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Biomedical subjects

M J Lew

Publications and source records attributed to M J Lew.

32 records · Page 2Linked to original sources
PubMed1987

Selective manipulation of neurohumoral control of the cardiac pacemaker by drugs given intrapericardially.

A technique of intrapericardial administration of beta-adrenoceptor and muscarinic cholinergic receptor antagonist drugs has been tested in conscious rabbits. Intrapericardial propranolol or atenolol (50 micrograms/kg) had the same effect on isoprenaline heart rate dose-response curves and on the sympathetic component of the arterial baroreceptor-heart rate reflex as did conventional, 5-fold greater, intravenous doses of the drugs. The action of intrapericardial propranolol was attributable to its (-)isomer. Intrapericardial propranolol (50 micrograms/kg) had little effect on ventricular contractility. Plasma levels of propranolol and atenolol after intrapericardial administration were, respectively, 7- and 40-fold less than after the usual intravenous doses. Intrapericardial hyoscine methyl bromide (10 micrograms/kg) abolished baroreflex vagal effects on heart rate as effectively as did the conventional, 5-fold greater, intravenous dose. The duration of receptor blockade by both classes of drugs when given intrapericardially was at least 2 hr. We conclude that the rapid diffusion of beta-adrenoceptor and muscarinic cholinergic receptor blocking drugs from the pericardial sac to receptors on the sinoatrial cardiac pacemaker, and their prolonged actions, provides a useful technique for preventing the actions of the sympathetic and vagus nerves, and of circulating catecholamines, on the chronotropic functions of the heart.

Adrenergic beta-Antagonists↗
PubMed1987

Release of endogenous ATP during sympathetic nerve stimulation.

1 Vas deferens from guinea-pig was stimulated with a suction electrode and both contractions and release of endogenous ATP monitored 2 Release of ATP was tetrodotoxin-sensitive and increased when the number of stimuli was increased. 3 Release of ATP was not due to contraction of the muscle and persisted following block of contractions with prazosin and alpha, beta-methylene ATP. 4 These results indicate that stimulation of the sympathetic nerves in the vas deferens releases endogenous ATP presynaptically, supporting a cotransmitter function for ATP with noradrenaline.

4-Aminopyridine↗
PubMed1987

Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit.

The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.

Adrenergic alpha-Antagonists↗
PubMed1984

Reversible inhibition of neuronal uptake by benextramine, an irreversible presynaptic alpha-adrenoceptor antagonist.

Benextramine, a covalently binding alpha-adrenoceptor blocking agent, potentiated the action of noradrenaline but not isoprenaline in guinea pig isolated right atria. This potentiation was probably caused by inhibition of neuronal uptake. When the benextramine was washed from the tissues for 60 min, no potentiation of the action of noradrenaline was observed. This easily reversed inhibition of neuronal uptake by benextramine contrasts with the effects of desipramine and phenoxybenzamine because the potentiating effect of these drugs was unaffected by 60 min of washing. The presence of benextramine also caused a small tachycardia in both rabbit and guinea pig right atria which was probably due to the release of endogenous noradrenaline. Clonidine a presynaptic alpha 2-adrenoceptor agonist, inhibited the responses to electrical field stimulation. Pretreatment with benextramine greatly diminished the effect of clonidine. This alpha 2-adrenoceptor antagonism was not reversed by washing the benextramine from the tissue for 240 min. We conclude that benextramine is a readily reversible inhibitor of neuronal uptake and an irreversible antagonist of presynaptic alpha 2-adrenoceptors.

Adrenergic alpha-Antagonists↗
PubMed1984

Phentolamine--an unexpected agonist in the rabbit.

Phentolamine (0.1-10 microM) caused an anomalous rightward shift of the relationship between the number of electrical field pulses and tachycardia in the rabbit isolated right atrium. Phentolamine was apparently acting as a presynaptic agonist on sympathetic nerve endings to inhibit transmitter release. The effect was prevented by benextramine treatment and antagonized 10 fold by yohimbine (1 microM) but not by prazosin (0.1 microM). In ganglion-blocked (mecamylamine) conscious or anaesthetized rabbits, phentolamine (3-1000 micrograms kg-1) caused a dose-related rise in blood pressure that was antagonized by yohimbine (1 mg kg-1). These pressor and inhibitory cardiac sympathetic nerve effects of phentolamine are not found in similar preparations from the guinea-pig or rat. Therefore, these rabbit-specific agonist effects of phentolamine at sites similar to alpha 2-adrenoceptors make this drug unsuitable as an alpha-adrenoceptor antagonist in rabbits.

Adrenergic alpha-Agonists↗
PubMed1983

Disadvantages of cocaine as a neuronal uptake blocking agent: comparison with desipramine in guinea-pig right atrium.

Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria. Right atria were equilibrated with the irreversible alpha-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic alpha-adrenoreceptors. Cumulative (-) noradrenaline concentration-response curves were shifted to the left by DMI (0.01-1 microM) without significant change in the resting or maximum rates. Cocaine (1-100 microM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 microM. Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 microM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 microM respectively. The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discourage its use in experiments on sympathetic transmission.

Animals↗
PubMed1983

Characterization of the clonidine receptor site.

At the cardiac sympathetic nerve terminal the alpha 2-adrenoceptor is presynaptic and appears to be located at an extrasynaptic site. This is suggested by (1) absence of evidence of autoinhibitory feedback at physiologic stimulus levels up to about 50 percent of the maximum chronotropic response in the isolated guinea pig right atrium, and (2) absence of significant competition between clonidine and synaptically released noradrenaline (NA) for the presynaptic site. In the central nervous system (CNS) cardiovascular alpha 2-receptors are probably located at a postsynaptic site in bulbospinal regions of the brain, since they produce effects identical to those of synaptic release of NA. Experiments with the clonidine analog alinidine (ST 567) suggest that there are differences in central receptor type subserving clonidine-mediated baroreflex heart rate and blood pressure changes.

Animals↗
PubMed1982

Apparent frequency-dependent effect of clonidine on cardiac sympathetic transmission: the role of neuronal uptake.

Decreases in spontaneous atrial period (tachycardia) were measured in atropinized guinea-pig isolated right atria in response to intramural sympathetic nerve stimulation. Electrical field pulses were applied only during the atrial refractory period to avoid arrhythmias. Responses to one to six field pulses delivered in a single refractory period were substantially reduced by clonidine (1-1000 nM), providing no evidence that the blocking action of this drug is frequency-dependent. Stimulus-response lines to one, two or four field pulses delivered as one field pulse per refractory period (1/1) were displaced to the right by clonidine (0.1 microM) such that 8, 16 and 32 field pulses were required to obtain responses equivalent to control. Stimulus-response lines to lower frequencies of field stimulation (one field pulse every 4 or 8 refractory periods) were lower in slope and displaced further by clonidine, whereas lines from stimuli at higher frequencies (2/1 and 4/1) were steeper and less displaced by clonidine. These findings confirmed an "apparent" frequency-dependent blocking action of clonidine. However, substantial blockade of neuronal uptake by desipramine (0.1 microM) raised the slope and decreased the shift of the stimulus-response lines to the lower frequencies of field stimulation compared with 2/1 or 4/1. Clonidine (0.1 microM) had no effect on the responses to exogenous norepinephrine. We suggest that clonidine reduces the amount of norepinephrine released per field pulse independently of the frequency of stimulation. The decrease in tissue response may appear to be frequency-dependent under some circumstances due to the effects of uptake and removal processes.

Animals↗
PubMedDate not supplied

Synthesis and biological characterization of a series of analogues of omega-conotoxin GVIA.

The 27-residue polypeptide omega-conotoxin GVIA (omega-CgTx), from the venom of the cone shell Conus geographus, blocks N-type neuronal calcium channels. It contains three disulphide bridges. We report here the synthesis and biological characterization of a series of analogues in which one disulphide has been replaced by substitution of appropriate Cys residues with Ser, viz. [Ser1,16]-omega -CgTx, [Ser8,19]-omega-CgTx, [Ser15,26)-omega-CgTx, [Ser16]-omega-CgTx8-27 and [Ser15]-omega-CgTx1-19. All syntheses were conducted manually using either Boc or Fmoc methodology. Deprotected peptides were oxidized to their bridged forms using either aerial oxidation or aqueous dimethyl sulphoxide. Peptides were purified using RP-HPLC, and their purity and identity were checked by RP-HPLC, capillary electrophoresis and mass spectrometry. Inhibition of neuronal N-type calcium channels was assessed as the inhibition of the twitch responses of rat vas deferens stimulated with single electrical pulses at 20 second intervals. None of these analogues was biologically active, suggesting that the disulphides play an important role in maintaining biological activity.

Amino Acid Sequence↗
PubMedDate not supplied

Examination of adenosine receptor-mediated relaxation of the pig coronary artery.

1. The adenosine receptors mediating relaxation of porcine isolated left anterior descending coronary arteries (LAD) and the effects of the level and type of preconstriction on the responses to adenosine analogues were examined in the present study. 2. Relaxation responses to the non-selective adenosine receptor agonist N-ethylcarboxamidoadenosine (NECA) were endothelium independent. N-Ethylcarboxamidoadenosine, GR 79236 (A1 receptor selective) and 8-cyclopentyl-1,3-dipropylxanthine (CGS 21680) (A2A receptor selective) produced full relaxation in LAD precontracted to 50% of the response to potassium depolarization with the thromboxane receptor agonist U46619. The order of potency was CGS 21680 = NECA > GR 79236, consistent with that defining the A2A receptor subtype. 3. 3,7-Dimethyl-1-propargylxanthine (DMPX; A2 receptor selective) competitively antagonized NECA and CGS 21680 with pKB values of 4.95 +/- 0.09 and 5.06 +/- 0.22, respectively. The A1 receptor selective antagonist 1,3-[3H]-dipropyl-8-cyclopentylxanthine (DPCPX) had no effect on NECA relaxation, even in the presence of DMPX. 4. The sensitivity to relaxation by NECA was dependent on the precontracting agent. Arteries precontracted with endothelin (ET)-1 were most sensitive to NECA, U46619-precontracted arteries were intermediate and KCl-precontracted arteries were least sensitive. 5. The potency of NECA was reduced when the preconstriction level was increased from 50 to 90% of maximum in U46619-precontracted arteries (pEC50 7.94 +/- 0.12 and 7.35 +/- 0.04, respectively) and, in KCl-precontracted arteries, both the potency and maximum effect of NECA were reduced when the preconstriction level increased from 50 to 80% of maximum (pEC50 7.52 +/- 0.13 and 6.91 +/- 0.26, respectively; maximum responses 82.5 +/- 10.2 and 23.9 +/- 3.6%, respectively, of the preconstricted tone). Relaxation responses to NECA were independent of the level of precontraction in ET-1-precontracted arteries. 6. In porcine LAD, relaxation responses to adenosine analogues were endothelium independent and were mediated via A2A adenosine receptors. Responses to NECA were dependent on both the level and type of preconstriction.

Adenosine↗
PubMedDate not supplied

Access of blood-borne vasoconstrictors to the arteriolar smooth muscle.

In vitro experiments have shown that luminally applied water-soluble vasoactive materials have limited access to arteriolar smooth muscle cells, and as a result, the responses to such agents applied luminally are less than the responses to those applied adventitially. To determine the extent to which this 'compartmentation' influences arteriolar responsiveness to blood-borne water-soluble vasoconstrictors in vivo, we applied phenylephrine, vasopressin and angiotension II to arterioles in the hamster cheek pouch both by luminal perfusion, and by topical application to the arteriolar smooth muscle via micropipettes. The arterioles were about 2 orders of magnitude more sensitive to these water-soluble vasoconstrictors when they were applied topically than when they were applied luminally. In contrast, the arterioles were almost equally sensitive to the lipid-soluble alpha 1-adrenoceptor agonist SKF 89748-A applied by either route. The venular wall appears to be much less effective as a barrier than the arteriolar endothelium. Phenylephrine and vasopressin both elicited large arteriolar constrictions when perfused through venules in close proximity to the arteriole, and these constrictions were larger than those observed when the drug was applied to the arteriole's own lumen. Our observations confirm that the arteriolar endothelium can inhibit the direct access of water-soluble blood-borne agents to the arteriolar smooth muscle in vivo, and they suggest that the capillaries and venules could be the primary routes of access for water-soluble agents from the blood to the arteriolar smooth muscle.

Angiotensin II↗
PubMedDate not supplied

Wall thickness to lumen diameter ratios of arteries from SHR and WKY: comparison of pressurised and wire-mounted preparations.

Passive properties (diameter, wall-to-lumen ratio and axial length) of small mesenteric arteries from SHR and WKY rats were measured with the artery segments cannulated and pressurised, or mounted on wires in a myograph. The measurements were made with a range of distending pressures (or calculated equivalent distending pressures when wire-mounted) from 0 to 180 mm Hg. The axial length of artery segments increased with increasing distending pressure when cannulated, but not when wire-mounted. The axial extension was greater for arteries from WKY (up to 105%) than for arteries from SHR (up to 65%). The arteries from SHR had significantly smaller diameters and greater wall-to-lumen ratios than the arteries from WKY. However, the diameters calculated for the arteries when wire-mounted were less than the measured diameters, and the wall-to-lumen ratio was always greater when wire-mounted than when cannulated because of the underestimated diameter and the absence of axial extension. Wall-to-lumen ratios decreased with increased distending pressure; values at 180 mm Hg were only 18 and 25% of those at 0 mm Hg for WKY and SHR arteries, respectively. The large degree of variability of wall-to-lumen ratios obtained from the two different preparations and the large range of values that are obtained from a single artery at different distending pressures must call into question the validity of characterising vascular hypertrophy by any single estimation of this parameter.

Animals↗
PubMedDate not supplied

Alpha 1- and alpha 2-adrenoceptor-mediated pressor responses: are they differentiated by calcium antagonists or by functional antagonism?

We investigated the effect of cardiovascular depression on the pressor responses to the alpha 1-adrenoceptor selective agonist methoxamine, and the alpha 2-adrenoceptor selective agonist B-HT 920 in anesthetized ganglion-blocked rats. The calcium channel blocking drug nifedipine preferentially inhibited the effect of B-HT 920, as has been reported by other authors. Lowering the starting blood pressure by hemorrhage, by nitroprusside infusion, or by additional pentobarbitone also preferentially inhibited the pressor effect of B-HT 920. These selective effects of vascular depression on B-HT 920 are consistent with predicted interactions between functional antagonists and a partial (low-efficacy) agonist. This was tested in part by reducing the maximum effect of methoxamine by phenoxybenzamine treatment. Under these conditions, methoxamine behaved like B-HT 920 in that it was sensitive to inhibition by nitroprusside infusion. By analogy, the vasodepressive effect of calcium channel blocking drugs could be responsible for the preferential inhibition of the vasoconstrictor responses to alpha 2-adrenoceptor agonists. It is concluded that a differential reliance on influx of extracellular Ca2+ by alpha 1- and alpha 2-adrenoceptors may not be the only explanation of the selective effect of calcium channel blocking drugs.

Animals↗