Biomedical subjects

N G Turner

Publications and source records attributed to N G Turner.

5 recordsLinked to original sources

PubMed1998

Determination of the pH gradient across the stratum corneum.

The objective of this work was to determine the pH gradient profile across hairless mouse stratum corneum (SC) using ratiometric laser scanning confocal microscopy imaging and direct pH measurement with a flat surface electrode. Dual-emission ratiometric imaging used the fluorophore, carboxy seminaphthorhodafluor-1, which displays a pH-dependent shift in its emission spectrum. The assay developed was unsuccessful, however, because (i) the pKa of seminaphthorhodafluor (approximately 7.5) makes it insufficiently sensitive to the normal pH gradient (4-7.4) across mammalian SC, and (ii) the unexpectedly high buffering capacity of the skin precluded a meaningful calibration of the system. In the second method, pH measurements with a flat surface electrode were recorded as the hairless mouse SC was progressively tape stripped. In the superficial SC, the pH remained relatively constant (approximately 5.9); further removal of the barrier resulted in a steady increase in pH to approximately 7. In conclusion, improved methodology is clearly required to characterize precisely the pH profile across the SC.

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PubMed1998

Visualization and quantitation of iontophoretic pathways using confocal microscopy.

Laser scanning confocal microscopy (LSCM) has been used to visualize and quantitate the penetration of a model, anionic, fluorescent compound (calcein) along the iontophoretic transport pathways within hairless mouse skin. The LSCM technique permits optical sectioning of full-thickness, unfixed tissue, thereby avoiding poor image quality due to blurring from out-of-focus fluorescence, and obviating artifactual redistribution of the permeant during processing. Simple measurements of the approximate flux of the probe across hairless mouse skin were also made using standard in vitro diffusion cell methodology and a fluorometric assay. Most importantly, LSCM imaging strategies were developed to overcome depth-dependent sensitivity problems. These visualization studies showed that iontophoresis of calcein into hairless mouse skin enhanced delivery, particularly via follicular structures, to significant depths into the barrier. Nonfollicular transport was also apparent, especially at more superficial levels. Quantitative analyses of the LSCM images showed that, although significant nonfollicular transport occurs, the efficiency of the follicular pathway, when the relative surface area is taken into account, is considerable. Overall, therefore, this work contributes significantly to the ultimate goal of fully comprehending the mechanism(s) of iontophoretic drug delivery across the skin.

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PubMed1997

Iontophoretic transport pathways: dependence on penetrant physicochemical properties.

The objective of this work was to investigate how the preferred iontophoretic transport pathways of a molecule depend on its physicochemical properties. Laser scanning confocal microscopy (LSCM) was used to visualize in hairless mouse skin the distribution of two fluorescent penetrants: calcein, a multiply charged (-4), hydrophilic molecule; and nile red, a lipophilic, neutral compound. Iontophoresis and passive delivery of nile red showed that the percutaneous transport of this compound occurred via (inter- and intracellular) pathways that were clearly distinct from those followed by calcein. Although the distribution of nile red was influenced somewhat by the passage of current relative to the passive control, there was relatively little enhancement of the penetration of this compound into the skin. Calcein, on the other hand, did not passively enter the skin. However, with iontophoresis, greatly enhanced transport, with an important contribution from follicular structures, was observed. Sequential (dual) transport of the two fluorophores illustrated clearly the different pathways followed and reflected the transport and visualization studies of the individual species. It may be concluded, therefore, that the iontophoretic pathways followed across the skin are dictated by the physicochemical properties of the penetrant and by its affinity for the different environments available.

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PubMed1997

Iontophoresis of poly-L-lysines: the role of molecular weight?

PURPOSE: (1) To determine the extent of iontophoretic transport as a function of molecular weight (MW) of the penetrant; and (2) to visually and quantitatively characterize the iontophoretic transport pathways (follicular (F) versus nonfollicular (NF) of the fluorescently-labeled poly-L-lysines employed. METHODS: A series of fluorescently-labeled poly-L-lysines (FITC-PLLs) [4 KDa, 7 KDa and 26 KDa] were used to study the extent and distribution of iontophoretic skin penetration as a function of MW using laser scanning confocal microscopy (LSCM). RESULTS: It was found that, relative to the passive controls, and under the electrical conditions considered, iontophoresis greatly enhanced the penetration of the 4 KDa analog, slightly elevated the delivery of the 7 KDa FITC-PLL, but had no effect on the transport of the larger 26 KDa FITC-PLL. Quantitative analyses of LSCM images revealed that iontophoresis increased transport via F pathways only slightly more than that through NF pathways for the 4 KDa and 7 KDa FITC-PLL molecules. CONCLUSIONS: It is visually apparent that the iontophoretic transport pathways taken are importantly determined by the physicochemical properties (including size and charge) of the penetrant. The results presented here demonstrate an inverse dependence of iontophoretic delivery upon the MW of the penetrant.

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PubMed1997

The effect of current on skin barrier function in vivo: recovery kinetics post-iontophoresis.

PURPOSE: The objective of this study was to determine the extent to which current passage perturbed the skin's intrinsic permeability, and to quantify how quickly and to what extent the barrier properties recovered from the effects of iontophoresis. METHODS: Laser scanning confocal microscopy (LSCM) and impedance spectroscopy (IS) were employed, respectively, to visualize and quantify the recovery kinetics. RESULTS: LSCM images were obtained following passive calcein diffusion through pre-iontophoresed HMS skin in vivo that had been allowed to recover for progressively longer periods of time. IS was used to quantify the rate and extent of skin permeability recovery following current pretreatment. Impedance spectra were recorded 0, 3, 5, 7, 9 and 18 hrs after current termination. CONCLUSIONS: Enhanced calcein permeability as assessed by confocal microscopy persisted for up to 24 hrs following current passage. Consistent with these LSCM findings, IS indicated that the time required for the impedance of hairless mouse skin to return to pre-iontophoresis levels (following 2-hr current passage at 0.5 mA/cm2) was at least 18 hrs.

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