Effects of a peripherally acting alpha 2-adrenoceptor antagonist (L-659,066) on hemodynamics and plasma levels of catechols in conscious rats

Potentiation of Glibenclamide Hypoglycaemia in Mice by MK-467, a Peripherally Acting Alpha2-Adrenoceptor Antagonist

Pharmacological antagonism and genetic depletion of pancreatic α2A-adrenoceptors increase insulin secretion in mice and enhance the insulinotropic action of glibenclamide, a representative of the sulphonylurea class of insulin secretagogues used in the therapy of type 2 diabetes. Antagonism of α2-adrenoceptors in the central nervous system (CNS) causes tachycardia and hypertension, making generalized α2-adrenoceptor blockade unfavourable for clinical use despite its potential to decrease blood glucose levels. The purpose of this study was to test the acute effects of the peripherally acting α2-adrenoceptor antagonist MK-467 alone and in combination with glibenclamide in non-diabetic C57BL/6N mice. Cardiovascular safety was assessed in freely moving mice with radiotelemetry. Dose-dependent decreases in blood glucose and increases in plasma insulin concentrations were seen with the combination of MK-467 and glibenclamide; the combinations were much more potent than glibenclamide or MK-467 alone. Furthermore, MK-467 had no effect on mean arterial pressure or heart rate in freely moving mice and did not prevent the centrally mediated hypotensive effect of the α2-adrenoceptor agonist medetomidine. Thus, peripheral blockade of α2-adrenoceptors does not evoke the same cardiovascular adverse effects as antagonism of CNS α2-adrenoceptors. The current results indicate that the combined use of small doses of a peripherally acting α2-adrenoceptor antagonist with a sulphonylurea drug could provide a novel option for the treatment of type 2 diabetes, especially in patients with increased tonic α2-adrenoceptor-mediated inhibition of insulin secretion.

Haemodynamic interactions of medetomidine and the peripheral alpha-2 antagonist MK-467 during step infusions in isoflurane-anaesthetised dogs

The haemodynamic interactions of a step infusion with medetomidine (MED) and the peripherally acting alpha-2 antagonist MK-467 (MK) were compared with MED infused alone in isoflurane-anaesthetised dogs. Eight purposely-bred Beagles were used in a randomised crossover study. Anaesthesia was induced with propofol intravenously (IV) and maintained with isoflurane in oxygen. Dogs received 1.25 µg/kg MED as a 1 min loading dose IV, along with a step-down MED infusion at rates of 8.0 µg/kg/h (step 1: 0-20 min), 5.5 µg/kg/h (step 2: 20-40 min) and 4.0 µg/kg/h (step 3: 40-95 min). Five minutes after starting the MED infusion, the dogs received MK-467 in a step-up infusion at rates of 100 µg/kg/h (step 1: 5-35 min), 200 µg/kg/h (step 2: 35-65 min) and 500 µg/kg/h (step 3: 65-95 min). Heart rate (HR), systolic (SAP) and mean arterial (MAP) blood pressures and arteriovenous oxygen content differences (a-vO2 diff) were calculated. Plasma drug concentrations were analysed. Repeated-measures general linear mixed models with Bonferroni correction were used for statistical analyses. MED infusion alone increased SAP maximally by 24.9%, MAP by 34.7% and a-vO2 diff by 222.5%, and reduced HR by 32.3%, but these changes were significantly attenuated by MK-467. Most MED effects returned to baseline during step 2 of MK-467 infusion and step 3 of MED infusion (MED/MK-467 ratio 1:18 to 1:50). Plasma concentrations of MED tended to be lower with the addition of MK-467. The use of step infusions helped to narrow down the therapeutic range for the MED/MK-467 infusion dose ratio during isoflurane anaesthesia in dogs.

Prolonged activation of the baroreflex decreases arterial pressure even during chronic adrenergic blockade

Previous studies suggest that prolonged electric activation of the baroreflex may reduce arterial pressure more than chronic blockade of alpha(1)- and beta(1,2)-adrenergic receptors. To determine whether central inhibition of sympathetic outflow has appreciable effects to chronically reduce arterial pressure by actions distinct from well-established mechanisms, we hypothesized that chronic baroreflex activation would lower arterial pressure substantially even during complete alpha(1)- and beta(1,2)-adrenergic receptor blockade. This hypothesis was tested in 6 dogs during adrenergic blockade (AB; 18 days) with and without electric activation of the carotid baroreflex (7 days). During chronic AB alone, there was a sustained decrease in the mean arterial pressure of 21+/-2 mm Hg (control: 95+/-4 mm Hg) and an approximately 3-fold increase in plasma norepinephrine concentration (control: 138+/-6 pg/mL), likely attributed to baroreceptor unloading. In comparison, during AB plus prolonged baroreflex activation, plasma norepinephrine concentration decreased to control levels, and mean arterial pressure fell an additional 10+/-1 mm Hg. Because of differences in plasma norepinephrine concentration, we also tested the acute blood pressure-lowering effects of MK-467, a peripherally acting alpha(2)-antagonist. After administration of MK-467, there was a significantly greater fall in arterial pressure during AB (15+/-3 mm Hg) than during AB plus prolonged baroreflex activation (7+/-3 mm Hg). These findings suggest that reflex-induced increases in sympathetic activity attenuate reductions in arterial pressure during chronic AB and that inhibition of central sympathetic outflow by prolonged baroreflex activation lowers arterial pressure further by previously undefined mechanisms, possibly by diminishing attendant activation of postjunctional alpha(2)-adrenergic receptors.

A novel alpha 2-adrenoceptor antagonist attenuates the early, but preserves the late cardiovascular effects of intravenous dexmedetomidine in conscious dogs

OBJECTIVES

To test the hypothesis that L-659,066, a peripherally acting alpha 2-adrenoceptor agonist, will abolish the early pressor response but preserve the late depressor action of intravenous dexmedetomidine in conscious, unsedated dogs.

DESIGN

A prospective investigation.

SETTING

A laboratory research.

PARTICIPANTS

Nine chronically instrumented dogs.

INTERVENTIONS

Dogs received dexmedetomidine, 5 micrograms/kg intravenously, in the presence or absence of L-659,066, 0.1, 0.2, or 0.4 mg/kg intravenously, pretreatment in a random fashion determined with a Latin square design on different experimental days.

MEASUREMENTS AND MAIN RESULTS

Systemic and coronary hemodynamics were assessed under control conditions, 30 minutes after administration of L-659,066 and 5 and 60 minutes after intravenous administration of dexmedetomidine. Dexmedetomidine alone acutely increased mean arterial pressure (106 +/- 3 to 175 +/- 4 mmHg; p < 0.05), left ventricular (LV) systolic and end-diastolic pressures, systemic vascular resistance (3,400 +/- 350 to 13,360 +/- 2,290 dyne.s.cm-5; p < 0.05), and coronary vascular resistance (2.69 +/- 0.19 to 4.18 +/- 0.43 mmHg.Hz-1.10(-2); p < 0.05) and decreased LV +dP/dtmax and cardiac output (2.6 +/- 0.3 to 1.3 +/- 0.2 L/min; p < 0.05). Dexmedetomidine alone decreased heart rate, mean arterial pressure, and LV systolic pressure and caused sustained reductions in +dP/dtmax and cardiac output up to 60 minutes after administration. L-659,066 alone increased heart rate, +dP/dtmax, cardiac output, and coronary blood flow velocity and decreased systemic vascular resistance. Mean arterial and LV pressures and coronary vascular resistance were unchanged. Pretreatment with L-659,066 abolished the acute dexmedetomidine-induced increases in mean arterial pressure, LV pressures, systemic and coronary vascular resistance and decreases in +dP/dtmax and cardiac output. In contrast, reductions in mean arterial pressure and LV systolic pressure observed 60 minutes after administration of dexmedetomidine were preserved in dogs receiving L-659,066. Cardiac performance, systemic vascular resistance, and coronary hemodynamics were also maintained to a greater degree 60 minutes after dexmedetomidine administration in the presence of L-659,066.

CONCLUSION

L-659,066 prevents the immediate pressor effects of 5 micrograms/kg of intravenous dexmedetomidine but preserves the majority of the late beneficial cardiovascular effects of this selective alpha 2-adrenoceptor agonist in conscious dogs.

The central effects of moxonidine on intraocular pressure and its antagonism by L-659, 066 and L-657, 743 in the rabbit

1) The imidazoline, moxonidine (MOX), injected icvt into the anterior lateral ventricle of NZW rabbits induced ocular hypotension (> 7.0 mmHg) that persisted for two hrs. 2) L-659, 066 injected i.v. or icvt inhibited MOX-induced ocular hypotension, significantly. 3) L-657, 743, injected icvt at 100-fold lower concentration than icvt L-659, 066, significantly inhibited MOX-induced ocular hypotension. 4) Alpha-2-adrenoceptors, located in the CNS, play a role in MOX-induced ocular hypotension, as evidenced by the ability of the relatively selective alpha-2 antagonists, L-659, 066 and L-657, 743 to inhibit icvt MOX-induced ocular hypotension.

Increased cardiac production of dihydroxyphenylalanine (DOPA) during sympathetic stimulation in anaesthetized dogs

Entry of dihydroxyphenylalanine (DOPA) into plasma from specific organs may reflect regional activity of tyrosine hydroxylase, the enzyme responsible for the immediate synthesis of DOPA and rate-limiting for subsequent formation of catecholamines. Therefore, cardiac spillovers of DOPA, noradrenaline and the intraneuronal metabolite of noradrenaline, dihydroxyphenylglycol (DHPG), were examined during two periods of graded electrical stimulation of the sympathetic nerves to the heart in anesthetized dogs. Responses were examined before and after neuronal uptake blockade with desipramine. Cardiac spillover of DOPA increased by 1.8- and 4.4-fold during sympathetic stimulation before desipramine and by 1.6- and 3.3-fold after desipramine. Fold increases in cardiac spillover of DOPA were much lower than but positively related with fold increases in noradrenaline spillover (5.9- and 13.8-fold increases before and 9.0- and 15.8-fold increases after desipramine). Increases in cardiac spillover of DHPG (1.5- and 2.3-fold increases) were blocked by desipramine so that fold changes in spillover of DOPA were greater than and poorly related to changes in spillover of DHPG. Fold increases in cardiac spillover of DOPA showed a close one-to-one positive relationship with fold increases in the sum of cardiac spillovers of noradrenaline and dihydroxyphenylglycol before and after desipramine. For a given fold increase in noradrenaline release, transmitter turnover is increased fractionally and noradrenaline synthesis need also only increase fractionally to maintain transmitter stores constant. The close relationship between fold increases in cardiac spillover of DOPA and combined spillovers of noradrenaline and DHPG is consistent with regulation of tyrosine hydroxylase activity to match changes in noradrenaline synthesis with changes in noradrenaline turnover. Changes in cardiac spillover of DOPA appear to reflect local changes in tyrosine hydroxylase activity.

Evidence for functional alpha 2-adrenoceptors on vascular sympathetic nerve endings in the human forearm

The role of alpha 2-adrenoceptors on vascular sympathetic nerve endings in modulating release of the sympathetic neurotransmitter norepinephrine (NE) in humans was examined by measuring the regional rate of appearance of NE in forearm venous plasma (forearm NE spillover [FSO]) in 32 healthy volunteers during intra-arterial infusion of drugs acting at adrenoceptors or directly on vascular smooth muscle. Simultaneous intra-arterial infusions of tracer amounts of [3H]NE were used to calculate the extraction rate of NE in the forearm. Methoxamine or propranolol with epinephrine (PRO + EPI) was used to stimulate alpha-adrenoceptors, yohimbine was used to inhibit alpha-adrenoceptors, and sodium nitroprusside (NIP) was used to produce increases in forearm blood flow directly. Sympathetic efferent activity was manipulated by systemic intravenous infusions of NIP or trimethaphan. Yohimbine and NIP increased and PRO + EPI and methoxamine decreased NE FSO, without effects on systemic blood pressure, heart rate, or arterial levels of catechols. Changes in FSO were flow dependent; therefore, the slope of the relation between the changes in FSO and forearm blood flow was used to evaluate the effects of each drug on regional sympathoneural activity. During administration of yohimbine, the mean slope of the relation between the change in estimated FSO and the change in forearm blood flow was about four times that of the mean slope during administration of NIP (F = 6.35, p less than 0.05). The slopes of the relations between changes in FSO and forearm blood flow were unaffected by systemic trimethaphan or NIP infusion, indicating that the activity of alpha 2-adrenoceptors was not altered during inhibition or reflexive stimulation of sympathetic outflow. The results suggest that alpha 2-adrenoceptors modulate release of NE from vascular sympathetic nerve endings in humans and that the function of these receptors is unchanged during acute changes in junctional NE concentrations.

Sympathoneural and skeletal muscle contributions to plasma dopa responses in pithed rats

Dihydroxyphenylalanine (DOPA) in plasma has been thought to originate from sympathetic nerve endings and to reflect catecholamine biosynthesis, because changes in DOPA levels follow pharmacologically- or environmentally-induced manipulations that alter turnover of the sympathetic neurotransmitter, norepinephrine (NE). Skeletal muscle may be an additional, non-neural source of circulating DOPA. In the present study we examined sympathoneural and skeletal muscle contributions to DOPA in arterial plasma in pithed rats. Electrical stimulation of the spinal cord causes discharges of sympathetic post-ganglionic neurons, with attendant release of NE into the bloodstream, and discharges of spinal motoneurons, which causes diffuse contraction of skeletal muscle. Stimulation of the spinal cord rapidly elevated arterial plasma concentrations of NE, dihydroxyphenylglycol (DHPG), and DOPA. Pre-treatment with curare, a skeletal muscle relaxant, did not affect the NE and DHPG responses but attenuated the DOPA responses by about 50%. Administration of chlorisondamine, a ganglionic blocker, abolished NE and DHPG responses to cord stimulation, and DOPA responses were decreased by about 90%. Adrenal-demedullation did not affect the stimulation-induced DOPA responses. The results demonstrate that in pithed rats undergoing spinal cord stimulation, DOPA is released into the bloodstream. Since this response is markedly inhibited after ganglionic blockade and also attenuated after skeletal muscle paralysis, the results provide indirect evidence that DOPA formed in sympathetic neurons can be stored in a non-neuronal pool and released during skeletal muscle contraction.

Simultaneous measurement of plasma and brain extracellular fluid concentrations of catechols after yohimbine administration in rats

The present study examined whether systemic injection of the alpha 2 adrenoceptor blocker, yohimbine, affects concentrations of norepinephrine (NE) and its metabolites in extracellular fluid in the brain and in blood. Microdialysis probes were inserted into the posterior hypothalamus, medulla, and caudate/putamen in rats. Microdialysate and arterial blood were sampled after intravenous administration of yohimbine. In the hypothalamus yohimbine produced significant increases in extracellular fluid concentrations of NE, its intraneuronal metabolite, dihydroxyphenylglycol (DHPG), and methoxyhydroxyphenylglycol (MHPG), a major neuronal and extraneuronal metabolite of NE. The increases in these levels were small or absent in the caudate/putamen, where dopamine is the primary catecholamine transmitter. During systemic infusion of tracer amounts of [3H]NE, little if any radioactive NE or DHPG appeared in the microdialysate, whereas substantial levels of [3H]MHPG were present and increased as plasma [3H]MHPG levels rose. The results support the view that alpha 2 adrenoceptor blockade in the brain increases hypothalamic and medullary release, reuptake, and metabolism of NE. The findings cannot be explained by disruption of the blood-brain barrier for catecholamines by insertion of the microdialysis probes. Enhanced sympathetic outflow and peripheral release of NE when alpha 2 adrenoceptors are blocked appears to be attended by enhanced central NE release, presumably as a result of presynaptic alpha 2 adrenoceptor blockade at noradrenergic terminals in the brain. This is consistent with the hypothesis that central noradrenergic NE release is regulated by presynaptic alpha 2 adrenoceptors.