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H L Fung

Publications and source records attributed to H L Fung.

At least 37 records · Page 2Linked to original sources
PubMed1999

In vivo disposition of 3-nitro-L-tyrosine in rats: implications on tracking systemic peroxynitrite exposure.

In many pathological conditions such as inflammatory and neurodegenerative diseases, the in vivo toxicity of nitric oxide has been attributed to the toxic oxidant peroxynitrite. Interaction of peroxynitrite with biological molecules can modify tyrosine residues on the proteins at the ortho position resulting in the formation of the stable end-product, 3-nitro-L-tyrosine (3-NT). Recent investigations indicate that changes in the circulating concentrations of 3-NT in pathological conditions may reflect the extent of nitric oxide-dependent oxidative damage and peroxynitrite toxicity. In the present study, we examined the in vivo disposition characteristics of 3-NT in rats after either a single i.v. bolus dose (10 mg/kg) or a loading and maintenance infusion at 10 or 30 mg/kg. Plasma concentrations of 3-NT were analyzed by a reversed-phase HPLC method. After a single bolus dose of 3-NT at 10 mg/kg, the average half-life of the elimination phase for the drug was 68.5+/-18.4 min (n = 5). Infusions of 3-NT at two different doses (10 and 30 mg/kg) indicated that the pharmacokinetic properties of 3-NT below plasma concentrations of 100 microM were both linear and stationary. Urinary excretion of unchanged 3-NT was minimal, but two distinct metabolites of 3-NT were identified in the urine collected throughout the study. These findings may be useful in the interpretation of the plasma and urine 3-NT concentrations as possible indices of systemic peroxynitrite exposure.

Animals↗
PubMed1998

Effects of nitro-L-arginine on blood pressure and cardiac index in anesthetized rats: a pharmacokinetic-pharmacodynamic analysis.

PURPOSE: Nitric oxide synthase (NOS) inhibitors such as Nitro-L-arginine (L-NA) are being considered for the management of hypotension observed in septic shock. However, little information is available regarding the pharmacokinetic and pharmacodynamic properties of these agents. Our objective was to examine the relationships between L-NA plasma concentration and various hemodynamic effects such as cardiac index (CI), mean arterial pressure (MAP), and heart rate (HR) elicited by L-NA administration in rats. METHODS: L-NA was infused at doses between 2.5-20 mg/kg/hr in anesthetized rats over one hour. Hemodynamic effects and plasma L-NA levels were determined. RESULTS: Infusion of L-NA resulted in dose-dependent increases in MAP and systemic vascular resistance (SVR), decreases in CI, and minimal change in HR. The relationships between the hemodynamic effects and plasma L-NA levels were not monotonic, and hysteresis was observed. Using nonparametric analysis, the equilibration half-time (t1/2,keo) between plasma L-NA and the hypothetical effect site was determined to be 51.5 +/- 6.6 min, 42.4 +/- 10.1 min, 43.4 +/- 9.0 min for MAP, CI, and SVR, respectively (n = 14). The Emax and EC50 values obtained were + 32.5 +/- 8.4 and 2.6 +/- 1.3 microg/ml for MAP and -52.9 +/- 15.6 and 3.7 +/- 1.8 microg/ml for CI, respectively. CONCLUSIONS: Although L-NA can bring about beneficial elevation of MAP, such effect is always accompanied by a stronger effect on CI depression. Dose escalation of L-NA may bring about detrimental negative inotropic effect and loss of therapeutic efficacy.

Anesthesia↗
PubMed1998

Contribution of vascular tissue to the antiplatelet activity of sodium nitroprusside.

We tested whether or not platelet inhibition by sodium nitroprusside (SNP) was enhanced by vascular tissue production of nitric oxide (NO) and calcitonin gene-related peptide (CGRP) release. Platelet aggregation was determined with whole blood impedance aggregometry after incubations of SNP in the presence or absence of rat aortic tissue (AT) or AT + CGRPS(8-37) (a specific CGRP antagonist). SNP alone had no effect on platelet aggregation until 100 microM was used (2.3 + 1.5 omega vs. control aggregation of 9.9 +/- 2.0 omega; p < 0.001). Co-incubation of AT with SNP significantly enhanced platelet inhibition at 1 (1.6 +/- 1.3 omega; p < 0.001), 10 (0.7 +/- 0.4 omega; p < 0.001), and 100 microM (0.3 +/- 0.3 omega; p < 0.001). CGRP(8-37) did not significantly antagonize aggregation by SNP + AT (p > 0.05). The inhibition of platelet aggregation by 10 microM SNP was inhibited by methylene blue (MB) (9.0 +/- 1.7 omega at 10 microM; 11.7 +/- 2.4 omega at 100 microM; p < 0.001) but not by 30 microM L-N(upsilon)-monomethyl-L-arginine (L-NMMA; 2.9 +/- 1.8 omega; p > 0.05). These results indicate that vascular tissue significantly contributes to the ability of SNP to inhibit platelet aggregation, probably through greater vascular enzymatic production of NO, but not by releasing CGRP, in contrast to nitroglycerin.

Animals↗
PubMed1997

Application of pharmacodynamic modeling for designing time-variant dosing regimens to overcome nitroglycerin tolerance in experimental heart failure.

PURPOSE: Prolonged continuous administration of nitroglycerin (NTG) leads to hemodynamic tolerance. We used a previously developed pharmacokinetic-pharmacodynamic (PK/PD) model of NTG tolerance in experimental heart failure to test whether dosage regimens, designed from this model, may allow avoidance of tolerance development upon continuous NTG infusion. METHODS: Simulation experiments (using ADAPT II) were performed to evolve a time-variant infusion regimen that would theoretically provide sustained hemodynamic effect (30% reduction in left ventricular end-diastolic pressure, LVEDP) throughout 10 hours of drug dosing. A computer controlled infusion pump was utilized to deliver this time-variant input. Infusion experiments were then conducted in CHF rats to challenge the predictability of the applied PK/PD model. RESULTS: Simulations showed that exponentially increasing input functions provided more sustained LVEDP effects when compared to linear or hyperbolic input functions delivering the same total NTG dose. A computer-selected infusion regimen of 6.56e0.00156 x minutes micrograms/min was anticipated to provide the desired hemodynamic profile in our animal model. Experiments conducted in rats with congestive heart failure (n = 4) confirmed the prediction of sustained hemodynamic effect without tolerance (28 +/- 4% reduction in LVEDP at 10 hrs). CONCLUSIONS: These findings support the utility of our PK/PD model of NTG tolerance in predicting NTG action, and serve as an example of therapeutic optimization through PK/PD considerations.

Animals↗
PubMed1997

Nitroglycerin-inhibited whole blood aggregation is partially mediated by calcitonin gene-related peptide -- a neurogenic mechanism.

1. The role of the vasculature and calcitonin gene-related peptide (CGRP) in nitroglycerin (NTG)-mediated platelet inhibition was studied. 2. In vitro incubations of CGRP in whole blood induced a dose-dependent inhibition of platelet aggregation with an IC50 of 62.1 nM. 3. The platelet inhibition induced by CGRP was blocked by co-incubation of 0.53 microM CGRP8-37, as well as 30 microM N(G)-nitro-monomethyl-L-arginine (L-NMMA). 4. In a separate group of experiments, 100 nM NTG in rat whole blood (WB) induced platelet inhibition of 6.0 +/- 1.3% (mean +/- s.d.), which was enhanced to 77.6+/-3.5% by the addition of rat aortic tissue (AT) (P<0.001). The inclusion of CGRP8-37 with NTG and AT in WB reduced platelet inhibition to 31.6+6.8% (P<0.01). Incubation of WB and AT with 30 microM L-NMMA reduced NTG-induced inhibition of platelet aggregation to 26.4+/-4.2% (P<0.001). 5. It is concluded that vascular tissue contributes to the antiplatelet mechanism of action of NTG. Furthermore, NTG apparently evokes the release of CGRP from vascular tissue and this neuropeptide contributes to the antiplatelet actions of NTG. 6. The antiplatelet activity of CGRP in whole blood is mediated primarily through the activation of nitric oxide synthase.

Adenosine Diphosphate↗
PubMed1997

Vascular and hemodynamic differences between organic nitrates and nitrites.

Because nitroglycerin (NTG, an organic nitrate) and isoamyl nitrite have similar chemical structures and a common mechanism of vascular relaxation (i.e., conversion to nitric oxide in vascular tissues and activation of guanylyl cyclase), it has often been assumed that organic nitrates and nitrites have identical pharmacologic actions. Because recent studies have shown that the vascular enzymes responsible for nitric oxide generation from organic nitrates and nitrites are distinct, we hypothesized that the in vitro vascular actions, in vivo hemodynamic effects and tolerance properties (both in vitro and in vivo) would be different as well. Isolated blood vessel studies showed that NTG provided more stable relaxation effects than ISAN, was more potent and caused greater in vitro vascular tolerance. Because the mechanism(s) of vascular tolerance in vitro may not be the same as those occurring in vivo, we also compared the left ventricular hemodynamic effects and tolerance properties of NTG vs. isoamyl nitrite and in congestive heart failure rats. Constant NTG infusion (10 micrograms/min) caused initial reductions in left ventricular end-diastolic pressure of 45 to 55%, which returned to baseline within 10 hr (tolerance development). In contrast, isobutyl nitrite and isoamyl nitrite (45 micrograms/min) caused initial reductions in left ventricular end-diastolic pressure similar to NTG (42-58%), but these hemodynamic effects of organic nitrites were maintained even when infusions were carried out to 22 hr. These results show that organic nitrites and organic nitrates are not pharmacologically identical (in vitro or in vivo), and may suggest a therapeutic advantage for organic nitrites in the treatment of some cardiovascular diseases.

Amyl Nitrite↗
PubMed1996

Specific binding of nitroglycerin to coronary artery microsomes. Evidence of a vascular nitrate binding site.

The vasodilating action of organic nitrates, such as nitroglycerin (NTG), is thought to be mediated through metabolic conversion to nitric oxide (NO) in vascular smooth muscle. Although the pertinent enzyme(s) that carries out this crucial step has not been identified, previous studies have shown that the primary enzymatic site is located within cellular membrane fractions. In these studies, we examined the binding of [14C]NTG to microsomal fractions from bovine coronary arteries. Specific binding was linearly related to protein concentration, and binding equilibrium was reversible, reached equilibrium within 1 hr, and remained stable for 4 hr at 25 degrees. Competition experiments with unlabeled NTG demonstrated the presence of two binding sites of differing affinities (high-affinity site: Bmax 24.1 +/- 0.9 pmol/mg protein, Kd 554 +/- 22 pM; low-affinity site: Bmax 79.0 +/- 2.9 pmol/mg protein, Kd 151 +/- 3 microM). Both of the thiol alkylators 1-chloro-2,4-dinitrobenzene and N-ethylmaleimide were found to inhibit [14C]NTG binding, as well as enzymatic generation of NO from NTG, in a concentration-dependent manner. Competition of [14C]NTG was also observed with five other organic nitrate vasodilators, and the degree of competition was linearly related to the in vitro vaso-relaxing potencies of these agents. Parallel experiments also showed that in the absence of thiol cofactor, the enzymatic production of NO from NTG was antagonized competitively by less potent organic nitrates. Intact blood vessel experiments using rat aorta also showed that the presence of isosorbide dinitrate (ISDN), at concentrations that did not induce relaxation alone, caused a slight but significant shift in the relaxation potency of NTG (EC25 9 +/- 2 versus 28 +/- 7 nM, in the presence and absence of 0.3 microM ISDN, respectively; P < 0.05). These results demonstrate the presence of specific binding of organic nitrates to microsomal proteins in vascular smooth muscle, and the observed binding is apparently related to enzymatic conversion to NO and the vasodilating properties of these compounds.

Animals↗
PubMed1996

Reversed-phase high-performance liquid chromatography method for the analysis of nitro-arginine in rat plasma and urine.

Nitro-L-arginine (L-NNA) is an inhibitor of the enzyme nitric oxide synthase (NOS). We developed a simple, sensitive and reproducible reversed-phase high-performance liquid chromatographic method for detection of nitro-arginine (L- and D-enantiomer) in rat plasma and urine. Samples were treated with perchloric acid, neutralized and eluted through a C8 reversed-phase column with a mobile phase of 18.5 mM heptanesulfonic acid-10% methanol in water, using theophylline as an internal standard. Plasma recovery for both isomers was complete, and the sensitivity limit was 0.5 micrograms/ml. This method may be used for disposition studies of L-NNA in small animals.

Animals↗
PubMed1996

Pharmacokinetic and pharmacodynamic optimization of intraperitoneal chemotherapy.

Intraperitoneal drug administration has been utilized to increase chemotherapeutic exposure to tumors confined to the peritoneal cavity, such as those found in ovarian, gastric, colo-rectal and mesotheliomal cancers. Extensive pre-clinical and clinical experimentation has been conducted to assess the pharmacokinetic and therapeutic benefits of this mode of therapy. Pharmacokinetic studies have shown that the barrier function of the peritoneal membrane may be utilized to produce large, favorable concentration gradients between peritoneal perfusate and blood. However, most clinical studies so far have demonstrated minimal increases in drug efficacy or decreases in drug toxicities from intraperitoneal drug therapy alone. This paper reviews the application of adjunctive therapies that have been rationally conceived to optimize intraperitoneal drug therapy through the alteration of antineoplastic pharmacokinetics and pharmacodynamics.

Antineoplastic Agents↗
PubMed1996

Inverse targeting of peritoneal tumors: selective alteration of the disposition of methotrexate through the use of anti-methotrexate antibodies and antibody fragments.

We have hypothesized that antidrug antibodies (ADAb) may be employed to impart site-specific alterations in the disposition of drug molecules, potentially allowing for targeted drug therapy. We are specifically interested in minimizing systemic exposure to free drug and systemic toxicities resultant from regional chemotherapy through the intravenous administration of ADAb. In this report, we present the production and purification of anti-methotrexate Fab fragments, and we present investigations of the effects of anti-methotrexate Fab and anti-methotrexate immunoglobulin G on the disposition of methotrexate in the rat. Pharmacokinetic studies revealed that intravenous anti-methotrexate immunoglobulin G (anti-MTX IgG) and anti-methotrexate Fab (anti-MTX Fab) administration produced dramatic alterations in the plasma pharmacokinetics of methotrexate (MTX), following intraperitoneal MTX administration (area under the total MTX concentration vs time curve for anti-MTX IgG relative to control, 420 +/- 90 (p < 0.05); for anti-MTX Fab relative to control, 46 +/- 6.1 (p < 0.05); area under the free MTX concentration vs time curve for anti-MTX IgG relative to control, 0.64 +/- 0.16; for anti-MTX Fab relative to control, 0.45 +/- 0.20 (p < 0.05)). Additional studies conducted in anesthetized rats revealed no significant alterations in the area under the total peritoneal MTX concentration vs time curves, free MTX peritoneal concentration vs time curves, or peritoneal exit rate of MTX in anti-MTX Fab treated animals relative to controls. Therefore, our pharmacokinetic studies demonstrate that ADAb may produce site-specific alterations in drug pharmacokinetics, potentially enhancing the site specificity of drug distribution and drug action following regional chemotherapy.

Animals↗
PubMed1996

Arterial versus venous metabolism of nitroglycerin to nitric oxide: a possible explanation of organic nitrate venoselectivity.

Nitroglycerin (NTG) and other organic nitrates are predominant venodilators in vivo and in vitro. This selectivity is an important determinant of their ability to relieve angina and congestive heart failure symptoms, but the mechanism of this phenomenon is unknown. Because organic nitrate vasodilation occurs through metabolism to nitric oxide (NO), we tested the hypothesis that their venoselectivity is related to an enrichment of the pertinent enzyme in venous tissue. Enzymatic conversion of NTG to NO was examined in microsomal fractions from bovine aorta as compared with vena cava tissues. NTG (150, 450, or 900 microM) was incubated with 1 mg microsomal protein and glutathione (13 microM), and cumulative NO production was measured for 5 h. When enzyme velocities were normalized to microsomal protein, statistical significance was not observed between fractions from aorta and those from vena cava. However, when enzyme activity was normalized to tissue weight or total homogenate protein, statistically higher activity was observed in the venous tissue (p < 0.05). These differences were greatest (two- to three-fold higher in vena cava at all three NTG concentrations, p < 0.01) when enzyme velocity was normalized to the initial cellular content of the homogenates (i.e., homogenate DNA concentrations). These results suggest that organic nitrate venoselectivity may be at least partly explained by enrichment of the bioactivating enzyme in venous smooth muscle cells.

Animals↗
PubMed1996

Sustained antiplatelet properties of nitroglycerin during hemodynamic tolerance in rats.

Organic nitrates possess important antiplatelet actions that are useful in the treatment of unstable angina and myocardial infarction, but the susceptibility of platelets to nitrate tolerance has not been extensively studied. In normal conscious rats, we showed that continuous infusion of nitroglycerin (NTG) at 10 micrograms/min had no significant effect on mean arterial pressure (MAP) as compared with control, but hemodynamic tolerance could be demonstrated by MAP response to a bolus intravenous (i.v.) NTG challenge. By this criterion, continuous 8-h NTG infusion produced hemodynamic tolerance (a decrease in MAP response of 45.7 +/- 19.9%, p < 0.05), whereas D5W control and S-nitroso-N-acetylpenicillamine (SNAP) infusions did not. During NTG infusion, platelet-rich plasma (PRP) cyclic GMP was increased by 41.4 +/- 13.6% as compared with control and remained increased throughout the infusion (p < 0.05). Bleeding time during a 2-h infusion of NTG was 8.9 +/- 1.2 min as compared to 3.8 +/- 0.4 min in controls (p < 0.05). After 8-h of NTG infusion, the bleeding time was 10.2 +/- 1.4 min versus 4.4 +/- 0.4 min in controls (p < 0.05). NTG also decreased the PRP platelet concentration by 30% in 8 h, whereas D5W had no effect. In vitro experiments showed that platelets in themselves do not produce significant amounts of cyclic GMP. These data indicate that the biochemical and antiaggregation effects of NTG on platelets are not diminished during hemodynamic tolerance and that these effects may be dependent on extraplatelet production of nitric oxide (NO).

Animals↗
PubMed1996

Pharmacokinetics and steady-state tissue distribution of L- and D-isomers of nitroarginine in rats.

Nitric oxide synthase (NOS) inhibitors, such as nitro-L-arginine (L-NNA), have been used in vivo as mechanistic probes of the NOS system and as potential therapeutic agents for reversing the hypotension developed in septic shock. Little information is available regarding the pharmacokinetic and biodistribution pattern of these compounds. We have examined the in vivo disposition, as well as steady-state biodistribution, of NNA isomers in rats. Plasma and tissue concentrations of L-NNA were determined by HPLC. After intravenous administration of a bolus dose of 20 mg/kg in rats, plasma concentrations of both L- and D-NNA declined biexponentially, with average half-lives of 12 min and 20 hr for L-NNA, and 15 min and 15 hr for the D-enantiomer, respectively. In contrast to L-NNA, the D-isomer had a higher systemic clearance (170 +/- 20 vs. 70.9 +/- 8.2 ml/hr/kg; p = 0.0004) and shorter mean residence time (17.3 +/- 3.7 vs. 31.7 +/- 3.7 hr; p = 0.0039). Based on these pharmacokinetic characteristics, steady-state plasma concentrations of NNA isomers were achieved within 6-8 hr through the use of a loading dose and maintenance infusion. NNA concentrations achieved in many tissues exceeded plasma concentrations, indicating binding of the drug to tissue components. The tissue-to-plasma distribution coefficient (Kp) for L-NNA was variable among various tissues and ranged from 0.67 for testes to 3.17 for liver. Both kidneys and skeletal muscle had Kp values higher than 2, whereas distribution to the cerebrospinal fluid was minimal (Kp = 0.09). Steady-state distribution of the D-isomer of NNA was similar to that of L-NNA.

Animals↗
PubMed1995

High-affinity rabbit antibodies directed against methotrexate: production, purification, characterization, and pharmacokinetics in the rat.

We describe the production, purification, characterization, and disposition of rabbit polyclonal methotrexate antibodies. These antibodies are prepared for subsequent testing of a drug delivery approach to reduce systemic toxicity upon regional administration of methotrexate. The polyclonal antibodies were raised in New Zealand white female rabbits immunized with a methotrexate-bovine serum albumin conjugate. The anti-methotrexate antibodies were sequestered from rabbit serum through the use of a protein-G affinity column which allowed for purification of up to 100 mg of rabbit IgG in 30-40 min. The extent of purification was demonstrated through SDS-PAGE and calculation of specific binding activity relative to total protein concentration. The purified antibodies have been shown to have high affinity (Keq range: 1.8 x 10(8) to 8.75 x 10(9) M-1) and high selectivity for methotrexate. Preliminary pharmacokinetic studies of the purified antibodies in the rat following a 6 mg/kg intravenous infusion (n = 4) indicate a steady state volume of distribution of 38.0 +/- 11.2 mL kg-1, a systemic clearance of 0.92 +/- 0.67 mL kg-1 h-1 and an elimination half life of 28.9 +/- 7.9 h (mean +/- SD).

Animals↗
PubMed1995

Nitric oxide donors: biochemical pharmacology and therapeutics.

The NO donors are a diverse group of agents with unique chemical structures and biochemical requirements for generation of NO. The differences in biochemistry and metabolism may, in turn, cause differences in their pharmacology and therapeutic actions. A thorough understanding of the biochemical pharmacology of NO donors and factors controlling their therapeutic activity would facilitate the optimal use of these agents as chemical carriers of NO, and the development of newer agents than can selectively modulate the many physiological actions of NO.

Animals↗
PubMed1994

Mechanisms of nitrate tolerance.

There is now little dispute that clinical tolerance of organic nitrates occurs, particularly when these drugs are used by themselves to treat patients with stable angina pectoris and congestive heart failure. Classical hypotheses of nitrate tolerance suggest the phenomenon to result from vascular depletion of critical sulfhydryl groups, which are necessary to bring about vasorelaxation from nitrates. While this mechanism of nitrate tolerance probably operates when isolated blood vessels are exposed to high concentrations of nitrate in vitro, there is little evidence to suggest that it contributes to clinical nitrate tolerance. Instead, emerging data suggest that nitrates can cause significant shifts in fluid distribution and secretion of neurohormonal factors that can modulate their vasorelaxant effects. Use of angiotensin converting enzyme inhibitors and diuretics in conjunction with nitrates may alleviate the development of tolerance, but the experience has not been universally favorable. Other receptor-effector systems that affect cardiovascular function, such as the adrenergic system, may also be affected by nitrate tolerance. The mechanisms of nitrate tolerance are therefore likely to be multifactorial, involving vascular biochemical changes, physiologic compensation, and possibly receptor regulation.

Angina Pectoris↗
PubMed1994

Photochemical generation of nitric oxide from nitro-containing compounds: possible relation to vascular photorelaxation phenomena.

We examined the production of nitric oxide (NO) from various nitro-containing compounds (500 microM and 100 microM solutions in Krebs buffer, pH 7.4). Sealed vials containing solutions of NaNO2, N-nitro-L-arginine, 4-nitrophenol, BAY K 8644, or N-nitro-L-arginine methyl ester were stored in the dark, under normal room light, or were exposed to ultraviolet light (365 nm), for 30 min (24 degrees C). NO was measured in the vial headspace after 30 min, using a sensitive assay previously established in our laboratory. Production of NO was found to be dependent on the intensity of light exposure for all compounds, and the highest degree of light-induced production of NO was found for NaNO2 and BAY K 8644 solutions. Since NO is a relaxant of smooth muscle, these results help explain the increased sensitivity to relaxation by UV light of vascular and other types of smooth muscle in the presence of NaNO2, BAY K 8644 and N-nitro-L-arginine, as observed by other investigators.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗