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Biomedical subjects

R R Burnette

Publications and source records attributed to R R Burnette.

15 recordsLinked to original sources
PubMed2000

Statistical properties of thermodynamic quantities for cyclodextrin complex formation.

Literature values of DeltaG degrees (change in Gibbs free energy), DeltaH degrees (change in enthalpy), and TDeltaS degrees (temperature times change in entropy) for 1:1 complex formation by alpha-, beta-, and gamma-cyclodextrins constitute normally distributed populations with the following statistical parameters (all energy quantities in kcal mol(-1); n is the number of data points; mu is the population mean; sigma is the standard deviation): for alpha-cyclodextrin, n = 512, micro(DeltaG) = -2.85, sigma(DeltaG) = 1.23, micro(DeltaH) = -4.77, sigma(DeltaH) = 2.98, micro(TDeltaS) = -1.96, and sigma(TDeltaS) = 2.72; for beta-cyclodextrin, n = 415, micro(DeltaG) = -3.67, sigma(DeltaG) = 1. 37, micro(DeltaH) = -4.24, sigma(DeltaH) = 2.89, micro(DeltaS) = -0. 56, and sigma(TDeltaS) = 2.63; for gamma-cyclodextrin, n = 42, micro(DeltaG) = -3.71, sigma(DeltaG) = 1.19, micro(DeltaH) = -3.10, sigma(DeltaH) = 3.39, micro(TDeltaS) = +0.69, and sigma(TDeltaS) = 3. 29. The temperature is 298.15 K. The mean DeltaG degrees values correspond to binding constants of 123, 490, and 525 M(-1) for alpha-, beta-, and gamma-cyclodextrins, respectively.

Cyclodextrins↗
PubMed1997

Characterization of the effects of amphotericin B on ion channels in MDCK cells using the patch-clamp technique.

Cultured Madin-Darby Canine Kidney cells were used as a model to study the mechanism of nephrotoxicity of amphotericin B using the patch-clamp technique. At the whole-cell level, amphotericin B altered potassium conductances in two types of these cells categorized on the basis of whole-cell potassium currents. The first cell type, classified as Type I, exhibited no significant whole-cell potassium currents. The second type, Type II, exhibited depolarization-induced outward potassium currents that rundown over time. In both of these subpopulations, exposure to amphotericin B at a concentration of 68 nM for a prolonged period of time (approximately 30-45 min) led to an increased whole-cell potassium conductance. In Type I cells, it increased by a factor of 16 and in Type II cells, by a factor of 3.5. Furthermore, the potassium currents observed in Type I cells following amphotericin B treatment bore no resemblance to currents through pores formed by amphotericin B in artificial membranes. At the single-channel level, incubation with amphotericin B led to a significantly higher potassium channel activity in both inside-out and outside-out patches. Kinetic studies in inside-out patches revealed that the increases in channel activity were associated with a decrease in the mean closed time and an overall increase in the mean open time. In summary, our data suggest that the direct toxicity of amphotericin B is primarily related to its ability to disturb normal ion channel functioning rather than to formation of pores in cell membranes.

Amphotericin B↗
PubMed1996

Computer simulation of human blood flow and vascular resistance.

Organ blood flows and associated vascular resistances have been investigated through the use of a Microsoft Windows-compatible computer program which employs Monte Carlo simulations based on the system's principal components. This approach replicates the system's behavior by maintaining proper correlations between all variables in the system as well as allowing for modulation of the system by its inherent uncertainties. By applying various external constraints, such as a specific age, weight, height and/or blood pressure, the simulations allow for insights to be obtained about the behavior of individual patients. In particular, patient specific organ blood flows and associated vascular resistances can be determined as a function of a patient's age, weight, height and blood pressure.

Adult↗
PubMed1993

The effect of amphotericin B on the K-channel activity of MDCK cells.

By using the whole-cell patch technique, it is shown that the total outward current is increased, as a function of time, after the addition of amphotericin B to the bathing solution. The whole-cell current is shown to be primarily a K-channel current by the blockage of this current upon application of TEA to the bathing solution. Single K-channel studies, using the outside-out patch-clamp technique, reveal that the single K-channel opening probability increases by a factor of six after the addition of amphotericin B. In addition, single K-channel voltage dependent studies, using the inside-out patch-clamp technique, demonstrate that this increase in opening probability is due to an increase in the amplitude of Po(v). In contrast to the present belief that amphotericin B simply creates pores in a cell's membrane, these results suggest that amphotericin B can also influence the function of the cell's K-channel proteins.

Amphotericin B↗
PubMed1992

Fundamental pharmacokinetic limits on the utility of using a sinusoidal drug delivery system to enhance therapy.

Clinically, it is known that some disease states respond to drug treatment in a cyclic manner. This has resulted in qualitatively, or empirically, determined cyclically varying drug treatment studies which have been shown to improve therapeutic response in some cases. A theory is developed, for drugs that can be described by pure catenary pharmacokinetic models, which enables one to quantitatively determine at what time a cyclic infusion of drug should be initiated, what the frequency of infusion should be, and what the amplitude of the infusion should be to obtain maximum therapeutic benefit at steady state. Also, the theory allows one to determine quantitatively a priori if a drug's pharmacokinetics precludes the possibility of any real advantage to be gained by cyclically infusing the drug. To implement the theory, it is assumed that the drug obeys linear pharmacokinetics and that the desired pharmacological response is rapid and approximately proportional to a pharmacokinetic compartmental concentration. In particular, a linear system analysis approach is applied to drugs obeying linear pharmacokinetics. It is found that at steady state the amplitude of the sinusoidally varying component of drug's compartmental concentration can be expressed as the amplitude of the rate of infusion times the magnitude of the compartment's transfer function. In addition, an expression for the shift in phase (lag time) of the compartmental drug concentration, relative to the input infusion, is obtained. For a one-compartment model, or for a compartment containing the site of infusion, the amplitude of the sinusoidally varying component ultimately declines in direct proportion to the period (T) of oscillation and the lag time increases from 0 to -0.25T as the period decreases. At a short enough cyclic infusion period, the lag time increments by an additional value of -0.25T, and the attenuation in sinusoidal amplitude decreases by an additional factor of T, for each compartment sequentially connected down the chain from the compartment receiving the infusion. This theory is then applied to the drugs, 5-fluorouracil, KS1/4-DAVLB, theophylline, and adriamycin to see if sinusoidal modulation of the infusion rate would be of therapeutic benefit. The theoretical predictions are then compared to clinically determined empirical results and shown to be consistent. In general, it is shown that the micro rate constants describing the drug's pharmacokinetics must be large (i.e., the system must be able to respond rapidly) for sinusoidal infusion to be of value.

Animals↗
PubMed1991

Evidence that oleic acid exists in a separate phase within stratum corneum lipids.

Oleic acid is known to be a penetration enhancer for polar to moderately polar molecules. A mechanism related to lipid phase separation has been previously proposed by this laboratory to explain the increases in skin transport. In the studies presented here, Fourier transform infrared spectroscopy (FT-IR) was utilized to investigate whether or not oleic acid exists in a separate phase within stratum corneum (SC) lipids. Per-deuterated oleic acid was employed allowing the conformational phase behavior of the exogenously added fatty acid and the endogenous SC lipids to be monitored independently of each other. The results indicated that oleic acid exerts a significant effect on the SC lipids, lowering the lipid transition temperature (Tm) in addition to increasing the conformational freedom or flexibility of the endogenous lipid alkyl chains above their Tm. At temperatures lower than Tm, however, oleic acid did not significantly change the chain disorder of the SC lipids. Similar results were obtained with lipids isolated from the SC by chloroform:methanol extraction. Oleic acid, itself, was almost fully disordered at temperatures both above and below the endogenous lipid Tm in the intact SC and extracted lipid samples. This finding suggested that oleic acid does exist as a liquid within the SC lipids. The coexistence of fluid oleic acid and ordered SC lipids, at physiological temperatures, is consistent with the previously proposed phase-separation transport mechanism for enhanced diffusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Effect of dipolar aprotic permeability enhancers on the basal stratum corneum.

The effect of dimethyl sulfoxide, dimethyl formamide, and dimethyl acetamide on the basal stratum corneum of excised nude mouse skin was investigated. All of these dipolar aprotic solvents caused a swelling of the basal stratum corneum cells and a disruption of the normal keratin pattern. This behavior suggests that dipolar aprotic solvents might alter the barrier properties of the basal stratum corneum cells. To test this hypothesis, the distribution of topically applied, electron-dense divalent metal ions (Hg2+ and Ni2+) was studied in excised nude mouse skin which had been perturbed by the application of dipolar aprotic solvents, and in controls which had not been so treated. In control skin membranes, Hg2+ and Ni2+ were located almost exclusively in the intercellular space of the stratum corneum. However, with the application of a dipolar aprotic solvent, Hg2+ and Ni2+ were found in the intercellular spaces and inside the basal stratum corneum cells, where they appeared to be primarily associated with the cytoplasmic filaments. Sulfide precipitation allowed for the localization of Hg2+ and Ni2+, and subsequent chemical identification by energy-dispersive X-ray microanalysis. The spatial resolution of X-ray microanalysis studies was approximately 0.5-0.75 micron. The spatial alteration in mercury and nickel precipitate distribution, which occurs when the skin is pretreated with a dipolar aprotic solvent, is consistent with the hypothesis that the pathway of Hg2+ and Ni2+ diffusion through the basal stratum corneum has also been modified.

Acetamides↗
PubMed1988

Characterization of the pore transport properties and tissue alteration of excised human skin during iontophoresis.

Pores through which charged carriers move during iontophoresis were demonstrated by the use of the cathodic (-) iontophoretic transport of fluorescein from the epidermis to the dermis. Both dermatomed (0.8-mm) human cadaver skin and full-thickness female human breast skin were investigated. The density of pores, as visualized by fluorescein transport, was approximately 2-5 cm-2. A set of microelectrodes rastered across the visualized pore gave a maximal response when directly above the pore, demonstrating that the pore was a locus of charge transport. Fluorescein was also sometimes observed at the diffusion cell-tissue interface. This indicates that edge damage had occurred as the result of clamping the tissue in a diffusion cell. Studies were conducted to determine if tissue damage occurred during iontophoretic transport. The electrical resistance across excised skin was measured at 0.2 Hz and found to decrease initially by approximately an order of magnitude after the application of an iontophoretic current of 0.16 mA/cm2 for 1 h. The electrical resistance then increased, reaching a plateau value which was lower than the original tissue resistance before application of an iontophoretic current. Controls were carried out to demonstrate that the observed electrical resistance changes were not just due to tissue hydration effects. These results imply that the passage of current through excised human skin at clinically acceptable current densities can lead to tissue damage which is not fully reversible.

Adult↗
PubMed1988

Influence of constant current iontophoresis on the impedance and passive Na+ permeability of excised nude mouse skin.

The impedance of excised nude mouse skin was determined over the frequency range 0.2-2500 Hz. Impedance versus frequency plots were obtained for skin which had undergone progressive hydration over a period of 8 h, and for similarly hydrated skin which, during the hydration period, was also exposed to a current density of 0.16 mA cm-2 for 1 h. The parallel frequency-dependent skin resistance and capacitance were then calculated from the impedance data. The skin resistance, at 0.2 Hz, for skin exposed to the iontophoretic current decreased by a factor of approximately 5 compared with that observed for skin which had undergone hydration, implying that the current had altered the ion conducting pathways of the skin. In addition, the Na+ flux was measured and shown to be linearly correlated (r = 0.99) with the inverse of the impedance of the skin at 0.2 Hz. This implies that the low frequency impedance of the skin is a measure of the passive ion permeability of the skin, and that this technique can be used as a noninvasive way to assess the relative effects of different types of iontophoretic current on the skin.

Animals↗
PubMed1987

Characterization of the permselective properties of excised human skin during iontophoresis.

The iontophoretic and passive transport of [3H]mannitol, 22Na+, 36Cl-, and 45Ca++ across excised human cadaver skin was studied using diffusion cells. The anode (+) was placed in the side of the diffusion cell facing the epidermis and the cathode (-) was placed in the side facing the dermis, and current densities at 0, 0.078, 0.16, and 0.23 mA.cm-2 were investigated. The results showed that mannitol and Na+ were transported preferentially by anodal (+) iontophoresis, Cl- was transported by cathodal (-) iontophoresis, and all respective fluxes were approximately proportional to the applied current density. When the skin was present as a membrane barrier between the two diffusion cell chambers the voltage induced flux of Na+ was found to be higher than its free solution value, and that of Cl- was lower. Taken together these results suggest that the skin is a permselective membrane and exists with an "apparent" net negative charge at the free solution pH of 7.4. During iontophoresis this permselectivity leads to current-induced volume flow, which provides a primary mechanism for the transport for a polar uncharged molecule such as mannitol. When Ca++ is substituted for Na+ on the side of the diffusion cell facing the epidermis, the Cl- flux from the dermal side is enhanced with a portion of the remaining charge being carried by Ca++. The mannitol flux from the epidermal side was decreased under these conditions. This implies that Ca++ alters the anion/cation flux ratio in the excised tissue, possibly by binding to fixed negative charges in the membrane, with the result that the volume flow is decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Comparison between the iontophoretic and passive transport of thyrotropin releasing hormone across excised nude mouse skin.

Thyrotropin releasing hormone [L-proglutamyl-L-histidyl-L-proline amide (TRH)], a tripeptide with molecular weight of 362 and a pKa of 6.2, was used as a model peptide for in vitro passive and iontophoretic diffusion cell studies using excised dorsal nude mouse skin. The results indicate that both the charged and uncharged TRH fluxes across the excised tissue were greater than those obtained by passive diffusion alone. The steady-state flux of both the uncharged and charged TRH was directly proportional to the applied current density, with flux being greater for the uncharged TRH. Additional studies on the transport of methylene blue indicate that transport may be occurring through pores, and that positive ions are preferentially passed through the skin. These results imply that the steady-state flux of TRH is primarily due to a direct, electrically induced ion motion and convection. A practical implication of these results is that it may be possible to enhance and control the transdermal delivery of peptides.

Animals↗
PubMed1985

Synthesis and evaluation of a prototypal artificial red cell.

A new process allows microencapsulation of purified human hemoglobin and 2,3-diphosphoglycerate to form neohemocytes. The microcapsule membrane is composed of phospholipids and cholesterol. Neohemocytes are substantially smaller than erythrocytes, contain 15.1 grams per decaliter of hemoglobin, and have a P50 value (the partial pressure of oxygen at which the hemoglobin is half-saturated) of 24.0 torr. All rats given 50-percent exchange transfusions survived with only limited evidence of reversible toxicity. Normal serum glutamate-pyruvate-transaminase values at 1, 7, and 30 days after transfusion were consistent with minimal hepatotoxicity. The concentration of blood urea-nitrogen was elevated by 35 percent after 1 day but returned to normal by day 7. However, histopathology revealed normal kidneys on day 1 as well as on days 7 and 30. Neohemocytes cleared from the circulation of transfused rats with an apparent half-life of 5.8 hours.

Alanine Transaminase↗
PubMed1983

Neohemocytes.

The neohemocytes described in this report average approximately 0.4 microns in diameter. The microcapsule membrane is composed of biodegradable lipids, including phospholipids, and has a structure similar to liposomes. In a suspension having an apparent hematocrit of 0.5, the neohemocytes occupy 50% of the volume, the neohemocyte membrane accounts for approximately 2%, and the hemoglobin suspension accounts for about 48% of the volume. The encapsulated hemoglobin suspension averages 15.8g%. The P50 averages 26, the Hill Number averages 2.1, and methemoglobin is typically less than 5%. Transfusions in rats of neohemocyte suspensions, where the RBC hematocrit is lowered below 0.03, consistently give a fivefold-or-better increase in survival time relative to transfusions of equal concentrations of unencapsulated hemoglobin. Many of the problems of hemoglobin microencapsulation have been overcome, and the results strongly indicate that neohemocytes may become the functional component of a resuscitative fluid for use in man.

Animals↗
PubMed1980

Spectroscopic analysis of the equilibrium and kinetic DNA binding properties of several actinomycin analogs.

Experiments are described that measure DNA dissociation kinetics and thermal denaturation temperatures for a series of actinomycin analogs containing, in the 3' amino acid position, pipecolic acid, proline or azetidine-2-carboxylic acid. Also included are studies on actinomycin C3. Analysis of the temperature dependence of the slowest rate constant for DNA dissociation shows that both the enthalpy and entropy of ativation increase as the ring size of the 3' amino acid decreases from six to five to four. All compounds increase the DNA melting temperature to the same extent except for the analog containing pipecolic acid, which shows a smaller effect. These results are discussed in terms of a possible role for conformational changes in the actinomycin pentapeptide lactone rings in determining the slow DNA dissociations rates for this class of intercalators. It is suggested that cis-trans isomerization of proline may be important in this regard.

Binding Sites↗