Hypotensive action of captopril in spontaneously hypertensive and normotensive rats. Interference with neurogenic vasoconstriction.

The effects of captopril and angiotensin II on adrenergic neurotransmission have been studied in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). In a pithed rat preparation, vasoconstrictor responses evoked by spinal stimulation were greater in SHR than WKY (p less than 0.01). Captopril reduced responses to electrical stimulation and this reduction was greater in the SHR (p less than 0.001). Bilateral nephrectomy reduced the vasoconstrictor responses to nerve stimulation in both strains of rat and abolished the effects of captopril. In an isolated perfused mesenteric artery WKY (p less than 0.05). Angiotensin II potentiated responses from both strains of rat, however the amplitude of the potentiation was greater in preparations from the SHR than those from WKY (p less than 0.002). Captopril (30 mg/kg by mouth) reduced blood pressure in conscious SHR over a 5-day dosing period. In WKY rats, no hypertensive action of captopril was observed. However, in another normotensive strain, the Alderley Park Wistar rat (APW), captopril lowered blood pressure. Plasma renin activity was not significantly different among these three strains of rat. The APW have previously been shown to be very sensitive to the adrenergic potentiating actions of angiotensin II. Captopril thus lowers blood pressure in SHR and APW, and both these strains are sensitive to the adrenergic potentiating actions of angiotensin II. It does not lower blood pressure in WKY, which is relatively insensitive to these actions of the octapeptide. Therefore, the hypotensive action of captopril in the rat may be due to its interference with the adrenergic potentiating effect of angiotensin II.

The cardiovascular pharmacology of ICI 170777 ((6RS)-6-methyl-5-(pyrid-4-yl)-3H,6H-1,3,4- thiadiazin-2-one) a novel compound with positive inotropic and vasodilator effects.

1. This paper describes the cardiovascular effects of ICI 170777, a novel compound which enhances cardiac contractility and causes arterial and venous dilatation. 2. The positive inotropic effects of ICI 170777 on the heart were demonstrated by an increase in left ventricular dP/dtmax in the anaesthetized and conscious dog, and by an increase in tension development in isolated papillary muscles from the cat. 3. In the anaesthetized dog, the positive inotropic effects of ICI 170777 and of isoprenaline were attenuated by atenolol (5 mg kg-1, i.v.). Atenolol displaced the dose-response curve to ICI 170777 to the right by 4 fold but displaced the isoprenaline dose-response curve to the right by 247 fold. In vitro, however, atenolol (10 microM) had no significant effect on the positive inotropic response to ICI 170777. In the ganglion-blocked anaesthetized dog, infusion of a low dose of ICI 170777 which had no effect on the basal left ventricular dP/dtmax, selectively potentiated the positive inotropic effects of isoprenaline. These results indicate that ICI 170777 has both a non-adrenoceptor-mediated positive inotropic effect on the heart and also facilitates the beta-adrenoceptor-mediated control of contractility. 4. In the denervated and perfused hind-limb of the dog, ICI 170777 reduced arterial perfusion pressure and increased limb circumference at a constant arterial flow and venous pressure. This indicates that ICI 170777 has direct dilator actions on both arterial and venous vessels. In this preparation, diazoxide exerted an arterial selective vasodilator effect and sodium nitroprusside was a relatively selective venous dilator. ICI 170777 exhibited a balanced arterial and venous dilator effect which was intermediate in profile between that of diazoxide and that of sodium nitroprusside. 5. In the conscious dog, low doses (2-5 mgkg -, orally) of ICI 170777 evoked an increase in left ventricular dP/dt,,, with no significant effect on heart rate or blood pressure. At a higher dose (10mg kg 1, orally) it also reduced blood pressure and caused a significant increase in heart rate. The duration of the positive inotropic effect of 5mg kg- (orally) of ICI 170777 was 10-12 hours. This response did not diminish following repeated administration of the compound. 6. The positive inotropic action and balanced arterial and venous dilator effect of ICI 170777 indicate that the compound may be useful in the treatment of congestive heart failure, a disorder that is characterized by decreased cardiac contractility and enhanced arterial and venous constrictor tone.

Evidence that the positive inotropic effects of the alkylxanthines are not due to adenosine receptor blockade.

We investigated the possibility that the positive inotropic effects of the alkylxanthines are due to adenosine receptor blockade. The potency of 8-phenyltheophylline, theophylline and enprofylline as adenosine antagonists was assessed in vitro, using the guinea-pig isolated atrium, and in vivo, using the anaesthetized dog. The order of potency of the alkylxanthines as antagonists of the negative inotropic response to 2-chloroadenosine in vitro, and of the hypotensive response to adenosine in vivo was 8-phenyltheophylline greater than theophylline greater than enprofylline. The order of potency of the alkylxanthines as positive inotropic and chronotropic agents in the anaesthetized dog was enprofylline greater than theophylline greater than 8-phenyltheophylline. The results of this study indicate that the inotropic effects of the alkylxanthines in the anaesthetized dog are not due to adenosine receptor blockade.

The in vitro pharmacology of ZM 241385, a potent, non-xanthine A2a selective adenosine receptor antagonist.

1. This paper describes the in vitro pharmacology of ZM 241385 (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin- 5-yl amino]ethyl) phenol), a novel non-xanthine adenosine receptor antagonist with selectivity for the A2a receptor subtype. 2. ZM 241385 had high affinity for A2a receptors. In rat phaeochromocytoma cell membranes, ZM 241385 displaced binding of tritiated 5'-N-ethylcarboxamidoadenosine (NECA) with a pIC50 of 9.52, (95% confidence limits, c.l., 9.02-10.02). In guinea-pig isolated Langendorff hearts, ZM 241385 antagonized vasodilatation of the coronary bed produced by 2-chloroadenosine (2-CADO) and 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) with pA2 values of 8.57 (c.l., 8.45-8.68) and 9.02 (c.l., 8.79-9.24) respectively. 3. ZM 241385 had low potency at A2b receptors and antagonized the relaxant effects of adenosine in the guinea-pig aorta with a pA2 of 7.06, (c.l., 6.92-7.19). 4. ZM 241385 had a low affinity at A1 receptors. In rat cerebral cortex membranes it displaced tritiated R-phenylisopropyladenosine (R-PIA) with a pIC50 of 5.69 (c.l., 5.57-5.81). ZM 241385 antagonized the bradycardic action of 2-CADO in guinea-pig atria with a pA2 of 5.95 (c.l., 5.72-6.18). 5. ZM 241385 had low affinity for A3 receptors. At cloned rat A3 receptors expressed in chinese hamster ovary cells, it displaced iodinated aminobenzyl-5'-N-methylcarboxamido adenosine (AB-MECA) with a pIC50 of 3.82 (c.l., 3.67-4.06). 6. ZM 241385 had no significant additional pharmacological effects on the isolated tissues used in these studies at concentrations three orders of magnitude greater than those which block A2a receptors. At 10 microM it displayed only minor inhibition of the bradycardic effects in guinea-pig atria to some concentrations of carbachol. At 10 microM, ZM 241385 had a small inhibitory effect on relaxant effects of isoprenaline in guinea-pig aortae but no effect on sodium nitrite-induced relaxation. ZM 241385(100 microM) was without effect on phenylephrine-induced tone in guinea-pig aortae.7. ZM 241385 (10 microM) had no inhibitory effect on rat hepatocyte phosphodiesterase types I, II, III and IV but caused a small inhibition of the calcium calmodulin-activated type I enzyme.8. ZM 241385 is the most selective adenosine A2a receptor antagonist yet described and is therefore a useful tool for characterization of responses mediated by A2 adenosine receptors.

In vivo characterisation of ZM 241385, a selective adenosine A2A receptor antagonist.

The in vivo characterisation of ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-+ ++ylamino] ethyl)phenol), a novel, non-xanthine, selective adenosine A2A antagonist is described. In anaesthetised dogs ZM 241385 (i.v.) was 140-fold more potent in attenuating vasodilator responses to exogenous adenosine in the constant flow perfused hind limb than the bradycardic effects. In pithed rats in which blood pressure was supported by angiotensin II infusion, ZM 241385 (10 mg kg-1, i.v.) did not inhibit the hypotensive or bradycardic effects of the A3/A1 receptor agonist N(6)-2-(4-amino-3-iodophenyl)ethyladenosine (APNEA). In conscious spontaneously hypertensive rats, ZM 241385 (3-10 mg kg-1, p.o.) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine. No inhibition of the bradycardic effects of adenosine was observed following these doses of ZM 241385. The results indicate that ZM 241385 can be used to evaluate the role of adenosine A2A receptors in the action of adenosine in vivo.

Diuretic and saliuretic effects of 1,3-dipropyl-8-cyclopentylxanthine, a selective A1-adenosine receptor antagonist.

We have previously shown that 8-phenyltheophylline (8-PT), a non-selective antagonist at adenosine A1- and A2-receptors, has a diuretic effect. In this study, the diuretic and adenosine antagonist effects of the A1-receptor selective compound 1,3-dipropyl-8-cyclopentylxanthine (CPX) have been examined in the conscious rat. CPX (0.1 and 0.3 mg kg-1 i.v.) significantly attenuated bradycardic but not hypotensive responses evoked by adenosine. In contrast, 8-PT (3 mg kg-1 i.v.) significantly antagonized both adenosine-induced bradycardia and hypotension. CPX (0.1 and 0.3 mg kg-1 i.v.) evoked a dose-related diuretic and saliuretic response in the conscious rat. These results indicate that the diuretic effects of adenosine antagonists are associated with blockade of the A1-receptor sub-type.

The adenosine receptor antagonist, 8-phenyltheophylline, causes diuresis and saliuresis in the rat.

The diuretic and adenosine antagonist actions of two alkylxanthines have been compared in the conscious rat. 8-Phenyltheophylline (10 mg kg-1) antagonized adenosine-induced bradycardia in the rat for at least 3 h whereas enprofylline (10 mg kg-1) had no effect on this response. 8-Phenyltheophylline (10 mg kg-1) evoked diuresis and saliuresis in the rat whereas enprofylline (10 mg kg-1) had no effect on excretory parameters. These results indicate that the diuretic action of some alkylxanthines may be related to adenosine antagonism.

Pharmacodynamics of ZM 241385, a potent A2a adenosine receptor antagonist, after enteric administration in rat, cat and dog.

4-(2-[7-Amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5] triazin-5-ylamino]ethyl)phenol (ZM 241385) is currently the most selective for the A2a adenosine receptor antagonist. This paper describes the in-vivo activity of ZM 241385 after administration by both oral and intraduodenal routes. In conscious spontaneously hypertensive rats, ZM 241385 (1-10 mg kg-1) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine (1 mg kg-1 min-1, i.v.) by up to 45% after oral administration. Activity of ZM 241385 was maintained for at least 6 h after 3 and 10 mg kg-1 (p.o.). In conscious normotensive cats, ZM 241385 attenuated the blood pressure responses to adenosine (0.6-1.0 mg kg-1, i.v.) by 94% after 10 mg kg-1 (p.o.) and by up to 74% after 0.3 mg kg-1 (i.v.). Duration of action of ZM 241385 up to 12 h (36% inhibition) was observed after 3 mg kg-1 (p.o.). In anaesthetized dogs and cats, ZM 241385, after intraduodenal administration (1-10 mg kg-1), produced a rapid (dose ratio 100-fold 15 min after administration of 10 mg kg-1 in the cat) and prolonged (dose ratio of 14 at 6 h after administration of 10 mg kg-1) attenuation of the vasodilatation responses to adenosine receptor stimulation. When administered by this route ZM 241385 was six times more potent than theophylline in the cat and at least twice as potent as theophylline in the dog. In conclusion, ZM 241385 is a potent, selective A2a adenosine receptor antagonist which is orally active, with a good duration of action by the enteric route in cat, rat and dog. It could therefore be used to evaluate the role of adenosine A2a receptors in the action of adenosine in-vivo.

Captopril attenuates adrenergic vasoconstriction in rat mesenteric arteries by angiotensin-dependent and -independent mechanisms.

1. Angiotensin-converting enzyme inhibitors can attenuate reflex sympathetic vasoconstriction in vivo. We have investigated the effects of captopril (SQ 14 225) on adrenergic vasoconstrictor mechanisms in isolated, Krebs-Ringer solution perfused, rat mesenteric arteries. 2. Low concentrations of captopril (2 X 10(-6) mol/l) did not alter the vasoconstrictor response evoked by sympathetic nerve stimulation. 3. Exogenous angiotensin I and II did not have a direct vasoconstrictor effect, but caused dose-related increases in the amplitude of responses induced by nerve stimulation. 4. The potentiating effect of angiotensin I was antagonized by captopril (6.7 X 10(8)-2 X 10(-6) mol/l) and by saralasin (10(-8) mol/l). The potentiating effect of angiotensin II was antagonized by saralasin only. 5. In the absence of exogenous peptides high concentrations of captopril (1 X 10(-4)-3 X 10(-4) mol/l) antagonized vasoconstrictor responses evoked by sympathetic nerve stimulation and exogenous noradrenaline, but not those evoked by potassium chloride. 6. These results indicate that captopril can have two types of inhibitory effect at the adrenergic neuro-effector junction. High concentrations antagonize responses to noradrenaline and nerve stimulation. This effect is independent of peptide hormones and is unlikely to occur in vivo. Lower concentrations block the local vascular conversion of angiotensin I into II. As angiotensin II is an important peripheral amplifier of adrenergic vasoconstriction, this effect will also reduce sympathetic vasoconstrictor tone. This latter interaction could explain the inhibitory effect of converting enzyme inhibitors on sympathetic reflexes.

A method for screening diuretic agents in the rat.

A reproducible screening method for diuretic agents has been developed based upon the selection of a group of homogenous rats with a similar excretory pattern of water and electrolytes. Normal saline (4% body weight) was used as a hydrating fluid to select appropriate rats and to validate the diuretic activity of standard diuretics. The method required only a small number of rats for screening compounds and produced consistent responses in increasing water and electrolyte excretion.

Open fronted safety cabinets in ventilated laboratories.

Open fronted Class I and II microbiological safety cabinets (MSCs) are required by the British Standard 5726 to provide similar levels of operator protection (viz. 10(5). In laboratories that are naturally ventilated large numbers of both types of cabinets have been shown to exceed this requirement consistently over a number of years. The designs of some mechanically ventilated laboratories, however, produce excessive turbulence and draughts that can prejudice containment at the front aperture. On-site commissioning tests to determine operator protection factor are now well established and are recognized as being essential to the setting up of all open fronted cabinets in both ventilated and unventilated laboratories. This paper shows that where environmental conditions induce unsatisfactory cabinet containment, adjustments to air supply and exhaust systems can be made which will enable both Class I and II cabinets to produce operator protection factors in excess of 10(5). When compatibility is achieved between the local environment and the cabinets it is demonstrated that disturbances at the front aperture, caused by operator working procedures or by disturbances due to personnel movement within the room, have similar effects on both Class I and II cabinets. Once performance levels have been satisfactorily achieved, regular containment testing has shown that consistent performance can be maintained. These aspects of open fronted safety cabinet performance are discussed in relation to ventilated laboratories suitable for work with the human immunodeficiency virus (HIV). Of paramount importance in the future is the necessity to design laboratory air systems that will be compatible with satisfactory safety cabinet performance--a relatively new requirement in ventilation system specifications.

ICI 147,798: a novel diuretic agent with beta adrenoceptor blocking activity.

ICI 147,798 is a novel compound which has both diuretic and beta adrenoceptor blocking properties in a single molecule. The natriuretic activity at 30 mg/kg p.o. was about 65% of the hydrochlorothiazide value at 10 mg/kg p.o. in saline-loaded rats; the corresponding kaliuretic activity was 42%. The natriuretic and the kaliuretic activity of ICI 147,798 in dogs were similar to that of hydrochlorothiazide over the doses 1 to 20 mg/kg p.o., although significantly less kaliuresis was obtained with ICI 147,798 at 1 mg/kg p.o. In the toad bladder preparation (analogous to the distal mammalian nephron), ICI 147,798 inhibited Na+ transport with mucosal and serosal IC50 values of 56 and 120 microM, respectively. ICI 147,798 inhibited isoproterenol-induced tachycardia in rats, cats, guinea pigs and dogs; these effects were associated with antagonism of isoproterenol vasodepressor responses. The pKB values of ICI 147,798 were 9.1 and 8.8 in isolated right atria and trachea of guinea pigs, respectively. ICI 147,798 did not exhibit local anesthetic activity in rabbit cornea and intrinsic sympathomimetic activity in catecholamine-depleted dogs and rats. Duration of beta blockade after a p.o. dose of 1 mg/kg in dogs followed the sequence: nadolol greater than ICI 147,798 greater than atenolol greater than timolol greater than propranolol. It is concluded that ICI 147,798 is a novel diuretic agent with nonselective beta blockade, and it appears to have the potential of a direct tubular action.