The effects of thermally induced activity in vivo upon the levels of sodium, chlorine and potassium in the epithelia of the equine sweat gland.

Evaporation of moisture from the skin surface is an important means of thermoregulatory heat loss in Equidae, which respond to thermal stress with a smooth increase in the rate of sweat output to a plateau level which may be maintained for several hours (Allen & Bligh, 1969; Robertshaw & Taylor, 1969; Montgomery, Jenkinson & Elder, 1982). This pattern is similar to that which occurs when humans are exposed to a warm environment (Montgomery et al. 1984). The mode of sweat formation in both the horse and man is similar, involving cell death, fluid transport and the loss of vesicles both by exocytosis and microapocrine secretion (Montgomery etal. 1982, 1984). However, the secretions produced by the two glands differ markedly in electrolyte composition. Human sweat is a hypotonic fluid containing sodium chloride as its principal solute (Schulz, 1969; Sato, 1977), whereas equine sweat is hypertonic and contains a high concentration of potassium (Soliman & Nadim, 1967; Kerr & Snow, 1983). This could be a reflection of differences in the underlying mechanism of ionic transport within the gland. Recent X-ray microanalytical studies of atrichial (eccrine) sweat glands from both the human back (McWilliams et al. 1987) and rat footpad (McWilliams et al. 1988) have demonstrated changes in intracellular elemental concentration upon thermal stimulation which suggest that sodium influx and potassium efflux are central to the secretory process. However, the changes in intracellular ionic concentration associated with fluid secretion by the epitrichial (apocrine) equine sweat gland, have not been investigated. In this study, the elemental compositions of the epithelia of both resting and active equine sweat glands have been investigated and compared with those found in man (McWilliams et al. 1987). Four Shetland ponies (2 mares and 2 geldings) were exposed to a hot, humid environment (40°C dry bulb, 23°C wet bulb) in a climatic chamber for 4h and the