A series of sulfonamide compounds, para-aminohippurate, and inulin were used to study the permeability of the epithelium of human sweat glands.
Inulin was not excreted in the sweat. The ratios of the concentrations in sweat to the concentrations in plasma, S/P, of sulfanilamide, sulfapyridine, sulfathiazole, sulfadiazine, and para-aminohippurate were found to be 0.69, 0.58, 0.13, 0.11, and 0.02 respectively, independent of the plasma concentrations and the sweating rates. The fact that the S/P ratios are thus unaffected by the absolute number of molecules transported suggests that these compounds enter into the sweat by simple diffusion and not via a specific secretory mechanism which could become saturated by increasing load. If this is so, the difference in the S/P ratios must be explained by an unequal permeability of the epithelium of the sweat gland to the various compounds and some explanation for these differences in the rate of excretion must exist in terms of physicochemical properties of the compounds.
A comparison between the S/P ratios and the pK values of the various sulfonamides indicates that the differences in their rates of excretion in the sweat depend upon the degree of ionization of the various compounds at the physiological pH. Compounds which are mainly non-ionized are excreted with high S/P ratios, whereas ionized compounds appear with low ratios. A quantitative relationship was shown to exist between the S/P ratio for each compound and the percentage of the compound which is non-ionized at pH 7.4.