The effects of vasoactive amines found in the equine hindgut on digital blood flow in the normal horse

Reasons for performing study: Disturbances of digital blood flow are thought to be fundamental to the pathophysiology of acute laminitis. However, factors linking the initiating events in the equine hindgut with these disturbances in the foot remain to be determined. Hypothesis: Amine compounds, formed by bacteria in the equine hindgut, have digital vasoconstrictor effects in vivo. Methods: Tryptamine (1.6 mug/kg/min) and phenylethylamine (2.13 mug/kg/min) were infused i.v. into standing nonsedated horses. Digital blood flow was measured by Doppler ultrasound and foot surface temperature was monitored. Plasma 5-hydroxytryptamine (5-HT) concentrations were measured by HPLC. Results: Tryptamine and phenylethylamine infusions had no effect on systemic arterial blood pressure or heart rate, but caused significant decreases in digital arterial blood now (mean +/- s.e. 29.2 +/- 8.5 and 18.4 +/- 6.8%, respectively). Both amines also caused decreases in dorsal hoof wall temperature (0.6 +/- 0.1 and 0.5 +/- 0.1degreesC for tryptamine and phenylethylamine, respectively) and concomitant increases in plasma 5-HT concentration. Conclusions: Tryptamine and phenylethylamine caused reduction of digital blood flow, effects which may have been mediated, in part, via displacement of 5-HT from platelets. Potential relevance: Amine compounds occurring in the equine hindgut, if released into the circulation following carbohydrate overload, could contribute to selective digital vasoconstriction. Further work in ponies and horses, with naturally occurring laminitis, is necessary to determine whether amines represent a therapeutic target in this disease.