Angiotensin converting enzyme inhibition prevents trophic and hypertensive effects of an antagonist of adenosine receptors

The continuous infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, causes hypertension and marked cardiovascular structural changes in Wistar rats. Adenosine inhibits noradrenaline and renin release. We investigated the effects of sympathetic denervation, evaluated renin activity and the influence of angiotensin converting enzyme inhibition in DPSPX-treated rats. Captopril was given (30 or 100 mg kg(-l) day(-l); p.o.) from day -l to day 28. On day 0, constant infusions of DPSPX (90 microg kg(-l) h(-l); i.p.) or vehicle were started. On day 28, fragments of the left ventricle, mesenteric and tail arteries were processed for morphological studies. Plasma renin activity was increased in DPSPX-treated animals. Sympathetic denervation delayed and partially prevented blood pressure rise. Angiotensin converting enzyme inhibition prevented DPSPX-induced hypertension and morphological changes. Our results, although pointing to the involvement of the sympathetic system, suggest that other mechanisms are involved. We could not differentiate between the trophic and anti-hypertensive effects of angiotensin converting enzyme inhibition.     

Sympathetic denervation caused by long-term noradrenaline infusions; prevention by desipramine and superoxide dismutase.

1. The effects of continuous intravenous infusion of noradrenaline (0.01 and 0.1 microgram kg-1 h-1) were studied in both the infused lateral saphenous vein and the contralateral saphenous vein of normal dogs. Noradrenaline, saline, noradrenaline + desipramine or noradrenaline + superoxide dismutase were infused using Alzet osmotic minipumps. 2. After a 5 day infusion period, the noradrenaline content in plasma and in both saphenous veins was determined, and the venous tissues submitted to light microscope morphometry and ultrastructural study and used for the determination of their O-methylation capacity (with [3H]-isoprenaline as a substrate). 3. Noradrenaline caused dose-dependent damage to the sympathetic nerve endings of the lateral saphenous veins. Concomitant changes in extraneuronal structure and function were observed (hypertrophy of smooth muscle cells, nuclear dysmorphy, thickening of the vessel wall, impairment in O-methylation capacity). 4. Desipramine and superoxide dismutase prevented or reduced the effects of noradrenaline on both the morphological and the biochemical parameters; the protection afforded by superoxide dismutase was more marked than that by desipramine. 5. It is concluded that moderately high doses of noradrenaline exert a 6-hydroxydopamine-like effect and that this chemical sympathectomy is partially or totally prevented by desipramine or superoxide dismutase. The data suggest that a substance derived from noradrenaline, in the formation of which free oxygen radicals are involved and which is subject to neuronal uptake, is the chemical entity responsible for the neurotoxic effect observed.     

ACTH increases noradrenaline release in pithed rabbits with electrically stimulated sympathetic outflow

ACTH 0.03-1 microgram/kg per min i.v. increased the noradrenaline spillover rate (the rate at which endogenous noradrenaline enters into plasma) and the plasma noradrenaline concentration in pithed rabbits with electrically stimulated sympathetic outflow. ACTH 0.1 and 1 microgram/kg per min decreased the mean arterial pressure (MAP). The effects of ACTH persisted in animals treated with propranolol. Corticosterone 10 micrograms/kg per min had no effect on the neurochemical and circulatory parameters. ACTH 0.03 and 1 microgram/kg per min increased plasma corticosterone and cortisol concentrations; the two doses of ACTH had approximately the same effect. The plasma corticosterone concentration reached after infusion of corticosterone 10 micrograms/kg per min was about twice that obtained after ACTH 0.03 or 1 microgram/kg per min. In a second series of experiments, a pressor dose of noradrenaline (1 or 2 micrograms/kg per min) was infused i.v. into pithed rabbits. ACTH 0.03 and 1 microgram/kg per min decreased blood pressure and increased heart rate in these animals. The results suggest that high doses of ACTH increase noradrenaline release by an action on postganglionic sympathetic neurons. The effect is probably not mediated through adrenal steroids. In addition, ACTH seems to decrease MAP and to increase heart rate through postsynaptic vascular and myocardial effects.     

Structural and functional alterations caused at the extraneuronal level by sympathetic denervation of blood vessels

Summary The lateral saphenous vein of the dog and the rabbit ear artery were surgically denervated, by clamping the vessel or by removal of the superior cervical ganglion, respectively. Both procedures resulted in denervation of the vessels.The denervated, lateral saphenous vein was supersensitive to exogenous noradrenaline and inactivation of the amine (in oil immersion experiments) was slower in denervated vein strips than in control strips treated with cocaine. Incubation experiments with ³H-noradrenaline confirmed that denervated strips formed considerably fewer metabolites than control ones (in the absence or presence of cocaine) and that O-methylation of noradrenaline was reduced by about 50%. When the strips were incubated with ³H-isoprenaline, the denervated ones accumulated and metabolized isoprenaline to a lesser degree than control strips. Hydrocortisone did not reduce the accumulation of isoprenaline in the denervated vein and had only minor effects on O-methylation. The metabolism of noradrenaline and isoprenaline gradually recovered with time.In the ear artery, denervation was accompanied by a marked reduction in O-methylation, but not in accumulation, of isoprenaline. In both vessels there was a highly significant positive correlation between noradrenaline content and O-methylating capacity; in the saphenous vein accumulation of isoprenaline was also positively correlated to noradrenaline content.Morphological changes observed in the denervated vessels consisted essentially in dedifferentiation of smooth muscle cells (which attained larger dimensions, had an indented, large nucleus, augmented euchromatin and an increased amount of ribosomes), abundance of extracellular material and fibroblasts. Mast cells were present in denervated veins (but not in controls) and the histamine content was increased in the former. Structural alterations were homogeneously distributed in the saphenous vein but restricted to the adventitio-medial area in the rabbit ear artery. Depletion of endogenous noradrenaline by reserpine pretreatment did not cause the alterations seen after denervation. On the other hand, continuous intravenous infusion of noradrenaline during 5 days did not prevent, and even worsened, the alterations caused by denervation. It was concluded that noradrenaline does not appear to be the factor the lack of which is exclusively responsible for the impairment of the extraneuronal system, in the denervated tissue.Taken together, the data show that the sympathetic innervation of blood vessels exerts a regulatory function on extraneuronal events; the disappearance of innervation results in marked impairment of the corticosteroid-sensitive O-methylating system and in morphological changes of both smooth muscle cells and fibroblasts.Part of the results were presented at 3rd Joint Meeting of the French and Spanish Pharmacological Societies in Toulouse, 1981 (Branco et al. 1982)     

EFFECTS OF THE α2-ADRENOCEPTOR AGONIST UK14304 ON PRESSOR RESPONSES IN PITHED RATS

1. Intravenous infusions of UK14304 (0.3-10 micrograms/kg per min) in pithed rat produced dose-dependent pressor responses which were not affected by prazosin (10 micrograms/kg) but were reduced by yohimbine (0.3 mg/kg). 2. Pressor responses to noradrenaline (0.1 micrograms/kg), phenylephrine (1 micrograms/kg) and vasopressin (10 mU/kg) were enhanced during infusions of UK14304 (0.03-1 micrograms/kg per min). Likewise, pressor responses to spinal sympathetic stimulation were enhanced during infusions of low concentrations of UK14304 (0.03-0.3 microgram/kg per min) but were reduced during infusion of a higher concentration of UK14304 (10 micrograms/kg per min). 3. After administration of yohimbine (0.3 mg/kg) or the calcium channel blocking drug diltiazem (infused at 50 micrograms/kg per min), pressor responses to noradrenaline and UK14304 were reduced, and responses to noradrenaline during infusion of UK14304 were not enhanced. 4. Prazosin (10 micrograms/kg) revealed a secondary depressor component in the response to sympathetic stimulation which is due to beta-adrenoceptor activation, since it was abolished by ICI 118551 (0.3 mg/kg). In the presence of ICI 118551 plus prazosin, pressor responses to sympathetic stimulation were enhanced during infusions of UK14304. 5. The depressor response to nitroprusside and the depressor component of responses to sympathetic stimulation after prazosin were enhanced during infusions of UK14304 at concentrations that increased the blood pressure. 6. The findings show that alpha 2-adrenoceptor activation enhanced the pressor responses to sympathetic nerve stimulation, noradrenaline, phenylephrine and vasopressin in the pithed rat and beta-adrenoceptor activation produced depressor responses which increased with increasing blood pressure.     

Author index to volume 60

The effects of adenosine, ATP and adenosine beta, gamma-methylene-5'-triphosphate (APPCP) were compared in the saphenous vein of the dog. Adenosine and ATP in concentrations which did not affect the response to exogenous noradrenaline, depressed that to nerve stimulation; this effect was antagonized by theophylline. APPCP increased tension in unstimulated veins and in those made to contract with noradrenaline and nerve stimulation. The results suggest that the inhibitory effect of ATP on adrenergic neurotransmission is due to its rapid breakdown to adenosine, which acts on prejunctional P1 purinoceptors.     

Superoxide dismutase partially prevents sympathetic denervation by 6-hydroxydopamine

Summary The effects of superoxide dismutase (S.O.D.) in two models of chemical denervation induced by 6-hydroxydopamine (6-OHDA) were studied.To evaluate the effects of S.O.D. on in vitro 6-OHDA-induced denervation, fragments of the lateral saphenous veins of mongrel dogs were pre-incubated in oxygenated Krebs-Henseleit solution with or without S.O.D. and then incubated under control conditions, with 6-OHDA or with 6-OHDA + S.O.D. Following the incubation period the fragments were repeatedly washed with Krebs solution and then used for determination of noradrenaline and for morphological study. 6-OHDA produced a profound depletion of noradrenaline. This depletion was significantly reduced although not prevented by S.O.D. The protective effect of S.O.D. was concentration-dependent. The ultrastructural study confirmed the 6-OHDA-induced sympathetic nerve degeneration as well as the protective effect afforded by S.O.D.In order to evaluate the effects of S.O.D. on in vivo 6-OHDA-induced denervation, male Wistar rats were anaesthetized and the tail vein cannulated. Saline or S.O.D. were intravenously delivered. 6-OHDA was injected five minutes after the beginning of infusions. Fragments of the left ventricle and vasa deferentia were used for determination of noradrenaline and for morphological study. 6-OHDA produced a significant depletion of noradrenaline in the left ventricle and vas deferens (to 8% and 18% of control values respectively). This depletion was reduced, though not prevented by S.O.D. Morphological data confirmed the neurotoxic effect of 6-OHDA and a protective role for S.O.D.In the concentration shown to afford protection against in vitro 6-OHDA-induced denervation, S.O.D. neither chemically inactivated 6-OHDA, nor did it exert any blocking effect on the neuronal uptake of ³H-noradrenaline. Thus, the protection afforded by S.O.D. against chemical denervation by 6-OHDA appears to be due to the free radical scavenging effect of S.O.D. Send offprint requests to A. Albino-Teixeira at the above address     

Central gastric antisecretory action of adenosine in the rat

The effects of intracerebroventricularly (i.c.v.) administered adenosine and some of its analogues on gastric secretion were studied in rats. The compounds inhibited the gastric output of acid, pepsin and fluid in pylorus-ligated rats in a dose-dependent manner with an order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) greater than (-)N6-phenylisopropyladenosine (R-PIA) greater than (+)N6-phenylisopropyladenosine (S-PIA) greater than adenosine. Pretreatment with 10 and 30 mg/kg of theophylline i.v. or 5 mg/kg of 8-phenyltheophylline s.c. did not modify the antisecretory effect of 0.1 microgram of NECA i.c.v. NECA injected i.c.v. did not affect the secretion induced by carbachol in awake rats subjected to vagotomy or in anaesthetized rats with intact vagi. NECA i.c.v. had no effect on the serum concentration of gastrin. The depletion of brain monoamines (noradrenaline, dopamine and serotonin) with 6-OHDA i.c.v. significantly attenuated the inhibitory action of NECA. Pretreatment with 10 mg/kg of naloxone i.v. or indomethacin s.c. did not modify the antisecretory effect of NECA. The results indicate that adenosine inhibits gastric secretion in rats by a decrease in the stimulatory vagal impulses to the stomach, and that it acts in the brain via receptors insensitive to xanthines. Brain biogenic monoamines, but not opioid peptides or prostaglandins seem to be involved in the central gastric antisecretory action of adenosine.     

MODIFICATION BY DILTIAZEM, A CALCIUM ANTAGONIST, OF THE PULMONARY VAGAL AND CARDIAC SYMPATHETIC CHEMOREFLEXES IN THE DOG

1. Experiments were performed on anaesthetized, open-chest dogs to determine the effects of diltiazem on: the pulmonary vagal chemoreflex evoked by intravenous (i.v.) injection of capsaicin; cardiac sympathetic chemoreflexes activated by epicardial application of bradykinin or capsaicin; and baroreflex-mediated changes in heart rate resulting from both pressor and depressor effects produced by i.v. injections of noradrenaline and bradykinin, respectively. 2. Diltiazem infused i.v. at a rate of 10-30 micrograms/kg per min (mean cumulative dose 0.53 +/- 0.05 mg/kg, n = 9), did not affect basal heart rate, despite significant (P less than 0.001) reduction of resting blood pressure. 3. Diltiazem treatment did not affect the pressor responses to i.v. noradrenaline (0.3 micrograms/kg) or the hypotensive effects of i.v. bradykinin (0.3 micrograms/kg), but reduced significantly both the baroreflex-mediated bradycardia (P less than 0.01) and tachycardia (P less than 0.05) occurring with noradrenaline and bradykinin, respectively. 4. In contrast, diltiazem greatly enhanced reflex bradycardia (P less than 0.001) and systemic hypotension (P less than 0.01) resulting from activation of the afferent vagal pulmonary receptors by i.v. capsaicin (3-5 micrograms/kg). 5. Reflex pressor responses evoked by activation of the afferent cardiac sympathetic neurons by epicardial application of bradykinin (1 microgram) or capsaicin (10-20 micrograms) were not affected by diltiazem, but the corresponding reflex increases in heart rate evoked by both substances were significantly (P less than 0.01) reduced. 6. The results indicate that diltiazem, while reducing the influence of sinoaortic baroreceptors on heart rate, facilitates the reflex vagal control of the cardiac pacemaker by the afferent cardiopulmonary vagal receptors. These nervous reflex mechanisms, which include attenuation of positive chronotrophic effects that may result from ischaemia-induced activation of the afferent cardiac sympathetic neurons, may play an important role in the protective action of diltiazem in ischaemic heart disease.     

Evidence for postsynaptic alpha 1- and alpha 2-adrenoceptors mediating catecholamine-induced negative chronotropic ventricular responses in the conscious dog

1. Adrenaline (0.25-1 microgram/kg), noradrenaline (0.125-0.5 microgram/kg) and dopamine (25-100 micrograms/kg) given in the conscious dog with chronic atrio-ventricular block after beta-adrenoceptor blockade, increased ventricular cycle length (VCL) and mean blood pressure (MBP). 2. Atropine (muscarinic receptor blocker) reduced the catecholamine-induced effects on VCL without modifying their hypertensive effects. 3. Phenoxybenzamine or phentolamine (alpha-adrenoceptor blockers) only decreased the effects of adrenaline on VCL but suppressed those of noradrenaline and dopamine. They only reduced the effects of adrenaline and noradrenaline on MBP, but reversed that of dopamine. 4. Yohimbine (alpha-adrenoceptor blocker) suppressed the catecholamine-induced effects on VCL, and reduced strongly the hypertensive effects of adrenaline and noradrenaline and reversed that of dopamine. 5. Thus, these results show the existence of negative chronotropic postsynaptic alpha-adrenoceptors in the ventricles.     

The effects of drugs and denervation on removal and accumulation of noradrenaline in the perfused hind-limb of the dog

Summary The removal of noradrenaline by the autoperfused hind-limb of dogs anaesthetized with pentobarbital, as well as the accumulation of noradrenaline in the saphenous vein were studied. Sensitivity of the perfused vascular area was determined by the response of the perfusion pressure to infusions of noradrenaline.The removal of noradrenaline declined very slowly during infusions lasting for up to 2 h, but edema of the perfused limb occurred after 45 to 60 min; therefore, the duration of infusion was limited to 30 min. During this period, noradrenaline was infused in rates increasing by a factor of 2 and ranging from 0.5 to 16 g/kg per minute. Accumulation capacity was saturated at 1 g/kg · min^–1, but the amount removed increased until a four-to eightfold rate was reached and then levelled off.At a rate of 1 g/kg · min^–1 the influence of drugs and of surgical denervation was investigated in other experimental series. Cocaine, nialamide, phenoxybenzamine and pretreatment with reserpine reduced removal (by 50, 45, 40 and 35%, respectively). Cortexone had no detectable influence on removal with this rate of infusion, but blocked it effectively when 4 g/kg · min^–1 were infused. Accumulation of noradrenaline in the vein was prevented by cocaine or reserpine, slightly reduced by phenoxybenzamine and enhanced by nialamide. The effects of nialamide plus cocaine did not differ significantly from those of cocaine alone, but cortexone plus cocaine completely blocked removal and accumulation. Surgical denervation reduced removal by about 70% and abolished accumulation; reserpine plus nialamide had similar effects. In the case of nialamide, removal progressively diminished during the infusion period and this time dependence of effects was accompanied by a prolongation of noradrenaline washout.Cocaine, reserpine and denervation caused supersensitivity of the perfused vessels to noradrenaline, whereas nialamide and cortexone had no such effect and phenoxybenzamine caused subsensitivity.The pronounced ability of the perfused vessels of the hind-limb to remove noradrenaline from the circulating blood is attributed primarily to neuronal uptake and intraneuronal oxidative deamination; extraneuronal uptake and inactivation seem to play an important role when neuronal mechanisms are saturated (infusion of higher noradrenaline doses) or impaired (after cocaine or denervation).Supported by Instituto de Alta Cultura (Research Project PMC/2). Part of this work was presented at the Fifth International Congress on Pharmacology (S.Francisco, July 23–28, 1972).     

The role of presynaptic alpha-adrenoceptors in the regulation of blood pressure in the conscious rabbit.

1 Changes in mean arterial pressure, heart rate and plasma noradrenaline after alpha-adrenoceptor blockade with several alpha-adrenoceptor antagonists have been studied in the conscious rabbit in order to investigate the possible role of presynaptic alpha-adrenoceptors in cardiovascular regulation. 2 Prazosin (0.05-2 mg/kg) and phentolamine (0.5-20 mg/kg) produced dose-dependent falls in mean arterial pressure and rises in plasma noradrenaline. These changes were related to the degree of postsynaptic alpha-adrenoceptor blockade determined by the pressor response to intravenous phenylephrine. 3 Similar changes in mean arterial pressure and plasma noradrenaline were observed after administration of the direct vasodilators hydralazine (1-10 mg/kg) and nitroprusside (2.5-55 microgram kg-1 min-1). 4 After baroreceptor deafferentation by sinoaortic denervation the falls in mean arterial pressure were much greater and the rise in plasma noradrenaline was markedly attenuated. 5 Yohimbine (1 mg/kg) increased mean arterial pressure and plasma noradrenaline but it was not possible to exclude the possibility that central nervous effects of yohimbine underlay the increased sympathetic activity. 6 The magnitude of the baroreflex response to changes in pressure make it unlikely that the functional significance of the presynaptic alpha-adrenoceptor can be readily determined by measurement of plasma noradrenaline in intact animals.     

Cutaneous venous dysfunction studied in vivo in the LPS-treated rabbit: implication of NO in saphenous vein hyporeactivity

Superficial vein pathology involves both mechanical (hyperpressure and distension) and inflammatory mechanisms. Conflicting results exist about the role of NO in the venous hyporeactivity induced by inflammation. In order to clarify this point, we aimed to investigate the effects of sepsis on cutaneous vein responsiveness in vivo and the possible contributions of constitutive and inducible NOS to the changes of venous contractility. Saphenous vein diameter was recorded by an ultrasonic echo-tracking device in pentobarbital-anaesthetised rabbits. Bacterial lipopolysaccharide (LPS) was administered i.v. at 20 mg/kg/15 min, inducing a progressive fall in mean arterial blood pressure after 2-3 h. The effects of LPS on saphenous vein responsiveness to noradrenaline (2 microg/kg i.v.) were measured simultaneously. In some rabbits, veins were removed for immunochemistry to detect iNOS staining. The venoconstriction to noradrenaline was already significantly reduced at 30 min after LPS (6+/-1% instead of 19+/-1% before LPS) and was completely abolished 3 h after LPS. A reduction of the venoconstriction induced by sumatriptan, a 5-HT(1B/D) agonist, (100 microg/kg, 11+/-1% after saline n=5) was also observed 180 min after LPS infusion (3+/-1%, n=4). The venodilatations induced by acetylcholine or sodium nitroprusside injected locally into the vein were not altered by LPS. When administered 90 min after LPS infusion, the NOS inhibitor L-NAME but not the selective iNOS inhibitor L-NIL (10 mg/kg) induced a recovery of the venoconstriction. Preventive perfusion with L-NAME (10 mg/kg/2 h) reduced the initial hyporeactivity to noradrenaline (30 to 60 min), but accelerated the lethal fall in MAP. L-NIL (10 mg/kg/2 h), to a lesser extent than L-NAME, also reduced the initial hyporeactivity to noradrenaline; in contrast to L-NAME, L-NIL also delayed the complete loss of noradrenaline constriction and improved animal survival. In control animals, neither L-NAME nor L-NIL modified the venoconstriction induced by noradrenaline. iNOS staining was observed in the saphenous vein endothelium after LPS. The experimental model developed in these experiments allows the study of venous responsiveness during sepsis in vivo. Our results show that LPS administration reduces saphenous vein contractility to both adrenergic and serotoninergic constrictor agents. The data suggest that both endothelial and inducible NO are involved in the loss of venous reactivity but these enzymes exert contrasting effects on blood pressure changes.     

Myometrial responses in situ to nicergoline, acebutolol, phentolamine and noradrenaline of the rat in proestrus

The effects of two adrenoceptor antagonists, nicergoline (alpha 1) and acebutolol (beta 1), on the contraction of myometrium in the proestrous rat were compared to those of noradrenaline and phentolamine. The spontaneous myometrial contractions of Wistar rats on the day of proestrus were recorded isometrically and the data were analysed using Wilcoxon non-parametric statistics. All drugs were administered i.v. and the doses are expressed as microgram/kg body weight. Noradrenaline (1200 micrograms/kg per h) induced a 32.5% reduction (P less than 0.001) of the uterine contraction amplitude. Nicergoline did not alter uterine motility significantly when administered alone at doses ranging from 400 to 1600 micrograms/kg. However, successive injections of nicergoline in the same range given during noradrenaline infusion at 600 micrograms/kg per h potentiated the relaxing action of the latter (38%, P less than 0.01). Phentolamine (120 micrograms/kg) reduced myometrial activity by 25% (P less than 0.05). This inhibitory response rose to 65% (P less than 0.001) when the dose of phentolamine was increased to 960 micrograms/kg. When a single injection of nicergoline (400 micrograms/kg) was followed by the administration of increasing doses of acebutolol (120, 1200, 2400 micrograms/kg) the slight inhibitory effect on uterine motility observed after administration of each of the two agents separately became more pronounced (P less than 0.05). It appears from these results that combining noradrenaline with nicergoline and nicergoline with acebutolol leads to potentiation of their relaxing effects. Furthermore the results confirm that nicergoline is a partial alpha-blocker.     

Antagonist pharmacology of adenosine A2B receptors from rat,guinea pig and dog

We have sought evidence for species differences between adenosine A2B receptors by comparing the potencies of eight adenosine receptor antagonists, representing four different chemical classes, at the native adenosine A2B receptors which mediate relaxation of smooth muscle from rat colon, guinea pig aorta and dog saphenous vein. In all three assays, the antagonists caused parallel rightward shifts in the concentration-response curves to NECA and there was no depression of the maximum responses. There were highly significant correlations between the pKB values on each of the three receptors. However, the pKB values of 8-SPT (8-p-(sulphophenyl)theophylline), XAC (8-[-[[[[(2-aminoethyl)amino]-carbonyl]methyl]oxy]phenyl]-1,3-dipropylxanthine), CGS 15943 (9-chloro-2,2-(furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine) and CGH 2473 N-[4-(3,4-dichloro-phenyl)-5-pyridin-4-yl-thiazol-2-yl]-acetamide) for the dog receptor exceeded by at least 0.5 log units the pKB values at the rat and guinea pig sites. Our data indicate species differences between the rat and guinea pig adenosine A2B receptors on the one hand and the dog adenosine A2B receptor on the other with respect to antagonist pharmacology.     

Dual effect of adrenaline on noradrenaline release in the pithed rabbit

We examined the effects of adrenaline on the noradrenaline release rate and plasma catecholamine levels in the pithed rabbit with electrically stimulated sympathetic outflow (3 Hz). Adrenaline (0.06 micrograms/kg/min) increased the rate of noradrenaline release into the plasma. This increase was prevented by propranolol (0.2 mg/kg + 0.1 mg/kg/h) and probably involves activation of presynaptic beta-adrenoceptors. A higher dose of adrenaline (1.0 micrograms/kg/min) significantly reduced the noradrenaline release rate. The reduction was "reversed" to a facilitatory effect by phenoxybenzamine (4 mg/kg). Propranolol alone slightly inhibited the noradrenaline release rate. After pretreatment with desipramine (1.0 mg/kg + 0.2 mg/kg/h), the inhibitory effect of propranolol on noradrenaline release was more pronounced and blood pressure was also lowered. However, in rabbits pretreated with captopril (1 mg/kg) in addition to desipramine, the sympathoinhibitory effect of propranolol was not observed. These results suggest that adrenaline can activate either presynaptic beta-adrenoceptors to increase noradrenaline release or, in higher doses, presynaptic alpha-adrenoceptors to inhibit noradrenaline release in vivo. The decrease in the noradrenaline release rate produced by propranolol alone may not be due to blockade of facilitatory presynaptic beta-adrenoceptors, but rather to depression of renin secretion. This would decrease angiotensin II formation and hence decrease the presynaptic release-enhancing effect of angiotensin II.     

Inhibition of cardiac sympathetic neurotransmission by histamine in the dog is mediated by H1-receptors.

1 The role of histamine H1- and H2-receptors in mediating prejunctional inhibition of cardiac sympathetic neurotransmission and histamine-induced coronary vasodilatation were investigated in perfused dog hearts in situ. 2 Intra-arterial injections of histamine into the right coronary artery during the resting state caused slightly positive chronotropic responses in doses larger than 1 microgram. 3 Histamine in doses of 0.1 to 10 micrograms into the right coronary artery reduced the tachycardia resulting from electrical stimulation of the cardiac sympathetic nerves. 4 Intra-coronary infusions of chlorpheniramine (300 micrograms/min) significantly reduced the histamine-induced depression of cardiac nerve stimulation. The effects of cimetidine (300 micrograms/min) and metiamide (300 micrograms/min) were less pronounced. 5 Histamine (1 to 10 micrograms) further increased heart rate resulting from the continuous intra-coronary infusion of noradrenaline (1 or 3 micrograms/min). 6 Intra-arterial injections of histamine (0.1 to 10 micrograms) caused an increase in coronary blood flow in a dose-dependent manner. This was partially inhibited by intra-coronary infusion of chlorpheniramine (10 to 300 micrograms/min) and by cimetidine (10 to 300 micrograms/min). The combination of both drugs (10 to 100 micrograms/min of each) caused a larger inhibition. 7 The present results suggest that the histamine-induced depression of heart rate during cardiac sympathetic nerve stimulation is due to a prejunctional effect mediated mainly by H1-receptors. Histamine-induced coronary vasodilatation in the dog is mediated both by H1- and H2-receptors.     

Comparison of activity of alpha-adrenoceptor agonists and antagonists in dog and rabbit saphenous vein

Summary The alpha adrenoceptors present on the saphenous vein of the dog and rabbit were characterised in vitro using the selective alpha₁ antagonist prazosin and the alpha₂ antagonists rauwolscine and yohimbine. In the dog saphenous vein, prazosin and rauwolscine competitively antagonised contractile responses to phenylephrine and UK-14,304 respectively. Noradrenaline was competitively blocked by prazosin only. In contrast, phenylephrine and methoxamine-induced responses in the rabbit saphenous vein were insensitive to prazosin and corynanthine. Rauwolscine competitively blocked responses to UK-14,304, noradrenaline and phenylephrine and pA₂ values were similar against each agonist. Noradrenaline and UK-14,304 were equipotent agonists on the rabbit saphenous vein while in the dog vein noradrenaline has a lower potency than UK-14,304. These results suggest that the dog saphenous vein has a mixed population of alpha adrenoceptors, while the alpha adrenoceptors on the rabbit vein are homogeneous, with characteristics of the alpha₂ subtype.     

Simultaneous measurement of [3H]noradrenaline release and neurogenic contraction under identical conditions, to determine the prejunctional inhibitory effects of SKF 99101H and BRL 56905 in dog saphenous vein

Using a tissue bath system which allowed the simultaneous measurement of electrically-induced [³H]nor-adrenaline release and neurogenic contraction under identical conditions, we investigated the prejunctional inhibitory activity of the selective 5-HT_1D/1B receptor agonists BRL 56905 ((±)-3-amino-6-carboxamido-1,2,3,4-tetrahydrocarbazole) and SKF 99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate), compared to sumatriptan and 5-HT. Transmural electrical stimulation (2 Hz) of dog saphenous vein induced consistent increases in [³H]noradrena- line release as well as reproducible contractile responses (<10% decrease over four stimulation periods). BRL 56905, SKF 99101H, sumatriptan and 5-HT (60 nM – 6 μM) inhibited electrically-evoked [³H]noradrenaline release and neurogenic contractile responses in dog saphenous vein. However, despite being measured under identical conditions, the inhibition of [³H]noradrenaline release was consistently greater than the inhibition of neurogenic contraction induced by a particular concentration of agonist, suggesting that neurogenic contractile responses in dog saphenous vein result from the combined release of noradrenaline and other non-noradrenergic neurotransmitters. Under the present assay conditions, since the agonists produced only small (BRL 56905, sumatriptan and 5-HT) or marginal (SKF 99101H) contractile responses, it is unlikely that this is the cause of the discrepancy observed between inhibition of release and inhibition of contraction. The inhibitory effects of BRL 56905, sumatriptan and 5-HT were blocked by the 5-HT_1D/1B receptor antagonist methiothepin, consistent with the involvement of canine ca-5-HT_1D/1B receptors in inhibiting neurotransmitter release and subsequent smooth muscle contraction in dog saphenous vein. The present results show that the novel 5-HT_1D/1B receptor agonists BRL 56905 and SKF 99101H are at least as potent as sumatriptan and 5-HT, at activating prejunctional inhibitory ca-5-HT_1D/1B heteroreceptors on sympathetic axon terminals in dog saphenous vein. In addition, when measured simultaneously in the same tissue preparation, [³H]noradrenaline release was inhibited to a much greater extent than neurogenic contraction by any particular agonist. Received: 7 October 1996 / Accepted: 23 December 1996     

Role of G-proteins and adenylate cyclase in antinociception produced by intrathecal purines

The effects of pertussis toxin, forskolin and phosphodiesterase inhibitors on the antinociceptive action of intrathecal purines were examined to investigate the possible involvement of adenylate cyclase in spinal antinociception. Pretreatment with pertussis toxin (0.25 and 0.5 microgram) inhibited the antinociceptive action of L-phenyl-isopropyladenosine (L-PIA), N6-cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamide adenosine (NECA) in the tail flick and hot plate tests. Forskolin (10-30 micrograms) reduced the effect of CHA and NECA in the hot plate test. Ro 201724 (30 micrograms) and Rolipram (20 micrograms) inhibited CHA in the tail flick and hot plate tests, but did not affect NECA in either test. These results suggest (1) spinal antinociception by purines is mediated by interactions with G-proteins (Gi linked to adenylate cyclase and/or Go linked to ion channels) (2) spinal antinociception by CHA is due to inhibition of adenylate cyclase (3) a separate mechanism, which does not involve stimulation of adenylate cyclase, may be involved in the spinal action of NECA.