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Biomedical subjects

J A Angus

Publications and source records attributed to J A Angus.

At least 37 records · Page 2Linked to original sources
PubMed1998

Human vascular to cardiac tissue selectivity of L- and T-type calcium channel antagonists.

1. Voltage-operated calcium channel (VOCC) antagonists are effective antihypertensive and antianginal agents but they also depress myocardial contractility. 2. We compared four L-type calcium channel antagonists, felodipine, nifedipine, amlodipine and verapamil and a relatively T-type selective calcium channel antagonist, mibefradil, on human and rat isolated tissue assays to determine their functional vascular to cardiac tissue selectivity (V/C) ratio. 3. The V/C ratio was calculated as the ratio of the IC50 value of the antagonist that reduced (by 50%) submaximally contracted (K+ 62 mM) human small arteries from the aortic vasa vasorum (vascular, V) mounted in a myograph and the IC50 value of the antagonist that reduced (-)-isoprenaline (6 nM) submaximally stimulated human right atrial trabeculae muscle (cardiac, C) mounted in organ chambers. 4. The average pIC50 values (-log IC50 M) for the human vascular preparations were felodipine 8.30, nifedipine 7.78, amlodipine 6.64, verapamil 6.26 and mibefradil 6.22. The average pIC50 values for the cardiac muscle were felodipine 7.21, nifedipine 6.95, verapamil 6.91, amlodipine 5.94, and mibefradil 4.61. 5. The V/C ratio calculated as antilog [pIC50V-pIC50C] is thus mibefradil 41, felodipine 12, nifedipine 7, amlodipine 5 and verapamil 0.2. 6. In rat small mesenteric arteries the pIC50 values for the five drugs were similar to the values for human vasa vasorum arteries contracted by K+ 62 mM. However for methoxamine (10 microM) contraction in the rat arteries the pIC50 values were lower for felodipine 7.24 and nifedipine 6.23, but similar for verapamil 6.13, amlodipine 6.28 and mibefradil 5.91. 7. In conclusion. in the human tissue assays, the putative T-channel antagonist mibefradil shows the highest vascular to cardiac selectivity ratio; some 3 fold higher than the dihydropyridine, felodipine, and some 200 fold more vascular selective than the phenylalkylamine, verapamil. This favourable vascular to cardiac selectivity for mibefradil, from a new chemical class of VOCC antagonist, may be explained by its putative T-channel selectivity.

Animals↗
PubMed1998

Endogenous angiotensin II and bradykinin delay and attenuate the hypotension after N-type calcium channel blockade in conscious rabbits.

The effects of N-type calcium channel inhibition with omega-conotoxin GVIA (omega-CTX) on cardiovascular parameters and vagally mediated autonomic reflexes and the role of the renin-angiotensin system were assessed in conscious rabbits. Omega-CTX (10 microg/kg, i.v.) resulted in hypotension, tachycardia, and attenuation of the sympathetic and vagal components of the baroreceptor-heart rate reflex (baroreflex). In the control group (no pretreatment), the peak decrease in mean arterial pressure (MAP) of 13 +/- 3 mm Hg from 72 +/- 2 mm Hg occurred after 33 +/- 3 min, with a corresponding tachycardia of 80 +/- 20 beats/min (n = 6). The tachycardia was due to vagal withdrawal, as a similar increase in heart rate (84 +/- 8 beats/min) after omega-CTX was observed after pretreatment with the beta-adrenoceptor antagonist, propranolol (n = 6). Angiotensin-converting enzyme (ACE) inhibition with enalaprilat revealed a larger, more rapid decrease in MAP in response to omega-CTX (-19 +/- 4 mm Hg from 65 +/- 1 mm Hg after 18 +/- 2 min; n = 6) compared with the control group. Similar larger decreases in MAP were also observed in the presence of the AT1-receptor antagonist, losartan, or the bradykinin B2 receptor antagonist, HOE-140 (n = 5-6). Pretreatment with enalaprilat, losartan, or HOE-140 caused a 50% decrease in the reflex tachycardia after omega-CTX compared with that observed in the control group, and omega-CTX caused a greater attenuation of the vagal component of the baroreflex and a decrease in the bradycardia evoked by the Bezold-Jarisch-like reflex. Also, there was a significant decrease in the bradycardia induced by the nasopharyngeal reflex after omega-CTX in the presence of ACE inhibition and HOE-140. Thus in the conscious rabbit, angiotensin II and bradykinin have a role in attenuating and slowing the hypotensive effect of N-type calcium channel inhibition. Vagolytic effects of omega-CTX on the baroreflex are augmented, and on other vagal reflexes are unmasked, via inhibition of the renin-angiotensin system. The complexity and mechanism of the interaction between N-type calcium channels and the renin-angiotensin system remain to be elucidated.

Adrenergic beta-Antagonists↗
PubMed1997

Structure-function relationships of omega-conotoxin GVIA. Synthesis, structure, calcium channel binding, and functional assay of alanine-substituted analogues.

The structure-function relationships of the N-type calcium channel blocker, omega-conotoxin GVIA (GVIA), have been elucidated by structural, binding and in vitro and in vivo functional studies of alanine-substituted analogues of the native molecule. Alanine was substituted at all non-bridging positions in the sequence. In most cases the structure of the analogues in aqueous solution was shown to be native-like by 1H NMR spectroscopy. Minor conformational changes observed in some cases were characterized by two-dimensional NMR. Replacement of Lys2 and Tyr13 with Ala caused reductions in potency of more than 2 orders of magnitude in three functional assays (sympathetic nerve stimulation of rat isolated vas deferens, right atrium and mesenteric artery) and a rat brain membrane binding assay. Replacement of several other residues with Ala (particularly Arg17, Tyr22 and Lys24) resulted in significant reductions in potency (<100-fold) in the functional assays, but not the binding assay. The potencies of the analogues were strongly correlated between the different functional assays but not between the functional assays and the binding assay. Thus, the physiologically relevant assays employed in this study have shown that the high affinity of GVIA for the N-type calcium channel is the result of interactions between the channel binding site and the toxin at more sites than the previously identified Lys2 and Tyr13.

Alanine↗
PubMed1997

Prolonged cardiovascular effects of the N-type Ca2+ channel antagonist omega-conotoxin GVIA in conscious rabbits.

omega-Conotoxin GVIA (omega-CTX) is an N-type Ca2+ channel antagonist that is considered to be only partially reversible in vitro. In vivo, its effects after 24 h are unknown. To assess the duration of action of this peptide in vivo, the effects of a single intravenous injection of omega-CTX on mean arterial pressure (MAP), heart rate (HR), postural adaptation, and the baroreflex were investigated in conscious rabbits. MAP, HR, the baroreflex induced by i.v. glyceryl trinitrate (0.4-20 micrograms/kg) and phenylephrine (0.1-15 micrograms/kg) and orthostatic responses to 1 min 90 degrees head-up tilt were assessed before (0 h) and 2-168 h after administration of omega-CTX (10 micrograms/kg i.v. bolus: n = 6-9) or vehicle (0.9% saline; n = 6). Acute phase I: By 2 h after omega-CTX administration, MAP had decreased from 75 +/- 3 mm Hg to 60 +/- 2 mm Hg; HR increased from 220 +/- 7 beats/min to 249 +/- 5 beats/min (n = 9). There was marked attenuation of the baroreflex curve (HR range decreasing by 61%). By 24 h. MAP and HR had returned to control values, but the HR range was still 18% less than that of control. Phase II: MAP and HR then decreased steadily over the next 96 h to significantly lower values by 120 h after omega-CTX administration (delta-8 +/- 2 mm Hg and -29 +/- 2 beats/min, respectively; n = 6). Thereafter, MAP and HR values increased and by 168 h these parameters, and the baroreflex, were similar to control values. In response to 90 degrees tilt, there was no change in MAP at 0 h; however, 1 h after omega-CTX, significant postural hypotension was observed with decreases of 14 +/- 1 mm Hg(n = 9). Smaller orthostatic responses were still observed 48 h after omega-CTX administration: however, by 72 h, head-up tilt no longer induced a significant change in MAP. In the vehicle-treatment group, there were no changes in cardiovascular parameters during 0-168 h. Thus omega-CTX (10 micrograms/kg i.v.) causes acute hypotension, as well as postural hypotension, and has sympatholytic and vagolytic effects that are mostly reversed after 48 h in the conscious rabbit. However, a second hypotensive and bradycardic phase lasting a further 96 h ensues, suggesting that other prolonged effects from central neural or hormonal mechanisms or fluid shifts may occur.

Animals↗
PubMed1997

New insights into vascular reactivity: from altered structure to neural control.

1. The present review covers two aspects of the author's research into the pharmacology of vascular reactivity of isolated vessels and in the intact circulation. First, how 'normal' reactivity is altered by injury or disease and, second, how novel drugs have allowed insight into the role of the cotransmitter neuropeptide Y and 'N' type calcium channels in neurotransmitter release. 2. Acute endothelium removal in the femoral artery of the anaesthetized dog confirmed the obligatory role of these cells in the dilatation response to intra-arterial acetylcholine (ACh). After 4 weeks, conduit arteries respond with a thickened neointima following acute endothelial injury but, provided macrophage-derived foam cells are absent, the artery relaxes normally to ACh. 3. In the dog coronary vasculature, stable collateral arteries have a marked neointima of non-contractile smooth muscle cells that are lined with endothelium. Reactivity to vasodilator stimuli is normal while that to vasoconstrictor stimuli is impaired. 4. In the conscious rabbit, superficial femoral artery (SFA) occlusion stimulates profound angiogenesis but, despite these changes to the hindlimb vasculature, reactivity to vasodilator and vasoconstrictor agents from day 1 to 6 months following SFA is unaltered. 5. Endothelial dysfunction is discussed in relation to hypertension, hypercholesterolaemia and congestive heart failure. 6. The novel "N' type calcium channel antagonist omega-conotoxin GVIA, was used to explore the role of "N' type channels in cardiac and vascular neurotransmitter release in conscious rabbits. 7. The novel putative Y 1-selective neuropeptide Y antagonist 1229U91 was shown to inhibit nerve-mediated contractions of isolated mesenteric, but not femoral, artery segments in the rat. This regional difference in a possible cotransmitter role of the peptide is discussed.

Animals↗
PubMed1996

Distribution of N-type Ca2+ channel binding sites in rabbit brain following central administration of omega-conotoxin GVIA.

Central administration of the N-type Ca2+ channel blocker omega-conotoxin GVIA in conscious rabbits has previously been shown to result in a slowly developing hypotensive and sympatholytic effect, with peak changes observed after 48 h. The aim of the current study was to examine the distribution of [125I] omega-conotoxin GVIA binding in rabbit brain alone or following a prior i.c.v. injection of omega-conotoxin GVIA to determine the site(s) of action of centrally administered omega-conotoxin GVIA. Brains were removed from rabbits 2 or 48 h after central administration of vehicle or non-labelled omega-conotoxin GVIA (30 pmol/kg, i.c.v.). Brain sections were then incubated with [125I] omega-conotoxin GVIA (50 pM) and the density of specific [125I] omega-conotoxin GVIA binding measured in dpm/mm2 was determined by quantitative receptor autoradiography. In the vehicle group, highest densities of [125I] omega-conotoxin GVIA binding sites (> 20 dpm/mm2) were detected in cortex, caudate, putamen, and the stratum oriens and stratum radiatum of the hippocampus. Prior (48 h) i.c.v. injection of omega-conotoxin GVIA resulted in a decrease in specific binding of [125I] omega-conotoxin GVIA, particularly in cortex and some portions of the caudate and hippocampus. Lesser effects were observed with a prior (2 h) i.c.v. injection of omega-conotoxin GVIA. Central administration of omega-conotoxin GVIA may be acting to disrupt neurotransmission in higher brain regions which may, in turn, affect cardiovascular control mechanisms in the rabbit.

Animals↗
PubMed1996

Role of the endothelium in the genesis of cardiovascular disease.

1. Endothelial cells release nitric oxide (NO) and the putative endothelium-derived hyperpolarizing factor (EDHF) in response to an increase in shear stress and receptor stimulation. 2. Tests of endothelial function have principally used acetylcholine (ACh)-mediated relaxation of precontracted isolated blood vessels or increases in forearm blood flow measured by venous occlusion plethysmography. Basal NO release is tested by a rise in resistance during infusion of the NO synthase inhibitor L-NMMA. Potential traps for investigators looking to evoke endothelial dysfunction following reduced ACh responses are discussed. 3. Endothelial dysfunction appears to occur in large but not small arteries in human and animal hypertension. Patients with long-standing congestive heart failure have endothelial dysfunction in buttock skin resistance arteries and there is coronary artery endothelial dysfunction following coronary ischaemia. 4. Remodelled arteries from neointimal thickening as a result of coronary collateral development in dog heart and new angiogenic vessel growth following large artery occlusion in the rabbit hindlimb appear to have normal endothelial function in relation to NO release. 5. Development of specific NO synthase inhibitors, antagonists of EDHF and the constrictor peptide endothelin, will clarify the role of these endothelium-derived factors in the cause or maintenance of vascular dysfunction. Defining redundancy and hierarchy of importance of these vascular factors are areas for future resolution.

Animals↗
PubMed1996

Synthesis and characterization of a selective peptide antagonist of neuropeptide Y vascular postsynaptic receptors.

1. A cyclic dimeric nonapeptide neuropeptide Y (NPY) receptor antagonist, 1229U91, was synthesized by Fmoc chemistry and dimerised in solution. Its effects were assayed in mesenteric arteries from rats and mice, and in rat vas deferens. 2. Mesenteric arteries were cannulated and pressurised to 55 mmHg and the external diameters continuously measured. NPY, PYY, Leu31Pro34NPY and NPY(13-36) each caused concentration-related contractions with the order of potency PYY > or = Leu31Pro34NPY = NPY > NPY (13-36), consistent with the Y1 receptor subtype. 3. 1229U91 had no agonist activity in the arteries but caused a concentration-related rightward shift of NPY (mouse arteries) or Leu31Pro34NPY (rat) concentration-response curves. The antagonism was competitive with pKBS of 7.69 +/- 0.15 and 7.47 +/- 0.13 in the mouse and rat arteries, respectively. 4. Sympathetic nerves in the vas deferens were stimulated with a single electrical field pulse every 20 s and the twitch responses recorded. NPY, PYY, Leu31Pro34NPY and NPY(13-36) inhibited the twitches with the order of potency PYY > NPY > NPY(13-36) >> Leu31Pro34NPY, consistent with the Y2 receptor subtype. 5. 1229U91 inhibited the vas deferens twitch with a shallow concentration-response curve and a time-course of inhibition distinct from that of NPY. 1229U91 (30 microM) did not cause a rightward shift of the NPY concentration-response curve. 1229U91 is at least 5 orders of magnitude less potent in the vas deferens than in rat brain Y2 binding assays reported by others, suggesting that the brain and vas deferens Y2 receptors are different. 6. It is concluded that 1229U91 is a competitive antagonist of NPY Y1 vascular receptors and has additional properties that inhibit the electrically evoked twitch of the rat vas deferens.

Analysis of Variance↗
PubMed1996

Baroreflex resetting but no vascular tolerance in response to transdermal glyceryl trinitrate in conscious rabbits.

1. We investigated whether acute (5 h) and chronic (3 days) transdermal glyceryl trinitrate (GTN) patches could cause the development of tolerance in terms of haemodynamics and vascular reactivity in the conscious rabbit. The effects of haemodynamic tolerance were assessed on arterial pressure, heart rate and the baroreflex control of heart rate, while hindquarter vascular reactivity in response to dilator and constrictor drugs and reactive hyperaemia were used to assess vascular tolerance. 2. Seven days prior to experiments, an inflatable cuff, a pulsed Doppler flow probe and an indwelling intra-aortic catheter (for i.a. agonist infusions) were implanted around the lower abdominal aorta. 3. In acute experiments, the effects of 0-5 h treatment with transdermal GTN (0 Sham), 10 or 20 mg 24 h-1) on MAP, HR and the baroreflex were examined. Chronic experiments were performed on three separated days (days 0 - before, 4 - with GTN patch and 8 - recovery). On each day, the baroreflex, reactive hyperaemic responses and hindquarter vascular dose-response curves to i.a. GTN, adenosine, acetylcholine, S-nitroso-N-acetylpenicillamine (SNAP) and methoxamine were assessed. On days 1-4, GTN was administered transdermally via a patch(es) (10 mg 24 h-1 (low dose) or 20 mg 24 h-1 (high dose); renewed every 24 h). 4. Acute treatment with 20 mg GTN 24 h-1, but not with 0 (n = 4) or 10 mg GTN 24 h-1 (n = 4), caused a significant fall in MAP (8 +/- 1 mmHg; n = 4) and resetting of the baroreflex by 5 h. Chronic GTN caused a significant fall in MAP of 8 +/- 2 and 8 +/- 2 mmHg on day 4 with low (n = 8) and high dose (n = 8), respectively, with no change in HR. There was no significant change to hindquarter vascular reactivity to i.a. infusion of GTN, nor were there any significant differences in the reactivity to i.a. adenosine, acetylcholine, SNAP or methoxamine with either low or high doses of GTN. 5. Chronic GTN treatment with low and high dose patches caused a parallel leftward shift ('resetting') of the baroreflex on day 4. By day 8, the baroreflex had still not recovered from this leftward shift 6. In the rabbit, chronic exposure to clinical nitrate patches caused haemodynamic compensation and baroreflex resetting but no evidence of vascular reactivity tolerance. Novel NO donor drugs and delivery regimens which provide intermittent dosing may prevent the development of haemodynamic resetting rather then preventing vascular tolerance, a commonly perceived difficulty in chronic nitrate therapy.

Administration, Cutaneous↗
PubMed1996

Effects of N-, P- and Q-type neuronal calcium channel antagonists on mammalian peripheral neurotransmission.

1. The effects of N-, P- and Q-type neuronal voltage-operated calcium (Ca2+) channel antagonists on neurotransmission were determined in a range of cardiovascular and urogenital tissues, as well as the diaphragm, isolated from rat or mouse. 2. The pharmacological tools chosen were omega-conotoxin GVIA (CTX GVIA), a selective N-type Ca2+ channel antagonist, the P-type channel blocker (< or = 100 nM) omega-agatoxin IVA (AGA IVA) and omega-conotoxin MVIIC (CTX MVIIC), a non-selective antagonist of N-, P- and Q-type channels. The effects of these antagonists on nerve-mediated responses were assessed in right atria, vasa deferentia, phrenic nerve-hemidiaphragms and small mesenteric arteries. 3. Rat mesenteric artery contractile responses to perivascular nerve stimulation were concentration-dependently inhibited by CTX GVIA (1-10 nM); inhibition was 92% with 10 nM. CTX MVIIC was > 100 fold less potent and only caused an inhibition of 46% at the highest concentration (1000 nM). AGA IVA (100 nM) had no effect. 4. In rat vas deferens stimulated at 0.05 Hz, CTX GVIA (10 nM) completely inhibited the twitch response and CTX MVIIC, about 100 fold less potent, caused total inhibition at 1000 nM. AGA IVA did not affect the twitch. In rat preparations stimulated at 20 Hz, a CTX GVIA-resistant (< or = 1000 nM) twitch response of 25% was apparent which could be blocked by 1000 nM AGA IVA or CTX MVIIC. In mouse vas deferens (20 Hz stimulation), CTX GVIA 10 nM caused an 87% inhibition of the twitch, the remainder being resistant to CTX GVIA, 100 nM. CTX MVIIC was only 10 fold less potent than CTX GVIA and completely inhibited the response at 1000 nM. AGA IVA (100 nM) inhibited the twitch by 55%. 5. The twitch response of the mouse phrenic nerve-hemidiaphragm was concentration-dependently inhibited by AGA IVA (1-100 nM); inhibition was 92% at 100 nM. CTX MVIIC was about 10 fold less potent than AGA IVA with an inhibition of 80% at 1000 nM. CTX GVIA was without effect. In the rat diaphragm preparation, AGA IVA (< or = 100 nM) and CTX GVIA (< or = 1000 nM) had little effect on the twitch response. CTX MVIIC (1000 nM) inhibited the twitch by 57%. 6. In rat and mouse right atria, sympathetic responses were concentration-dependently inhibited by CTX GVIA with almost complete block at 10-100 nM. CTX MVIIC was 100 fold less potent and caused complete inhibition at 1000 nM in the mouse preparation. AGA IVA did not affect atrial sympathetic responses. 7. These data suggest that N-type Ca2+ channels predominate in the control of sympathetic transmission in the mesenteric artery, vas deferens and right atrium. In the mouse vas deferens (and rat tissue at high stimulus frequency), P- and Q-type channels also mediate Ca2+ influx. P- and Q-type Ca2+ channels control neurosecretion at the motor endplate, with no role for N type channels.

Animals↗
PubMed1996

Analysis of relaxation and repolarization mechanisms of nicorandil in rat mesenteric artery.

1. The mechanisms by which nicorandil causes relaxation of rat isolated small mesenteric arteries mounted on a Mulvany myograph was investigated by use of a combination of putatively mechanism-specific antagonists. 2. In arteries precontracted by the thromboxane-mimetic, U46619, the EC50 for cromakalim and levcromakalim-induced relaxation curves were raised by 36 and 17 fold by glibenclamide (3 microM) while the EC50 for nicorandil-induced relaxation was unaffected by either glibenclamide or methylene blue (10 microM). A combination of these antagonists raised the EC50 for nicorandil by 8 fold. 3. In U46619-contracted arteries, nifedipine (100 nM) did not affect the cromakalim relaxation curve but it raised the EC50 for nicorandil by 5 fold. The combination of methylene blue, glibenclamide and nifedipine further inhibited the maximum relaxation to nicorandil. 4. In separate experiments, membrane potential (Em) and force responses were measured simultaneously. In arteries depolarized and contracted by U46619 both nicorandil and cromakalim repolarized (delta Em, 35 mV) and relaxed (100%) the vessels. Glibenclamide (3 microM) did not alter the relaxation-concentration curve to nicorandil but reduced the maximum repolarization to delta 10.8 mV. In contrast to Em and relaxation-response curves to cromakalim were shifted to the right by glibenclamide by 30-100 fold. 5. In unstimulated arteries, nicorandil (but not cromakalim) -induced hyperpolarization was significantly antagonized by methylene blue (10 microM) (6.6 fold rightward shift) or nifedipine (100 nM) (2.6 fold). In depolarized arteries (U46619), nifedipine but not methylene blue inhibited the nicorandil-induced hyperpolarization. 6. In arteries precontracted to 50% tissue maximum by either KCl or U46619, nifedipine (100 nM) relaxed the artery but failed to repolarize the Em. Presumably voltage-operated calcium channels (VOCC) were blocked preventing contraction but the artery remained depolarized, presumably through non VOCC mechanisms. 7. These data suggest that nicorandil may relax small arteries through 3 parallel pathways, (i) NO-donor mediated stimulation of guanylate cyclase and increase in cyclic GMP, (ii) K+ATP channel opening, and (iii) nifedipine-sensitive VOCC inhibition. Em data suggest that nicorandil-induced repolarization is caused principally through opening K+ATP channels. Blockade of this hyperpolarization by glibenclamide is not sufficient to alter the relaxation, indicating dissociation of nicorandil-induced changes in membrane potential and relaxation. 8. These results highlight the 'chameleon' actions of nicorandil where there is no apparent association of Em repolarization with relaxation, in contrast to the parallel responses for cromakalim.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed1996

Selectivity of omega-conotoxin GVIA for n-type calcium channels in rat isolated small mesenteric arteries.

1. The selectivity of omega-conotoxin GVIA (omega-CTX) for prejunctional N-type voltage-operated calcium channels (VOCC) was examined in rat isolated small mesenteric arteries mounted in a Mulvany-Halpern myograph. Contractile responses to perivascular nerve stimulation, noradrenaline (NA) and potassium (K+) were obtained before and after treatment with omega-CTX. The effects of omega-CTX were compared with those of felodipine, an L-type VOCC blocker. 2. Omega-CTX (3 nmol/L-10 mu mol/L) inhibited contractions to electrical field stimulation by up to 94%, compared with the corresponding time control group. Felodipine (0.1 mu mol/L) had little effect on the contractions to electrical stimulation compared with the vehicle-treated vessels. 3. Concentration-response curves to exogenous NA (0.1 mu mol/L) and contractions to a submaximal concentration of K+ (50 mmol/L) were unaffected by omega-CTX (3 nmol/L-10 mu mol/L). In contrast, the maximum contraction to NA in vessels exposed to felodipine (0.1 mu mol/L) was reduced by 37%, and the contraction to K+ (62 mmol/L) was reduced by 84% compared with vehicle-treated arteries. 4. The results indicate that even at concentrations up to 10 mu mol/L (10 000-fold higher than required to inhibit prejunctional N-type VOCC), omega-CTX inhibits only neurotransmitter release. Its effects are clearly different to felodipine as omega-CTX has no effect on post-junctional alpha1-adrenoceptor-mediated vasoconstriction or direct smooth muscle depolarization considered to be mediated by L-type VOCC. Therefore, at least at the vascular neuroeffector junction, omega-CTX appears to be highly selective for N-type VOCC with no effect on L-type VOCC.

Animals↗
PubMed1995

Analysis of competitive agonist-antagonist interactions by nonlinear regression.

The rigorous estimation of a dissociation constant (Kb) for antagonists in functional assays has been sought by pharmacologists using a variety of techniques ever since the regression method of Arunlakshana and Schild in 1959. Here, Michael Lew and James Angus describe a simplified global regression method with improved accuracy compared to Schild analysis. The method is suitable for personal computers with standard graphing and statistical software. The accuracy of the predicted pKb values and confidence intervals has been tested by comparing examples of published data, and by mathematical (bootstrap) simulations.

Animals↗
PubMed1995

Hemodynamic and autonomic reflex effects of chronic N-type Ca2+ channel blockade with omega-conotoxin GVIA in conscious normotensive and hypertensive rabbits.

The effects of chronic administration of omega-conotoxin GVIA (omega-CTX), an N-type Ca2+ channel blocker, on hemodynamics and autonomic reflexes were studied in conscious normotensive (sham) and hypertensive (wrap) New Zealand white rabbits. During surgery, a pulsed Doppler-flow probe was implanted around the lower abdominal aorta, and both kidneys were wrapped in cellophane (wrap) or left undisturbed (sham). Rabbits were studied 4 weeks later on 5 consecutive days. On days 1-4, hemodynamics, the baroreceptor-heart-rate (HR) reflex induced by drugs and the Bezold-Jarisch-like reflex evoked by serotonin, were measured before and 2 h after administration of omega-CTX (10 micrograms/kg i.v. bolus). On day 5, hemodynamics and reflexes were again assessed, but no further omega-CTX was given. On day 1, omega-CTX caused falls in mean arterial pressure (MAP) of 17 +/- 3 and 27 +/- 5 mm Hg in sham (n = 6) and wrap (n = 11) rabbits, respectively, with tachycardia. Hindquarter vascular conductance (HVC) increased > 35% in both groups. On days 2-4, omega-CTX still caused falls in MAP; however it was progressively less than on day 1. HR and HVC did not change. After omega-CTX on day 1, there was marked attenuation of the sympathetic components and decrease in the vagal components of the baroreceptor-HR reflex curves, with HR range decreasing by > 61% in sham and wrap rabbits. These curves remained similar over the next 4 days, and were unaffected by further omega-CTX. However, omega-CTX had no effect on the vagally mediated Bezold-Jarisch-like reflex. In auxiliary experiments in vitro, omega-CTX (1 nM) inhibited sympathetic, but not vagal, responses to electrical nerve stimulation of rabbit-isolated right atria. Thus, omega-CTX is a potent hypotensive agent in normotensive and hypertensive rabbits, predominantly via a peripheral sympatholytic action with no effect on vagal-cardiac efferent activity. However, it may affect the vagal component of the baroreceptor-HR reflex by an unknown central mechanism.

Animals↗
PubMed1995

Collateral development and angiogenesis after major artery ligation does not alter hindquarter vascular reactivity in conscious rabbits.

We tested whether vasodilator and vasoconstrictor responses of the hindquarter vasculature in conscious rabbits were altered 1 day, 2 weeks, and 6 months after bilateral superficial femoral artery ligation (SFAL). With pharmacological autonomic blockade, hindquarter flow (Doppler flowmeter) was restored to 84% of control values 1 day postligation (n = 5). Responses to aortic balloon inflation (5-80 s), and intraaortic infusion of norepinephrine, angiotensin II, serotonin, acetylcholine, sodium nitroprusside, and adenosine were similar to pre-SFAL responses. Two weeks post-SFAL, acrylic casts showed an extensive collateral network originating from branches of internal iliac and deep femoral arteries. Acetylcholine-induced dilatation was attenuated postligation (n = 7) relative to controls (n = 13). Serotonin caused constriction in two rabbits postligation but dilatation in all others; however responses to all other agents tested were similar to controls. Reactive hyperemia and vascular reactivity were similar in rabbits 6 months post-SFAL (n = 5) and controls (n = 5). Thus, despite extensive vascular remodeling after SFAL, global hyperemic flow responses of the rabbit hindquarter vasculature appeared normal. We found only minimal changes in vascular reactivity to constrictor and dilator stimuli. This model of peripheral vascular disease does not reflect the clinical syndrome.

Acetylcholine↗
PubMed1995

Synthesis and biological characterisation of a series of iberiotoxin analogues.

We report here the synthesis of iberiotoxin (IbTX), a 37-amino acid peptide containing three disulfide bridges, and a series of mono-looped analogues. All syntheses were conducted using Fmoc chemistry. Synthesis of IbTX gave a product which was indistinguishable from a reference sample in both its physico-chemical properties and its biological activity. A series of three mono-looped analogues, in which four of the six cysteines were replaced by alanine, were synthesised to give [Ala7,13,28,33]-IbTX, [Ala13,17,33,35]-IbTX and [Ala7,17,28,35]-IbTX. Oxidation of the linear form of [Ala7,17,28,35]-IbTX to form the Cys13 to Cys33 disulfide bridge proceeded more slowly than that of the other two analogues. None of these analogues was biologically active, indicating that no single loop is the mediator of channel blocking activity.

Amino Acid Sequence↗