Quantification of the response of equine apocrine sweat glands to beta(2)-adrenergic stimulation

The aim of the present study was to characterise the quantitative sweating response of the horse to beta (2)-adrenergic stimulation. The sweating resp onses of 6 horses to the randomised infusion of 8 different adrenaline conc entrations (0.025, 0.05, 0.075, 0.1, 0.2, 0.4, 1.0 or 2.0 mug/kg bwt/min), was investigated. Sweating rate (SR) and skin temperature (T-SK) on the nec k (N) and gluteal region (G), and plasma adrenaline and noradrenaline conce ntrations were measured. Peak SR was similar to 15 (N) and similar to9 g/m( 2)/min (G) during infusion of both 1.0 and 2.0 mug/kg bwt/min adrenaline. S weat produced per nmol/l plasma adrenaline peaked during the infusion of 0. 075 mug/kg bwt/min adrenaline. Higher adrenaline infusion concentrations re sulted in a progressive decrease in the amount of sweat produced per nmol/l plasma adrenaline and a plateau of 6 g/m(2)/(nmol/l) plasma adrenaline was reached for infusions between 1.0 and 2.0 mug/kg bwt/min. Peak SR were far lower than we have previously reported during exercise. There was no evide nce of sweat gland fatigue or vasoconstriction during infusion, suggesting saturation of sweat gland beta (2) receptors. We conclude that sweating in the horse is under dual control from a combination of hormonal and neural m echanisms.