Effect of preheating on the permeability of neonatal rat stratum corneum to alkanols.

We heated flat sheets of neonatal rat stratum corneum for various times at temperatures between 40 and 90 degrees C before determining the permeability coefficient (Kp) of propanol and/or hexanol from water. Below 70 degrees C, Kp remained constant; at 75 degrees C, Kp increased linearly with exposure time; at 80 degrees C and above, there was a large increase in under 2 h, with no further increase on longer heating. There was a 15-fold increase in 6-h Kp between 70 degrees C and 80 degrees C, values being constant above 80 degrees C but at a figure less than for lipid-extracted stratum corneum. Thermal analysis showed that the increase in Kp corresponds to changes in the 80 degrees C lipid endotherm, suggesting that the increased Kp is due to a disordering of the lipid structures. The effect of treating preheated stratum corneum with dimethylsulfoxide (DMSO) vapor for 16 h was also studied. Below 70 degrees C, Kp was increased five-fold, but between 70 and 80 degrees C this difference was eliminated, so that above 80 degrees C the Kp was the same as with heat treatment alone. We concluded that both heat and DMSO affect the lipid structures of stratum corneum. DMSO produced a small, reversible structural change, while the effect of heat is irreversible and produces a greater degree of disorder in the lipid structures, but the lipid still contributed to the barrier effect of stratum corneum.