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Biomedical subjects

M G Collis

Publications and source records attributed to M G Collis.

At least 37 records · Page 2Linked to original sources
PubMed1987

Effect of 8-phenyltheophylline, enprofylline and hydrochlorothiazide on glycerol-induced acute renal failure in the rat.

The adenosine antagonist 8-phenyltheophylline (8-PT) is a diuretic in normal rats and can ameliorate glycerol-induced acute renal failure (ARF) in this species. To define which action of 8-PT is important in its salutary effect in ARF, we have compared its effects with those of enprofylline (a xanthine with little affinity for adenosine receptors) and with those of the tubular diuretic hydrochlorothiazide. In one series of experiments, groups of rats with ARF of 24 h duration were given a single dose of drug or vehicle. Only 8-PT enhanced urine volume when compared with the vehicle-treated group. In a second set of experiments, groups of glycerol-injected rats received drug or vehicle treatment (i.p.) twice daily for 2 days. Rats which received a course of 8-PT treatment had significantly lower plasma urea and creatinine concentrations, a higher glomerular filtration rate, a lower kidney weight and improved kidney morphology when compared with vehicle-treated rats. The only beneficial effect noted after enprofylline treatment was an improved kidney morphology. Hydrochlorothiazide treatment compared with vehicle treatment did not ameliorate any index of renal function but resulted in significant elevations in plasma urea and creatinine levels. The inability of enprofylline or hydrochlorothiazide to mimic the effects of 8-PT in ARF indicate that the effects of 8-PT are probably associated with adenosine receptor blockade and not with a tubular diuretic action.

Acute Kidney Injury↗
PubMed1986

Evidence that the intracellular effects of adenosine in the guinea-pig aorta are mediated by inosine.

Previous studies have demonstrated that high concentrations of adenosine interact with both a cell surface receptor and with an intracellular site to evoke relaxation of the guinea-pig aorta. The intracellular action of adenosine was investigated in the present study. The purine sensitive 'P-site' did not appear to be involved since other P-site agonists did not consistently evoke relaxation. A major interaction with intracellular S-adenosylhomocysteine hydrolase also appeared unlikely since 1-homocysteine had only minor effects on adenosine-evoked responses. Inhibition of adenosine deaminase attenuated responses evoked by high concentrations of adenosine. The deaminated metabolite of adenosine, inosine, also evoked aortic relaxation. These responses were mediated solely via an intracellular site since they were blocked by an inhibitor of nucleoside-facilitated diffusion but were unaffected by an adenosine receptor antagonist. These results indicate that a major part of the intracellular effect of adenosine is mediated by its deaminated metabolite inosine.

Adenosine↗
PubMed1986

Effect of the adenosine antagonist 8-phenyltheophylline on glycerol-induced acute renal failure in the rat.

8-Phenyltheophylline (8-PT)(10 mg kg-1) or its vehicle(1 ml kg-1) were administered intravenously or intraperitoneally twice daily over 48 h to rats with acute renal failure (ARF) induced by intramuscular (i.m.) injection of glycerol. Rats treated with 8-PT i.v. had significantly lower plasma urea and creatinine levels at 24 and 48 h compared to untreated animals. The vehicle also reduced plasma urea and creatinine when compared to untreated controls. However, plasma urea levels in 8-PT-treated rats were significantly lower than in vehicle-treated animals at 24 and 48 h after both i.v. and i.p. administration. Plasma creatinine concentrations also tended to be lower in the 8-PT-treated group. [3H]-inulin clearance at 48 h after i.m. glycerol was significantly greater in rats dosed i.p. with 8-PT compared to either untreated or vehicle treated rats. Examination of kidneys taken from rats 48 h after i.m. glycerol showed that 8-PT treatment significantly reduced renal damage and kidney weight compared to the untreated or vehicle-treated groups. In a 7 day study all the rats which received 8-PT i.p. survived whilst in the vehicle and untreated groups the mortality rates were 12 and 21% respectively. In a separate series of experiments 8-PT (10 mg kg-1, i.v. or i.p.) was found to antagonize adenosine-induced bradycardia in conscious rats for up to 5 h. There is no clear explanation for the partial protection afforded by the vehicle but it may be related to either its alkalinity or an osmotic effect produced by the polyethylene glycol component. 9 The protective effect of 8-PT in rats with ARF was probably the result of adenosine antagonism.

Acute Kidney Injury↗
PubMed1986

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.

Adenosine↗
PubMed1985

Comparison of the potency of 8-phenyltheophylline as an antagonist at A1 and A2 adenosine receptors in atria and aorta from the guinea-pig.

The potency of 8-phenyltheophylline as an antagonist at A1 adenosine receptors in guinea-pig atria and at A2 adenosine receptors in the guinea-pig aorta has been investigated. 8-Phenyltheophylline was an apparently competitive antagonist of the negative chronotropic effect of adenosine, 2-chloroadenosine, L-N6-phenyl-isopropyl adenosine (L-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) on atria and of the relaxant effect of adenosine, 2-chloroadenosine and NECA on the aorta. The pA2 values for 8-phenyltheophylline ranged from 6.4 to 6.6 and were not significantly different, irrespective of the agonist or tissue used. These results indicate that 8-phenyltheophylline is a relatively potent antagonist at adenosine receptors but does not exhibit selectivity for either of the putative sub-types in isolated tissues.

Animals↗
PubMed1984

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.

Animals↗
PubMed1983

Adenosine relaxes the aorta by interacting with an A2 receptor and an intracellular site.

The purpose of this study was to determine whether the adenosine receptor that mediates relaxation of the noradrenaline-contracted guinea-pig aorta is of the A1 or A2 subtype. 5'-N-ethylcarboxamide adenosine (NECA) and 5'-N-cyclopropylcarboxamide adenosine (NCPCA) were about 100 times more potent as relaxants of the aorta than L-N6-phenylisopropyladenosine (L-PIA) and N6-cyclohexyladenosine. L-PIA was 3 times more potent than D-PIA. These relaxations were not altered by the purine transport inhibitor dipyridamole, but were attenuated by the cell surface adenosine receptor antagonist 8-phenyltheophylline. Adenosine and 2-chloroadenosine differed from NECA and NCPCA since they evoked greater maximal relaxations and their submaximal responses were less sensitive to blockade by 8-phenyltheophylline. These differences were abolished by dipyridamole which indicates that they were due to an intracellular action of adenosine and 2-chloroadenosine. The intracellular 'P-site' agonist, 9-beta-D-xylofuranosyladenine evoked small relaxations that were attenuated by dipyridamole but were unaffected by 8-phenyltheophylline. These results indicate that adenosine can relax the aorta via interactions with a cell surface A2 receptor and with an intracellular site.

Adenosine↗
PubMed1983

Adenosine A1 receptor mediated inhibition of nerve stimulation-induced contractions of the rabbit portal vein.

The aim of this study was to determine whether the adenosine receptor that inhibits adrenergic neurotransmission in the rabbit portal vein is of the A1 or the A2 subtype. Isometric contractions of the isolated vein were evoked by electrical field stimulation and by exogenous noradrenaline. Low concentrations of adenosine, and a number of analogues inhibited the response evoked by field stimulation but had no effect on those evoked by noradrenaline. The order of inhibitory potency was: L-N6-phenylisopropyladenosine (L-PIA) = N6-cyclohexyladenosine (CHA) = 5'-N-cyclopropylcarboxamide adenosine (NCPCA) greater than or equal to 5'-N-ethylcarboxamide adenosine (NECA) = 2-chloroadenosine greater than adenosine greater than D-PIA. The difference in potency between the stereoisomers L- and D-PIA was about 60 fold. The purine transport inhibitor dipyridamole potentiated the inhibitory effect of adenosine but not that of its analogues. The inhibitory responses evoked by adenosine and its' analogues were attenuated by the adenosine antagonist theophylline. These results indicate that adenosine selectively inhibits contractions of the rabbit portal vein evoked by adrenergic nerve stimulation via activation of an adenosine A1 receptor.

Adenosine↗
PubMed1983

Evidence for an A1-adenosine receptor in the guinea-pig atrium.

1 The purpose of this study was to determine whether the adenosine receptor that mediates a decrease in the force of contraction of the guinea-pig atrium is of the A1- or A2-sub-type. 2 Concentration-response curves to adenosine and a number of 5'- and N6-substituted analogues were constructed and the order of potency of the purines was: 5'-N-cyclopropylcarboxamide adenosine (NCPCA) = 5'-N-ethylcarboxamide adenosine (NECA) greater than N6cyclohexyladenosine (CHA) greater than L-N6-phenylisopropyl adenosine (L-PIA) = 2-chloroadenosine- greater than adenosine greater than D-N6-phenylisopropyl adenosine (D-PIA). 3 The difference in potency between the stereoisomers D- and L-PIA was over 100 fold. 4 The adenosine transport inhibitor, dipyridamole, potentiated submaximal responses to adenosine but had no significant effect on those evoked by the other purines. 5 Theophylline antagonized responses evoked by all purines, and with D-PIA revealed a positive inotropic effect that was abolished by atenolol. 6 The results indicate the existence of an adenosine A1-receptor in the guinea-pig atrium.

Acetylcholine↗
PubMed1982

Can ATP stimulate P1-receptors in guinea-pig atrium without conversion to adenosine?

The aim of this study was to determine whether ATP must be hydrolysed to adenosine in order to activate the P1-purinoceptor. Isometric contractions of electrically paced guinea-pig isolated left atria were recorded. Purines evoked negative inotropic responses that were competitively antagonised by theophylline. The order of agonist potency was 2-chloroadenosine greater than adenosine greater than beta, gamma-methylene ATP greater than ATP. Adenosine deaminase alone, or combined with 5'-nucleotidase, attenuated responses to adenosine and 5' AMP, respectively, but did not decrease those to ATP or beta, gamma-methylene ATP. Inhibition of 5'-nucleotidase did not alter responses to ATP. Dipyridamole potentiated responses to ATP both in the absence and in the presence of adenosine deaminase. Alpha, beta-methylene ATP had little agonist activity, however this was not due to its resistance to hydrolysis as the stable beta, gamma-methylene isostere of ATP was a potent agonist. These results indicate that hydrolysis of ATP to adenosine or 5' AMP is not a pre-requisite for activation of the P1-receptor in the guinea-pig atrium.

Adenosine↗
PubMed1982

Evidence for an A2/Ra adenosine receptor in the guinea-pig trachea.

1 An attempt was made to determine whether the extracellular adenosine receptor that mediates relaxation in the guinea-pig trachea is of the A(1)/R(i) or A(2)/R(a) subtype.2 Dose-response curves to adenosine and a number of 5'- and N(6)-substituted analogues were constructed for the isolated guinea-pig trachea, contracted with carbachol.3 The 5'-substituted analogues of adenosine were the most potent compounds tested, the order of potency being 5'-N-cyclopropylcarboxamide adenosine (NCPCA) > 5'-N-ethylcarboxamide adenosine (NECA) > 2-chloroadenosine > L-N(6)-phenylisopropyladenosine (L-PIA) > adenosine > D-N(6)-phenylisopropyladenosine (D-PIA).4 The difference in potency between the stereoisomers D- and L-PIA on the isolated trachea was at the most five fold.5 Responses to low doses of adenosine and its analogues were attenuated after treatment with either theophylline or 8-phenyltheophylline. The responses to 2-chloroadenosine were affected to a lesser extent than were those to the other purines.6 Adenosine transport inhibitors, dipyridamole and dilazep, potentiated responses to adenosine, did not affect those to NCPCA, NECA, L-PIA and D-PIA but significantly reduced the responses to high doses of 2-chloroadenosine.7 Relaxations evoked by 9-beta-D-xylofuranosyladenosine which can activate intracellular but not extracellular adenosine receptors, were attenuated by dipyridamole but unaffected by 8-phenyltheophylline.8 The results support the existence of an extracellular A(2)/R(a) subtype of adenosine receptor and an intracellular purine-sensitive site, both of which mediate relaxation.

2-Chloroadenosine↗
PubMed1981

Adenosine contracts the isolated rat tail artery by releasing endogenous 5-hydroxytryptamine.

Adenosine (10(-4)-10(-3) M) contracted the isolated rat tail artery. This effect exhibited rapidly developing tachyphylaxis. Methysergide (10(-7) M) prevented the contractile response to adenosine and 5-hydroxytryptamine (5-HT) without affecting that to noradrenaline. Pretreatment of rats with parachlorophenylalanine (PCPA) abolished the contraction to adenosine (5 X 10(-4) M). Responses to 5-HT and noradrenaline were not significantly affected. These results indicate that the contractile response of the rat tail artery induced by adenosine is probably mediated by endogenous 5-HT.

Adenosine↗
PubMed1981

Calcium dependence of prejunctional inhibitory effects of adenosine and acetylcholine on adrenergic neurotransmission in canine saphenous veins.

In canine blood vessels acetylcholine and adenosine inhibit the exocytotic release of norepinephrine during nerve stimulation. The present experiments were designed to determine the Ca2+ dependence of these prejunctional effects. Segments of canine saphenous veins were mounted for isometric tension recording in organ chambers filled with Krebs-Ringer or Tyrode solution. Altering the Ca2+ concentration of the solution did not affect the inhibitory response to acetylcholine during nerve stimulation; the prejunctional potency of adenosine was inversely related to the Ca2+ concentration of the bath content. the ionophore A23187 caused contractions which were inhibited by phentolamine, verapamil, and adenosine but were augmented by acetylcholine. Helical strips of dog saphenous veins were incubated in [3H]norepinephrine and mounted for superfusion and determination of [3H]norepinephrine in the superfusate. A23187 increased the overflow of [3H]norepinephrine. Acetylcholine augmented this efflux; by contrast adenosine decreased the release induced by the ionophore. The results demonstrate that the prejunctional effect of acetylcholine was not due to direct interference with the availability of Ca2+ for the electro-secretory process in adrenergic nerve terminals and suggest that adenosine interferes either with the coupling role of the activator ion or its extrusion from the neuroplasm.

Acetylcholine↗
PubMed1981

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.

Adrenergic Fibers↗
PubMed1981

Action of lithium on the adrenergic nerve ending.

The effect of lithium on adrenergic neurotransmission was investigated using the lateral saphenous vein of the dog as a model of the neuroeffector process. Helically cut vein strips were mounted in either an organ bath where tension was recorded or in a superfusion system where both tension and the overflow of [3H]norepinephrine and its metabolites were determined. Lithium was used at both therapeutic (0.5-1.5 mEq/l) and toxic (2.5-14.4 mEq/l) concentrations. Lithium had no effect on basal tension or overflow [3H]norepinephrine. At therapeutic concentrations, the neuronal amine uptake mechanism was augmented, whereas at toxic concentrations monoamine oxidase was inhibited. In therapeutic concentrations, lithium attenuated the response of the adrenergic nerve ending to electrical stimulation (0.5-10 Hz). This response was due in part to the augmentation of the neuronal amine uptake mechanism. At toxic concentrations, responses to low frequency (0.2-1.0 Hz) electrical stimulation were augmented, whereas those at higher frequencies (5-10 Hz) were attenuated. Inhibition of monoamine oxidase by lithium accounted for only a part of the augmented overflow of transmitter and subsequent contractile response since inhibition of monoamine oxidase did not prevent the augmentation by lithium of [3H]norepinephrine overflow. The mechanism by which lithium in both therapeutic and toxic concentrations reduces transmitter overflow and subsequently contractile responses remains to be elucidated. Thus, lithium alters the disposition and release of norepinephrine and, as a result, affects adrenergic neurotransmission.

Animals↗
PubMed1980

Pre- and post-junctional adrenergic mechanisms and hypertension.

1. The ability of the blood vessel wall to synthesize noradrenaline is augmented at the early stages of genetic hypertension in animals. It usually is normal, or reduced in chronic hypertension. 2. The exocytotic release of noradrenaline is greater than normal, in the early stages of a number of experimental models of hypertension. 3. Postjunctional receptors for noradrenaline are not uniform throughout the vascular tree. Their sensitivity changes during the development of high blood pressure. 4. In chronic hypertension neuronal uptake and the extraneuronal disposition of released noradrenaline is depressed in the heart, but accelerated in the blood vessel wall. 5. The adrenergic neuro-effector interaction undergoes a diverging long-term adaptation in the heart and the blood vessels of hypertensive animals. In the former this tends to increase, but in the latter to reduce, the efficiency of sympathetic nervous control.

Angiotensin II↗