerts an effect on renal release of prostaglandin E₂ (PGE₂). In the first group, intravenous infusion of ANF (50 ng min^-1kg^-1body wt) reduced basal PGE₂ release from 4.4 ± 0.8 pmol min^-1to 1.8 ± 0.7 pmol min^-1. In the second group, intrarenal infusion of an α-adrenoceptor agonist, phenylephrine (2.5–6.75 μg min^-1), raised PGE₂ release from 2.7 ± 0.5 pmol min^-1to 7.5 ± 1.3 pmol min^-1. During continuous α₁-adrenergic stimulation, intravenous infusion of ANF (100 ng min^-1kg^-1body wt) reduced PGE₂ release to 3.5 ± 1.0 pmol min^-1. These results demonstrate that ANF reduces basal and α₁-adrenergic stimulated renal PGE₂ release.     

Role of endothelium-derived nitric oxide in the regulation of tonus in large-bore arterial resistance vessels, arterioles and veins in cat skeletal muscle

The role of endothelium-derived nitric oxide in the regulation of vascular resistance (tonus) in cat skeletal muscle was studied with the use of NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide formation from L-arginine. The study was performed with a whole-organ technique which permits simultaneous, continuous and quantitative recordings of resistance reactions in the whole vascular bed (RT) and in its three consecutive sections: large-bore arterial resistance vessels (greater than 25 microns; Ra,prox), small arterioles (less than 25 microns; Ra,micro) and veins (Rv). NG-monomethyl-L-arginine (3-100 mg kg-1 tissue, i.a.) induced a dose-dependent increase in resistance that was preferentially, but not selectively, confined to the large-bore arterial resistance vessels. At a maximally effective dose (100 mg kg-1), the nitric oxide inhibitor caused a marked constriction, within 5 min, on average increasing RT by 99%, Ra,prox by 138%, Ra,micro by 18% and Rv by 23%. The constrictor response to NG-monomethyl-L-arginine was long-lasting but disappeared gradually over a period of about 1 h. However, it could be abruptly abolished by excess L-arginine (300 mg kg-1, i.a.). The vasodilator response (RT) to acetylcholine was significantly attenuated in the presence of NG-monomethyl-L-arginine compared with the control response. The results suggested that nitric oxide formation from L-arginine by the vascular endothelium plays a fundamental role in the regulation of vascular resistance (tone) in vivo, with its main site of action located in the large-bore arterial resistance vessels.     

Effects on renal sodium and potassium excretion of vasopressin and oxytocin in conscious dogs

Renal effects of arginine vasopressin and oxytocin were studied in conscious dogs, made water-diuretic by a waterload equivalent to 2% of body weight. Body water and content of sodium were maintained by separate servo-controlled infusions. Peptides were infused for 60 min at rates of 50 pg kg^-1 min^-1 (arginine vasopressin) or 1 ng kg^-1 min^-1 (oxytocin), either separately or combined. Infusions increased plasma arginine vasopressin to 1.9 ± 0.2 (arginine vasopressin alone) and 1.8 ± 0.3 pg kg^-1 (arginine vasopressin plus oxytocin and plasma oxytocin to 72 ± 5 (oxytocin alone) and 77 ± 8 pg ml^-1 (oxytocin plus arginine vasopressin). Arginine vasopressin or arginine vasopressin plus oxytocin increased urine osmolality similarly by a factor of 13, decreased urine flow to between 5 and 7% of control and decreased free water clearance. Oxytocin reduced urine flow and free water clearance and increased urine osmolality by a factor of 2. Oxytocin and arginine vasopressin separately increased excretion of sodium from 4 ± 2 to 15 ± 6 μmol min^-1 and from 7 ± 4 to 25 ± 13 μmol min^-1, respectively. Arginine vasopressin plus oxytocin led to a pronounced natriuresis (13 ± 4 to 101 ± 27 μmol min^-1). Arginine vasopressin and arginine vasopressin plus oxytocin increased the excretion of potassium by a factor of 2.5. Oxytocin and arginine vasopressin plus oxytocin increased urinary Na⁺/K⁺ ratio by a factor of 3.7. It is concluded, that oxytocin at plasma concentrations of 70–80 pg ml^-1 has modest antidiuretic and natriuretic effects and that the combined action of arginine vasopressin oxytocin may elicit supra-additive natriuretic effects.     

Adenosine triphosphate treatment for meconium aspiration-induced pulmonary hypertension in pigs

To investigate the pulmonary haemodynamic effects of meconium aspiration and subsequent adenosine triphosphate (ATP) treatment, 12 anaesthetized and ventilated pigs (wt 24-28 kg) received either ATP or an equal volume of saline into the right heart in doses of 0.02 to 0.80 lmol kg^-1 min^?1 after intratracheal administration of 2 mL kg^?1 of human meconium. Meconium instillation induced significant increases in pulmonary vascular pressures and total and postarterial resistances calculated from pulmonary artery occlusion studies, but did not affect the systemic haemodynamics, except for a fall in heart rate and increase in central venous pressure. Infusion of ATP at the lowest doses (0.02 and 0.08 µmol kg^?1 min^?1) selectively decreased the pulmonary arterial pressure and vascular resistance and at 0.32 and 0.80 µmol kg^?1 min^?1 reduced both the pulmonary and systemic resistances. In the lung circulation the increasing doses of ATP reduced preferably the arterial, but also the postarterial resistance. Withdrawal of ATP infusion led to a significant rebound effect especially in the postarterial segment of the lung circulation. Meconium aspiration thus induces an acute, predominantly postarterial obstruction in the lung circulation and infusion of ATP at low doses selectively dilates the pulmonary vascular bed and may help to preclude elevation of capillary pressures in meconium aspiration-induced pulmonary hypertension.     

α 2 –Adrenoceptor activation may trigger the increased production of endothelium–derived nitric oxide in skeletal muscle during acute haemorrhage

Our previous studies indicated that acute haemorrhage leads to a pronounced increase in the release of endothelium-derived nitric oxide (EDNO) graded in relation to the magnitude of the blood loss. The EDNO-induced vasodilatation, confined selectively to the arterial `feeder' vessels, attenuates the concomitant reflex adrenergic constriction and thereby prevents deleterious reduction of blood flow. The present study aimed at investigating whether the reflex release of blood-borne catecholamines might trigger this EDNO release via activation of endothelial α₂-adrenoceptors. The study was performed on the sympathectomized vascular bed of cat skeletal muscle with a technique permitting quantitative recordings of resistance (tone) in consecutive vascular sections. Selective α₂-adrenoceptor blockade with idazoxan applied at steady state vasoconstriction after a 35% blood loss evoked an initial generalized dilator response (attributable to inhibition of post-synaptic smooth muscle α₂-adrenoceptors), followed by a constrictor response selectively in the arterial feeder vessels, the latter compatible with the hypothesis of reduced EDNO release by α₂-adrenoceptor blockade. More direct evidence for the hypothesis was obtained from studies of the vascular response to EDNO blockade ( L -NAME) after haemorrhage in the presence and absence of α₂-adrenoceptor blockade. The constrictor response to EDNO blockade, which is a measure of the pre-existing EDNO dilator influence (EDNO production), was significantly smaller (P < 0.01) in the presence than absence of α₂-adrenoceptor blockade. The results indicate that blood-borne catecholamines, via activation of endothelial α₂-adrenoceptors, trigger the increase in the EDNO release in acute haemorrhage, implying a functionally important negative feedback in the integrated control of vascular tone in bleeding.     

Systemic and centrally mediated angiotensin II effects in the horse

The primary aim of the study was to evaluate the potential value of intravenous (i. v.) infusion of angiotensin II (AII) for phonocardiographic differential diagnosis of equine valvular insufficiency. Ten-minute AII infusions at 4.5–33 pmol kg^-1 min^-1 induced clear-cut dose-dependent rises in systemic arterial blood pressure (aBP), whereas the pulmonary aBP remained largely unaffected. It implies that i. v.infusion of All at about 10 pmol kg^-1 min^-1 could be a valuable tool for the acoustic differentiation between mitral and tricuspid valvular dysfunction in the horse. The infusion at, and above 9 pmol kg^-1 min^-1 caused increased heart rate. This chronotrophic effect was not strictly dose-dependent and exhibited significant tachyphylaxis. Angiotensin II administration at, or above 9 pmol kg^-1 min^-1 was needed to induce an urge to drink, suggesting that angiotensin-induced thirst does not appear in the euhydrated horse until the octapeptide reaches supraphysiological blood concentration. Determinations of plasma aldosterone concentration (PA) revealed comparatively high morning control values (269 ± 46 pmol^-1).Three consecutive AII infusions with 10-min intervals and at increasing dosages caused a cumulative, almost fourfold elevation of PA.The PA pattern indicated that AII-induced hypersecretion of aldosterone continued for several minutes after the end of the infusions, but also showed that the metabolic clearance of the hormone took precedency of the secretion within 20 min post-infusion. In two of the horses a fall in PA occurred during a fourth, final infusion, indicating that in these instances the previous AII administration had impoverished the store of aldosterone available for release from the adrenal cortex.