Synthesis and structure-activity relationships of 2-sulfonamido-1,3,4,6,7,11b alpha-hexahydro-2H-benzo[a]quinolizines as alpha 2-adrenoceptor antagonists.

A series of 2-sulfonamido-1,3,4,6,7,11b alpha-hexahydro-2H-benzo[a]quinolizines were synthesized and examined for alpha 2- and alpha 1-adrenoceptor antagonist activity on the rat vas deferens and anococcygeus muscle, respectively. A number of compounds in this series were shown to be potent and selective alpha 2-adrenoceptor antagonists. Studies on the resolved enantiomers of compounds 6, 10, and 16 showed that alpha 2-adrenoceptor antagonist activity resided primarily in the 2R,11bS isomers, related to the absolute configuration of the alpha 2-antagonist yohimbine, such that the benzene ring and sulfonamide groups in this series were superimposable on the pyrrole and ester groups of yohimbine.

Differential class III and glibenclamide effects on action potential duration in guinea-pig papillary muscle during normoxia and hypoxia/ischaemia.

1. Microelectrode recording techniques were used to study the effects of several potassium channel blockers which are considered to be Class III antiarrhythmic compounds. The effects of (+)-sotalol, UK-66,914, UK-68,798 and E-4031 on action potential duration (APD) were determined in guinea-pig isolated papillary muscles. The compounds were evaluated under normoxic or hypoxic/ischaemic conditions at 36.5 degrees C and compared to glibenclamide, which is considered to be a blocker of ATP-dependent potassium channels. Prolongation of action potential duration at 90% repolarization (APD90) was taken as an indirect measure of potassium channel blockade. 2. Under normoxic conditions, the Class III compounds prolonged APD in a concentration-dependent manner. According to EC15 values, the order of potency of the Class III compounds was found to be UK-68,798 > E-4031 > UK-66,914 > (+)-sotalol. Glibenclamide did not significantly prolong APD90 under normoxic conditions. 3. Perfusion with an experimental hypoxic or ischaemic bathing solution produced qualitatively similar effects on action potentials. Over a period of 20-25 min in either of the experimental solutions, there was a small decrease in action potential amplitude (APA) and a prominent shortening of APD. The ischaemic solution also depolarized the resting membrane potential by about 15 mV. 4. (+)-Sotalol and UK-66,914 did not reverse the shortening of APD induced by perfusion with hypoxic Krebs solution. High concentrations of glibenclamide (10 microM) and UK-68,798 (30 and 60 microM) partially reversed the hypoxia-shortened APD. Glibenclamide was more potent and exhibited a greater time-dependent action than UK-68,798. 5. During experimental ischaemia, the Class III compound E-4031 (10 microM, n = 7) produced small, but significant, increases in the APD90 (11 +/-3 ms after 20 min) which were not clearly time-dependent(14 +/- 4 ms after 30 min). UK-68,798 (10 microM) also produced a small, but insignificant, increase in APD90(12 =/-6 ms at 20 min, n = 4). Higher concentrations of UK-68,798 (30 and 60 microM, n = 4) did not produce a consistently significant increase in APD90 during ischaemia: significance was only attained after 20 min in the presence of 60 microM UK-68,798 (24 +/- 12 ms). However, in marked contrast to the effects of the Class III compounds, glibenclamide (10 microM) produced large time-dependent increases in ischaemic APD90 (34 +/- 11 ms at 7 min, n = 9) which were significant 15 min or more after drug addition(52 +/- 12 ms at 20 min, n = 7; 74 +/- 5 ms at 30 min, n = 6).6. The present microelectrode data suggest that blockers of ATP-dependent potassium channels, such as glibenclamide, might prove to be more effective than Class III compounds against ischaemia-induced shortening of cardiac action potentials.

The effects of meptazinol in comparison with pentazocine, morphine and naloxone in a rat model of anaphylactic shock.

The actions of meptazinol, pentazocine, morphine and naloxone on the cardiovascular changes accompanying anaphylactic shock were evaluated in ovalbumin-sensitized anaesthetized rats. Pretreatment with meptazinol and pentazocine prevented the fall in mean arterial pressure associated with antigen challenge, whereas morphine and naloxone attenuated but did not completely prevent, this change. None of the drugs significantly altered the antigen-induced decreases in heart rate. All the drugs partially reversed the fall in mean arterial pressure when given after antigen challenge although the activity of naloxone was less marked. Pretreatment with reserpine prevented the restoration of blood pressure by all drugs. Additional experiments with meptazinol showed that pretreatment with phentolamine prevented its pressor action. In pithed non-sensitized rats the frequency-pressor response curve to splanchnic stimulation was shifted to the left by meptazinol and shifted to the right by pentazocine, but the changes were small Morphine and naloxone had no significant effects. It was concluded that opioid mixed agonist-antagonists reverse the cardiovascular changes associated with anaphylactic shock. These effects appear to be mediated by facilitation of sympathetic neurotransmission.

The evaluation of the novel pressor activity of gamma-piperidinobutyramide (WY 20051, DF480).

1 gamma-Piperidinobutyramide (Wy 20051, DF480) injected intravenously evoked pressor responses in the anaesthetized ganglion blocked rat preparation over the dose range 2.4 x 10(-6)-3.0 x 10(-4) mol/kg. 2 High doses (greater than 3.8 x 10(-5) mol/kg) or even repeated submaximal doses (1.9 x 10(-5) mol/kg) of Wy 20051 caused tachyphylaxis of this pressor response. 3 The noradrenaline pressor-response curve was shifted significantly to the right of the control curve following a dose of Wy 20051 (1.5 x 10(-4) mol/kg cumulative). 4 The dose-response curve for the pressor action of Wy 20051 was potentiated in reserpine-treated anaesthetized rats. In contrast, tyramine-induced pressor responses were abolished. 5 Wy 20051 contracted the guinea-pig isolated aortic spiral preparation (3.8 x 10(-5)-6.0 x 10(-4) mol) and evoked constrictor responses in the perfused mesenteric vasculature preparation of the rat (5.9 x 10(-7)-1.2 x 10(-5) mol). At higher doses the responses were reduced. 6 Wy 20051-induced constrictor responses of the perfused mesentery were unaffected by blockade of alpha-adrenoceptors or by tachyphylaxis of 5-hydroxytryptamine receptors. 7 The time for abolition of Wy 20051-induced constrictor responses of the mesentery in a calcium-free medium was not significantly different from that required for noradrenaline, but was significantly greater than that for KCl (P less than 0.001). 8 Wy 20051 and noradrenaline, but not KCl, evoked constrictor responses in the depolarized rat mesenteric vasculature. 9 The results indicate that Wy 20051 evokes pressor responses which have some of the characteristics of those of noradrenaline. However, the responses are not elicited by an alpha-adrenoceptor mechanism.

An investigation into the selectivity of a novel series of benzoquinolizines for alpha 2-adrenoceptors in vivo.

The potencies and selectivities of a novel series of benzoquinolizines for the alpha 2-adrenoceptor have been investigated in the rat in comparison with yohimbine and indoramin. Peripheral postjunctional alpha 2- and alpha 1-adrenoceptor blockade was measured as the reversal of B-HT 933 and methoxamine-induced pressor responses, respectively, in the pithed rat. Peripheral prejunctional alpha 2-adrenoceptor blockade was measured as the reversal of B-HT 933-induced inhibition of an electrically evoked tachycardia in the pithed rat. Central alpha 2-adrenoceptor blockade was measured as a reversal of the hypotension induced in anaesthetized rats by central (i.c.v.) administration of clonidine. Wy 25309, Wy 26392, Wy 26703 and yohimbine (0.3-3 mg kg-1 i.v.) evoked dose-dependent shifts to the right of the dose-response curves to B-HT 933 whilst having minimal effects on the methoxamine dose-response curve. The selectivity for alpha 2-adrenoceptors increased with the dose of antagonist administered. In general, the order of selectivity was Wy 25309 greater than Wy 26392 greater than Wy 26703 greater than yohimbine. Indoramin (1 mg kg-1 i.v.) shifted the methoxamine pressor dose-response curve to the right without affecting the B-HT 933 dose-response curves, confirming its selective alpha 1-antagonist activity. Peripheral administration of all three benzoquinolizines (1-100 micrograms kg-1 i.v.) led to a dose-dependent reversal of the hypotension evoked by central administration of clonidine (500 ng i.c.v.). The reversal was incomplete, higher doses causing a further decrease in blood pressure. A similar degree of hypotension induced by the ganglion blocking agent chlorisondamine (1 mg kg- I i.v.) was not reversed by the benzoquinolizines. 9 It is concluded that Wy 25309, Wy 26392 and Wy 26703 are selective alpha 2-adrenoceptor antagonists which readily penetrate the CNS.

Comparison of the cardiovascular effects of meptazinol and naloxone following haemorrhagic shock in rats and cats.

The cardiovascular effects of the opioid mixed agonist-antagonist, meptazinol, and the opioid antagonist, naloxone, have been evaluated in conscious rats, anaesthetized rats and anaesthetized cats following the induction of haemorrhagic shock. The mean arterial pressure of conscious rats decreased by 17-29 mmHg following a haemorrhage of 20% of blood volume. Meptazinol (17 mg kg-1, i.m.) administered after haemorrhage evoked a rapid and sustained increase in mean arterial pressure to pre-haemorrhage levels. Naloxone (10 mg kg-1, i.v.) also increased mean arterial pressure to a level significantly higher than post-haemorrhage values. Neither haemorrhage nor subsequent drug treatments evoked significant changes in the heart rates of conscious rats. In anaesthetized rats, 20% haemorrhage evoked decreases in mean arterial pressure, heart rate and cardiac output. Blood flow to the heart, skin, skeletal muscle, kidneys, spleen and liver (arterial) was decreased. Meptazinol and naloxone increased blood pressure and total peripheral resistance, but did not significantly alter heart rate or cardiac output. Hepatic arterial flow decreased further in both drug and vehicle treated groups. In addition meptazinol slightly reduced skeletal muscle flow. In anaesthetized cats 40% haemorrhage decreased mean arterial pressure by 46 +/- 3 mmHg. An intravenous infusion of either meptazinol or naloxone (cumulative 2 mg kg-1, i.v.) partially restored blood pressure. In experimental animal models of haemorrhagic shock, meptazinol has a similar cardiovascular profile to naloxone. The established analgesic activity of meptazinol may confer an advantage in some shock states.

Investigation into the cardioregulatory properties of the alpha 1-adrenoceptor blocker indoramin.

The cardioregulatory properties of the alpha 1-adrenoceptor blocker indoramin have been compared with those of prazosin in the anaesthetized rat. The effects of autonomic blockade on heart rate responses evoked by these two agents and their effects on blood pressure and heart rate after peripheral or central administration have been compared. Cumulative administration of indoramin (0.8-25.6 mg kg-1 i.v.) evoked significant decreases in arterial blood pressure and a concomitant bradycardia. Pithing or autonomic blockade, by pretreatment with a combination of practolol and bilateral vagotomy, prevented the bradycardia evoked by indoramin (0.8-3.2 mg kg-1 i.v.). Atropine sulphate pretreatment abolished the bradycardia until a cumulative dose of 25.6 mg kg-1(i.v.) of indoramin had been reached. Bilateral vagotomy, intravenous administration of atropine methylnitrate or practolol pretreatment attenuated the bradycardia. Prazosin (0.02-0.64 mg kg-1 i.v.) evoked a fall in arterial blood pressure of similar magnitude to that observed following indoramin. A bradycardia was evoked only at a relatively high dose (0.64 mg kg-1 i.v.). Intracisternal injection of indoramin or prazosin evoked bradycardia and hypotension at a dose which had no effect after intravenous injection (25 micrograms). Intracerebroventricular injection of indoramin (25 micrograms) had no significant effect on heart rate or blood pressure compared to control values, whereas prazosin (25 micrograms) evoked a significant tachycardia and hypotension. It is concluded that the bradycardia evoked by indoramin in the rat is not due to a direct action on the heart except possibly at high doses. Central alpha 1-adrenoceptor blockade, possibly in the brainstem region, results in a bradycardia and this may explain the lack of reflex tachycardia following the administration of indoramin.

Evaluation of the bronchodilator properties of Ro 31-6930, a novel potassium channel opener, in the guinea-pig.

1. Ro 31-6930 (0.001-0.3 microM), cromakalim (0.03-3.0 microM), salbutamol (0.001-0.3 microM) and theophylline (0.3-100 microM) evoked dose-related reductions in guinea-pig spontaneous tracheal tone with IC50 values of 0.044, 0.20, 0.021 and 21.0 microM respectively. All four agents also relaxed tone supported by betahistine, carbachol, 5-hydroxytryptamine (5-HT), leukotriene D4 (LTD4), U46619 and prostaglandin D2 (PGD2). The order of potency of tracheal relaxants was always salbutamol greater than Ro 31-6930 greater than cromakalim greater than theophylline. 2. All four agents evoked dose-related reductions in 5-HT- and histamine-induced bronchoconstriction in pithed vagotomised guinea-pigs. The dose of Ro 31-6930 producing 50% inhibition of a 5-HT bronchoconstriction was 11.6 micrograms kg-1 and the dose producing 50% inhibition of a histamine bronchoconstriction was 4.4 micrograms kg-1. Salbutamol was approximately 4-5 times more potent than Ro 31-6930 whilst cromakalim was approximately 10 times less potent than Ro 31-6930 as a bronchodilator. Theophylline was markedly less potent than any of the other agents. 3. Ro 31-6930, cromakalim, salbutamol and theophylline each protected conscious guinea-pigs from histamine-induced respiratory distress. Ro 31-6930 and salbutamol were each effective at oral doses of 1.0 and 3.0 mg kg-1 whilst cromakalim was effective at oral doses of 3.0 and 10.0 mg kg-1. Theophylline showed activity only at 300 mg kg-1 p.o. 4. Ro 31-6930 is a novel potassium channel opener which is a potent relaxant of guinea-pig tracheal smooth muscle in vitro and a bronchodilator in vivo.

A comparison of the haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine in anaesthetised normotensive rats.

The regional haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine were compared using pulsed Doppler flowmetry in the anaesthetised rat. In order of potency, Ro 31-6930 (0.1-300 micrograms/kg), cromakalim (1-300 micrograms/kg) and nifedipine (1-1000 micrograms/kg) produced dose related falls in mean arterial pressure. The hypotensive effects of Ro 31-6930 and cromakalim were accompanied by reflex tachycardia. All three agents reduced renal vascular resistance by 30-50%. Cromakalim exerted a selective action on this vascular bed. Similar maximal reductions in mesenteric vascular resistance (37-50%) were observed; however, cromakalim was the least potent on this vascular bed. Maximal reductions in iliac vascular resistance (65-78%) were observed, with an order of potency as observed on mean arterial pressure. Qualitative differences in the regional haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine are evident from this study. The different profiles of Ro 31-6930 and cromakalim may reflect structural differences between the pharmacophores of these compounds.

Assessment of myeloperoxidase activity in renal tissue after ischemia/reperfusion.

We have shown that a photometric assay of myeloperoxidase derived from rat blood polymorphonucleocytes employing 3,3',5,5'-tetramethylbenzidine as substrate is more sensitive than an established assay employing o-dianisidine. We went on to demonstrate that rat renal tissue is capable of inhibiting peroxidase activity. This activity approached 100% when the rat renal supernate was incubated at 60 degree C for 2 h and the assay was conducted in the presence of a 10-fold higher concentration of hydrogen peroxide (H2O2). Rat kidneys undergoing 45 min ischaemia and 1,3 and 6 h reperfusion in vivo, exhibited significant increases in myeloperoxidase activity, indicating tissue polymorphonucleocyte accumulation. Monoclonal antibodies against rat intercellular adhesion molecule 1 (ICAM-1) and CD18 of beta 2-integrins administered both 5 min before a period of 45 min renal ischaemia (20 micrograms/kg i.v.) and at the commencement of 1 h reperfusion (20 micrograms/kg i.v.) reduced renal tissue polymorphonucleocyte accumulation. However, similar treatment with the parent murine antibody immunoglobulin G1 (IgG1) and an unrelated murine antibody, IgG2a, also significantly reduced renal tissue polymorphonucleocyte accumulation. In conclusion, we demonstrate that the rat renal suppression of peroxidase activity can be overcome by a combination of heat inactivation and the provision of excess assay H2O2. In addition, the available evidence suggests that murine monoclonal antibodies against rat adhesion molecules may exert non-specific actions in our model of renal ischaemia/reperfusion in vivo.

The inhibitory effects of Ro 31-6930 and BRL 38227 on cholinergically-mediated bronchoconstriction in the guinea-pig.

This study compares the effects of two K(+0-channel openers, Ro 31-6930 and BRL 38227, on cholinergically-evoked contraction of guinea-pig airways to examine whether either compound acts through prejunctional inhibition of the release of acetylcholine. In the isolated trachea, Ro 31-6930 and BRL 38227 evoked concentration-dependent inhibition of tone generated by electrical field stimulation with pD2 values of 7.03 (6.77-7.29) and 6.26 (5.91-6.61) respectively and of that elicited by acetylcholine with pD2 values of 7.38 (6.52-8.24) and 6.65 (6.16-7.13). Neither compound was more potent against responses to electrical field stimulation than against acetylcholine. In the anaesthetised guinea-pig, Ro 31-6930 inhibited the bronchoconstriction evoked by bilateral vagus nerve stimulation and intravenous acetylcholine with ID50 values of 12.9 +/- 3.9 and 3.6 +/- 1.3 micrograms/kg i.v. respectively. The corresponding values for BRL 38227 were 356 +/- 157 and 37.9 +/- 13.4 micrograms/kg i.v. respectively. Thus, in vivo, both compounds were more potent against acetylcholine than against vagal stimulation. These results provide indirect evidence that K(+)-channel openers do not inhibit the release of acetylcholine from parasympathetic nerves in guinea-pig airway smooth muscle.

The effects of some alpha-adrenoceptor antagonists on the responses of the canine saphenous vein to B-HT 933, UK-14304 and methoxamine.

The benzoquinolizines Wy 25309, Wy 26703 and Wy 27127, previously reported as potent antagonists at presynaptic alpha 2-adrenoceptors were also potent antagonists of B-HT 933 in isolated saphenous veins of the dog confirming their activity at post synaptic alpha 2-adrenoceptors. Yohimbine was a more potent antagonist of B-HT 933 in isolated saphenous vein than were the Wy compounds or idazoxan contrasting with the reported potencies of these compounds at presynaptic sites in rat vas deferens and raising the possibility of differences between pre- and postsynaptic alpha 2-adrenoceptors. Contractions of the saphenous vein were observed with high concentrations of idazoxan.

The effect of age on the sensitivity of pre- and postsynaptic alpha-adrenoceptors to agonists and antagonists in the rat.

Age-related changes in presynaptic alpha-2 and postsynaptic alpha-1 adrenoceptors have been determined using the rat isolated vas deferens and the thoracic aorta, respectively. The IC50 values of clonidine, B-HT 933 and UK 14,304 for inhibition of the electrically evoked contractions of the vas deferens were significantly higher in 50 week old rats when compared with rats of 5 weeks. Similarly, EC50 values for the contraction of the thoracic aorta by noradrenaline, methoxamine and phenylephrine were significantly increased in 50 week old rats compared with 5 week old rats. No age-related changes in the potency of the selective alpha-2 adrenoceptor antagonists yohimbine and Wy 26392 were detected in the vas deferens. Similarly, there were no age-related changes in the alpha-1 adrenoceptor antagonist potency of indoramin or prazosin on the aorta. The results of the present study suggest that the potency of both alpha-1 and alpha-2 adrenoceptor agonists, as measured by their respective EC and IC50 values decreases with increasing age.

Alpha 2-adrenoceptor antagonism and other pharmacological antagonist properties of some substituted benzoquinolizines and yohimbine in vitro.

Comparison of pA2 values for antagonism of clonidine induced inhibition of the electrically evoked contraction of the rat isolated vas deferens (alpha 2-adrenoceptor) and antagonism of contractions to methoxamine on the rat isolated anococcygeus (alpha 1-adrenoceptor) showed a group of substituted benzoquinolizines (Wy 25309, 26392 and 26703) to be more potent and more selective alpha 2-adrenoceptor antagonists than yohimbine. The benzoquinolizines and yohimbine enhanced stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [3H]-noradrenaline, as expected for alpha 2-adrenoceptor antagonists. In contrast to the results on the rat vas deferens, yohimbine was more potent than the benzoquinolizines. At higher concentrations all the alpha-adrenoceptor antagonists reduced the stimulation-evoked contraction of the pulmonary artery. The benzoquinolizines were competitive antagonists of 5-hydroxytryptamine on the rat isolated ileum. Wy 25309 showed only weak activity (pA2 = 5.21) whereas Wy 26703 was more potent (pA2 = 7.25). Yohimbine was a potent antagonist of 5-hydroxytryptamine. Wy 26703 was the only compound to have histamine antagonist effects in the guinea pig isolated ileum and to antagonise the chronotropic effect of isoprenaline on the isolated atria of the guinea pig and in both instances activity was weak (pA2 values 5.3 and 5.5 respectively). Yohimbine reduced the spontaneous beating of the atria at 3 X 10(-6) M. No compound at 10(-5) M antagonised acetylcholine on the guinea pig ileum. These novel substituted benzoquinolizines should be useful experimental compounds for the study of alpha 2-adrenoceptor mediated responses.

Cardioregulatory properties of indoramin in the rat.

The antihypertensive action of the competitive alpha-adrenoceptor antagonist indoramin is not accompanied by reflex tachycardia in animals or man. The possibility that the established local anaesthetic property of indoramin is involved in its cardioinhibitory action has been investigated. Indoramin evoked a dose-dependent bradycardia in anaesthetized/pithed rats. The decrease in heart rate was slightly greater than that evoked in anaesthetized intact animals suggesting that indoramin had a direct action on the heart. Reflex tachycardia was simulated in pithed rats be increasing the frequency of cardiac nerve stimulation from 0.3 to 1 Hz. Indoramin and the local anaesthetic agents, lignocaine and procaine, reduced the positive chronotropic response without markedly altering the basal rate. The response curves were parallel. In contrast, phentolamine decreased the positive chronotropic response, but only at high doses which were associated with a marked decrease in the basal rate. Thymoxamine and prazosin had so significant effects on the chronotropic response. These experiments suggest that the cardioregulatory action of indoramin is attributable to its local anaesthetic property had this action further distinguishes it from the other alpha-adrenoceptor antagonists tested.