Glucagon secretory response to hypoglycaemia,adrenaline and carbachol in streptozotocindiabetic rats

Glucagon response to insulin-induced hypoglycaemia is impaired in diabetes, but the mechanism is not established. Pancreatic A cell hyporesponsiveness to adrenergic or cholinergic stimulation could contribute to the impairment. We therefore compared the plasma glucagon responses to intravenous infusion of adrenaline (1200 ng kg^-1 min^-1 for 20 min) or to intravenous injection of the cholinergic agonist carbachol (50 μ kg^-1) in chloral hydrate-anaesthetized rats made diabetic with the use of streptozotocin (80 mg kg^-1 subcutaneously) 6 weeks before and in anaesthetized control rats. Insulin was infused intravenously to reduce plasma glucose levels to below 1.8 mmol L^-1. As expected, the plasma glucagon response was reduced by ～ 45% in streptozotocindiabetic rats compared with controls (P= 0.045). During adrenaline infusion, plasma glucagon levels increased by 277 ± 92 pg mL^-1 in controls (P= 0.009) and by 570 ± 137 pg mL^-1 in the diabetic rats (P= 0.002). Thus, the plasma glucagon response to adrenaline was approximately doubled in the diabetic rats (P= 0.045). Following carbachol injection, plasma glucagon levels were raised by 1211 ± 208 pg mL^-1 (P < 0.001) in controls but only by 555 ± 242 pg mL^-1 in the diabetic rats (P= 0.049). Thus, the plasma glucagon response to carbachol was impaired by ～ 58% in the diabetic rats (P= 0.028). We conclude that carbachol-stimulated glucagon secretion is impaired concomitantly with the impaired glucagon response to hypoglycaemia in streptozotocin-diabetic rats, whereas adrenaline-induced glucagon secretion is exaggerated. We suggest that a reduced pancreatic A cell responsiveness to cholinergic stimulation could contribute to the impairment of the glucagon response to insulininduced hypoglycaemia in diabetes.     

Glucagon-like peptide-1 reduces hepatic glucose production indirectly through insulin and glucagon in humans

The effect of glucagon-like peptide-1 (GLP-1) on hepatic glucose production and peripheral glucose utilization was investigated with or without infusion of somatostatin to inhibit insulin and glucagon secretion in 13 healthy, non-diabetic women aged 59 years. After 120 min 3-³H-glucose infusion, GLP-1 was added (4.5 pmol kg^?1 bolus + 1.5 pmol kg^?1 min^?1). Without somatostatin (n = 6), GLP-1 decreased plasma glucose (from 4.8 ± 0.2 to 4.2 ± 0.3 mmol L^?1, P = 0.007). Insulin levels were increased (48 ± 3 vs. 243 ± 67 pmol L^?1, P = 0.032), as was the insulin to glucagon ratio (P = 0.044). The rate of glucose appearance (R_a) was decreased (P = 0.003) and the metabolic clearance rate of glucose (MCR) was increased during the GLP-1 infusion (P = 0.024 vs. saline). Also, the rate of glucose disappearance (R_d) was reduced during the GLP-1 infusion (P = 0.004). Since R_a was reduced more than R_d, the net glucose flow was negative, which reduced plasma glucose. Somatostatin infusion (500 μg h^?1, n = 7) abolished the effects of GLP-1 on plasma glucose, serum insulin, insulin to glucagon ratio, R_a, R_d, MCR and net glucose flow. The results suggest that GLP-1 reduces plasma glucose levels mainly by reducing hepatic glucose production and increasing the metabolic clearance rate of glucose through indirectly increasing the insulin to glucagon ratio in healthy subjects.     

Acute effects of C-peptide on the microvasculature of isolated perfused skeletal muscles and kidneys in rat

The C-peptide has recently been suggested to have beneficial effects in several organs and improve glycaemic control in human type I diabetes, while there were no such effects in healthy controls. The exact mechanisms behind these effects are, however, not clear. In an attempt to study the actions of C-peptide on the microvasculature in normal rats during more controlled conditions, isolated rat hindquarters and kidneys were perfused with albumin solutions in order to obtain low basal concentrations of C-peptide. In rat hindquarters, infusion of C-peptide significantly increased the capillary filtration coefficients (CFC) from 0.035±0.002 to 0.044±0.002 mL min^-1 100 g^-1 mmHg^-1 (P<0.001, n=9) and the permeability surface area product (PS) for vitamin B₁₂ from 3.48±0.29 to 4.02±0.37 mL min^-1 100 g^-1 (P<0.01, n=6). Addition of C-peptide to the perfusate during infusion of sodium nitroprusside did not induce any additional alteration of CFC or PS. The vascular resistance was slightly decreased from 2.74±0.17 to 2.64±0.17 mmHg min 100 g mL^-1 (P<0.01, n=9). These effects of C-peptide are compatible with increases in capillary surface area without alteration of the permeability per se. In isolated rat kidneys perfused at low temperature (8 °C) prepared to inhibit all metabolic processes, C-peptide induced no changes in glomerular filtration rate, total vascular resistance or fractional albumin clearance. Therefore, C-peptide causes active vasodilation of the normothermic microvasculature and hence recruitment of capillaries. These findings support the previous observations in man that C-peptide indeed has biological effects.     

No role of interstitial adenosine in insulin-mediated vasodilation

The mechanisms behind the vasodilatory effect of insulin are not fully understood, but nitric oxide plays an important role. We have investigated the possibility that insulin mediates vasodilatation in the human skeletal muscle via an increase in extracellular adenosine concentrations. In eight healthy subjects (H) and in four subjects with a complete, high (C5–C6/7) spinal cord injury (SCI) a hyperinsulinaemic (480 mU min^–1 kg^–1), isoglycaemic clamp was performed. SCI subjects were included as it has been proposed that adenosine and adenine nucleotides may be released from nerve endings in the skeletal muscle. Adenosine concentrations in the extracellular fluid (ECF) of skeletal muscle in the thigh were measured by means of the microdialysis technique. Leg blood flow (LBF) was measured by termodilution. In response to insulin infusion, LBF always increased (P < 0.05) (from 228 ± 25 and 318 ± 18 mL min^–1 to 451 ± 41 and 530 ± 29 mL min^–1, SCI and H, respectively [mean ± SEM]). Concentrations of adenosine in the muscle ECF did not change with infusion of insulin and did not differ between groups (before: 147 ± 55 [SCI] and 207 ± 108 [H] nmol L^–1; during: 160 ± 36 [SCI] and 165 ± 74 [H] nmol L^–1). No significant correlation between concentrations of adenosine and corresponding LBF rates was achieved (LBF=[–0.0936 · Adenosine] + 475. R=–0.092, P=0.22, number of samples=181, number of subjects=12). Conclusion: the mechanism by which insulin mediates an increase in skeletal muscle blood flow is not associated with adenosine in the ECF.     

The effect of hyperglycaemia on regional cerebral glucose oxidation in humans studied with [1–11C]-D-glucose

The effect of hyperglycaemia on regional cerebral glucose utilization was studied in five healthy males fasted over-night using positron emission tomography. Selectively labelled glucose, [1–¹¹C]-D -glucose, was used as a tracer. After correction for the small loss of [¹¹C]CO₂ from the tissue, this tracer measures the rate of glucose oxidation rather than the total rate of glucose metabolism. Each subject was investigated twice: during normoglycaemia (plasma glucose 5.3 ± 0.3 μmol mL^?1) and at the end of a 2-h period of hyperglycaemia (plasma glucose 13.8 ± 0.7 μmol mL^?1). Assuming unchanged rate constant for loss of labelled CO₂ at normo- and hyperglycaemia the oxidative metabolic rate of glucose was found to be slightly larger at combined hyperglycaemia and hypersulinemia (0.30 ± 0.01 mmol mL^?1 min^?1) than at normal glucose and insulin levels (0.25 ± 0.01 mmol mL^?1 min^?1). This suggests that the process of glucose phosphorylation might not be fully saturated in the human brain or, alternatively, that the glycogen deposition increases during short-term hyperglycaemia. The relative increase of oxidative metabolic rate was considerably larger (≈50%) in white matter than in the brain as a whole (20%). The brain glucose content was found to increase non-linearly with increasing plasma glucose. Together with data from previous studies these results suggest that the free glucose in the human brain is close to zero when the plasma glucose is below 2 μmol mL^?1.     

Optimal glycaemic control in unrestrained diabetic dogs using programmed compound squarewave insulin infusions

A glycaemic control identical with the normal has been achieved in unrestrained totally depancreatised dogs using a portable open-loop insulin delivery system. The device consisted of a battery power pack with a flow-rate controller, an insulin reservoir and a peristaltic pump from which pulses of insulin were delivered every 90 seconds into the inferior vena cava through an exteriorised indwelling catheter. Insulin was infused at the basal rate of 0.45±0.03 mUkg⁻¹ min⁻¹ (Mean±s.e.m.) in the postabsorptive state resulting in peripheral IRI and plasma glucose levels of 12±1 μU ml⁻¹ and 86±7 mg dl⁻¹. In the postprandial period the infusion rate was enhanced sevenfold to the rate of 3.16±0.21 mU kg⁻¹min⁻¹ for 7h and then reduced to 1.05±0.07 mU kg⁻¹ min⁻¹ for an additional 2.25 h. A weight-maintaining constant diet was provided and the resulting glycaemic profiles were similar to age, sex and weight-matched healthy controls. Fasting peripheral insulin levels in the infused diabetic dogs were not significantly different from non-diabetic controls (10±1μUml⁻¹). However, in the postprandial period of enhanced delivery, insulin levels in the diabetic dogs were 3.1 times higher than the controls. With the compound square waveforms of preprogrammed insulin infusion found appropriate in this study unaccountable low or high plasma glucose levels did not occur but hyperinsulinism accompanied the glycaemic normalisation following a mixed meal.     

吡格列酮对脂质灌注大鼠糖代谢和PPAR-γ表达的影响

目的：探讨吡格列酮 (Pio) 对游离脂肪酸诱导的胰岛素抵抗大鼠糖代谢和PPAR-γ表达的影响。方法:采用扩展正糖钳夹实验和［3-3H］标记葡萄糖示踪技术，观察了4 h脂质灌注导致大鼠血浆游离脂肪酸（FFA）升高引起糖代谢和脂肪组织PPAR-γ表达变化及Pio处理后的影响。 结果:在钳夹稳态期，对照组（N组）血浆FFA水平明显降低，而脂质灌注组（L组）和吡格列酮+脂质组（P/L组）FFA水平明显升高。 P/L组葡萄糖输注率(GIR)较N组明显降低（P<0.01）, 而L组又明显低于P/L组（P<0.01）；N组和P/L组肝糖输出 (HGP) 与基础值相比被明显抑制达85%（均P<0.01），在L组，胰岛素对HGP的抑制作用受到明显障碍（仅抑制8.7%）。L组和P/L组葡萄糖清除率（GRd）明显低于N组（P<0.01）。P/L组脂肪组织PPAR-γ表达明显增加。 结论:脂质灌注诱导了大鼠胰岛素抵抗。吡格列酮干预使大鼠脂肪组织PPAR-γ表达明显增加，并抑制了内源性肝糖产生，从而部分逆转了脂质诱导的胰岛素抵抗。     

Systemic effects of angiotensin III in conscious dogs during acute double blockade of the renin-angiotensin-aldosterone-system

Aims: The study was designed to determine (i) whether the effects of angiotensin III (AngIII) are similar to those of angiotensin II (AngII) at identical plasma concentrations and (ii) whether AngIII operates solely through AT₁‐ receptors. Methods: Angiotensin II (3 pmol kg^?1 min^?1–3.1 ng kg^?1 min^?1) or AngIII (15 pmol kg^?1 min^?1–14 ng kg^?1 min^?1) was infused i.v. during acute inhibition of angiotensin converting enzyme (enalaprilate; 2 mg kg^?1) and of aldosterone (canrenoate; 6 mg kg^?1 plus 1 mg kg^?1 h^?1). Arterial plasma concentrations of angiotensins were determined by radioimmunoassay using a cross‐reacting antibody to AngII. During ongoing peptide infusion, candesartan (2 mg kg^?1) was administered to block the AT₁‐receptors. Results: Angiotensin immunoactivity in plasma increased to 60 ± 10 pg mL^?1 during infusion of AngII or infusion of AngIII. AngII significantly increased mean arterial blood pressure (+14 ± 4 mmHg) and plasma aldosterone by 79% (+149 ± 17 pg mL^?1) and reduced plasma renin activity and sodium excretion (?41 ± 16 mIU L^?1 and ?46 ± 6 μmol min^?1 respectively). AngIII mimicked these effects and the magnitude of AngIII responses was statistically indistinguishable from those of AngII. All measured effects of both peptides were blocked by candesartan. Conclusion: At the present arterial plasma concentrations, AngIII is equipotent to AngII with regard to effects on blood pressure, aldosterone secretion and renal functions, and these AngIII effects are mediated through AT₁‐ receptors. The metabolic clearance rate of AngIII is five times that of AngII.     

Reducing plasma free fatty acids by acipimox improves glucose tolerance in high‐fat fed mice

To study whether free fatty acids (FFAs) contribute to glucose intolerance in high‐fat fed mice, the derivative of nicotinic acid, acipimox, which inhibits lipolysis, was administered intraperitoneally (50 mg kg^?1) to C57BL/6J mice which had been on a high‐fat diet for 3 months. Four hours after administration of acipimox, plasma FFA levels were reduced to 0.46 ± 0.06 mmol L^?1 compared with 0.88 ± 0.10 mmol L^?1 in controls (P < 0.001). At this point, the glucose elimination rate after an intravenous glucose load (1 g kg^?1) was markedly improved. Thus, the elimination constant (K_G) for the glucose disposal between 1 and 50 min after the glucose challenge was increased from 0.54 ± 0.01% min^?1 in controls to 0.66 ± 0.01% min^?1 by acipimox (P < 0.001). In contrast, the acute insulin response to glucose (1–5 min) was not significantly different between the groups, although the area under the insulin for the entire 50‐min period after glucose administration was significantly reduced by acipimox from 32.1 ± 2.9 to 23.9 ± 1.2 nmol L^?1 × 50 min (P=0.036). This, however, was mainly because of lower insulin levels at 20 and 50 min because of the lowered glucose levels. In contrast, administration of acipimox to mice fed a normal diet did not affect plasma levels of FFA or the glucose elimination or insulin levels after the glucose load. It is concluded that reducing FFA levels by acipimox in glucose intolerant high‐fat fed mice improves glucose tolerance mainly by improving insulin sensitivity making the ambient islet function adequate, suggesting that increased FFA levels are of pathophysiological importance in this model of glucose intolerance.     

The acute effects of insulin on the cardiorespiratory responses to hypoxia in streptozotocin‐induced diabetic rats

Aims: We examined whether or not streptozotocin (STZ)‐induced diabetic rats, which have a lower heart rate (HR, beats min^?1) than control rats, could maintain hypoxic ventilatory response. Methods: Twenty‐six Wistar rats, which had been injected with STZ (60 mg kg^?1, EXP) or vehicle (0.1 m citrate buffer, CONT) intraperitoneally at 9 weeks of age, had their cardiorespiratory responses to normoxia and 12%O₂ examined after 5 weeks. Results: Compared with CONT rats, EXP rats had a higher blood glucose [24 ± 3 vs. 5 ± 1 (mean ± SD) mmol L^?1], a lower body weight (320 ± 23 vs. 432 ± 24 g), lower HR (303 ± 49 vs. 380 ± 44 in normoxia, and 343 ± 56 vs. 443 ± 60 in hypoxia) and a lower mean arterial blood pressure (MAP) (89 ± 6 vs. 102 ± 10 mmHg in hypoxia). In contrast, both groups had similar values in ventilation (), –metabolic rate (MR) ratio and arterial blood gases (ABGs). In EXP rats, with an acute insulin supplement (i.v., 0.75 U h^?1 for 1.5–2 h), not only blood glucose, but also HR, and MAP were normalized as those obtained in CONT rats, and in hypoxia further increased without affecting –MR ratio and ABGs. Such acute cardiorespiratory stimulating effects of insulin could not be obtained in non‐diabetic rats (n = 7, 355 ± 24 g), in which euglycaemia (mean 6.4 mmol L^?1) was maintained during the measurements. Conclusions: Our results suggest that, in STZ‐induced diabetic rats: (1) ventilation is hardly suppressed by hyperglycaemia, (2) cardiorespiratory responses can be acutely stimulated by short insulin injection, and (3) the effects, including those through acute blood glucose normalization, are possibly specific for the diabetic impairments.     

Pancreatic islet blood flow during euglycaemic, hyperinsulinaemic clamp in anaesthetized rats

Aims: Previous studies have demonstrated that pancreatic islet blood flow is crucially dependent on blood glucose concentration. Thus, hyperglycaemia increases and hypoglycaemia decreases islet blood perfusion, by a combination of nervous and metabolic signals. The aim of the present study was to evaluate if hyperinsulinaemia, without associated hypoglycaemia, affects islet blood flow. Methods: Thiobutabarbital‐anaesthetized Wistar–Furth rats were subjected to an euglycaemic, hyperinsulinaemic clamp, that is they were infused for 60 min with either saline, insulin (18 mU kg^?1 min^?1), glucose (27 mg kg^?1 min^?1) or both glucose and insulin. This was followed by islet blood flow measurements with a microsphere technique. Results: Animals receiving only glucose doubled their blood glucose and serum insulin concentrations, whereas rats receiving only insulin had blood glucose concentrations <2 mmol L^?1 and a 10‐fold increase in serum insulin concentrations. Animals given simultaneous glucose and insulin had normal blood glucose concentrations but a 10‐fold increase in serum insulin concentrations. Total pancreatic blood flow was unaffected in all animals. Islet blood flow was increased in hyperglycaemic and decreased in hypoglycaemic rats compared with control rats. Islet blood flow did not differ between clamped and control rats. Conclusions: Serum insulin concentration per se does not affect islet blood flow, whereas the ambient blood glucose concentration is of major importance in this context.     

Global cerebral blood flow during infusion of adenosine in humans: assessment by magnetic resonance imaging and positron emission tomography

Adenosine, an endogenous vasodilator, induces a cerebral vasodilation at hypotensive infusion rates in anaesthetized humans. At lower doses (< 100 μg kg^?1 min^?1), adenosine has shown to have an analgesic effect. This study was undertaken to investigate whether a low dose, causing tolerable symptoms of peripheral vasodilation affects the global cerebral blood flow (CBF). In nine healthy volunteers CBF measurements were made using axial magnetic resonance (MR) phase images of the internal carotid and vertebral arteries at the level of C2–3. Quantitative assessment of CBF was also obtained with positron emission tomography (PET) technique, using intravenous bolus []> ¹⁵O]butanol as tracer in four of the subject at another occasion. During normoventilation (5.4 ± 0.2 kPa, mean ± s.e.m.), the cerebral blood flow measured by magnetic resonance imaging technique, as the sum of the flows in both carotid and vertebral arteries, was 863 ± 66 mL min^?1, equivalent to about 64 ± 5 mL 100 g^?1 min^?1. The cerebral blood flow measured by positron emmission tomography technique, was 59 ± 4 mL 100 g^?1 min^?1. All subjects had a normal CO₂ reactivity. When adenosine was infused (84 ± 7 μg kg^?1 min^?1) the cerebral blood flow, measured by magnetic resonance imaging was 60 ± 5 mL 100 g^?1 min^?1. The end tidal CO₂ level was slightly lower (0.2 ± 0.1 kPa) during adenosine infusion than during normoventilation. In the subgroup there was no difference in cerebral blood flow as measured by magnetic resonance imaging or positron emission tomography. In conclusion, adenosine infusion at tolerable doses in healthy volunteers does not affect global cerebral blood flow in unanaesthetized humans.     

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.     

Extracellular distribution of insulin in muscle of rats exposed to long-term hyperinsulinaemia

The importance of increased capillary density for the regulation of insulin sensitivity by transcapillary delivery of insulin to muscle cells in insulin-exposed rats was investigated by direct microdialysis measurements of interstitial [¹²⁵I]insulin concentrations in the femoral muscle during an euglycaemic hyperinsulinaemic clamp. In insulin-exposed rats plasma insulin was ～25% (P<0.05) higher than that in control animals during the first 100 min and reached their maximal concentrations after 100 min. After a nitroprusside infusion given at 100 min both groups had similar concentrations of insulin in plasma as well as in muscle interstitial fluid. However, mean glucose infusion rate during the first clamp hour was 20.5±2.3 and 12.6±5.2 mg kg^-1 min^-1 (P<0.05) in insulin-exposed and control animals, respectively. During the second clamp hour the corresponding figures were 21.1±2.4 and 13.9±2.6 (P<0.05). It may be concluded that capillarization and/or nitroprusside affected plasma insulin concentrations without altering either the interstitial insulin levels or the insulin effect on glucose consumption. The data suggest that the elevated insulin sensitivity after chronic insulin exposure is dependent on other than transcapillary transport events and demonstrate the different kinetics for insulin distribution in plasma and in the interstitial fluid.     

Hyperglycaemia is responsible for the inhibited gastrointestinal transit in the early diabetic rat

The effect of plasma glucose levels on the gastrointestinal motility of the rat was studied. Chronic hyperglycaemia was induced by i. v. injection of streptozotocin 1 week before the motility experiment. Some rats received additional daily insulin therapy (1.25, 2.5 or 10 IU kg^-1) after induction of diabetes mellitus. Acute hyperglycaemia was induced by the continuous i. v. infusion of glucose solution (11, 22, 44 or 88 mg kg^-1 min^-1) 10 min before the motility experiments. The rats were killed 15 min after successful orogastric feeding of a charcoal-contained suspension. Gastrointestinal transit was calculated as the percentage of the overall lenght of the small intestine to which the charcoal moved during this time period. The diabetic rats were found to have delayed transit compared with controls (meanpLSEM: 32.2pL2.1% vs. 42.9pL4.2%, P<0.05). Correction with moderate doses of insulin therapy failed to inhibt transit, whereas hypoglycaemia induced by high-dose insulin treatment enhanced transit, whereas hypoglycaemia induced by high-dose insulin treatment enhanced transit. High doses of glucose elicited acute hyperglycaemia and delayed transit when compared with saline infused non-diabetic rats. In early diabetes, hyperglycaemia probably mediates the inhibited gastrointestinal transit, since correction of hyperglycaemia usually restores the delayed transit.     

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.     

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.     

Beta-endorphin infusion alters pancreatic hormone and glucose levels during exercise in rats

β-Endorphin (BE) infusion at rest can influence insulin and glucagon levels and thus may affect glucose availability during exercise. To clarify the effect of BE on levels of insulin, glucagon and glucose during exercise, 72 untrained male Sprague-Dawley rats were infused i.v. with either: (1) BE (bolus 0.05?mg?·?kg^?1 +0.05?mg?·?kg^?1?·?h^?1, n?=?24); (2) naloxone (N, bolus 0.8?mg?·?kg^?1?+?0.4?mg?·?kg^?1, n?=?24); or (3) volume-matched saline (S, n?=?24). Six rats from each group were killed after 0, 60, 90 or 120 min of running at 22?m?·?min^?1, at 0% gradient. BE infusion resulted in higher plasma glucose levels at 60?min [5.93 (0.32)?mM] and 90?min [4.16 (0.29)?mM] of exercise compared to S [4.62 (0.27) and 3.41 (0.26?mM] and N [4.97 (0.38) and 3.44 (0.25)?mM]. Insulin levels decreased to a greater extent with BE [21.5 (0.9) and 18.3 (0.6) uIU?·?ml^?1] at 60 and 90?min compared to S [24.5 (0.5) and 20.6 (0.6)?uIU?·?ml^?1] and N [24.5 (0.4) and 21.6 (0.7)?uIU?· ml^?1] groups. Plasma C-peptide declined to a greater extent at 60 and 90?min of exercise with BE infusion compared to both S and N. BE infusion increased glucagon at all times during exercise compared to S and N. These data suggest that BE infusion during exercise influences plasma glucose by augmenting glucagon levels and attenuating insulin release.     

Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

Aim: The aim of the present study was to compare the coronary flow (CF) reserve of ex vivo perfused hearts from type 2 diabetic (db/db) and non‐diabetic (db/+) mice. Methods: The hearts were perfused in the Langendorff mode with Krebs–Henseleit bicarbonate buffer (37 °C, pH 7.4) containing 11 mmol L^?1 glucose as energy substrate. The coronary reserve was measured in response to three different interventions: (1) administration of nitroprusside (a nitric oxide donor), (2) administration of adenosine and (3) production of reactive hyperaemia by short‐term ischaemia. Results: Basal CF was approximately 15% lower in diabetic when compared with non‐diabetic hearts (2.1 ± 0.1 vs. 2.6 ± 0.2 mL min^?1). The maximum increase in CF rate in response to sodium nitroprusside and adenosine was significantly lower in diabetic (0.6 ± 0.1 and 0.9 ± 0.1 mL min^?1 respectively) than in non‐diabetic hearts (1.2 ± 0.1 and 1.4 ± 0.1 mL min^?1 respectively). Also, there was a clear difference in the rate of return to basal CF following short‐term ischaemia between diabetic and non‐diabetic hearts. Thus, basal tone was restored 1–2 min after the peak hyperaemic response in non‐diabetic hearts, whereas it took approximately 5 min in diabetic hearts. Conclusion: These results show that basal CF, as well as the CF reserve, is impaired in hearts from type 2 diabetic mice. As diabetic and non‐diabetic hearts were exposed to the same (maximum) concentrations of NO or adenosine, it is suggested that the lower coronary reserve in type 2 diabetic hearts is, in part, because of a defect in the intracellular pathways mediating smooth muscle relaxation.     

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.,