Sweating responses in the horse

The sweat glands of the horse are very sensitive to L-adrenaline, 0·5 to 1·0 μ g/kg, i-v. being enough to cause general sweating. Given intradermally, the threshold dose is usually about 0·1 μ g; this causes local sweating, lasting for an hour or more, and spreading as a clear-cut track along superficial lymphatic vessels for long distances. This striking result has not previously been recognized. The action of adrenaline is not inhibited by any of seven adrenaline antagonists which were tried. No piloerection is caused within a few hours by adrenaline. L- nor -Adrenaline, by either route causes no sweating, or only a little locally an hour or two after i-d. injection; but tracks of hair erection identical with the adrenaline sweating tracks appear, and on i-v. injection in doses of 5 mg or over, general piloerection occurs. The piloerection is inhibited by dibenamine. Parasympathomimetic substances (acetylcholine, Carbachol) cause sweating when given in suitable doses, either i-d. or i-v.; their action is inhibited by atropine, and augmented by anticholinesterases. The sweating on i-v. injection is probably due to adrenaline release from the adrenals, that on i-d. injection to skin vaso-dilatation. Factors which increase the skin circulation favour, or cause, sweating, and vice versa. Hence sympathetic denervation leads to prolonged and profuse sweating, and subsequently to an increased tendency to sweat, lasting for more than 3 months. There is little evidence to favour the view that the sweat glands in the horse normally respond to, or are provided with the necessary innervation for, the action of nerves, sym­pathetic or otherwise, but strong evidence that they normally respond to an increase in the amount of adrenaline in the blood, or to increased blood supply. The latter is under nervous control. After i-d. injections of adrenaline the skin is anhidrotic for some days. This may throw light on the condition of tropical anhidrosis.