Effects of dexmedetomidine on kidney and brain tissue microcirculation and histology in ovine cardiopulmonary bypass: a randomised controlled trial.

Cardiac surgery requiring cardiopulmonary bypass is associated with postoperative acute kidney injury and neurocognitive disorders, including delirium. Intra-operative inflammation and/or impaired tissue perfusion/oxygenation are thought to be contributors to these outcomes. It has been hypothesised that these problems may be ameliorated by the highly selective α2 -agonist, dexmedetomidine. We tested the effects of dexmedetomidine on renal and cerebral microcirculatory tissue perfusion, oxygenation and histology in a clinically relevant ovine model. Sixteen sheep were studied while conscious, after induction of anaesthesia and during 2 h of cardiopulmonary bypass. Eight sheep were allocated randomly to receive an intravenous infusion of dexmedetomidine (0.4-0.8 µg.kg-1 .h-1 ) from induction of anaesthesia to the end of cardiopulmonary bypass, and eight to receive an equivalent volume of matched placebo (0.9% sodium chloride). Commencement of cardiopulmonary bypass decreased renal medullary tissue oxygenation in the placebo group (mean (95%CI) 5.96 (4.24-7.23) to 1.56 (0.84-2.09) kPa, p = 0.001), with similar hypoxic levels observed in the dexmedetomidine group (6.33 (5.33-7.07) to 1.51 (0.33-2.39) kPa, p = 0.002). While no differences in kidney function (i.e. reduced creatinine clearance) were evident, a greater incidence of histological renal tubular injury was observed in sheep receiving dexmedetomidine (7/8 sheep) compared with placebo (2/8 sheep), p = 0.041. Graded on a semi-quantitative scale (0-3), median (IQR [range]) severity of histological renal tubular injury was higher in the dexmedetomidine group compared with placebo (1.5 (1-2 [0-3]) vs. 0 (0-0.3 [0-1]) respectively, p = 0.013). There was no difference in cerebral tissue microglial activation (neuroinflammation) between the groups. Dexmedetomidine did not reduce renal medullary hypoxia or cerebral neuroinflammation in sheep undergoing cardiopulmonary bypass.

Clonidine and dexmedetomidine increase the pressor response to norepinephrine in experimental sepsis: a pilot study.

OBJECTIVE: During septic shock, vasopressors are a cornerstone of therapy. In septic shock, very high doses of vasopressors sometimes have to be used due to vascular desensitization, the mechanisms of which are poorly understood. This study assesses whether α-2 agonists increase pressor responsiveness following lipopolysaccharide administration. DESIGN: Parallel groups of animals (n = 7 per group) subjected to pharmacologic interventions. SETTING: Physiology laboratory. SUBJECTS: Rats. INTERVENTIONS: In anesthetized rats, the pressor responses to increasing doses of norepinephrine (norepinephrine-systolic pressure curve) were assessed during a baseline period, after injection of saline or lipopolysaccharide, and after subsequent injection of saline, dexmedetomidine (100 µg/kg IV), or clonidine (200 µg/kg IV). MEASUREMENTS AND MAIN RESULTS: Differences in the slopes of the norepinephrine-pressure curves were assessed across drug treatments and intervals. The pressor dose of norepinephrine necessary to increase systolic pressure by 33 and 100 mm Hg (pressor dose 33 and pressor dose 100) was determined. Pressor responsiveness to norepinephrine decreased slightly over time in the saline-saline group (saline 1 or 2 vs baseline: mean decrease of the slope, 2 mm Hg/µg/kg norepinephrine; p < 0.05), whereas there was a large decrease after lipopolysaccharide (lipopolysaccharide vs baseline: mean decrease of the slope, 7.2; p < 0.001). Clonidine alone had no effect, but when administered following lipopolysaccharide, it caused a striking increase in pressor responsiveness (mean slope after lipopolysaccharide, 10.7 [95% CI, 9.9-11.6]; after clonidine, 17.5 [95% CI, 16.7-18.4]). Similarly, dexmedetomidine administered after lipopolysaccharide caused a large increase in pressor responsiveness above lipopolysaccharide values. Accordingly the pressor dose 33 and pressor dose 100 values were lowered following lipopolysaccharide and restored by α-2 agonists. CONCLUSIONS: The pressor response to norepinephrine was reduced following lipopolysaccharide and increased to baseline levels following α-2 agonists.

Specific control of sympathetic nerve activity to the mammalian heart and kidney.

There is a large body of evidence indicating that sympathetic nerves to individual organs are specifically controlled, but only few studies have compared the control of cardiac sympathetic nerve activity (CSNA) with activity in other sympathetic nerves. In this review, changes in sympathetic activity to the heart and kidneys are described during increases in brain [Na+] and in heart failure (HF). In conscious sheep, increases in brain [Na+] increased CSNA and arterial pressure and, conversely, decreased renal sympathetic nerve activity (RSNA), promoting urinary sodium loss. These organ-specific effects are mediated via a neural pathway that includes an angiotensinergic synapse, the lamina terminalis and the paraventricular nucleus of the hypothalamus. There is also evidence of differential control of SNA in HF. In normal sheep, the resting burst incidence of CSNA was much lower than that of RSNA, whereas in HF they increased to similar, almost maximal levels in both nerves. Arterial baroreflex control of both these nerves was unchanged in HF, but the response of CSNA to changes in blood volume was almost absent. These data indicate that in HF the lower arterial pressure leads to reduced baroreflex inhibition of SNA, which, together with the lack of an inhibitory response to the increased volume and cardiac pressures, would contribute to the sympathoexcitation observed. These studies demonstrate differences in the control of CSNA and RSNA, enabling selective actions on the heart and kidney to restore fluid and electrolyte homeostasis in the case of elevated brain [Na+] and to increase cardiac output in HF.

Discharge properties of cardiac and renal sympathetic nerves and their impaired responses to changes in blood volume in heart failure.

Sympathetic nerve activity (SNA) consists of discharges that vary in amplitude and frequency, reflecting the level of recruitment of nerve fibers and the rhythmic generation and entrainment of activity by the central nervous system. It is unknown whether selective changes in these amplitude and frequency components account for organ-specific changes in SNA in response to alterations in blood volume or for the impaired SNA responses to volume changes in heart failure (HF). To address these questions, we measured cardiac SNA (CSNA) and renal SNA (RSNA) simultaneously in conscious, normal sheep and sheep in HF induced by rapid ventricular pacing. Volume expansion decreased CSNA (-62 +/- 10%, P < 0.05) and RSNA (-59 +/- 10%, P < 0.05) equally (n = 6). CSNA decreased as a result of a reduction in burst frequency, whereas RSNA fell because of falls in burst frequency and amplitude. Hemorrhage increased CSNA (+74 +/- 9%, P < 0.05) more than RSNA (+21 +/- 5%, P < 0.09), in both cases because of increased burst frequency, whereas burst amplitude decreased. In HF, burst frequency of CSNA (from 26 +/- 3 to 75 +/- 3 bursts/min) increased more than that of RSNA (from 63 +/- 4 to 79 +/- 4 bursts/min). In HF, volume expansion caused no change in CSNA and an attenuated decrease in RSNA, due entirely to decreased burst amplitude. Hemorrhage did not significantly increase SNA in either nerve in HF. These findings support the concept that the number of sympathetic fibers recruited and their firing frequency are controlled independently. Furthermore, afferent stimuli, such as changes in blood volume, cause organ-specific responses in each of these components, which are also selectively altered in HF.

Effect of central urotensin II on heart rate, blood pressure and brain Fos immunoreactivity in conscious rats.

Central administration of urotensin II (UII) increases heart rate (HR), cardiac contractility, and plasma levels of epinephrine and glucose. To investigate the mechanisms causing these responses we examined the effects of i.c.v. administration of rat UII (10 microg) on the sympatho-adrenal and pituitary-adrenal axes in conscious rats, and we mapped the brain sites activated by UII by immunohistochemically detecting Fos expression. In six conscious rats i.c.v. UII, but not vehicle, increased HR significantly 60-90 min after treatment and increased plasma glucose at 60 and 90 min, both indicators of increased epinephrine release. Plasma corticosterone levels were significantly elevated 90 min after i.c.v. UII. Conscious rats, given i.c.v. UII (n=12) and killed after 100 or 160 min, showed increased Fos-immunoreactivity (Fos-IR) in the nucleus of the solitary tract and the central nucleus of the amygdala (CeA) at both time points, compared with vehicle (n=11). In UII-treated rats, Fos-IR in the paraventricular nucleus of the hypothalamus (PVN) was significantly elevated at 160 min, but not 100 min, compared with vehicle. There were no increases in Fos-IR in the rostral ventrolateral medulla or the A5 cell group, areas associated with sympathetic outflow to the adrenal gland. In summary, i.c.v. UII increased HR and plasma glucose and corticosterone in conscious rats. UII increased Fos-IR in the CeA and PVN, but over a longer time course in the latter. These findings indicate that UII acts on specific brain nuclei to stimulate the hypothalamo-pituitary-adrenal axis and to stimulate adrenal sympathetic nerve activity.

Cortical Brain Stimulation with Endovascular Electrodes.

Electrical stimulation of neural tissue and recording of neural activity are the bases of emerging prostheses and treatments for spinal cord injury, stroke, sensory deficits, and drug-resistant neurological disorders. Safety and efficacy are key aspects for the clinical acceptance of therapeutic neural stimulators. The cortical vasculature has been shown to be a safe site for implantation of electrodes for chronically recording neural activity, requiring no craniotomy to access high-bandwidth, intracranial EEG. This work presents the first characterization of endovascular cortical stimulation measured using cortical subdural surface recordings. Visual stimulation was used to verify electrode viability and cortical activation was compared with electrically evoked activity. Due to direct activation of the neural tissue, the latency of responses to electrical stimulation was shorter than for that of visual stimulation. We also found that the center of neural activation was different for visual and electrical stimulation indicating an ability of the stentrode to provide localized activation of neural tissue.

Renal haemodynamics and oxygenation during and after cardiac surgery and cardiopulmonary bypass.

Acute kidney injury (AKI) is a common complication following cardiac surgery performed on cardiopulmonary bypass (CPB) and has important implications for prognosis. The aetiology of cardiac surgery-associated AKI is complex, but renal hypoxia, particularly in the medulla, is thought to play at least some role. There is strong evidence from studies in experimental animals, clinical observations and computational models that medullary ischaemia and hypoxia occur during CPB. There are no validated methods to monitor or improve renal oxygenation during CPB, and thus possibly decrease the risk of AKI. Attempts to reduce the incidence of AKI by early transfusion to ameliorate intra-operative anaemia, refinement of protocols for cooling and rewarming on bypass, optimization of pump flow and arterial pressure, or the use of pulsatile flow, have not been successful to date. This may in part reflect the complexity of renal oxygenation, which may limit the effectiveness of individual interventions. We propose a multi-disciplinary pathway for translation comprising three components. Firstly, large-animal models of CPB to continuously monitor both whole kidney and regional kidney perfusion and oxygenation. Secondly, computational models to obtain information that can be used to interpret the data and develop rational interventions. Thirdly, clinically feasible non-invasive methods to continuously monitor renal oxygenation in the operating theatre and to identify patients at risk of AKI. In this review, we outline the recent progress on each of these fronts.

Ring and peg electrodes for minimally-Invasive and long-term sub-scalp EEG recordings.

OBJECTIVE: Minimally-invasive approaches are needed for long-term reliable Electroencephalography (EEG) recordings to assist with epilepsy diagnosis, investigation and more naturalistic monitoring. This study compared three methods for long-term implantation of sub-scalp EEG electrodes. METHODS: Three types of electrodes (disk, ring, and peg) were fabricated from biocompatible materials and implanted under the scalp in five ambulatory ewes for 3months. Disk electrodes were inserted into sub-pericranial pockets. Ring electrodes were tunneled under the scalp. Peg electrodes were inserted into the skull, close to the dura. EEG was continuously monitored wirelessly. High resolution CT imaging, histopathology, and impedance measurements were used to assess the status of the electrodes at the end of the study. RESULTS: EEG amplitude was larger in the peg compared with the disk and ring electrodes (p<0.05). Similarly, chewing artifacts were lower in the peg electrodes (p<0.05). Electrode impedance increased after long-term implantation particularly for those within the bone (p<0.01). Micro-CT scans indicated that all electrodes stayed within the sub-scalp layers. All pegs remained within the burr holes as implanted with no evidence of extrusion. Eight of 10 disks partially eroded into the bone by 1.0mm from the surface of the skull. The ring arrays remained within the sub-scalp layers close to implantation site. Histology revealed that the electrodes were encapsulated in a thin fibrous tissue adjacent to the pericranium. Overlying this was a loose connective layer and scalp. Erosion into the bone occurred under the rim of the sub-pericranial disk electrodes. CONCLUSIONS: The results indicate that the peg electrodes provided high quality EEG, mechanical stability, and lower chewing artifact. Whereas, ring electrode arrays tunneled under the scalp enable minimal surgical techniques to be used for implantation and removal.

Cardiac actions of central but not peripheral urotensin II are prevented by beta-adrenoceptor blockade.

Urotensin II (UII) is a highly conserved peptide that has potent cardiovascular actions following central and systemic administration. To determine whether the cardiovascular actions of UII are mediated via beta-adrenoceptors, we examined the effect of intravenous (IV) propranolol on the responses to intracerebroventricular (ICV) and IV administration of UII in conscious sheep. Sheep were surgically instrumented with ICV guide tubes and flow probes or cardiac sympathetic nerve recording electrodes. ICV UII (0.2 nmol/kg over 1 h) caused prolonged increases in heart rate (HR; 33 +/- 11 beats/min; P < 0.01), dF/dt (581 +/- 83 L/min/s; P < 0.001) and cardiac output (2.3 +/- 0.4 L/min; P < 0.001), accompanied by increases in coronary (19.8 +/- 5.4 mL/min; P < 0.01), mesenteric (211 +/- 50 mL/min; P < 0.05) and iliac (162 +/- 31 mL/min; P < 0.001) blood flows and plasma glucose (7.0 +/- 2.6 mmol/L; P < 0.05). Propranolol (30 mg bolus followed by 0.5 mg/kg/h IV) prevented the cardiac responses to ICV UII and inhibited the mesenteric vasodilatation. At 2 h after ICV UII, when HR and mean arterial pressure (MAP) were increased, cardiac sympathetic nerve activity (CSNA) was unchanged and the relation between CSNA and diastolic pressure was shifted to the right (P < 0.05). The hyperglycemia following ICV UII was abolished by ganglion blockade but not propranolol. IV UII (20 nmol/kg) caused a transient increase in HR and fall in stroke volume; these effects were not blocked by propranolol. These results demonstrate that the cardiac actions of central UII depend on beta-adrenoreceptor stimulation, secondary to increased CSNA and epinephrine release, whereas the cardiac actions of systemic UII are not mediated by beta-adrenoreceptors and probably depend on a direct action of UII on the heart.

Amylin induces natriuresis by a central angiotensin-dependent mechanism.

This study provides evidence that amylin acts centrally to increase sodium excretion in the sheep. Amylin was infused at 8 mg/h into a carotid artery (IC), via a lateral ventricle (ICV), intravenously (IV) or intra-renally (IR) into conscious sheep (n=5 per group). Renal sodium excretion increased by at least 3-fold after 1 h of amylin infusion by ICV (66+/-14 to 367+/-35 mmol/min) and IC (78+/-14 to 244+/-22 mmol/min) routes of administration. Amylin infusion IV caused a 1.5-fold increase in sodium excretion while IR infusion did not have a significant effect. The natriuretic effect of ICV infused amylin was blocked by pre-treatment with the angiotensin AT1 receptor antagonist, losartan (1 mg/h). No changes in blood pressure or heart rate were recorded at this dose of amylin by any route of administration. Plasma renin concentration increased (1.32+/-0.22 to 2.55+/-0.73 pmol/Ang I/h; P<0.05) following IR infusion of amylin, and remained unchanged when amylin was infused by the other routes of administration. We conclude that amylin causes changes in sodium excretion in sheep through a central, angiotensin-dependent pathway and that amylin may increase renin secretion by a direct effect on the kidney.

Pressor Response to Noradrenaline in the Setting of Septic Shock: Anything New under the Sun-Dexmedetomidine, Clonidine? A Minireview.

Progress over the last 50 years has led to a decline in mortality from ≈70% to ≈20% in the best series of patients with septic shock. Nevertheless, refractory septic shock still carries a mortality close to 100%. In the best series, the mortality appears related to multiple organ failure linked to comorbidities and/or an intense inflammatory response: shortening the period that the subject is exposed to circulatory instability may further lower mortality. Treatment aims at reestablishing circulation within a "central" compartment (i.e., brain, heart, and lung) but fails to reestablish a disorganized microcirculation or an adequate response to noradrenaline, the most widely used vasopressor. Indeed, steroids, nitric oxide synthase inhibitors, or donors have not achieved overwhelming acceptance in the setting of septic shock. Counterintuitively, α 2-adrenoceptor agonists were shown to reduce noradrenaline requirements in two cases of human septic shock. This has been replicated in rat and sheep models of sepsis. In addition, some data show that α 2-adrenoceptor agonists lead to an improvement in the microcirculation. Evidence-based documentation of the effects of alpha-2 agonists is needed in the setting of human septic shock.

The brain renin-angiotensin system: location and physiological roles.

Angiotensinogen, the precursor molecule for angiotensins I, II and III, and the enzymes renin, angiotensin-converting enzyme (ACE), and aminopeptidases A and N may all be synthesised within the brain. Angiotensin (Ang) AT(1), AT(2) and AT(4) receptors are also plentiful in the brain. AT(1) receptors are found in several brain regions, such as the hypothalamic paraventricular and supraoptic nuclei, the lamina terminalis, lateral parabrachial nucleus, ventrolateral medulla and nucleus of the solitary tract (NTS), which are known to have roles in the regulation of the cardiovascular system and/or body fluid and electrolyte balance. Immunohistochemical and neuropharmacological studies suggest that angiotensinergic neural pathways utilise Ang II and/or Ang III as a neurotransmitter or neuromodulator in the aforementioned brain regions. Angiotensinogen is synthesised predominantly in astrocytes, but the processes by which Ang II is generated or incorporated in neurons for utilisation as a neurotransmitter is unknown. Centrally administered AT(1) receptor antagonists or angiotensinogen antisense oligonucleotides inhibit sympathetic activity and reduce arterial blood pressure in certain physiological or pathophysiological conditions, as well as disrupting water drinking and sodium appetite, vasopressin secretion, sodium excretion, renin release and thermoregulation. The AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and plays a role in memory mechanisms. In conclusion, angiotensinergic neural pathways and angiotensin peptides are important in neural function and may have important homeostatic roles, particularly related to cardiovascular function, osmoregulation and thermoregulation.

Cardiovascular effects of long-term central and peripheral administration of urocortin, corticotropin-releasing factor, and adrenocorticotropin in sheep.

The neuroendocrine hormones ACTH and corticotropin- releasing factor (CRF), which are involved in the stress response, have acute effects on arterial pressure. New evidence indicates that urocortin (UCN), the putative agonist for the CRF type 2 receptor, has selective cardiovascular actions. The responses to long-term infusions of these hormones, both peripherally and centrally, in conscious animals have not been studied. Knowledge of the long-term effects is important because they may differ considerably from their acute actions, and stress is frequently a chronic stimulus. The present experiments investigated the cardiovascular effects of CRF, UCN, and ACTH in conscious sheep. Infusions were made either into the lateral cerebral ventricles (i.c.v.) or i.v. over 4 d at 5 microg/h. UCN infused i.c.v. or i.v. caused a prolonged increase in heart rate (HR) (P < 0.01) and a small increase in mean arterial pressure (MAP) (P < 0.05). CRF infused i.c.v. or i.v. progressively increased MAP (P < 0.05) but had no effect on HR. Central administration of ACTH had no effect, whereas systemic infusion increased MAP and HR (P < 0.001). In conclusion, long-term administration of these three peptides associated with the stress response had prolonged, selective cardiovascular actions. The striking finding was the large and sustained increase in HR with i.c.v. and i.v. infusions of UCN. These responses are probably mediated by CRF type 2 receptors because they were not reproduced by infusions of CRF.

Differential regional hemodynamic effects of corticotropin in conscious sheep.

The regional hemodynamic effects of 5 days of intravenous infusion of corticotropin (ACTH) (5 micrograms/kg per day) were examined in conscious sheep (n = 8). Mean arterial pressure increased from 81 +/- 2 to 93 +/- 3 mm Hg (P < .001) on day 2 of ACTH and remained at this level during the infusion. Cardiac output increased from 5.13 +/- 0.19 to 6.06 +/- 0.33 L/min (P < .01) because of an increase in stroke volume from 65 +/- 4 to 79 +/- 8 mL per beat (P < .01); heart rate remained unchanged. ACTH did not alter total peripheral conductance but had differential effects on regional conductances. Mesenteric conductance fell from 5.8 +/- 0.2 to a minimum of 4.9 +/- 0.3 (mL/min)/mm Hg (P < .05), and renal conductance increased from 3.5 +/- 0.3 to 4.6 +/- 0.3 (mL/min)/mm Hg (P < .001). There was a small increase in iliac conductance (P < .05) and no change in coronary conductance. Mesenteric and iliac conductances fell progressively over 24 to 48 hours, whereas renal conductance increased rapidly after 3 hours of ACTH, reaching a maximum after 6 hours. Renal blood flow was increased during ACTH infusion from 278 +/- 18 to 403 +/- 23 mL/min (P < .001); mesenteric blood flow was unchanged; there was a small increase in iliac blood flow (P < .01); and coronary blood flow increased (P < .05), paralleling the change in cardiac output.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional hemodynamic and endocrine effects of aldosterone and cortisol in conscious sheep. Comparison with the effects of corticotropin.

We studied the cardiovascular responses to 5 days' infusion of aldosterone (10 micrograms/h) and cortisol (5 mg/h) to determine the possible contribution of mineralocorticoid and glucocorticoid actions to the regional hemodynamic changes caused by corticotropin. These infusion rates produce plasma levels similar to those seen during corticotropin stimulation. In five conscious sheep aldosterone progressively increased mean arterial pressure (P < .001) to a maximum of 11 mm Hg on day 5, whereas cortisol increased pressure by 5 mm Hg (P < .01) within 24 hours. Cardiac outputs on the control day and on day 5 of infusion were 4.4 +/- 0.3 and 4.9 +/- 0.3 L/min, respectively, for aldosterone and 4.3 +/- 0.4 and 5.0 +/- 0.4 L/min for cortisol. Neither steroid significantly altered total peripheral conductance, but they had different, nonuniform regional hemodynamic effects. Mesenteric conductance fell progressively with aldosterone from 7.14 +/- 0.35 (mL/min)/mm Hg to a minimum of 6.17 +/- 0.38 (P < .01) on day 5 of infusion. Mesenteric conductance was transiently reduced with cortisol, but this was not significant over the 5 days. Renal conductance was unchanged with aldosterone, but cortisol caused a rapid, sustained increase in renal conductance from 2.9 +/- 0.3 to 4.0 +/- 0.4 (mL/min) / mm Hg (P < .001) within 24 hours, similar to the increase caused by corticotropin. As with corticotropin there were only minor changes in the coronary and iliac vascular beds. In summary, these two endogenous steroids had contrasting, nonuniform regional hemodynamic effects, aldosterone causing mesenteric vasoconstriction, and cortisol causing renal vasodilatation.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional and autoradiographic studies to locate the sites at which clonidine acts to cause hyperglycaemia and inhibition of opiate-induced sympathetic outflow.

The effect of intracisternal administration of clonidine (1 microgram/kg) on the response to intravenous injection of morphine (4 mg/kg) was examined in conscious rabbits. Morphine acts on central opiate receptors to increase sympathetic outflow and cause hypertension. Clonidine, given intracisternally, prevented the morphine-induced rise in mean arterial pressure, fall in heart rate and increase in catecholamines in plasma. Using in vitro autoradiography, alpha 2-adrenoceptors were localised in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus and these may be two of the sites at which clonidine acts. Clonidine also causes hyperglycaemia after intravenous administration and the site of action was investigated by comparing the effects of intravenous and intracerebroventricular administration of clonidine (1, 2 and 2 micrograms/kg), given at intervals of 30 min. Similar increases in glucose occurred after intraventricular and intravenous administration of clonidine, indicating that it has both central and peripheral actions, which increase glucose by different mechanisms. Clonidine, given intraventricularly also reduced mean arterial pressure and heart rate but there were no effects after intravenous administration. These studies demonstrate that the inhibitory effect of clonidine on opiate-induced stimulation of sympatho-adrenal outflow is central, whereas the hyperglycaemic effect of clonidine depends on both central and peripheral mechanisms.

Effects of infusion of combinations of adrenocorticosteroids on systemic and regional haemodynamics in conscious sheep.

OBJECTIVE: To compare the cardiovascular, metabolic and endocrine responses to infusions of two combinations of adrenocorticosteroids and to determine their contribution to the haemodynamic effects of corticotrophin. METHODS: The effect of 5 day's infusion of combinations of seven corticosteroids (aldosterone, cortisol, corticosterone, 11-deoxycorticosterone, 11-deoxycortisol, 17 alpha-hydroxyprogesterone and 17 alpha-hydroxy-20 alpha-dihydroprogesterone and five corticosteroids (17 alpha-hydroxyprogesterone and 17 alpha,20 alpha-hydroxyprogesterone omitted) on arterial pressure, cardiac output (measured using electromagnetic flow probes) and regional blood flows (measured using transit-time flow probes) was determined in conscious sheep. RESULTS: Combined infusion of seven steroids increased mean arterial pressure from 79 +/- 2 to 91 +/- 2 mmHg (day 5). Cardiac output increased from 5.42 +/- 0.22 to 6.55 +/- 0.41 l/min owing to an increase in stroke volume. Mesenteric conductance fell from 6.3 +/- 0.4 to 5.4 +/- 0.5 ml/min per mmHg, and renal conductance increased from 3.1 +/- 0.1 to 4.0 +/- 0.1 ml/min per mmHg, resulting in no change in total peripheral conductance. There were only minor effects on the coronary and iliac vascular beds. Infusion of five steroids caused similar changes in mean arterial pressure (from 78 +/- 1 to 89 +/- 2 mmHg on day 5), cardiac output and regional blood flows. The cardiovascular, fluid, electrolyte and endocrine responses to both steroid treatments were similar to those with corticotrophin. CONCLUSIONS: Infusion of combinations of seven or five adrenocortical steroids reproduced the cardiovascular actions of corticotrophin, namely increases in arterial pressure, and cardiac output, mesenteric vasoconstriction and renal vasodilation. This contrasts with previous studies in which only an infusion of steroids including 17 alpha-hydroxyprogesterone and 17 alpha,20 alpha-OHP reproduced the pressor effect of corticotrophin fully, possibly because in these sheep, as has been proposed to be the case in humans, the dose-response curves for the hypertensinogenic, mineralocorticoid and glucocorticoid actions overlap.

Direct coronary vasodilator action of adrenomedullin is mediated by nitric oxide.

Increased circulating levels of adrenomedullin (ADM) cause peripheral vasodilatation and hypotension, accompanied by cardiac actions including tachycardia and increases in cardiac contractility, cardiac output, coronary conductance (CC) and coronary blood flow (CBF). It is unclear to what extent these cardiac effects are direct actions of ADM or secondary to the hypotension and altered cardiac loading. The direct cardiac actions of ADM were examined in conscious sheep previously implanted with aortic and coronary flow probes, and an indwelling left coronary artery cannula. Responses to infusion of ADM (0.5 microg kg(-1) h(-1) for 1 h) into the left coronary artery or jugular vein were compared (n=6). The effect of blockade of nitric oxide (NO) synthase with intracoronary (i.c.) N(omega)-nitro-l-arginine (l-NNA; 1.5 mg kg(-1) h(-1), infused for 2 h before and during ADM infusion, was assessed to determine whether the responses to ADM were mediated by NO (n=5). I.c. ADM caused large and sustained increases in CC (0.35+/-0.07-0.55+/-0.13 ml min(-1) mmHg-1, P<0.05) and CBF (28+/-6-42+/-9 ml min(-1), P<0.05), but had no effect on arterial pressure or indices of cardiac contractility (first differential of the upstroke of systole and peak aortic flow rate). Intravenous infusion of ADM had no effects. I.c. l-NNA, at a dose that abolished the coronary vasodilator action of acetylcholine, blocked ADM-induced coronary vasodilatation. In conclusion, ADM had a direct coronary vasodilator action that was mediated by release of endogenous NO and resulted in increased CBF. There was no evidence for a direct inotropic action of ADM.

Stimulation and suppression of renin release from incubations of rat renal cortex by factors affecting calcium flux.

Inhibition of renin secretion from incubations of rat kidney cortex by angiotensin II (AII), ouabain and K+ depletion, depended on the presence of external Ca2+. AII inhibition of isoprenaline-stimulated renin secretion was only partially dependent on external Ca2+. Ouabain and K+ depletion inhibited isoprenaline-stimulated renin release but only in the presence of external Ca2+. Since, in Ca2+-free medium, isoprenaline stimulated renin release when the Na+/K+-ATPase was blocked, isoprenaline probably does not act through the Na+/K+-ATPase. Lanthanum blocked the stimulation of renin release by isoprenaline. Ethylenediamine tetra-acetic acid (EDTA) and ethyleneglycol-bis-(beta-amino-ethyl ether) N,N'-tetra-acetic acid (EGTA) increased renin secretion to a similar degree in Ca2+- and Mg2+-free buffer. When Mg2+ was present the effect of EGTA, but not EDTA, was considerably reduced. Verapamil reduced the fall in basal renin secretion in normal but not Ca2+-free buffer. Verapamil did not block the inhibitory effects of AII or ouabain and did not alter the stimulation of renin secretion by isoprenaline. Bay K 8644 inhibited renin secretion from cortex incubated in medium containing 15 mM K+ and this was dependent on extracellular Ca2+. In normal buffer (5.9 mM K+) Bay K 8644 increased renin secretion.

Direct cardiac and vascular actions of adrenomedullin in conscious sheep.

1. Adrenomedullin (ADM) is a recently characterized circulating hormone which affects haemodynamic, renal and pituitary function in mammals. We have shown previously that in sheep, ADM produces vasodilatation together with increases in cardiac output and contractility. However, whether these effects are direct or mediated by autonomic reflexes is unclear. The present study examined the cardiovascular actions of an intravenous infusion of ADM in conscious, chronically instrumented sheep with either sympathetic, parasympathetic or autonomic ganglion blockade, to determine the role of the autonomic nervous system in mediating these cardiovascular changes. 2. Human ADM (1-52) was infused for 60 min at 2 micrograms kg-1 h-1 following: (1) saline control, (2) combined alpha/beta-adrenoceptor (sympathetic) blockade (proporanolol 0.4 mg kg-1 h-1 + phentolamine 0.15 mg kg-1 h-1 for 20 h), (3) muscarinic (parasympathetic) blockade (methscopolamine 0.05 mg kg-1 h-1 for 20 h) or (4) ganglion blockade (hexamethonium 3 mg kg-1 h-1 for 4 h). Measurements were made of mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral conductance (TPC), maximal aortic flow (Fmax) and maximal rate of change of aortic flow (dF/dt). 3. ADM reduced MAP by 3 +/- 1 mmHg, and increased CO (1.2 +/- 0.2 l min-1), HR (14 +/- 2 beats min-1), TPC (21 +/- 3 ml min-1 mmHg-1). Fmax (2.3 +/- 0.8 l min-1) and dF/dt (86 +/- 21 l min-1 s-1) in normal sheep. In animals with alpha/beta blockade, similar changes were observed with ADM. However, during muscarinic blockade, the increases in HR (32 +/- 4 beats min-1), CO (2.1 +/- 0.4 l min-1), TPC (31 +/- 4 ml min-1 mmHg-1). Fmax (4.0 +/- 0.6 l min-1), and dF/dt (150 +/- 12 l min-1 s-1) produced by ADM were enhanced. During ganglion blockade, ADM produced a greater reduction in MAP (-10 +/- 2 mmHg) compared to controls (-3 +/- 1 mmHg). However, there was no increase in HR. The changes in CO, TPC and contractility were similar to those observed in control animals. 4. These results suggest that the vasodilator effects of ADM on the periphery and its ability to increase CO and cardiac contractility are not mediated by the autonomic nervous system, but are probably the result of direct actions of ADM on the heart and vasculature.