Release and modulation of release of serotonin in rabbit superior colliculus

The release of previously incorporated [3H]serotonin and its presynaptic modulation were studied in slices of rabbit superior colliculus. Electrical stimulation at frequencies of 0.017-3 Hz greatly increased the outflow of tritiated compounds; this response was almost abolished by tetrodotoxin and in a low calcium medium. Unlabelled serotonin, when added in the presence of nitroquipazine, an inhibitor of high-affinity neuronal serotonin uptake, reduced the electrically evoked overflow of tritium, an effect antagonized by metitepin. Given alone, metitepin caused an increase. The evoked overflow was also decreased by clonidine, and the effect of clonidine was counteracted by phentolamine. Phentolamine itself increased the overflow response. However, this was probably not due to antagonism against an inhibitory effect of endogenous noradrenaline because, first, the selective alpha 2-adrenoceptor antagonist idazoxan did not share with phentolamine the overflow-enhancing effect, second, phentolamine continued to increase the overflow after noradrenergic axons had been destroyed by 6-hydroxydopamine, and third, the facilitatory effects of metitepin and phentolamine were not additive. Phentolamine, like metitepin, antagonized the presynaptic inhibitory effect of serotonin, indicating that it may increase the evoked overflow of tritium by blocking serotonin receptors rather than alpha-adrenoceptors. Ethylketocyclazocine decrease the electrically evoked overflow, and its effect was prevented by naloxone: peptides selective for opioid mu- or delta-receptors caused no change. Nicotine increased the basal outflow of tritium (in the absence of electrical stimulation); the increase was attenuated by hexamethonium and low calcium medium. No or minimal changes in tritium outflow were obtained with beta-adrenoceptor, dopamine receptor, muscarine receptor and GABA receptor ligands or with substance P and glutamate. In conjunction with our previous studies, these results indicate that serotonin is a neurotransmitter in the superior colliculus. Its release is modulated through presynaptic autoreceptors (probably 5-HT1), alpha 2-adrenoceptors, opioid kappa-receptors and nicotine receptors, of which only the autoreceptors receive an endogenous input, at least under the experimental conditions chosen. Each of the three groups of collicular monoamine axons that we have studied recently (cholinergic, noradrenergic, serotoninergic) possesses a specific pattern of presynaptic, release-modulating receptors. A physiological role seems likely only for the alpha 2-autoreceptors at the noradrenergic and the 5-HT1-autoreceptors at the serotoninergic axons.

Alpha-adrenoceptor subtypes in blood vessels: physiology and pharmacology

The existence of alpha 1- and alpha 2-adrenoceptors subtypes is now well established. Presynaptically, alpha 2-adrenergic autoreceptors modulate norepinephrine release. In vascular smooth muscle both alpha 1- and alpha 2-adrenoceptors exist postjunctionally, the alpha 1 subtype being innervated preferentially in most vascular beds. alpha 2-Adrenoceptors may be present on the smooth muscle cell as well as the endothelial cell where they may participate in the release of endothelium derived relaxing and constricting factors. Developing insights into the role of alpha-adrenoceptor subtypes in the cardiovascular system, and into the classification of subtypes of alpha 1- and alpha 2-adrenoceptors may be expected to have, if confirmed, significant therapeutic implications for drug discovery in the cardiovascular system.

Epinephrine stimulates inositol phospholipid metabolism by activating alpha-2 adrenergic receptors in human platelets

The metabolism of inositol phospholipids in response to epinephrine was investigated in intact human platelets. In platelets prelabelled with [3H]-myo-inositol in Ca2+-free HEPES buffer containing 10 mM LiCl, epinephrine caused an accumulation of inositol-1-phosphate in a concentration-dependent manner. The EC50 value for epinephrine was 5 microM. Yohimbine (1 microM), a selective alpha-2 adrenergic receptor antagonist, inhibited 88% of the epinephrine (10 microM) response, whereas prazosin (1 microM), a selective alpha-1 adrenergic receptor antagonist, failed to inhibit the response. Yohimbine inhibited the epinephrine (10 microM) response in a concentration-dependent manner. The inhibition constant (Ki) value for yohimbine was 60.3 nM. These data indicate that epinephrine stimulates phosphoinositide (PI) turnover by activating adrenergic receptors of the alpha-2 type in human platelets. In addition, this PI response elicited by epinephrine was found to be inhibited in a concentration-dependent manner by treatment of platelets with dibutyryl cyclic AMP and 8-bromo-cyclic GMP which are known as potent inhibitors for platelet activation, and may therefore be a useful biochemical index for the study of the function of human alpha-2 adrenergic receptors.

Pharmacological identification of the alpha-adrenergic receptor type which inhibits the beta-adrenergic activated adenylate cyclase system in cultured astrocytes

The adrenergic agonist norepinephrine can exert its influence on cell function by activating both alpha- and beta-adrenergic receptors. In astrocytes, the alpha-adrenergic receptor activity of norepinephrine is known to inhibit the cyclic AMP response elicited by its action at beta-adrenergic receptors. Pharmacological studies were conducted to identify the subtype of alpha-adrenergic receptor which mediates this inhibitory action. The alpha 2-adrenergic antagonist yohimbine potentiated the cyclic AMP response elicited by norepinephrine, whereas the alpha 1-adrenergic antagonist prazosin did not affect the response. The alpha 2-adrenergic agonist clonidine inhibited the cyclic AMP response elicited by the beta-adrenergic agonist isoproterenol and this inhibition could be blocked by yohimbine but not by prazosin. In contrast, the alpha 1-adrenergic agonist phenylephrine did not inhibit the cyclic AMP response to isoproterenol. These studies indicate that the inhibitory action of norepinephrine is mediated by its action at alpha 2-adrenergic receptors.

Alpha-adrenergic receptors in the regulation of liver functions

Rat liver contains 80% alpha 1 and 20% alpha 2 receptors. The role of adrenergic regulation of liver functions was studied by administration of antagonist and agonist pharmaca. Female rats were treated with the selective alpha 1 receptor blocker prazosin. A transitory inhibition of microsomal monooxygenases was observed after 5 days which was terminated at the 12 day. A 18.7% loss of glycogen was measured at this time indicating a slight adrenergic glycogenolytic effect. Dihyd 50-ergocryptime an equal lebocker of alpha 1 alpha 2 receptori preserved glycogen content and has not impaired liver functions. Agonist epinephrine inhibited monooxygenase functions and mobilised 75% of hepatic glycogen. Simultaneous administration of prazosin with epinephrine resulted into glycogen loss due to alpha 2 receptor stimulation. Dihydroergocryptine antagonised rather the metabolic actions of epinephrine. We have established that alpha 2 receptors are also involved in hepatic carbohydrate metabolism.

Alpha- and beta-adrenergic receptors of noradrenergic innervation modulate the lower limits of cerebral and cerebellar blood flow autoregulation in spontaneously hypertensive rats

The effects of alpha- and beta-adrenergic receptors on the lower limits of cerebral (CBF) and cerebellar blood flow (CeBF) autoregulation were examined in spontaneously hypertensive rats. CBF and CeBF were measured during hemorrhagic hypotension in rats treated with adrenergic blockers, phenoxybenzamine (PBZ) or propranolol (PPL), using a hydrogen clearance method. The lower limits for CBF beyond which blood flow was decreased steeply were 72% of resting values in the control, 44 in the PBZ and 80 in the PPL group. Similar tendency was also observed for CeBF. In the control group, the reducing rates of mean arterial pressure (MAP) to decrease CBF by 15 and 30% of the resting values were 33% (58 mm Hg) of the resting MAP and 46% (80 mm Hg), respectively, and those to reduce CeBF to the same extent were 35% (61 mm Hg) and 52% (92 mm Hg), respectively. In the PBZ group, the respective reducing rates of MAP were 52% (71 mm Hg) and 65% (88 mm Hg), respectively, for CBF and 50% (68 mm Hg) and 65% (88 mm Hg), respectively, for CeBF. In contrast, in the PPL group, those rates were 22% (39 mm Hg) and 32% (56 mm Hg), respectively, for CBF, and 23% (40 mm Hg) and 30% (52 mm Hg), respectively, for CeBF, being significantly smaller than those in the control and the PBZ groups (p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The roles of alpha- and beta-adrenoceptors in the chronotropic responses to norepinephrine in carp heart (Cyprinus carpio)

1. The chronotropic effect of norepinephrine was studied in isolated spontaneously beating atrial preparations of carp (Cyprinus carpio) heart. 2. Norepinephrine, 0.1 microM, caused a positive chronotropic effect, while at 1 microM it caused either a positive or a negative chronotropic effect. The positive chronotropic effect, observed in 13 preparations, was potentiated by phentolamine and almost completely blocked by propranolol. 3. The negative chronotropic effect observed in the other 5 preparations was greater in the presence of propranolol, reduced by phentolamine and not affected by atropine. 4. These results indicate that alpha- and beta-adrenoceptors may coexist, mediating the negative and positive chronotropic effects, respectively, in isolated atrial preparations of carp heart.

Effect of alpha-2 adrenergic receptor stimulation on short-circuit current across isolated skin of the toad Bufo arenarum

1. The effect of the alpha-2 adrenergic agonist clonidine on short-circuit current (SCC) across isolated skins of Bufo arenarum toads was investigated. 2. Clonidine inhibited basal SCC in a dose-dependent manner. 3. Blockade of the effect of clonidine on basal SCC by the selective alpha-2 antagonist yohimbine supports the hypothesis that the inhibitory effect is mediated by the stimulation of alpha-2 adrenergic receptors. 4. The fact that the inhibitory effect of clonidine is higher in skins with spontaneous positive SCC than in the negative ones, and that the alpha-2 agonist was unable to alter amiloride-induced negative SCC suggests that the inhibitory effect of clonidine may probably be mediated by inhibition of sodium transport.

Increased alpha-adrenergic responsiveness in idiopathic Raynaud's disease

In our study of 28 patients with idiopathic Raynaud's disease, the patients had significantly greater digital blood flow responses to intraarterial phenylephrine and clonidine than did normal control subjects. There were no group differences in finger blood flow responses to body heating, reflex cooling, digital ischemia, or to intraarterial tyramine or isoproterenol. There were also no group differences in blood pressure or heart rate during any procedure. These results suggest that patients with idiopathic Raynaud's disease have increased peripheral vascular alpha-adrenergic receptor sensitivity and/or density compared with normal persons.

Desensitization of alpha-1 adrenergic receptor mediated smooth muscle contraction in aorta from endotoxic rats

Desensitization of vascular smooth muscles in endotoxemia was studied using the aorta from intraperitoneally endotoxin-injected rats. The KCl- and phenylephrine-induced contractions were significantly decreased in the endotoxic aorta compared to the control. In the endotoxic aorta the phenylephrine-induced contracture showed a gradual tension decrease after reaching a plateau and was attenuated by prior exposure to high concentration of phenylephrine, while KCl produced a sustained contraction and it was not affected by prior exposure to phenylephrine. The phenylephrine- and KCl-induced contractures of the control aorta showed stable plateaus and were not affected by prior exposure to phenylephrine. Neither diminished contractile force nor in vitro desensitization of phenylephrine contracture of isolated aorta was prevented by pretreatment of endotoxic rats with an alpha-adrenergic antagonist, phentolamine. These findings suggest that the contractile response to phenylephrine is easily desensitized in the endotoxic aorta compared to the control and neither this in vitro desensitization nor the diminution of contractile force is caused by in vivo exposure of aorta to a high concentration of catecholamines during endotoxemia.

Role of cell calcium in alpha-1 adrenergic receptor control of arachidonic acid release from brown adipocytes

Exposure of brown fat cells to phenylephrine, an agonist of alpha-1 adrenergic receptors, activates a phospholipase A2 which releases arachidonic acid. Since receptor activation of phospholipase A2 requires calcium, experiments were undertaken to define more precisely the role played by calcium in the regulation of enzyme activity. In this study, adipocytes were loaded with the fluorescent calcium chelator quin2 in order to buffer intracellular calcium and block receptor stimulated changes in its concentration. When quin2 loaded adipocytes were incubated in buffer containing 0.10 mM calcium, the ability of phenylephrine to stimulate release of arachidonic acid was severely reduced. At an intracellular quin2 concentration of 6.6 mM stimulated arachidonic acid release was inhibited by more than 50% and at 13 mM it was completely blocked. In contrast, phenylephrine stimulation of inositol phosphate accumulation was unaffected by quin2. Quin2 also did not affect the liberation of arachidonic acid in response to exogenous phospholipase C, A23187 or forskolin. The intracellular calcium antagonist TMB-8 also inhibited phenylephrine-stimulation of arachidonic acid release and this effect was reversed by ionomycin. Basal phospholipase A2 activity was increased by introduction of high calcium concentrations into cells rendered permeable with digitonin, but phenylephrine still caused a further increase in enzyme activity. These findings show a selective inhibition of phenylephrine activation of phospholipase A2 by either the chelation of intracellular calcium with quin2 or by the calcium antagonist TMB-8 and suggest an essential role for intracellular calcium in alpha adrenergic stimulation of enzyme activity. However, because phenylephrine still stimulates enzyme activity in cells rendered permeable with digitonin, we suggest that the action of phenylephrine cannot be attributed solely to changes in intracellular calcium.

Changes in airway responsiveness and beta- and alpha-1-adrenergic receptors in the lungs of guinea pigs with experimental asthma

The effects of inhaled allergen on airway responsiveness and on beta- and alpha-1-adrenergic receptors on lung membrane were investigated in guinea pigs. After measuring the respiratory threshold to histamine (RT-HIS), one group of guinea pigs passively sensitized for ovalbumin was challenged by allergen inhalation (challenged group). Measurement of the RT-HIS 24 h following challenge revealed a significant decrease from 687 micrograms/ml (mean, n = 16) to 407 micrograms/ml (P less than 0.05). In addition the RT-HIS 24 h after challenge was also significantly lower in the challenged group than in controls (n = 9, P less than 0.05). The density of beta-adrenergic receptors on the lung membrane of the challenged group was 594 +/- 32 (mean +/- SE) fmol/mg protein (n = 11) compared with 712 +/- 24 fmol/mg protein (n = 9) in the controls, a statistically significant difference (P less than 0.05). A significant correlation was found between the RT-HIS and density of beta-adrenergic receptors. From these results, we concluded that the exaggerated airway responsiveness 24 h after allergen challenge is in part due to a decrease in the density of beta-adrenergic receptors. There was no difference in the density of alpha-1-adrenergic receptors nor a significant correlation between the RT-HIS and the number of alpha-1-adrenergic receptors in the challenged vs. the control groups.

Influence of Bay K 8644 on vascular responses mediated by alpha 1- and alpha 2-adrenoceptors

1. The calcium channel activator Bay K 8644 increased the potency of noradrenaline in cat middle cerebral (alpha 2-adrenoceptors) and mesenteric (atypical or mixed alpha 1- and alpha 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries (alpha 1-adrenoceptors). 2. In cat arteries, exposure to 15 mM K+ solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K 8644. 3. Bay K 8644 completely reversed the relaxation produced by nifedipine in K+-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K 8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the alpha-adrenoceptor subtype mediating the response.

Interaction between alpha 2-adrenergic and NPY receptor pathways in human erythroleukemia cells

Neuropeptide Y (NPY) appears to be a transmitter of the sympathetic nervous system, and its actions are similar to those of alpha 2-adrenergic receptor stimulation. In human erythroleukemia (HEL) cells, both NPY and epinephrine (acting through alpha 2-adrenergic receptors) inhibit adenylate cyclase and mobilize intracellular Ca++. We investigated possible interactions between NPY and epinephrine. In radioligand binding assays NPY did not alter antagonist or agonist binding to alpha 2-adrenergic receptors. NPY and epinephrine did not act synergistically to elevate intracellular Ca++. Neither agent alone, nor both together, affected the intracellular pH of HEL cells. Preincubation with NPY (like epinephrine) redistributed the alpha 2-adrenergic receptors away from the cell surface and into a sequestered pool.

Alpha 2-adrenoceptor stimulation inhibits cellular cyclic AMP production in microdissected human glomeruli

The physiological role of numerically predominant alpha 2-adrenoceptor in the kidney is still unknown. This study examined the effect of alpha 2-adrenoceptor stimulation on the production of cAMP from isolated human glomeruli. Unaffected portions of human kidneys which had been removed because of renal cell carcinoma were used for the study. Glomeruli were dissected manually under a stereo microscope. In these glomeruli, alpha 2-adrenoceptor stimulation with epinephrine in the presence of propranolol inhibited significantly parathyroid hormone-dependent increases in cAMP production. This inhibitory effect of epinephrine was removed by adding a specific alpha 2-adrenoceptor antagonist, yohimbine, indicating that the inhibitory effect of epinephrine on cAMP formation was due to alpha 2-adrenoceptor stimulation. Thus, alpha 2-adrenoceptors are involved in the inhibition of cAMP production in human glomeruli.

Marked blood pressure fluctuations during narcoleptic attacks alternating with abnormal wakefulness: effects of treatment with clonidine

A middle-aged man was admitted to our department because of sleep-wake cycle disorders (alternating hypersomnia and sleeplessness), bipolar behavioural disturbances and marked fluctuations in blood pressure and heart rate. Neither evident precipitating stimuli nor an obvious cause for his illness were found. When tests that normally activate intrinsic autonomic responses were performed, two distinct circulatory patterns were recognized. During hypersomnia (phase A), cardiovascular reflex activity was blunted or abolished and orthostasis could not be maintained. The clinical, biochemical, behavioural pictures and the observed decrease in sympathetic outflow resembled the effects of clonidine administration. On the contrary, during sleeplessness (phase B) the autonomic pathways were functionally integral and orthostatic hypotension was not detected. The clinical, biochemical, behavioural features and cardiovascular overactivity closely mimicked the abrupt withdrawal syndrome encountered with clonidine. Three hypothetical mechanisms are advanced to explain this intriguing case as well as the acute and chronic relief of our patient's clinical problem following institution of clonidine therapy (phase C). The role played by central alpha adrenoceptors in integrating sleep-wake, cardiovascular and behavioural functions is also suggested.

Modulation of adrenergic neurotransmission in the rat tail artery by dietary lipids

The effects of dietary lipids on prejunctional alpha 2-adrenoceptor function were investigated in perfused/superfused caudal arteries from adult rats. The investigation was designed to study the effects of diet supplemented with saturated fatty acids (coconut oil diet) or unsaturated fatty acids (sunflower oil diet) on alpha 2-adrenoceptor neuronal function in the proximal rat tail artery. Pregnant rats were fed Purina Rodent Chow (reference diet) or a semisynthetic diet containing 16% (wt/wt) of either sunflower oil or coconut oil. Neonatal pups were exposed to the diet via maternal milk and weaned rats were maintained on the same diet throughout adulthood. Artery segments (5-6 cm) were prelabeled with [3H]norepinephrine and perfused/superfused with Krebs-bicarbonate solution at 37 degrees C. The release of endogenous norepinephrine, total 3H, and [3H]norepinephrine was measured during field stimulation (supramaximal voltage, 5 Hz, 1 ms duration, for a total of 1,200 pulses). Both test diets caused a significant increase in norepinephrine content when compared with the reference diet (p less than 0.01). The results show that field stimulation-evoked release of norepinephrine from arteries obtained from rats fed coconut oil diet was significantly less than that exhibited by either the sunflower oil group or the reference group (p less than 0.05). Phentolamine (3 microM) caused a significant increase in percent release of endogenous norepinephrine and total 3H (p less than 0.05) in all groups. However, the increase above control values for the sunflower oil group was higher than the coconut oil group which suggested that dietary manipulations altered alpha2-adrenoceptor sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of epinephrine on glucose-1,6-bisphosphate and carbohydrate metabolism in skin

1. Injection of epinephrine induced in skin a decrease in the level of glucose-1,6-bisphosphate (Glc-1,6-P2), which was accompanied by correlated changes in the activities of several enzymes which are modulated by this regulator. 2. These effects were blocked by the alpha adrenergic blocker phentolamine, in contrast to muscle where the hormone increases Glc-1,6-P2, acting through beta receptors. 3. The changes in the enzymes' activities, as well as in glycogen and lactate content induced by epinephrine, reveal that the hormone causes, in skin, a stimulation of glycogenolysis and glycolysis, as well as an acceleration of pentose phosphate pathway. 4. The reduction in glycogen content induced by epinephrine, was blocked by the beta adrenergic blocker propranolol, whereas the hormone's effects on the other processes were mainly mediated through alpha receptors.

Amplifying effects of several vasoconstrictor agents of alpha 1-adrenoceptor-mediated contraction of isolated thoracic aortae of the guinea pig

In guinea-pig aorta phenylephrine and noradrenaline induced concentration-dependent contractions when beta-adrenoceptors were blocked. The contractile response to phenylephrine was amplified by previous application of subthreshold concentrations of 5-hydroxytryptamine, ouabain, B-HT 920, Bay K 8644, phorbol-12-myristate-13-acetate and arachidonic acid. Bay K 8644 also enhanced the contractile response to noradrenaline. The amplification was not substantially altered by removing the endothelium of the aorta. The increase in cytosolic calcium availability induced by the vasoconstrictor agents investigated may be responsible for the enhancement of the alpha 1-adrenoceptor-mediated contractions.

Absence of alpha-2 adrenergic effects on cAMP production in a genital tract smooth muscle cell line

This study sought to evaluate alpha-2 and beta adrenergic modulation of cAMP production in the DDT1 MF-2 transformed smooth muscle myocyte. After stimulation with forskolin or adrenergic agonists with or without subtype specific antagonists, cAMP production was determined. These experiments confirmed an increase of cAMP in response to forskolin, isoproterenol, epinephrine, and norepinephrine; the adrenergic stimulation was inhibited by propranolol. On the other hand, the alpha-2 agonist clonidine did not inhibit cAMP production. Likewise, alpha-2 receptor blockade did not increase cAMP production in response to epinephrine. These studies, therefore, suggest that the DDT1 MF-2 myocyte does not contain a significant population of functional alpha-2 adrenergic receptors.

The thermogenic actions of alpha 2-adrenoceptor agonists in reserpinized mice are mediated via a central postsynaptic alpha 2-adrenoceptor mechanism

1. The dose-related effects of the selective alpha 2-adrenoceptor agonists clonidine, UK-14,304 and B-HT 933 on the body temperature of untreated and reserpine-treated mice were investigated. 2. In untreated mice all three agonists induced a dose-related hypothermia. The highest doses of UK-14,304 and B-HT 933, 3 and 100 mg kg-1 respectively, elicited a marked (10 degrees C) hypothermia, whereas the maximal hypothermic effect of clonidine (5.5 degrees C) was less pronounced and reached a plateau at a dose of 0.5 mg kg-1 i.p. 3. Reserpine (2.5 mg kg-1, s.c.) induced a marked hypothermia in the mouse; 18 h after injection body temperature had decreased to only slightly (0.5-1.5 degrees C) above ambient (19 degrees C). 4. All three alpha 2-agonists produced a partial dose-related reversal of reserpine-induced hypothermia; maximal thermogenic responses (9-10 degrees C increases in body temperature) were elicited by doses of 0.2, 0.5 and 16 mg kg-1 i.p. of clonidine, UK-14,304 and B-HT 933 respectively, and the log dose-response curves for all 3 agonists were bell-shaped. 5. Following intracerebroventricular administration to reserpine-treated mice, the thermogenic response to clonidine was more rapid in onset, and the agonist was 20 fold more potent than when injected i.p. 6. The selective alpha 2-adrenoceptor antagonists, idazoxan (0.05-0.5 mg kg-1), Wy 26392 (0.3-5.0 mg kg-1) and yohimbine (0.1-1.6 mg kg-1) given orally attenuated the thermogenic responses to all 3 agonists in reserpinized mice in a dose-related manner. Pretreatment with a single dose of idazoxan (0.3 mg kg-1, orally) elicited a 6 fold parallel shift to the right in the dose-response curve to clonidine. 7. The selective alpha 1-adrenoceptor antagonists, prazosin (10 mg kg-1) and indoramin (3-10 mg kg-1), and the beta-adrenoceptor antagonist, propranolol (10 mg kg-1), only partially attenuated the thermogenic responses to the alpha 2-agonists in reserpinized mice. These effects were variable and not clearly dose-related. 8. Pretreatment of reserpinized mice with the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, markedly attenuated (60-95%) the thermogenic response to the noradrenaline uptake inhibitor, desipramine (0.13-12.5 mg kg-1, i.p.), but only slightly reduced (10-35%) that to clonidine (0.032-0.5 mg kg-1, i.p.). 9. These results suggest that alpha2-adrenoceptor agonists reverse reserpine-induced hypothermia via a central mechanism involving activation of postsynaptic alpha 2-adrenoceptors.

Ethanol exhibits alpha receptor blocking-like properties in anesthetized rats

The ability of ethanol to reduce alpha-adrenergic receptor-mediated pressor responsiveness in vivo was investigated in chloralose-anesthetized male Sprague-Dawley rats. Catheters were inserted in the jugular vein and the femoral artery of rats for the injection of drugs and the measurement of blood pressure, respectively. Dose-response curves for phenylephrine and norepinephrine were constructed by plotting the change in mean arterial pressure following a bolus dose of the agent against the dose of the pressor agent used. Following construction of an initial dose-response curve, animals were challenged with either a 1 g/kg dose of ethanol or an equivalent volume of saline (iv) and the dose-response curves were repeated. Using a similar protocol, pressor responsiveness was evaluated in animals pretreated with either yohimbine (1 mg/kg) or prazosin (3.9 micrograms/kg), a dose sufficient to produce partial blockade of alpha receptor-mediated pressor responsiveness, and then treated with ethanol. Ethanol produced a partial blockade of alpha receptors when the animals were challenged with either phenylephrine or norepinephrine. This blockade produced by ethanol was shown to be similar to that produced by the receptor blocking agents used in this study. To rule out any nonspecific effects of ethanol in reducing vascular reactivity, some animals were challenged with angiotensin II both before and after treatment with ethanol, yohimbine, or prazosin and after both drugs were administered together. Ethanol, as well as the alpha 1- and alpha 2-adrenergic blocking agents tested failed to have any significant effect on angiotensin II-pressor responsiveness, ruling out any nonspecific effect of ethanol on the vasculature. It is concluded, therefore, that ethanol has alpha receptor blocking-like activity in vivo.

[Dependence of hypoxic changes in macro- and micro-hemodynamics on the activity of adrenoreceptors in the skeletal muscle in hypothermia]

Dependence of hypoxic changes of macro- and microcirculation on the activity of adrenergic receptors in the cooled organism was studied on decentralized shank of cat under perfusion with constant blood flow. After cooling of cat (to 30 degrees C) and blockade of alpha-adrenoreceptors hypoxic hypoxia (10% O2 in N2) caused (a) much greater reduction of precapillary resistance of shank, (b) more striking (by 3 times) increase of capillary filtration coefficient and (c) the increase of capillary pressure and postcapillary resistance in contrast to their decrease to hypoxia under hypothermia before alpha-blockade. Beta-adrenoreceptor blockade had no influence on the changes of the resistance and exchange function of skeletal muscle blood vessels evoked by hypoxia under cooling.

Positive interaction between alpha-1 adrenergic and dopamine-2 receptors in locomotor activity of normo and supersensitive mice

In normosensitive mice either the D1 antagonist SCH 23390 or the D2 antagonist sulpiride inhibited the reversion of reserpine-induced akinesia elicited by the mixed D1/D2 agonist pergolide. In mice rendered supersensitive by a five days' reserpine treatment, sulpiride did not prevent the pergolide-induced reversal of akinesia while SCH 23390 disclosed two subpopulations of mice. One population responded to pergolide with marked locomotor activity whereas in the other subpopulation this response was absent. However, all mice challenged with pergolide failed to reverse reserpine-akinesia after alpha-methyl-p-tyrosine (AMPT) pretreatment. The alpha 1/alpha 2 agonist clonidine restored the ability of pergolide to overcome reserpine akinesia in supersensitive mice pretreated with SCH 23390. Clonidine reversed the akinesia in supersensitive mice but in normal animals it did not. However, in these last conditions, the combined use of clonidine plus the D2 agonist LY 171555 was effective to induce locomotion. Neither AMPT nor SCH 23390 inhibited this response whereas the alpha-adrenergic antagonists prazosin and yohimbine did prevent it. The alpha 2 agonist B-HT 920 failed to induce locomotor responses when given together with LY 171555. The same occurred with the D1 agonist SKF 38393 when given together with clonidine. The combined use of SCH 23390 plus prazosin in chronic reserpinized mice prevented pergolide-induced locomotion. Adrenergic stimulation, acting on alpha 1 receptors, could be an alternative to D1 stimulation as a necessary factor to obtain D2-induced motor responses under normo and supersensitive conditions.