Effect on renin release of inhibiting renal nitric oxide synthesis in anaesthetized dogs

Nitric oxide plays an important role in the regulation of basal renal blood flow. This study was performed to examine whether selective inhibiti± of renal nitric oxide synthesis affects renin release in vivo. Accordingly, in six barbiturate-anaesthetized dogs renin release was examined before and after intrarenal infusion of the selective inhibitor of nitric oxide synthesis, N^G-nitro-l -arginine (NOARG). NOARG was infused into the renal artery to yield a renal arterial blood concentration of 0.4 μmol ml^-1. NOARG did not change systemic arterial blood pressure and glomerular filtration rate, but reduced basal renal blood flow by 26 ± 2%. Urine flow, sodium and potassium excretion were reduced after inhibition of renal nitric oxide synthesis. Basal renin release (3 ± 2 μg AI min^-1) was not altered by NOARG infusion (1 ± 1 μg AI min^-1). To stimulate renin release the renal artery was constricted to a renal perfusion pressure of 50 mmHg. At this perfusion pressure infusion of NOARG reduced renin release significantly from 48 ± 11 μg AI min^-1to 14 ± 4 μg AI min^-1. In conclusion, inhibition of renal nitric oxide synthesis reduces basal renal blood flow and reduces renin release stimulated by renal arterial constriction. These findings indicate that renal nitric oxide modulates both renal blood flow and renin release in vivo.     

High afterload during 10 min of regional ischaemia affects diastolic creep but not systolic function in reperfused (stunned) myocardium

The effect of afterload during regional ischaemia on myocardial stunning was studied in 15 pentobarbital anaesthetized cats. 10 min occlusion of the left anterior descending artery (LAD) was followed by 60 min of reperfusion. Afterload was decreased by intravenous infusion of nitroglycerine 3–8 μg kg^-1 min^-1 in group I (n=8); left ventricular peak systolic pressure (LVSP) 84±4 mmHg (mean±SEM) during coronary artery occlusion. In group II (n=7) LVSP was increased to 188±10 mmHg by inflating an intraaortic balloon during coronary artery occlusion. Regional function in the LAD perfused region was evaluated by cross-oriented sonomicrometry. Myocardial tissue blood flow was evaluated by radio-labelled microspheres. Afterload alterations did not affect regional systolic shortening (10.8±2.0% vs. 11.0±1.5% in group I and II, respectively, after 60 min of reperfusion). However, increased end-diastolic dimensions (diastolic creep) in both the circumferential and longitudinal segments were markedly more pronounced in the high afterload group (group II). Also important, the markedly increased myocardial tissue blood flow during reperfusion in group II as compared with group I (2.30±0.18 vs.  1.34±0.08 mL min^-1 g^-1 and 2.58±0.23 vs. 1.49±0.07 mL min^-1 g^-1 in subepicardial and subendocardial layers in the LAD perfused region) suggests that increased diastolic creep increased metabolic demands. This study indicates that passive stretching of the ischaemic area during coronary artery occlusion is an important mechanism behind diastolic creep.     

Long-lasting coronary vasoconstrictor effects and myocardial uptake of endothelin-1 in humans

The effect of intravenous administration of the endothelium-derived vasoconstrictor peptide endothelin-1 (ET-1 0.2, 1 and 8 pmol kg^?1 min^?1) on coronary blood flow in relation to plasma ET-1 as well as blood lactate and glucose levels were investigated in six healthy volunteers. Coronary sinus blood flow was measured by thermodilution. Administration of ET-1 elevated arterial plasma ET 35-fold, dose-dependently increased mean arterial blood pressure from 95±5 mmHg to 110±6 mmHg (P<0.01) and reduced heart rate from 64±4 beats min^?1 to 58±4 beats min^?1 (P<0.05) at 8 pmol kg^?1 min^?1. Coronary sinus blood flow was reduced maximally by 23±4% (P<0.01) and coronary vascular resistance increased by 48±11% (P<0.01). Coronary sinus oxygen saturation decreased from 35±1% to 22±2% at 2 min after the infusion (P<0.01). A coronary constrictor response was observed at a 4-fold elevation in plasma ET. The reduction in coronary sinus blood flow lasted 20 min and coronary sinus oxygen saturation was still reduced 60 min after the infusion. Myocardial oxygen uptake or arterial oxygen saturation were not affected by ET-1. Myocardial lactate net uptake decreased by 40% whereas glucose uptake was unaffected. At the highest infusion rate there was a net removal of plasma ET by 24±3% over the myocardium (P<0.05). The results show that ET-1 induces long-lasting reduction in coronary sinus blood flow via a direct coronary vasoconstrictor effect in healthy humans observable at a 4-fold elevation in plasma ET-1. Furthermore, there is a net removal of circulating ET-1 by the myocardium.     

Hyperoxia reduces basal release of nitric oxide and contracts porcine coronary arteries

Aim: The purpose of the present study was to investigate whether changes in nitric oxide (NO) concentration is involved in hyperoxia‐induced vasoconstriction in porcine conduit coronary arteries. Methods: The effect of hyperoxia on NO release and vasoconstriction was evaluated by tension recording, microsensor measurements, and immunoblotting in porcine conduit coronary arteries contracted with U46619 or 5‐hydroxytryptamine. Results: In endothelium‐intact segments exchanging 20% O₂, 5% CO₂, 75% N₂ (normoxia) for 95% O₂, 5% CO₂ (hyperoxia) increased contraction. In segments without endothelium hyperoxia‐evoked contraction was abolished, but restored by an encircling donor segment with endothelium. An inhibitor of NOS, asymmetric dimethylarginine (ADMA, 300 μm ), reduced hyperoxic contraction and basal NO concentration by, respectively, 38 ± 12% and 46 ± 3% (P < 0.05, n = 9). A NO donor, S‐nitroso‐N‐acetylpenicillamine (SNAP), increased NO concentration and evoked relaxation to the same levels in normoxic and hyperoxic conditions. β‐actin and endothelial NO synthase (eNOS) protein expression was similar in normoxic and hyperoxic arterial segments. Phosphorylation of eNOS was unaltered in normoxia vs. hyperoxia, but phosphorylation of eNOS‐Ser¹¹⁷⁷ was increased and phosphorylation of eNOS‐Thr⁴⁹⁵ decreased by U46619. Blockers of ATP‐sensitive, voltage‐dependent and calcium‐activated K⁺ channels did not change hyperoxic contraction. However, high extracellular K⁺ concentration or a second and third exposure to hyperoxia decreased contraction. Conclusion: The present study provides direct evidence that hyperoxia reduces basal release of NO leading to depletable endothelium‐dependent vasoconstriction in porcine coronary arteries independent of changes in eNOS phosphorylation.     

Modulation of systolic and diastolic function by endothelin-1: relation to coronary flow

Different conclusions have been reached with regard to the effect of endothelin (ET-1) on cardiac contractility. We examined systolic and diastolic function in response to constant known concentrations of ET-1 with or without ET-1 induced reductions in coronary flow (CF). Rat hearts (n= 21) were buffer-perfused using constant coronary flow (cCF) or constant perfusion pressure (cPP). Left ventricular function was assessed isovolumically. Addition of ET-1 (10^-9 M) in the cCF group caused a gradual increase in PP from 61 ± 2 to 165±6mmHg (mean±SE) (P < 0.01). Within 10 min left ventricular systolic pressure (LVSP) increased from 111 ± 2 to a maximum of 134±4mmHg (P < 0.01) and [L\dP/dt] increased from 1640 ± 81 to a maximum of 2020 ± 92 mmHg s“¹ (P < 0.01). After 15 min left ventricular end diastolic pressure (LVEDP), a measure of diastolic stiffness (DS), also increased. With ET-1 (10 ⁸ M), similar haemodynamic alterations appeared more rapidly. In the cPP group, ET-1 (10”⁹ M) caused a sharp decrease in CF and LVSP fell from 115 ± 8 to 62±12 mmHg at 10 min (P < 0.001). Systolic function remained stable at a reduced level for 1 h. DS did not change. Thus, ET-1 possesses positive inotropic effects and increases diastolic stiffness. Both effects may be masked by vasoconstriction-induced ischaemia.     

The combination of L-arginine and ischaemic post-conditioning at the onset of reperfusion limits myocardial injury in the pig

Aim: To investigate whether ischaemic post‐conditioning (IPoC) combined with i.v. infusion of the nitric oxide (NO) substrate l ‐arginine at the onset of reperfusion exerts cardioprotective effect that is superior to either treatment given separately. Methods: Twenty‐six anesthetized pigs were subjected to coronary artery (left anterior descending artery, LAD) ligation for 40 min followed by 4 h reperfusion. The pigs were randomized into five different groups receiving either i.v. vehicle, i.v. l ‐arginine, IPoC 4 × 60 s together with i.v. vehicle or IPoC together with i.v. l ‐arginine and a group with IPoC 8 × 30 s. All infusions were started 10 min before reperfusion. Results: The infarct size of the vehicle group was 82 ± 4% of the area at risk. l ‐Arginine alone (79 ± 8%), IPoC 4 × 60 s vehicle (86 ± 3%) or IPoC 8 × 30 s vehicle (94 ± 7%) did not affect infarct size. l ‐Arginine together with IPoC significantly reduced infarct size to 59 ± 4% (P < 0.01). Except for higher LAD flow during early reperfusion in the IPoC l ‐arginine group, haemodynamic parameters did not differ between the four main groups. Heart rate and rate pressure product were lower during ischaemia and reperfusion in the IPoC 8 × 30 s vehicle group. In comparison with the vehicle group, there were no changes in the expression of Akt, phosphorylated Akt Ser⁴⁷³, inducible NO synthase, endothelial NO synthase (eNOS) or phosphorylated eNOS Ser¹¹⁷⁷ in the ischaemic/reperfused myocardium. Conclusion: l ‐Arginine given systemically at the onset of reperfusion protects the pig heart against ischaemia and reperfusion injury only when combined with IPoC. These results indicate that the combination of the two treatment strategies exerts cardioprotection.     

Nitric oxide and prostaglandin pathways interact in the regulation of hypercapnic cerebral vasodilatation

To test whether nitric oxide and prostaglandin pathways interact in hypercapnic cerebral vasodilatation, cerebral blood flow (CBF) was measured in enflurane anaesthetized Sprague–Dawley rats using the hydrogen clearance method. Isometric tension was measured in rat middle cerebral arteries in vitro. The neuronal NO synthase inhibitor 7-nitroindazole (7-NI 60 mg kg^–1 i.p.) reduced the hypercapnic CBF response by 62 ± 7% (but not the hypoxic response) and indomethacin (IMC 6 mg kg^–1 i.v.) reduced the hypercapnic CBF response by 60 ± 5%. Combined application caused only an 80 ± 1% reduction. The attenuation of hypercapnic CBF by IMC was diminished by 7-NI and similarly 7-NI had less effect in the presence of IMC. Spermine-NO (50 μM 0.5 μL min^–1 intracortically) increased eucapnic and hypercapnic CBF in the presence of IMC. In isolated middle cerebral arteries, combined application of sodium nitroprusside (SNP 3 n M ) and prostacyclin (30 n M ) had a synergistic vasodilatory effect. Milrinone (PDE-III inhibitor) also potentiated prostacyclin-mediated vasodilatation. Our results suggest that the NO- and IMC-sensitive pathways involved in the hypercapnic response are distinct, however, both may interact synergistically. A similar synergism was observed between the effects of SNP and prostacyclin.     

Sulfonylureas enhance GABAA synaptic potentials in rat midbrain dopamine neurones

Long-lasting myocardial ischaemia reduces the density of sarcolemmal L-type calcium channels (LCC). Ischaemic preconditioning protects the myocardium against development of infarction. The aim of this study was to investigate if ischaemia-induced loss in LCC is affected by ischaemic preconditioning. Specific (+)-[³H]isradipine binding to LCC was compared in membranes and homogenates from control and ischaemic regions of non-preconditioned and ischaemically preconditioned hearts [two 10 min left anterior descending coronary artery (LAD) occlusions, each followed by 30 min reperfusion]. Biopsies were sampled after 60 min mid LAD occlusion from ischaemic and control (supplied by circumflex artery) regions. Sixty min ischaemia reduced binding density of specific (+)-[³H]isradipine in membranes by 23±11% (n=7, P<0.05) in the non-preconditioned group and by 20±8% (n=6, P<0.05) in the preconditioned group. Binding density in homogenates was reduced by 36±5% (n=5, P<0.05) in the non-preconditioned group and by 21±5% (n=5, P<0.05) in the preconditioned group. The reductions in the two groups and reductions in membranes and homogenates were not statistically different. The dissociation constant of binding was similar in the groups. In conclusion, 60 min of ischaemia reduced the binding density of (+)-[³H]isradipine in membranes and homogenates by 20–36%. The reduction in density of binding sites was not caused by redistribution of sarcolemmal LCC to an intracellular compartment. Ischaemic preconditioning did not affect the decline in binding density as hypothesized.     

The novel non-peptide selective endothelin A receptor antagonist LU 135 252 protects against myocardial ischaemic and reperfusion injury in the pig

The aim of the study was to investigate the efficacy of the novel non-peptide selective endothelin A (ET_A) receptor antagonist LU 135 252 to limit the extent of myocardial ischaemic and reperfusion injury. Administration of LU 135 252 (1 and 5 mg kg^–1 i.v.) to anaesthetised pigs reduced mean arterial pressure (MAP) from 91 ± 4 to 79 ± 3 mmHg (P < 0.05) and 96 ± 3–82 ± 3 mmHg (P < 0.01), respectively. Heart rate, coronary blood flow and coronary vascular resistance were not affected by LU 135 252. The infarct size induced by 45-min ligation of the left anterior descending coronary artery (LAD) followed by 4-h reperfusion in pigs was 81 ± 5% of the area at risk in control animals given vehicle (n = 8). In pigs receiving 1 mg kg^–1 (n = 6) or 5 mg kg^–1 (n = 8) of LU 135 252 i.v. 20 min before ischaemia the infarct size was reduced to 64 ± 3% (P < 0.05) and 35 ± 4% (P < 0.001), respectively, of the area at risk. During the reperfusion period there was a non-significant trend towards a higher coronary blood flow and a lower coronary vascular resistance in the groups given LU 135 252 compared to controls. Myocardial overflow of ET-like immunoreactivity was increased during the reperfusion period but it was not affected by administration of LU 135 252. It is concluded that administration of the selective ET_A receptor antagonist LU 135 252 effectively protects the myocardium from ischaemia/reperfusion injury, indicating that the ET_A receptor subtype is involved in the development of ischaemia/reperfusion injury.     

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.     

Intimal injury in a transiently occluded coronary artery increases myocardial necrosis. Effect of aspirin

 This study tested the hypothesis that intimal injury in a transiently occluded coronary artery limits myocardial salvage. The effect of intimal injury on reactive hyperaemia was investigated in 17 pigs submitted to a 30-min occlusion of the left anterior descending coronary artery (LAD), not resulting in myocardial infarction. Catheter-induced intimal damage increased local platelet deposition (^99mTc) and reduced hyperaemia, but did not modify myocardial platelet or polymorphonuclear leucocyte content (myeloperoxidase activity) after 6 h reperfusion. To investigate the influence of intimal injury on the extent of myocardial necrosis secondary to a more prolonged coronary occlusion, and the role of platelets on this influence, 52 pigs were submitted to a double randomization (2×2 factorial design) to 250 mg i.v. aspirin vs. placebo and to coronary intimal injury vs. no coronary damage before a 48-min occlusion of the LAD and 6 h of reperfusion. After excluding 12 animals with reocclusion, coronary intimal injury was associated with larger infarcts (triphenyltetrazolium reaction) in animals receiving placebo (36.2±7.0% of the area at risk in animals with intimal injury vs. 10.8±3.9% in animals without coronary injury, P=0.006) but not in those receiving aspirin (20.3±6.5 vs. 21.7±6.5% of the area at risk in animals with and without intimal injury respectively). These results suggest that coronary intimal injury in the reperfused artery may have adverse effects on myocardial salvage by mechanisms other than reocclusion or embolization of platelet aggregates. Recieved: 19 December 1995 / Received after revision: 25 March 1996 / Accepted: 29 April 1996     

Mechanics of porcine coronary arteriesex vivo employing impedance planimetry: A new intravascular technique

Our objective was to evaluate methodological aspects of impedance planimetry, a new balloon catheter-based technique, for the investigation of coronary artery mechanical wall properties. We used a four ring-electrode electrical impedance measuring system that was located inside a balloon. Two of the electrodes were used for excitation and connected to a generator producing a constant alternating current of 250 mA at 5 kHz. The other two electrodes for detection were placed midway between the excitation electrodes. The balloon was distended with electrically conducting fluid through an infusion channel. The vessel cross-sectional area (CSA) was measured according to the field gradient principle by measuring the impedance of the fluid inside the balloon. Impedance planimetry was applied in the three major branches of the coronary arteries of seven extracted porcine hearts to assess luminal CSAs in response to internal pressurization. The biomechanical wall properties were evaluated by computing the strain [(r−r ₀)·r ₀ ⁻¹, wherer is the vessels inner radius computed as (CSA · π⁻¹)^? andr ₀ is the radius of the vessel at a minimal distension pressure], the tension [(r·dP), wheredP is the transmural pressure difference], and the pressure elastic modulus (ΔP·r·Δr ⁻¹). We found thatin vitro testing demonstrated that impedance planimetry was accurate and reproducible. The technique has controllable sources of crror. Measurements were performed with consecutively increasing pressures in the range 1–25 kPa (8–188 mmHg, 0.01–0.25 atm). The CSAs increased nonlinearly and were significantly larger in the left anterior descendent coronary artery (LAD) (1 kPa, mean 5.0 mm²; 25 kPa, mean 21.8 mm²) than in both the left circumflex (Cx) (4.5–16.0 mm²) and the right coronary artery (RCA) (2.8–15.6 mm²) (analysis of variance,P<0.001 for both). The circumferential wall tension-strain relation showed exponential behavior. For a given strain, tension values for LAD were significantly lower than those of Cx (P<0.01). The pressure elastic modulus-strain relation also was exponential, and values for Cx were significantly lower than values for LAD (P<0.001) and RCA (P<0.05). Impedance planimetry was applied to the study of coronary artery biomechanicsex vivo. The LAD had the largest CSA, and the Cx was the least compliant. Methodological aspects of anin vivo introduction of the method require additional evaluation.     

Cerebral blood flow decreases following microinjection of sodium nitroprusside into the nucleus tractus solitarii of anesthetized rats

The present study was undertaken to examine the effects of microinjection of sodium nitroprusside (SNP), which releases nitric oxide (NO) spontaneously, into the nucleus tractus solitarii (NTS) on cerebral circulation. Cerebral blood flow (CBF) was measured in urethane-anesthetized (1.5 g·kg^?1, i.p.), paralysed and artificially ventilated rats using labeled microspheres or laser Doppler flowmetry. The CBF was significantly decreased by microinjection of SNP (5 nmol, n=10, microsphere technique; 0.5 nmol, n=6, laser Doppler flowmetry) into the unilateral NTS. Microinjection of N ^G-monomethyl-L-arginine (L-NMMA), an inhibitor of the formation of NO, prevented cerebral vasoconstrictor responses induced by microinjection of l-glutamate into the NTS (n=10). Microinjection of N ^G-monomethyl-d-arginine (D-NMMA) had no effect on the cerebral vasoconstrictor responses induced by l-glutamate (n=11). Unilateral microinjections of L-NMMA into the NTS (n=9), of SNP into the area adjacent to the NTS (n=9), of vehicle solution into the NTS (n=10), and of light-inactivated SNP into the NTS (n=6) had no effect on cerebral circulation. Cerebral autoregulation was well maintained in our protocols (n=9). These results indicate that microinjection of SNP, an NO donor, into the NTS decreases CBF.     

Successful recanalisation of experimentally occluded coronary arteries in anaesthetised dogs with recombinant single-chain tissue-type Plasminogen activator (set-PA)

In anaesthetised closed chest dogs experimental occluding coronary thrombosis was induced by introducing a copper coil into a branch of the left circumflex coronary artery (LCX). LCX-occlusion and reperfusion after thrombolysic treatment were assessed by coronary angiography and typical ECG-signs.One hour after total LCX-occlusion vehicle alone (n=5), streptokinase (500 IU/kg/min; n=5) or single-chain tissue-type plasminogen activator (set-PA; 5000IU/kg/min; n=5) were intravenously infused. In no case spontaneous recanalisation could be observed in the control group within 150 min after onset of infarction. Streptokinase as well as sct-PA reopened the occluded LCX after an infusion period of 60 ± 23.7 min (x̄ ± SD) and 49.5 ± 14.6 min, respectively.Compared with prethrombolytic values no significant depletion of fibrinogen, α₂-antiplasmin, and plasminogen was observed in the dogs treated with streptokinase or single-chain tissue-type plasminogen activator.     

HSP20 phosphorylation and interstitial metabolites in hypoxia‐induced dilation of swine coronary arteries

Objective: Hypoxia induces coronary artery dilation, but the responsible mechanism is largely unknown. Many stimuli induce arterial smooth muscle relaxation by reducing ser¹⁹‐myosin regulatory light chain (MLC) phosphorylation. Other stimuli can induce smooth muscle relaxation without reductions in ser¹⁹‐MLC phosphorylation. This form of relaxation has been termed force suppression and appears to be associated with heat shock protein 20 (HSP20) phosphorylation on ser¹⁶. We investigated whether hypoxia‐induced sustained dilation in swine coronary arteries was promoted without ser¹⁹‐MLC dephosphorylation and associated with ser¹⁶‐HSP20 phosphorylation. Nitroglycerin vasodilation served as control. Methods: In a pressure myograph, the tunica media of intact pre‐contracted (PGF_2α; 10^?5 m ) porcine coronary artery segments were cannulated using a microdialysis catheter. Diameter responses and interstitial lactate/pyruvate ratios were studied during 90 min hypoxia, hypoxia + reoxygenation (60 min), nitroglycerin (100 μm , 90 min), and nitroglycerin + wash‐out (60 min). The arterial segments were snap‐frozen and analysed for ser¹⁶‐HSP20 phosphorylation and ser¹⁹‐MLC phosphorylation. Results: The normalized diameter responses to hypoxia (6.1 ± 4.3%) and nitroglycerin (12.6 ± 1.6%) were both significantly greater than normoxic control arteries (?10.5 ± 1.8%, anova , P < 0.05). Ser¹⁶‐HSP20 phosphorylation was increased with hypoxia and nitroglycerin treatment and ser¹⁶‐HSP20 phosphorylation correlated with changes in diameters (n = 29, r² = 0.64, P < 0.001). Ser¹⁹‐MLC phosphorylation was not significantly altered by hypoxia. The lactate/pyruvate ratio was significantly increased in hypoxic arteries but did not correlate with diameters or ser¹⁶‐HSP20 phosphorylation. Conclusion: Ser¹⁶‐HSP20 phosphorylation is a potential regulator of hypoxia‐induced dilation in coronary arteries.     

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.     

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.     

Influence of coronary venous retroinfusion and vasodilatation on regional myocardial blood flow measurement with microspheres. An analysis of ‘microsphere loss’ from ischaemic and reperfused porcine hearts

The influence of coronary venous retroinfusion and a vasoselective calcium antagonist felodipine on the microsphere loss in a porcine model of myocardial ischaemia and reperfusion was studied. Sixteen open-chest pigs underwent 45 min of myocardial ischaemia induced by occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Either felodipine (felo-retro group, 7 nmol kg^-1, n= 6) or the corresponding amount of vehicle (vehicle-retro group, n= 5) was infused retrogradely into the coronary veins over 30 min, starting 5 min before reperfusion. In a third group, the same amount of felodipine was administered intravenously (felo-iv group, n= 5). Myocardial regional blood flow was measured with radiolabelled microspheres (ø= 15 μm) injected before ischaemia to investigate a possible loss during ischaemia. In the felo-retro group, the apparent blood flow in the ischaemic areas, expressed as a percentage of the corresponding values in the non-ischaemic areas (%-flow), were 73±15, 73±11 and 75±19 in the subendocardial, midmyocardial and subepicardial layers, respectively. The corresponding percentage flows were 64±11, 70±11 and 62±9 in the vehicle-retro group and 75±18, 77±15 and 76±11 in the felo-iv group. The differences between the groups were not satistically significant. It is concluded that in this open-chest preparation microsphere loss observed in the ischaemic and reperfused myocardium is not increased by coronary venous retroinfusion or by a concomitantly administered vasodilative agent like felodipine.     

Effects of ATP on rat renal haemodynamics and excretion: role of sodium intake, nitric oxide and cytochrome P450

Aim: Adenosine‐5′‐triphosphate (ATP) affects intrarenal vascular tone and tubular transport via P2 receptors; however, the actual role of the system in regulation of renal perfusion and excretion remains unclear and is the subject of this whole‐kidney study. Methods: Effects of suprarenal aortic ATP infusion, 0.6–1.2 mg kg⁻¹ h⁻¹, were examined in anaesthetised rats maintained on low‐ (LS) or high‐sodium (HS) diet. Renal artery blood flow (RBF, transonic flow probe) and the perfusion (laser‐Doppler flux) of the superficial cortex (CBF) and outer and inner medulla (OM–BF, IM–BF) were measured, together with sodium and water excretion and urine osmolality. Results: Adenosine‐5′‐triphosphate did not change arterial pressure, RBF or CBF while the effects on medullary perfusion depended on sodium intake. In LS rats ATP increased IM–BF 19 ± 6%, the effect was prevented by inhibition of nitric oxide (NO) with N‐nitro‐l ‐arginine methyl ester. In HS rats ATP decreased OM–BF 16 ± 3% and IM–BF (7 ± 4%, not significant); previous inhibition of cytochrome P450 with 1‐aminobenzotriazol blunted the OM–BF decrease and reversed the previous decrease of IM–BF to a 13 ± 8% increase. Inhibition of P2 receptors with pyridoxal derivative (PPADS) abolished medullary vascular responses to ATP. In HS rats pre‐treated with PPADS, ATP increased tubular reabsorption, probably via adenosine formation and stimulation of P1 receptors. Conclusion: The data indicate a potential role of ATP in the selective control of renal medullary perfusion, different in sodium depleted and sodium replete rats. The action of ATP appears to be mediated by the NO system and the cytochrome P450 dependent vasoactive metabolites.     

Endotoxin‐induced shock in the pig – limited effects of low and high concentrations of inhaled nitric oxide

Aim: This study was carried out to study the prophylactic effects of inhalation of nitric oxide (NO) before and during the induction of endotoxic shock. Methods: Eighteen anaesthetized pigs received an infusion of 10–20 μg kg^?1 endotoxin during 2 h after pre‐treatment with the cortisol‐synthesis inhibitor metyrapone. Three groups were tested (n = 6 each) and received 0, 0.2 or 20 ppm inhaled NO from 30 min before start of endotoxin infusion until 4 h after start of endotoxin. Both 0.2 and 20 ppm NO were able to improve blood gas values. Results: Area above curve values of arterial P₂/FiO₂ from 0 to 4 h were 0.83 ± 0.09 kPa h (control), 0.78 ± 0.22 (0.2 ppm NO, non‐significant) and 0.31 ± 0.06 (20 ppm NO, P < 0.01, Mann–Whitney U‐test, compared to control). Area under curve values of PCO₂ from 0 to 4 h were 3.96 ± 0.66 kPa h (control), 1.20 ± 0.46 (0.2 ppm NO, P < 0.05, Mann–Whitney U‐test, compared to control) and 2.78 ± 1.06 (20 ppm NO group, non‐significant). The increase in pulmonary arterial pressure (PAP) was partly prevented by 20 ppm NO, but not by 0.2 ppm NO at 4 h. Inhaled NO did not affect the levels of BAL fluid total protein, tumour necrosis factor‐α, interleukin‐8 and neutrophil counts. Conclusions: The addition of a high (20 ppm), but not a low (0.2 ppm), concentration of NO to the inhaled air during endotoxin shock improves arterial oxygen tension and reduces pulmonary artery pressure. Neither dose affects lung mechanics or inflammatory indices, in spite of being given prophylactically.