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Keywords:

  • horse;
  • dexamethasone;
  • endothelin;
  • intradermal;
  • phenylephrine;
  • thermography;
  • vascular function

Summary

Reasons for performing study: In vitro, glucocorticoids potentiate vasoconstriction of equine digital vessels to catecholamines and this has been implicated as a mechanism of glucocorticoid-induced laminitis. This observation has never been confirmed in vivo.

Objectives: To study the effects of glucocorticoid therapy on vasoconstrictor responsiveness in the horse in vivo.

Methods: In a blinded, randomised cross-over experiment, 9 horses were treated with either dexamethasone (0.1 mg/kg bwt i.v. q. 24 h) or saline i.v. for 6 days. The changes in local average skin temperature before (baseline) and after intradermal injections of the α1-adrenoceptor agonist phenylephrine (PHE; 10−4, 10−5, 10−6, 10−7 and 10−8 mol/l), endothelin-1 (ET-1; 10−5, 10−6, 10−7, 10−8 and 10−9 mol/l) or ET-1 plus a blocker (BQ-123 10−6 mol/l; RES-701 10−6 mol/l; and L-NAME 10−4 mol/l) were investigated with a thermograph.

Results: Dexamethasone (DEX) decreased baseline skin temperatures, suggesting reduced blood flow as a consequence of an increase in vasomotor tone. This was accompanied by potentiation of the response to PHE as demonstrated by a left shift in the dose-response curve and a decrease in the EC50. Dexamethasone did not potentiate ET-1, but the interplay with the lower baseline temperature resulted in a significantly lower skin temperature for this vasoconstrictor after DEX. The different ET-1 blockers had no effect on ET-1 modulated skin temperatures.

Conclusions: Dexamethasone decreases skin perfusion. This is accompanied by a potentiated α1-adrenoceptor agonist response and a greater response to ET-1.

Potential relevance: Glucocorticoid therapy probably decreases perfusion of the equine hoof. During disease states that already are characterised by hypoperfusion and/or increased levels of circulating catecholamines, glucocorticoid therapy could, according to the vascular model of laminitis, tilt the balance in favour of laminitis.