Rat clonidine mydriasis model:Imidazoline receptors are not involved

The clonidine mydriasis model in rats has been widely applied in preclinical research to characterize a -adrenoceptor antagonistic properties of drugs. The present study was undertaken to pharmacologically determine if imidazoline I receptors are also involved in this model system. Sigmoid dose-response curves for pupillary dilation were produced in pentobarbital anesthetized rats by intravenous administration of increasing doses of agonists (guanabenz for a -adrenoceptors, clonidine for both a - adrenoceptors and imidazoline I receptors, and rilmenidine for imidazoline I receptors). Two antagonists (RS 79948 for a -adrenoceptors and efaroxan for imidazoline I receptors) were used to antagonize the mydriasis elicited by those three agonists, with antagonistic potencies calculated. In additional experiments, we examined the effect of the selective imidazoline I receptor antagonist, AGN 192403, on clonidine-induced mydriasis. The results showed that pupillary response curves elicited by guanabenz, clonidine and rilmenidine were competitively antagonized by both RS 79948 (0.03-1 mg/kg) and efaroxan (0.03-1 mg/kg) in a dose-related fashion. The potencies of either antagonist against the three agonists were not significantly different. AGN 192403 (5 mg/kg) did not significantly shift the clonidine mydriasis curve. These results suggest that imidazoline I receptors are not functionally involved in the rat clonidine mydriasis model and support this in vivo system as a useful model for studies of a -adrenoceptors. © 2004 Elsevier B.V. All rights reserved.

Evidence for the presence of G-proteins, adenylyl cyclase and phospholipase C activities in lymphatic smooth muscle cell membranes

In plasma membranes derived from bovine mesenteric lymphatic smooth muscle cells, guanine nucleotide and forskolin stimulated adenylyl cyclase (AC) activity in a concentration-dependent manner, indicative of the presence of the stimulatory G-protein G(s) linked to AC. There was no significant enzyme inhibition by low concentrations of guanine nucleotide and no effect on basal or guanine nucleotide-stimulated activity following pertussis toxin treatment of cells, suggesting the absence of G(1) linked to inhibition of AC. Furthermore, there was no effect of adrenaline, isoprenaline or clonidine on basal or forskolin-stimulated activities, nor was there any specific binding of the beta-adrenoceptor ligand [I-125]cyanopindolol to membranes, suggesting that cate-cholamine receptors do not modulate AC activity in these membranes. Pertussis toxin-mediated ADP ribosylation of membrane proteins and Western immunoblotting analysis revealed the presence of G-protein subunits G(alpha l2), G(alpha q), G(alpha 11) and G(beta 1). In experiments designed to identify a possible effector enzyme for these G-proteins, membranes were screened with a range of antibodies raised against phospholipase C (PLC) beta, gamma and delta isozymes. Though no evidence was obtained by Western blotting for any of these proteins, PLC activity was concentration-dependently stimulated by Ca2+, but not by AlF4-, GTP[S], or purified G(beta gamma) subunits. Finally, no specific binding to membranes of the alpha(1)-adrenoceptor ligand [H-3]prazosin or the alpha(2)-adrenoceptor ligand [H-3]yohimbine was obtained. In conclusion, this study provides evidence for a G(s)-dependent stimulation of AC, and for the presence of G(2) and G(q11), which do not appear to regulate a PLC activity also identified in lymphatic smooth muscle cell membranes. Furthermore, neither AC nor PLC appear to be associated with catecholamine receptors. Copyright(C) 1996 Elsevier Science Inc.

α-Adrenergic agonism enhances the growth hormone (GH) response to GH-releasing hormone through an inhibition of hypothalamic somatostatin release in normal men

The purpose of this study was to investigate the precise mechanism by which central a-adrenergic pathways modulate GH secretion in humans. In 10 normal subjects we compared the pattern of clonidine-induced GH release to that elicited by GH-releasing hormone (GHRH) given at a time of presumably similar responsiveness of the somatotrope. We also evaluated the effect of stimulation by GHRH (either endogenous, by administration of clonidine, or exogenous) on the GH response to a further exogenous GHRH stimulation. In 2 experiments the administration of clonidine (0.150 mg, orally) at 0 or 60 min was followed by a GHRH [GRF-(1-29); 1 µg/kg, iv] challenge at 180 min. In other experiments subjects received on separate occasions placebo or clonidine at 0 min, followed by GHRH at 60 min and again at 180 min. In a further experiment the administration of clonidine at 0 min was followed by 2 GHRH challenges (60 and 180 min later). The administration of clonidine 60 or 120 min, but not 180 min, before the GHRH bolus significantly (P <0.01) increased the GH responses to this challenge compared to those elicited by GHRH when given after placebo in a period of a similar somatotrope responsiveness. These, in turn, were significantly (P <0.05) higher than those elicited by clonidine alone. The close relationship between pre-GHRH plasma GH values and GHRH-elicited GH peaks, not observed for clonidine, was lost after pretreatment with this drug. These data indicate that clonidine was able to disrupt the intrinsic hypothalamic-somatotroph rhythm, suggesting that a-adrenergic pathways have a major inhibitory effect on somatostatin release. Our data also indicate that GH responses to a GHRH bolus administered 120 min after a prior GHRH challenge are dependent on two parameters: the intrinsic hypothalamic-somatotroph rhythm at the time of the second GHRH bolus, and the magnitude of GH secretion elicited by the previous somatotroph stimulation. In summary, a-adrenergic agonism appears to act primarily in GH control by inhibiting the hypothalamic release of somatostatin, rather than by stimulating GHRH secretion.

Alpha-2 agonists for sedation of mechanically ventilated adults in intensive care units: a systematic review

BACKGROUND: Care of critically ill patients in intensive care units (ICUs) often requires potentially invasive or uncomfortable procedures, such as mechanical ventilation (MV). Sedation can alleviate pain and discomfort, provide protection from stressful or harmful events, prevent anxiety and promote sleep. Various sedative agents are available for use in ICUs. In the UK, the most commonly used sedatives are propofol (Diprivan(®), AstraZeneca), benzodiazepines [e.g. midazolam (Hypnovel(®), Roche) and lorazepam (Ativan(®), Pfizer)] and alpha-2 adrenergic receptor agonists [e.g. dexmedetomidine (Dexdor(®), Orion Corporation) and clonidine (Catapres(®), Boehringer Ingelheim)]. Sedative agents vary in onset/duration of effects and in their side effects. The pattern of sedation of alpha-2 agonists is quite different from that of other sedatives in that patients can be aroused readily and their cognitive performance on psychometric tests is usually preserved. Moreover, respiratory depression is less frequent after alpha-2 agonists than after other sedative agents.

OBJECTIVES: To conduct a systematic review to evaluate the comparative effects of alpha-2 agonists (dexmedetomidine and clonidine) and propofol or benzodiazepines (midazolam and lorazepam) in mechanically ventilated adults admitted to ICUs.

DATA SOURCES: We searched major electronic databases (e.g. MEDLINE without revisions, MEDLINE In-Process & Other Non-Indexed Citations, EMBASE and Cochrane Central Register of Controlled Trials) from 1999 to 2014.

METHODS: Evidence was considered from randomised controlled trials (RCTs) comparing dexmedetomidine with clonidine or dexmedetomidine or clonidine with propofol or benzodiazepines such as midazolam, lorazepam and diazepam (Diazemuls(®), Actavis UK Limited). Primary outcomes included mortality, duration of MV, length of ICU stay and adverse events. One reviewer extracted data and assessed the risk of bias of included trials. A second reviewer cross-checked all the data extracted. Random-effects meta-analyses were used for data synthesis.

RESULTS: Eighteen RCTs (2489 adult patients) were included. One trial at unclear risk of bias compared dexmedetomidine with clonidine and found that target sedation was achieved in a higher number of patients treated with dexmedetomidine with lesser need for additional sedation. The remaining 17 trials compared dexmedetomidine with propofol or benzodiazepines (midazolam or lorazepam). Trials varied considerably with regard to clinical population, type of comparators, dose of sedative agents, outcome measures and length of follow-up. Overall, risk of bias was generally high or unclear. In particular, few trials blinded outcome assessors. Compared with propofol or benzodiazepines (midazolam or lorazepam), dexmedetomidine had no significant effects on mortality [risk ratio (RR) 1.03, 95% confidence interval (CI) 0.85 to 1.24, I (2) = 0%; p = 0.78]. Length of ICU stay (mean difference -1.26 days, 95% CI -1.96 to -0.55 days, I (2) = 31%; p = 0.0004) and time to extubation (mean difference -1.85 days, 95% CI -2.61 to -1.09 days, I (2) = 0%; p < 0.00001) were significantly shorter among patients who received dexmedetomidine. No difference in time to target sedation range was observed between sedative interventions (I (2) = 0%; p = 0.14). Dexmedetomidine was associated with a higher risk of bradycardia (RR 1.88, 95% CI 1.28 to 2.77, I (2) = 46%; p = 0.001).

LIMITATIONS: Trials varied considerably with regard to participants, type of comparators, dose of sedative agents, outcome measures and length of follow-up. Overall, risk of bias was generally high or unclear. In particular, few trials blinded assessors.

CONCLUSIONS: Evidence on the use of clonidine in ICUs is very limited. Dexmedetomidine may be effective in reducing ICU length of stay and time to extubation in critically ill ICU patients. Risk of bradycardia but not of overall mortality is higher among patients treated with dexmedetomidine. Well-designed RCTs are needed to assess the use of clonidine in ICUs and identify subgroups of patients that are more likely to benefit from the use of dexmedetomidine.

STUDY REGISTRATION: This study is registered as PROSPERO CRD42014014101.

FUNDING: The National Institute for Health Research Health Technology Assessment programme. The Health Services Research Unit is core funded by the Chief Scientist Office of the Scottish Government Health and Social Care Directorates.