Cardioprotective effects of carvedilol, a novel beta adrenoceptor antagonist with vasodilating properties, in anaesthetised minipigs: comparison with propranolol.

OBJECTIVE: The aim was to evaluate in a minipig model of acute myocardial infarction the cardioprotection provided by the beta adrenoceptor blocking and vasodilating activities present in carvedilol; comparison was made to the pure beta adrenoceptor antagonist, propranolol. METHODS: Experiments were performed in 25 Yucatan minipigs (9-12 kg), randomly assigned to receive vehicle (n = 7), carvedilol 0.3 mg.kg-1 (n = 6), carvedilol 1 mg.kg-1 (n = 6), or propranolol 1 mg.kg-1 (n = 6). Myocardial infarction was produced by occlusion of the left anterior descending coronary artery for 45 min followed by 4 h of reperfusion. Vehicle, carvedilol (0.3 and 1 mg.kg-1) or propranolol (1 mg.kg-1) were given intravenously 15 min before the coronary artery occlusion. At the end of the reperfusion period, infarct size was determined using Evans blue dye and triphenyltetrazolium chloride staining. Infarct volumes were visualised using computer assisted three dimensional image analysis of the stained myocardial tissue sections. Myeloperoxidase activity was measured in tissue samples removed from normal, infarcted, and at risk areas. RESULTS: Carvedilol (1 mg.kg-1) reduced infarct size by over 90% without producing pronounced changes in systemic haemodynamic variables. The ability of carvedilol to reduce infarct size was clearly dose dependent. Thus infarct size, which represented 27.5(SEM 2.3)% of the area at risk in the vehicle treated group, was only 13.1(4.0)% (p < 0.05) and 2.4(1.5)% (p < 0.01) in pigs treated with carvedilol at 0.3 and 1 mg.kg-1, respectively. In animals treated with propranolol (1 mg.kg-1), infarct size represented 10.9(2.4)% of the area at risk (p < 0.05). The 60% and 91% reductions in infarct size produced by propranolol (1 mg.kg-1) and carvedilol (1 mg.kg-1), respectively, were clearly evident upon three dimensional image analysis. The reduction in infarct size was significantly greater for carvedilol (1 mg.kg-1) compared to propranolol (1 mg.kg-1) at equivalent beta adrenoceptor blocking doses. Pretreatment with propranolol did not reduce the increases in myeloperoxidase activity observed in the area at risk or in the infarcted area. In contrast, carvedilol produced a dose dependent reduction in myeloperoxidase activity in these areas. CONCLUSIONS: Carvedilol limits myocardial necrosis resulting from coronary artery occlusion and reperfusion in a more pronounced manner than the pure beta adrenoceptor antagonist, propranolol. The cardioprotective effect of carvedilol, which reduced infarct size by 91%, may result from the combined effects of beta adrenoceptor blockade and vasodilatation, and possibly also from inhibition of intracellular calcium overload in cardiac cells resulting from antagonism of myocardial alpha 1 adrenoceptors and/or calcium channel blockade. The cardioprotection provided by carvedilol may ultimately be of benefit in hypertensive patients who are at risk for acute myocardial infarction.

Synthesis and antihypertensive activity of substituted trans-4-amino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-3-ols.

A series of novel substituted trans-4-amino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-3-ols was prepared and tested for antihypertensive activity in the conscious deoxycorticosterone acetate (DOCA)/saline treated hypertensive rat. Optimum blood pressure lowering activity requires 6-substitution by a strong electron-withdrawing group, together with a pyrrolidino or piperidino group at the 4 position. Exceptions to this were the 7-nitro-4-pyrrolidine analogue and the 6-nitro-3-chloropropylamine, which retained marked antihypertensive activity. All of these compounds were direct vasodilators and had comparable antihypertensive activity to hydralazine and to the calcium antagonist, nifedipine. The synthetic route to these compounds involves cyclization of of propargyl ethers to 2H-1-benzopyrans, followed by conversion via bromohydrins to 3,4-epoxides, which were ring opened with the appropriate amines. Meta-substituted propargyl ethers gave both 5- and 7-substituted benzopyrans on thermal cyclization, the former predominating. A new route to 2,2-dimethyl-7-nitrobenzopyran is described.

Synthesis and antihypertensive activity of 6,7-disubstituted trans-4-amino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-3-ols.

A series of novel 6,7-disubstituted trans-3,4-dihydro-2, 2-dimethyl-4-pyrrolidino-(or piperidino)-2H-1-benzopyran-3-ols was prepared and tested for antihypertensive activity in the conscious spontaneously hypertensive rat (SHR) and compared with certain of their monosubstituted analogues. The potent blood pressure lowering activity of the 6-monosubstituted compounds was enhanced by incorporation of an acetylamino or amino group at C(7) and that of the 7-nitro-substituted compound by incorporation of an amino (but not an acetylamino group) at C(6). The combination of 6-nitro or 6-cyano with 7-(acetylamino) or 7-amino groups and 6-amino with 7-nitro groups in trans-4-pyrrolidino- or -4-piperidino-2,2-dimethyl-2H-1-benzopyranols conferred superior antihypertensive activity to hydralazine and to the calcium antagonist, nifedipine, in SHR. The synthetic route to these compounds involves the conversion of 2H-1-benzopyrans to bromohydrins that were treated with pyrrolidine or piperidine. Preparation of the 6-cyano-7-amino analogue was accomplished when 6-cyano-7-[(trifluoroacetyl)amino]-2,2-dimethylbenzopyran was used as starting material.

Stimulation of the hypothalamo-pituitary-adrenal axis in the rat by three selective type-4 phosphodiesterase inhibitors: in vitro and in vivo studies.

1. Previous studies in our laboratory have shown that the synthetic xanthine analogue denbufylline, a selective type 4 phosphodiesterase (PDE-4) inhibitor, is a potent activator of the hypothalamo-pituitary-adrenal (HPA) axis when given orally or intraperitoneally (i.p.) to adult male rats. This paper describes the results of experiments in which well established in vivo and in vitro methods were used to compare the effects of denbufylline on HPA function with those of two other selective PDE-4 inhibitors, rolipram and BRL 61063 (1,3-dicyclopropylmethyl-8-amino-xanthine). For comparison, parallel measurements of the immunoreactive- (ir-) luteinising hormone (LH) were made where appropriate. 2. When injected intraperitoneally, rolipram (40 and 200 micrograms kg-1, P < 0.005), denbufylline (0.07-0.6 microgram kg-1, P < 0.05) and BRL 61063 (30 micrograms kg-1, P < 0.005) each produced marked rises in the serum ir-corticosterone concentrations. However, lower doses of rolipram (1.6 and 8 micrograms kg-1) and BRL 61063 (0.25-6 micrograms kg-1) were without effect (P > 0.05). By contrast, intracerebroventricular (i.c.v.) injection of rolipram (8 ng-1 micrograms kg-1) or denbufylline (50 ng-1 microgram kg-1) failed to influence the serum ir-corticosterone concentration. BRL 61063 (8-120 ng kg-1, i.c.v.) was also ineffective in this regard although at a higher dose (1 microgram kg-1, i.c.v.) it produced a small but significant (P < 0.05) increase in ir-corticosterone release. Denbufylline also increased the serum ir-LH concentration when given peripherally (0.2-0.6 microgram kg-1, i.p., P < 0.05) or centrally (100 ng kg-1, i.c.v., P < 0.05) but rolipram (1.6-200 micrograms kg-1, i.p. or 8 ng-1 microgram kg-1, i.c.v.) and BRL 61063 (0.25-30 micrograms kg-1, i.p. or 1 ng-1 microgram kg-1, i.c.v.) did not (P > 0.05). 3. In vitro rolipram (10 microM, P < 0.01), denbufylline (1 mM, P < 0.001) and BRL 61063 (1 and 10 microM, P < 0.05) stimulated the release of corticotrophin releasing hormone (ir-CRH-41) but lower concentrations of the drugs were without effect as also was BRL 61063 at 100 microM (P > 0.05); the rank order of potency was thus BRL 61063 > rolipram > denbufylline. The adenylyl cyclase activator forskolin (100 microM, P < 0.01) also stimulated the release of ir-CRH-41, producing effects which were additive with those of rolipram and denbufylline but not with those of BRL 61063. The secretory responses to forskolin (100 microM) were accompanied by a highly significant increase in the cyclic AMP content of the hypothalamic tissue (P < 0.005). Rolipram (10 microM) also significantly (P < 0.05) elevated the hypothalamic cyclic AMP but denbufylline (10 mM) and BRL 61063 (10 microM) did not. However, all three PDE-inhibitors potentiated the rise in cyclic AMP induced by forskolin (P < 0.05). None of the drugs tested, alone or in combination, modified the release of arginine vasopressin (ir-AVP) from the hypothalamus. 4. Rolipram (100 microM), denbufylline (100 microM) and BRL 61063 (100 microM) stimulated the release of corticotrophin (ir-ACTH) from pituitary tissue in vitro (P < 0.05) but in lower concentrations they were without significant effect. In addition, rolipram (10 microM, P < 0.05), denbufylline (0.1 microM, P < 0.05) and BRL 61063 (10 microM, P < 0.05) potentiated the significant (P < 0.05) rises in ir-ACTH secretion induced by forskolin (100 microM). Forskolin (100 microM) also produced a highly significant increase (P < 0.01) in the tissue cyclic AMP content which was further potentiated by rolipram (10 microM), denbufylline (10 microM) and BRL 61063 (10 microM) which, alone did not affect the cyclic AMP content of the tissue. 5. Since both denbufylline and BRL 61063 possess significant adenosine A1 receptor blocking activity, further studies examined the potential influence of these receptors on the secretion in vitro of CRH-41, AVP and ACTH. The release of ir-CRH-41 was increased significantly by adenosine deaminase (ADA, 5microml-1, P<0.05) and the A1-receptor antagonist, 1,3-dicyclopropyl-8-cyclopentylxanthine (DPCPX, 0.1-10nM, P<0.05). The responses to ADA were abolished by the A1 receptor agonist N6-cyclo-hexyladenosine (CHA, 100nM, P<0.05) which alone had no significant effect on ir-CRH-41 release. ADA (0.1-10microml-1) and DPCPX (1nM) had weak stimulant and inhibitory effects, respectively, on the release of ir-ACTH from the pituitary gland while CHA (0.1-10nM) was without effect. Ligand binding studies with [3H]-DPCPX as a probe demonstrated the presence of specific high affinity A1 binding sites in the hypothalamus (Kd=0.7nM; Bmax=367+/-32fmolmg-1 protein) and in the hippocampus (Kd=1nM; Bmax=1165 +/-145fmolmg-1 protein). In both tissues binding of the ligand was displaced by CHA (IC50=1nM (hypothalamus) and 2nM (hippocampus)), BRL 61063 (IC50=80nM (hypothalamus) and 100nM (hippocampus)) and denbufylline (IC50=5microM (hypothalamus) and 9microM(hippocampus)) but not by rolipram. 6.The results suggest that rolipram, denblufylline and BRL 61063 stimulate the HPA axis in the rat, acting at the levels of both the hypothalamus and the pituitary gland. Their actions may be explained, at least in part, by inhibition of PDE-4 but additional actions including blockade of hypothalamic adenosine A1 receptors by denbufylline and BRL 61063 cannot be excluded.

BRL 13776: a novel antihypertensive agent with interesting monoamine depleting properties.

1. Oral doses of 10-100 mg/kg of BRL 13776 lowered the blood pressure of both deoxycorticosterone acetate (DOCA)/NaCl-treated hypertensive rats and untreated normotensive rats. 2. BRL 13776 (100 mg/kg, orally) also reduced the blood pressure of renal hypertensive cats (cellophane perinephritis model). 3. No tolerance developed to the blood-pressure lowering action of BRL 13776 when an oral daily dose of 100 mg/kg was administered repeatedly for up to 15 days to hypertensive rats and cats. 4. The fall in blood pressure to BRL 13776 in rats was associated with a reduction of tissue catecholamines. 5. The catecholamine depletion occurred in all the peripheral tissues examined but in the brain was restricted to certain regions, these being the hind-brain on single dosing and the hind-brain, hypothalamus and mid-brain on repeated dosing. Catecholamine levels in the cerebral hemispheres were not affected by either single or repeated doses of BRL 13776. 6. BRL 13776 caused some reduction of the 5-hydroxytryptamine content of the heart but not of whole brain or any brain region. 7. Neither single doses (up to 900 mg/kg orally) nor repeated doses (100-300 mg/kg orally) of BRL 13776 produced any significant behavioural effects in animals. 8. BRL 13776 is a new type of agent to display both antihypertensive and monoamine-depleting properties. The reduction of noradrenaline in certain brain regions may be a cause of the antihypertensive response but depletion in the periphery could contribute in a major or minor way. The differential action on noradrenaline in the brain together with the lack of effect on 5-hydroxytryptamine might also explain the apparent absence of behavioural effects.

Roles for adenosine A1- and A2-receptors in the control of thyrotrophin and prolactin release from the anterior pituitary gland.

Adenosine has been implicated in various aspects of pituitary function but little is known of its role in the regulation of thyrotrophin (TSH) release. This study examined the effects of adenosine deaminase (ADA, which provokes adenosine breakdown) and selective adenosine-receptor ligands on the secretion of immunoreactive (ir-) TSH and prolactin (PRL) by rat anterior pituitary segments in vitro. ADA (5 U/ml) stimulated the release of both hormones (P<0.01) as also did the selective adenosine A1-receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.1 & 1 nM, P<0.01); the responses to ADA were inhibited by an A1-receptor agonist, N6-cyclohexyladenosine (0.1-10 nM, P<0.01). A non-selective A1/A2-receptor agonist, N-cyclopropylcarboxamidoadenosine (1-100 nM) had mixed effects on ir-TSH release. However, the A2A-receptor selective agonist, CGS 21680 (1-100 nM) increased ir-TSH (P<0.05) and ir-PRL release (P<0.01); its effects on ir-TSH were blocked by concentrations of DPCPX (100 nM, P<0.01) sufficient to antagonize A2-receptors. These data suggest that adenosine acts via A1-receptors to tonically suppress ir-PRL and ir-TSH release but that A2A-receptor activation enhances the release of both hormones.

Cyclic nucleotides and central cardiovascular control in the conscious cat.

When infused into the third ventricle of the brain of the conscious cat, the phosphodiesterase inhibitors, papaverine and aminophylline, increased blood pressure and heart rate whilst the phosphodiesterase stimulator, imidazole-4'-acetic acid, decreased blood pressure. Administered intravenously, these compounds caused the opposite effects indicating that the responses after third-ventricular administration were centrally mediated. Infused of dibutyryl cyclic 3',5'-AMP into the third ventricle increased blood pressure and heart rate, whereas dibutyryl cyclic 3',5'-GMP had little effect and antagonised and pressor response to dibutyryl cyclic 3',5'-AMP. ATP, 5'-AMP and adenosine induced similar changes to dibutyryl cyclic 3',5'-AMP, although the pressor responses to these compounds were of different onset and/or duration. When administered into the third ventricle both alpha- and beta-adrenoceptor blocking agents reduced the pressor response caused caused by the phosphodiesterase inhibitor, papaverine, given by the same route. These results demonstrate a central effect of adenosine and adenosine nucleotides to raise blood pressure. The possibility of a specific role for cyclic 3',5'-AMP in the central control of blood pressure is discussed.

BRL 8242 (2-[2-benzimidazolyl]-amino-2-imidazoline dihydrochloride), a new inhibitor of dopamine-beta-hydroxylase with antihypertensive activity.

BRL 8242 (2-[2-benzimidazolyl]-amino-2-imidazoline dihydrochloride) was found to inhibit dopamine-beta-hydroxylase in vitro and in vivo and to have antihypertensive activity. The effect on dopamine-beta-hydroxylase in vitro was shown by inhibition of the conversion of phenylethylamine to phenylethanolamine, using enzyme extracted from rat adrenals. In vivo, BRL 8242 inhibited 3H-noradrenaline but not 3H-dopamine biosynthesis from 3H-L-dopa in rat brain. Furthermore, the compound lowered endogenous noradrenaline levels in both rat brain and heart whilst increasing the concentration of brain dopamine. In both metacorticoid hypertensive and normotensive rats, BRL 8242 lowered blood pressure. This response was dose related and correlated well with the reduction of endogenous noradrenaline in the tissues examined. It is therefore suggested that the inhibition of dopamine-beta-hydroxylase by BRL 8242 may account for its blood pressure lowering activity.

Effects of alpha-methyldopa on blood pressure in the anaesthetized dog.

Intravenous infusion over 1 h of 20 mg kg(-1) of alpha-methyldopa produced hypotension in the anaesthetized dog. The magnitude of this effect was, however, inversely correlated with bodyweight. Either rapid intravenous injection of alpha-methyldopa or slow infusion of alpha-methyldopate, was less effective in lowering blood pressure than infusion of free alpha methyldopa in dogs of equivalent bodyweight. Infusion of alpha-methyldopa into a vertebral or internal carotid artery produced hypotensive responses but these were no greater and generally less than those obtained to intravenous infusion. alpha-Methyldopa was therefore capable of producing hypotension in the dog but no evidence was obtained for this being the result of an action within the brain.