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The effects of current density on the temperature dependence of the electrical properties of human stratum corneum were investigated in vitro at two different current densities: 13 and 130 μA cm −2 . To obtain information on the structural basis of the current-induced effects on the electrical resistances and capacitances, stratum corneum samples were subjected to three heating-and-cooling cycles (20-45-20 °C; 20-75-20 °C and 20-95-20 °C) which included four lipid phase transitions. At both current densities three characteristic temperature intervals could be distinguished: (1) A lower interval, from 20 to about 60 °C at the lower current density and from 20 to about 50 °C at the higher current density. In this interval a constant activation energy for ion transport and a gradual decrease of the resistances were found, whereas the capacitances were almost constant; all changes within this interval were thermo-reversible; (2) A middle interval, from 60 to about 75 °C at the lower and from 50 to about 75 °C at the higher current density. Within these temperature ranges, a rapid and thermo-irreversible decrease of the resistances was observed, accompanied by an increase of the capacitances; and (3) A higher interval, from 75 to 95 °C, within which the resistance did not decrease any further, although the capacitance increase continued. At a current density of 13 μA cm −2 , the activation energy of ion transport across human stratum corneum was 5.4 ± 0.7 kcal mol −1 , which suggested the presence of highly conductive pathways. The middle temperature interval at 13 μA cm −2 , 60–75 °C, in which irreversible changes occurred, corresponded with the temperature interval of the gel-liquid phase transition of stratum corneum lipids. The irreversible increase of the capacitances further continued from 75 to 95 °C. Hence, the capacitances are determined by free lipids as well as protein-bound lipids. At 130 μA cm −2 the temperature dependence of the electrical properties had the same features as observed at 13 μA cm −2 , but with two important differences: (1) A 15-fold decrease of the activation energy to 0.37 ± 0.03 kcal mol −1 ; and (2) A downward shift by almost 10 °C of the temperature, at which the thermal transition of the electrical properties starts. In conclusion, the thermal analysis of electrical properties has shown that the resistances of human stratum corneum are closely associated with the intercellular lipid lamellae, whereas the capacitances are determined by both the intercellular lipid lamellae and protein-bound lipids. Furthermore, under influence of an electrical field the lipid phase transition temperature is shifted downward, indicating that the electrical field is capable of modifying the arrangements of stratum corneum lipids. |
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