Biomedical subjects

C E Wright

Publications and source records attributed to C E Wright.

At least 55 records · Page 3Linked to original sources

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↗

PubMed1998

Cerebral glucose metabolism in women with panic disorder.

OBJECTIVE: This study was undertaken to clarify earlier inconsistent findings in brain metabolic topography in panic disorder patients at rest. METHOD: Positron emission tomography (PET) with [18F]fluorodeoxyglucose was used to determine cerebral metabolic activity in six female patients with a DSM-III-R diagnosis of panic disorder and in six healthy female volunteers. All patients with panic disorder were medication free and were sensitive to lactate infusion. RESULTS: A significant increase in glucose metabolism was found in the left hippocampus and parahippocampal area of the panic disorder subjects in comparison with that found in the healthy subjects. In addition, a significant decrease in metabolism was found in the right inferior parietal and right superior temporal brain regions of the panic disorder subjects in comparison with that of the normal subjects. There was no significant correlation between scores for the severity of panic disorder or for the severity of lactate-induced panic attack and the quantified PET abnormality. CONCLUSIONS: These data provide further support for the hypothesis of an abnormal brain metabolism in the hippocampal and parahippocampal area in individuals with panic disorder and also suggest other areas of aberrant brain metabolism in this disorder.

Adult↗

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

Pharmacokinetics and psychomotor performance of alprazolam: concentration-effect relationship.

The relationship between the pharmacokinetics of alprazolam and dose and the relationship between the concentration of alprazolam and psychomotor performance in healthy male volunteers were investigated in this double-blind, placebo-controlled, modified crossover study. Twenty-four volunteers received placebo in Phase I and then received single 2-mg, 4-mg, 8-mg, and 10-mg doses of a sustained-release formulation in Phases II through V, according to, a crossover design. Blood samples were collected at several times throughout each phase to 48 hours after the dose; the harvested plasma was assayed for concentrations of alprazolam, 4-hydroxyalprazolam, and alpha-hydroxyalprazolam by high-performance liquid chromatography. Sedation was rated at each blood-sampling time and psychomotor performance tests, consisting of digit-symbol substitution and card-sorting tasks, were conducted at several times after each dose. Area under the concentration-time curve and peak concentration for alprazolam increased proportionally with each higher dose; clearance did not differ significantly between treatments. The concentrations of 4-hydroxyalprazolam and alpha-hydroxyalprazolam increased proportionally with dose and the combined plasma concentration of the metabolites were less than 15% of unchanged concentrations of alprazolam for all doses. Maximum sedation increased with each increase in dose up to 8 mg, and psychomotor performance decreased with each increase in dose. Performance versus concentration curves for alprazolam exhibited a clockwise hysteresis loop in contrast to the counterclockwise hystereses previously reported for both intravenous and oral doses of immediate-release tablets. Data through 6 hours after dose were well described by a sigmoid Emax model. Alprazolam exhibits linear pharmacokinetics after single oral doses of sustained-release tablets between 2 mg and 10 mg. Reversal of the concentration-effect curve to a clockwise loop suggests the counterclockwise hystereses of rapidly absorbed doses was caused by the differing distribution rates into the systemic circulation and effect site and not by metabolite activity.

Adult↗

PubMed1997

Steady-state pharmacokinetic properties of pramipexole in healthy volunteers.

Pramipexole is a dopamine receptor agonist that has proved effective in the treatment of Parkinson's disease. The pharmacokinetic properties of pramipexole at steady-state concentrations were studied in 16 healthy men and women at four dose levels throughout the range recommended for Parkinson's patients. Plasma and urine samples collected within the four dose intervals were assayed for concentrations of pramipexole, using high-performance liquid chromatography. The total oral clearance for all participants was 419 mL/min. The mean volume of distribution and elimination half-life for all participants was 486 +/- 93.2 L and 12.9 +/- 3.27 hours. Concentrations of pramipexole were proportional to dose, although the drug's pharmacokinetic properties differed between men and women. The area under the concentration-time curve for each dose level was 35% to 43% greater in women, mainly because of a 24% to 27% lower oral clearance. The mean creatinine clearance in men and women was 112 +/- 12.8 mL/ min/1.73 m2 and 80.9 +/- 15.6 mL/min/1.73 m2, respectively. The renal clearance of pramipexole accounts for approximately 80% of oral clearance, and there was a significant correlation between renal and creatinine clearances. The influence of gender could not be distinguished from the influence of age and the resulting reduced creatinine clearance, but the measurement of pharmacokinetic properties produced linear results in both men and women.

Adult↗

PubMed1997

Human cytochromes mediating N-demethylation of fluoxetine in vitro.

Biotransformation of the selective serotonin reuptake inhibitor antidepressant, fluoxetine, to its principal metabolite, norfluoxetine, was evaluated in human liver microsomes and in microsomes from transfected cell lines expressing pure human cytochromes. In human liver microsomes, formation of norfluoxetine from R,S-fluoxetine was consistent with Michaelis-Menten kinetics (mean K(m) = 33 microM), with evidence of substrate inhibition at high substrate concentrations in a number of cases. The reaction was minimally inhibited by coincubation with chemical probes inhibitory for P450-2D6 (quinidine), -1A2 (furafylline, alpha-naphthoflavone), and -2E1 (diethyldithiocarbamate). Substantial inhibition was produced by coincubation with sulfaphenazole (Ki = 2.8 microM), an inhibitory probe for P450-2C9, and by ketoconazole (Ki = 2.5 microM) and fluvoxamine (Ki = 5.2 microM). However, ketoconazole, relatively specific for P450-3A isoforms only at low concentrations, reduced norfluoxetine formation by only 20% at 1 microM, and triacetyloleandomycin (> or = 5 microM) reduced the velocity by only 20-25%. Microsomes from cDNA-transfected human lymphoblastoid cells containing human P450-2C9 produced substantial quantities of norfluoxetine when incubated with 100 microM fluoxetine. Smaller amounts of product were produced by P450-2C19 and -2D6, but no product was produced by P450-1A2, -2E1, or 3A4. Cytochrome P450-2C9 appears to be the principal human cytochrome mediating fluoxetine N-demethylation. P450-2C19 and -3A may make a further small contribution, but P450-2D6 is unlikely to make an important contribution.

Aryl Hydrocarbon Hydroxylases↗

PubMed1997

Differential effects of omega-conotoxin GVIA on cholinergic and non-cholinergic secretomotor neurones in the guinea-pig small intestine.

1. Ussing chambers were used to study the effects of the specific N-type Ca2+ channel antagonist, omega-conotoxin GVIA, on neurally evoked secretion across isolated submucosa/mucosa preparations from the small intestine of the guinea-pig. 2. Cholinergic and non-cholinergic neurones were stimulated with 10 microM dimethylphenylpiperazinium (DMPP). Non-cholinergic secretomotor neurones were preferentially stimulated with 100 nM 5-hydroxytryptamine (5-HT), while cholinergic secretomotor neurones were preferentially stimulated with 3 microM 5-HT in the presence of the 5-HT2 receptor antagonist ketanserin (30 nM). 3. omega-Conotoxin GVIA (1 nM-1 microM) depressed the secretion evoked by DMPP in a concentration-dependent manner, but a substantial residual response was observed. Hyoscine (100 nM) significantly depressed secretion evoked by DMPP, but did not prevent further depression of secretion by omega-conotoxin GVIA. 4. The toxin was substantially more effective when the non-cholinergic secretomotor neurones were preferentially activated with 100 nM 5-HT, with a decrease in the response of more than 75% of the control value in the presence of 1 microM omega-conotoxin GVIA. 5. omega-Conotoxin GVIA (1 microM) was relatively ineffective against secretion evoked by preferential activation of cholinergic secretomotor nuerones with 3 microM 5-HT in the presence of 30 nM ketanserin, inhibiting the response by less than 33%. However, this inhibition was significant. Both 100 nM hyoscine and 300 nM tetrodotoxin abolished this effect of omega-conotoxin GVIA. 6. It is concluded that N-type Ca2+ channels play a major role in transmitter release from non-cholinergic secretomotor neurones, but are not important for release from cholinergic secretomotor neurones in the guinea-pig small intestine.

Animals↗

PubMed1997

Capsaicin and neurokinin A-induced bronchoconstriction in the anaesthetised guinea-pig: evidence for a direct action of menthol on isolated bronchial smooth muscle.

1. For many years menthol has been used in the treatment of respiratory disorders although, a bronchodilator effect of menthol has yet to be described. Using the bronchoconstrictors capsaicin (acting via stimulating the release of neuropeptides from sensory afferents) and neurokinin A (NKA) we have raised airways resistance in the guinea-pig (GP) and studied the effect of menthol on both capsaicin and NKA-induced bronchoconstriction in vivo. In vitro the effect of menthol on acetylcholine (ACh) and KCl precontracted GP bronchi was also studied. 2. GP (n = 13) were anaesthetized (urethane 1.5 g kg-1, i.p.) and a bolus injection of capsaicin (7.5 micrograms ml-1, i.v.) or infusion of NKA (1 microgram min-1, i.v.) was given either in the presence of air (0.81 min-1) or air impregnated with menthol vapour (7.5 micrograms l-1) freely breathed from a tracheal cannula via a T-piece. Airways resistance (Raw) and ventilation were measured throughout. Bronchi of mean internal diameter (1029 + 73.6 microns; n = 24) were removed from GP (n = 16) and mounted in the Cambustion myograph. Bronchial rings were maximally precontracted with 80 mM KCl or 2 mM ACh. Relaxation due to a cumulative dose of menthol (1- 3000 microM) was measured. 3. Menthol produced a significant (P < 0.05) 51.3% reversal of the capsaicin-induced increase in Raw, and also inhibited the significant (P < 0.05) reduction in minute ventilation (Ve) associated with the capsaicin-induced increased in Raw. Menthol also caused a significant (P < 0.05) 41% reversal of the NKA-induced increase in Raw. The NKA-induced decrease in Ve was again significantly (P < 0.05) reversed with menthol inhalation. Menthol caused a significant (P < 0.001) dose-dependent relaxation of KCl and ACh precontracted bronchi. 4. We have shown that menthol attenuates both capsaicin and NKA-induced bronchoconstriction in vivo and relaxes KCl and ACh preconstricted bronchi in vitro. Menthol inhibition of NKA and capsaicin-induced bronchoconstriction could be, in part, explained by a direct action of menthol on bronchial smooth muscle.

Airway Resistance↗

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

Alprazolam in young and elderly men: sensitivity and tolerance to psychomotor, sedative and memory effects.

This study was designed to determine whether age influences sensitivity to alprazolam and/or rate of acute tolerance development to the effects of alprazolam. Three treatments were each separated by 4 weeks. Twenty-five young (ages 22-35) and 13 elderly (ages 65-75) men received 2 mg of alprazolam/2 min i.v. Blood samples were obtained over 48 hr, and sedative, psychomotor and memory effects were assessed serially for 12 hr. Clearance was lower (P = .05) and elimination t[1/2] was longer (P = .005) in the elderly, but area under the concentration curve to 12 hr and maximum concentration did not differ by age group. Maximum impairment was greater in the elderly for all assessments. Mean EC50 values differed between the elderly (25.3 and 25.0 ng/ml) and the young (39.8 and 36.5 ng/ml) on card sorting and digit symbol substitution, respectively (P < .001). Bolus treatment data were used to individualize doses for the crossover of placebo and alprazolam; infusions were designed to maintain a plateau alprazolam concentration between 1 and 9 hr. Alprazolam concentrations through 12 hr did not differ between the young and elderly. Median t[1/2] for offset of effect for digit symbol substitution was 2.8 hr in the young and 4.9 hr in the elderly (P = .05). Therefore, aging decreases alprazolam clearance and increases sensitivity to effects of alprazolam through a mechanism other than pharmacokinetics; aging also decreases the rate of offset of effect of alprazolam. In addition, the data provide insight into the intensity of initial effect as a determinant of rate of tolerance development.

Adult↗

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

Midazolam hydroxylation by human liver microsomes in vitro: inhibition by fluoxetine, norfluoxetine, and by azole antifungal agents.

Biotransformation of the imidazobenzodiazepine midazolam to its alpha-hydroxy and 4-hydroxy metabolites was studied in vitro using human liver microsomal preparations. Formation of alpha-hydroxy-midazolam was a high-affinity (Km = 3.3 mumol/L) Michaelis-Menten process coupled with substrate inhibition at high concentrations of midazolam. Formation of 4-hydroxy-midazolam had much lower apparent affinity (57 mumol/L), with minimal evidence of substrate inhibition. Based on comparison of Vmax/Km ratios for the two pathways, alpha-hydroxy-midazolam formation was estimated to account for 95% of net intrinsic clearance. Three azole antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (Ki) versus alpha-hydroxy-midazolam formation were 0.0037 mumol/L for ketoconazole, 0.27 mumol/L for itraconazole, and 1.27 mumol/L for fluconazole. An in vitro-in vivo scaling model predicted inhibition of oral midazolam clearance due to coadministration of ketoconazole or itraconazole; the predicted inhibition was consistent with observed interactions in clinical pharmacokinetic studies. The selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine and its principal metabolite, norfluoxetine, also were inhibitors of both pathways of midazolam biotransformation, with norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro studies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair clearance of P450-3A substrates.

Anesthetics, Intravenous↗

PubMed1996

Association of invasive pneumococcal disease with season, atmospheric conditions, air pollution, and the isolation of respiratory viruses.

We examined the relation of invasive pneumococcal disease to season, atmospheric conditions, and the rate of respiratory virus isolation in a community-wide surveillance program in Houston. Among adults, the number of cases of pneumococcal bacteremia peaked in midwinter and declined strikingly in midsummer, indicating a high degree of inverse correlation with the ambient temperature. We detected significant correlations between the occurrence of pneumococcal disease and the isolation of respiratory syncytial virus (P < .001), influenza virus (P < .001), and all viruses except influenza virus (P < .001), as well as with air pollution, as measured by SO2 levels (P < .001). In contrast, the rate of invasive pneumococcal disease among infants and children was relatively more sustained from October through May, with a notable decrease in summer months; the incidence of pneumococcal disease was therefore less strongly correlated with cold weather and less closely associated with the isolation of respiratory syncitial virus or influenza virus. However, pneumococcal disease among infants and children was associated with isolation of these viruses after a 4-week lag period as well as with isolation of adenovirus and ragweed pollen counts. The finding, with regard to children, that correlations tended to be stronger for events that occurred 1 month previously than for those that occurred contemporaneously is consistent with the concept that viral or allergic events predispose to otitis media with effusion, which becomes suppurative and leads to pneumococcal bacteremia or meningitis. For adults, a more immediate predisposition to pneumococcal pneumonia and bacteremia because of viral infection or air pollution was suggested.

Adenoviruses, Human↗

PubMed1996

Inhibition of terfenadine metabolism in vitro by azole antifungal agents and by selective serotonin reuptake inhibitor antidepressants: relation to pharmacokinetic interactions in vivo.

Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (Ki) of 0.037 and 0.34 microM for desalkyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with Ki values of 0.28 and 2.05 microM, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro Ki values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons.

Antifungal Agents↗

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

Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole.

Biotransformation of the triazolobenzodiazepine triazolam to its hydroxylated metabolites, alpha-hydroxy (OH)- and 4-OH-triazolam, was studied in vitro using microsomal preparations of human liver. Mean values of Vmax (10.3 nM/min/mg of protein) and Km (304 microM) for the 4-OH pathway exceeded values for the alpha-OH pathway (2.4 and 74, respectively). However the mean Vmax/Km ratios for the two pathways were nearly identical, indicating that both contribute approximately equally to intrinsic clearance. Ketoconazole was a powerful inhibitor of triazolam biotransformation, having mean competitive Ki values of 0.006 and 0.023 microM for the alpha-OH and 4-OH pathways. This is consistent with the role of P450-3A isoforms in mediating triazolam biotransformation. The serotonin2 antagonist antidepressant nefazodone inhibited the alpha-OH and 4-OH pathways (Ki = 0.6 and 1.7 microM, respectively), but with considerably less activity than ketoconazole. Among six selective serotonin reuptake-inhibitor antidepressants, norfluoxetine was the most potent inhibitor (Ki = 2.7 and 8.0 microM) and fluoxetine itself was the weakest (Ki = 7.0 and 44.3 microM). In a double-blind clinical pharmacokinetic-pharmacodynamic study, administration of triazolam (0.125 mg) preceded by ketoconazole, compared to triazolam preceded by placebo, produced a nearly 9-fold reduction in apparent oral clearance of triazolam (41 vs. 337 ml/min) and a 4-fold prolongation of half-life (13.5 vs. 3.4 hr). Pharmacodynamic testing indicated enhancement of electroencephalographic beta activity, and enhanced decrements in digit-symbol substitution test performance, attributable to coadministration of ketoconazole. Plasma ketoconazole concentrations measured in the clinical study ranged from 0.02 microgram/ml (projected minimum) to 4.95 micrograms/ml (maximum). An in vitro-in vivo scaling model, using these plasma ketoconazole concentrations together with liver partition ratios and the in vitro Ki values, predicted a decrement of triazolam clearance due to ketoconazole coadministration that was consistent with the 88% decrement in clearance actually observed in vivo.

Adult↗

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.

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