A.W. Schindler

Plasma nitrite reflects constitutive nitric oxide synthase activity in mammals

Changes in plasma nitrite concentration in the human forearm circulation have recently been shown to reflect acute changes in endothelial nitric oxide synthase (eNOS)-activity. Whether basal plasma nitrite is a general marker of constitutive NOS-activity in vivo is yet unclear. Due to the rapid metabolism of nitrite in blood and the difficulties in its analytical determination literature data on levels of nitrite in mammals are largely inconsistent. We hypothesized that constitutive NOS-activity in the circulatory system is relatively uniform throughout the mammalian kingdom. If true, this should result in comparable systemic plasma nitrite levels in different species. Using three different analytical approaches we determined plasma nitrite concentration to be in a nanomolar range in a variety of species: humans (305 +/- 23 nmol/l), monkeys (367 +/- 62 nmol/l), minipigs (319 +/- 24 nmol/l), dogs (305 +/- 50 nmol/l), rabbits (502 +/- 21 nmol/l), guinea pigs (412 +/- 44 nmol/l), rats (191 +/- 43 nmol/l), and mice (457 +/- 51 nmol/l). Application of different NOS-inhibitors in humans, minipigs, and dogs decreased NOS-activity and thereby increased vascular resistance. This was accompanied by a significant, up to 80%, decrease in plasma nitrite concentration. A comparison of plasma nitrite concentrations between eNOS(-/-) and NOS-inhibited wild-type mice revealed that 70 +/- 5% of plasma nitrite is derived from eNOS. These results provide evidence for a uniform constitutive vascular NOS-activity across mammalian species.

Accuracy and reproducibility of long-term implanted transit-time ultrasound flow probes in dogs

Objective: To assess the accuracy and reproducibility of long-term implanted ultrasound transit-time flow probes for measuring cardiac output. Design: Prospective animal study. Settings: Animal research laboratory in a university department. Animals: Eleven anaesthetised dogs, 24–34 kg. Measurements and results: Flow probes (16–24 mm S-series, Transonic) were implanted around the pulmonary artery for a mean duration of 22 months (range 6–47 months). Comparisons (n = 147) were made between cardiac output thus obtained and that measured by the direct Fick principle using oxygen uptake (Deltatrac II Metabolic Monitor) and the arterial to mixed venous oxygen content difference measured by a galvanic cell (Lex-O2-Con-TL). Measurements were made either during baseline conditions or during pharmacologically altered cardiac output (range 22–180 ml · kg–1· min–1). Regardless of the intervention, the two methods yielded the same results in half of the dogs. In the others, however, cardiac output was underestimated by the flow probes by up to 38 % (probably because of non-perpendicular position of the probe towards the vessel). This difference was constant for the whole range of cardiac output studied and remained constant over the entire observation period for each individual dog, so that a correction factor was used. Thereafter, the mean difference between the two methods was –1.1 ml · kg–1· min–1 with a precision (SD) of 14.2 ml · kg–1· min–1 for all experiments. Conclusions: After in vivo calibration, ultrasound transit-time flow probes measure cardiac output precisely for several years, regardless of the intervention.

Desflurane increases heart rate independent of sympathetic activity in dogs

Background and objective: Desflurane has been shown to increase sympathetic activity and heart rate (HR) in a concentration-dependent manner. Nevertheless, desflurane, like all other volatile anaesthetics, increased HR in parallel to vagal inhibition in a previous study. Therefore, our hypothesis is that desflurane elicits tachycardia by vagal inhibition rather than by activation of the sympathetic nervous system. Methods: Six dogs were studied awake and during desflurane anaesthesia (1 and 2 MAC) alone, after pretreatment with propranolol (2 mg kg−1 followed by 1 mg kg−1 h−1), or after pretreatment with atropine (0.1 mg kg−1 followed by 0.05 mg kg−1 h−1). The effects on HR and HR variability were compared by an analysis of variance (P < 0.05). HR variability was analysed in the frequency domain as power in the high-(0.15-0.5 Hz, vagal activity) and low-frequency range (0.04-0.15 Hz, sympathetic and vagal activity). Results: HR increased during 2 MAC of desflurane from about 60 (awake) to 118 ± 2 beats min−1 (mean ± SEM) in controls and to 106 ± 3 beats min−1 in dogs pre-treated with propranolol. In contrast, pretreatment with atropine increased HR from 64 ± 2 to 147 ± 5 beats min−1 (awake) and HR decreased to 120 ± 5 beats min−1 after adding desflurane. High-frequency power correlated inversely with HR (r2 = 0.95/0.93) during desflurane alone and in the presence of β-adrenoceptor blockade, with no significant difference between regression lines. There was no correlation between these variables during atropine/desflurane. Conclusions: The increase in HR elicited by desflurane mainly results from vagal inhibition and not from sympathetic activation.

Endogenous Endothelin and Vasopressin Support Blood Pressure During Epidural Anesthesia in Conscious Dogs

We studied whether endogenous endothelin, like endogenous vasopressin, helps to maintain blood pressure during high epidural anesthesia when efferent sympathetic drive is diminished. On different days, six awake dogs underwent each of the following five interventions: blockade of vasopressin V1a receptors using [d (CH2)5Tyr(Me2)]AVP, (40 &mgr;g/kg) or endothelin receptors using tezosentan (3 mg/kg followed by 3 mg · kg−1 · h−1) with or without epidural anesthesia (1% lidocaine, intraindividual dose did not differ between experiments), and epidural saline (n = 5). The effects of endothelin- or vasopressin-receptor blockade were analyzed (means ± sem) and compared by an analysis of variance for repeated measures (paired Student’s t-test, &agr;-adjusted, P < 0.05). Vasopressin-receptor blockade decreased blood pressure (10 ± 2 mm Hg) only in the presence of epidural anesthesia, whereas endothelin-receptor blockade reduced blood pressure both in the presence and absence of epidural anesthesia (12 ± 3 versus 10 ± 1 mm Hg). During baseline and each intervention, plasma concentrations of vasopressin and big-endothelin were measured and compared by a Wilcoxon’s rank sum test;P < 0.05. Vasopressin concentrations increased during epidural anesthesia and after additional endothelin receptor blockade, but big-endothelin concentrations remained unchanged during each intervention. We conclude that vasopressin acts as a reserve system, as it stabilizes blood pressure specifically during epidural anesthesia, whereas the unchanged concentrations of big-endothelin indicate that the endothelin system is not specifically activated to support blood pressure during epidural anesthesia.

Fenoldopam--but not dopamine--selectively increases gastric mucosal oxygenation in dogs.

ObjectiveTo compare the effects of fenoldopam and dopamine on gastric mucosal and systemic oxygenation, and to identify the receptors involved. DesignRandomized controlled animal study. SettingUniversity research department of experimental anesthesiology. SubjectsSeven anesthetized dogs with chronically implanted ultrasound flow probes around the pulmonary artery for continuous measurement of cardiac output. InterventionsOn different days, the dogs received in random order either the selective DA1-agonist fenoldopam (0.1 and 1.0 &mgr;g·kg−1·min−1, with or without DA1-blocker pretreatment), dopamine (2.5 and 5.0 &mgr;g·kg−1·min−1, with or without &agr;1-blocker pretreatment), or saline (control). Measurements and Main ResultsWe continuously measured regional microvascular hemoglobin oxygen saturation (&mgr;HbO2) in gastric mucosa by reflectance spectrophotometry, and systemic oxygen delivery. Fenoldopam increased gastric mucosal &mgr;HbO2 by approximately 20%, and this effect was prevented by selective DA1-receptor blockade. In contrast, dopamine neither alone nor during &agr;1-blockade altered &mgr;HbO2. With respect to systemic measures of oxygen transport, fenoldopam had negligible effects, whereas dopamine (with and without &agr;1-blocker pretreatment) dose-dependently increased cardiac output and systemic oxygen delivery by approximately 30%. ConclusionsFenoldopam dose-dependently increased microvascular oxygenation of the gastric mucosa without changing systemic oxygen transport, i.e., this drug acted selectively on the splanchnic mucosa. The increase in gastric mucosal oxygenation was mediated by DA1-receptors. In contrast, dopamine markedly increased systemic oxygen transport, but did not affect microvascular oxygenation of gastric mucosa. This lacking effect on gastric mucosal oxygenation was not caused by &agr;1-mediated vasoconstriction. The regional effects of both catecholamines could not be deduced from systemic hemodynamics and oxygenation.

Dopamine under α1-blockade, but not dopamine alone or fenoldopam, increases depressed gastric mucosal oxygenation

ObjectiveTo compare the effects of dopamine, both in the presence and absence of &agr;1-blockade, and fenoldopam on microvascular gastric mucosal oxygenation and systemic oxygen transport under compromised circulatory conditions, both without and with fluid resuscitation. DesignRandomized controlled animal study. SettingUniversity department of anesthesiology. SubjectsEight anesthetized dogs with chronically implanted ultrasound flow probes around the pulmonary artery for continuous measurement of cardiac output. InterventionsOn different days, the dogs received in random order either dopamine (2.5 and 5.0 &mgr;g·kg−1·min−1, with or without &agr;1-blocker pretreatment), the selective DA1-agonist fenoldopam (0.1 and 1.0 &mgr;g·kg−1·min−1, with and without DA1-blocker pretreatment), or saline (control). These interventions were performed under compromised cardiocirculatory conditions (induced by ventilation with positive end-expiratory pressure [PEEP] of 10 cm H2O), both without and with fluid resuscitation. Measurements and Main ResultsWe continuously measured regional microvascular hemoglobin saturation (&mgr;HbO2) in gastric mucosa by reflectance spectrophotometry and systemic oxygen transport (&U1E0A;O2). Ventilation with PEEP significantly decreased &U1E0A;O2 (from 19 ± 2 to 9 ± 1 mL·kg−1·min−1, mean ± sem) and gastric mucosal &mgr;HbO2 (from 57 ± 2% to 37 ± 3%). Fluid resuscitation restored &U1E0A;O2 back to baseline (from 9 ± 1 to 19 ± 2 mL·kg−1·min−1) but only partially restored &mgr;HbO2 (from 37 ± 3% to 50 ± 4%). Under both conditions, dopamine with and without &agr;1-blockade significantly increased &U1E0A;O2 (by about 5 mL·kg−1·min−1 in the nonresuscitated state and 10 mL·kg−1·min−1 in the fluid resuscitated state, respectively), but only dopamine in the presence of &agr;1-blockade also significantly increased gastric mucosal &mgr;HbO2 (by 5 ± 1% and 7 ± 2% in the nonresuscitated and fluid resuscitated states, respectively). Fenoldopam under all study conditions did not significantly affect &U1E0A;O2 or &mgr;HbO2, either in the presence or absence of DA1-blockade. ConclusionsDuring compromised cardiocirculatory conditions, &agr;1-receptor activation during dopamine infusion prevented an increase in gastric mucosal oxygenation. Furthermore, selective DA1-stimulation (by fenoldopam) was insufficient to overcome the PEEP-induced depression of &mgr;HbO2. The responses of gastric mucosal oxygenation did not parallel changes in systemic oxygen transport. These findings were independent of fluid resuscitation.

Endogenous nitric oxide reduces the efficacy of the endothelin system to maintain blood pressure during high epidural anaesthesia in conscious dogs

Background and objective: During high epidural anaesthesia, endothelin only contributes minimally to blood pressure stabilization. This phenomenon could result from the inhibitory action of nitric oxide on the endothelin system. To clarify this, we studied the interaction between nitric oxide and endothelin during high epidural anaesthesia in conscious dogs, in comparison to the interaction of nitric oxide and vasopressin. Methods: Six animals were used in 45 individual experiments randomly arranged as follows: N‐ω‐nitro‐arginine‐methylester 0.3–10 mg kg−1 under physiological conditions or during high epidural anaesthesia (lidocaine 1%) and N‐ω‐nitro‐arginine‐methylester (l‐NAME) 0.3–10 mg kg−1 after preceding endothelin (Tezosentan®) or vasopressin (&bgr;‐mercapto‐&bgr;,&bgr;‐cyclo‐penta‐methylene‐propionyl‐O‐Me‐Tyr‐Arg‐vasopressin) receptor blockade under physiological conditions or during high epidural anaesthesia. During control experiments normal saline was injected either intravenously (n = 5) or into the epidural space (n = 4). Results: N‐ω‐nitro‐arginine‐methylester increased mean arterial pressure dose‐dependently in all groups. However, this effect was substantially reduced in the presence of the endothelin receptor antagonist compared to N‐ω‐nitro‐arginine‐methylester alone, both under control conditions (7 ± 3 vs. 21 ± 3 mmHg; P < 0.05) and during high epidural anaesthesia (17 ± 3 vs. 30 ± 1 mmHg; P < 0.05). Blockade of vasopressin showed no similar relationship with N‐ω‐nitro‐arginine‐methylester. Conclusions: The diminished increase in mean arterial pressure after injection of N‐ω‐nitro‐arginine‐methylester only during endothelin receptor blockade indicates that endogenous nitric oxide inhibits the action of endothelin during high epidural anaesthesia and might thus explain the reduced efficacy of endothelin in maintaining blood pressure during high epidural anaesthesia.