Sweat formation involves the secretion of an initial, isotonic, plasma-like fluid, followed by salt reabsorption. Reddy & Quinton (1994) recognised that a second reabsorptive mechanism, in addition to that provided by the basolaterally located, sodium pump-driven mechanism, was necessary to explain the acidity (down to pH 4) and very low salt concentration (down to 5 mM NaCl) found in normal sweat at low secretory rates. They proposed that HCO₃^–/Cl^– exchange brings Cl^– into the reabsorptive cells against its electrochemical gradient, with carbonic anhydrase (CA) and V-ATPase providing the energy to drive the exchanger.

Human eccrine sweat glands were isolated from skin samples from patients undergoing general surgery; local ethics committee approval and informed consent were obtained. Isolated glands from three patients were formalin fixed, wax embedded and sectioned using standard techniques (Bovell et al. 2000). Antigen retrieval was performed on 3 µm sections using the microwave/pressure method and immunohistochemically stained using the avidin-biotin-complex immuno-peroxidase staining method (Vector). A primary antibody with antigenicity for the 31 kDa subunit of the V-ATPase was used at 1:1600 dilution.

The 31 kDa subunit of the V-ATPase was confirmed to be present in the luminal duct cells, with particularly heavy staining at the apical membranes. Immunoreactivity was also observed in the cells of the secretory coil, and found to be particularly associated with the clear cells of the coil. The staining was not found to be particularly associated with the apical or basolateral membrane/pole of the cell but was distributed throughout the cytoplasm of the clear cells. The dark cells of the secretory coil were, by comparison, devoid of staining for the 31 kDa subunit.

The strong expression of V-ATPase in the reabsorptive duct is consistent with the hypothesis that the proton pump, in conjunction with carbonic anhydrase, provides the energy to pool Cl^– in the cell in exchange for HCO₃^–. The strong expression of V-ATPase in clear cells of the secretory coil is interesting as Na⁺-K⁺-2Cl^– co-transport and the basolateral Na⁺/K⁺-ATPase has hitherto seemed adequate to explain primary secretion. These mitochondria-rich cells are known to stain strongly for CAII and are the cells primarily responsible for electrolyte and fluid secretion. Secretion could operate as a ‘push-pull’ mechanism, the push provided by the sodium pump raising [Cl^–] and [K⁺] above their equilibrium levels via the co-transporter, and the ‘pull’ provided by the proton pump, a known energiser of luminal membranes (Wieczorek et al. 1999). Bicarbonate is known to be essential for normal human sweat gland function (Wilson et al. 1990), and other transport mechanisms are likely to be present in the luminal membranes to effect the production of primary sweat. Further work will address these speculations.

The authors would like to thank the patients and surgeons of Glasgow Royal Infirmary for their co-operation and Unilever plc for their financial support.

Bovell, D.L., Clunes, M.T., Roussa, E., Burry, J. & Elder, H.Y. (2000). Histochem. J. 32, 409-413.

Reddy, M.M. & Quinton, P.M. (1994). Am. J. Physiol. 267, C1136-1144.

Wieczorek, H., Brown, D., Grinstein, S., Ehrenfeld, J. & Harvey, W.R. (1999). Bioessays 21, 637-648.

Wilson, S.M., Bovell, D.L., Elder, H.Y., Jenkinson, D.M. & Pediani, J.D. (1990). Exp. Physiol. 75, 649-656.