Submitted on July 5, 2005
Accepted on November 2, 2005
Predisposition for Venoconstriction in the Equine Laminar Dermis: Implications in Equine Laminitis
John F. Peroni1, James N. Moore2, Erik Noschka1, Megan E. Grafton1, Maria Aceves-Avila1, Stephen J. Lewis3, and Tom P. Robertson3*
1 Department of Large Animal Medicine, The University iof Georgia, Athens, GA, USA
2 Department of Physiology and Pharmacology, The University iof Georgia, Athens, GA, USA; Department of Large Animal Medicine, The University iof Georgia, Athens, GA, USA
3 Department of Physiology and Pharmacology, The University iof Georgia, Athens, GA, USA
* To whom correspondence should be addressed. E-mail: troberts{at}vet.uga.edu.
Equine laminitis is a crippling condition associated with a variety of systemic diseases. Although it is apparent that the prodromal stages of laminitis involve microvascular dysfunction, little is known regarding the physiology of this vasculature. The aim of the present study was to determine the relative responses of equine laminar arteries and veins to the vasoconstrictor agonists phenylephrine (PE, 1nM - 10µM), 5-hydroxytryptamine (5-HT, 1nM - 10µM), prostaglandin PGF2
(PGF2 1nM - 100µM) and endothelin-1 (ET-1, 1pM - 1µM). We have determined that laminar veins were more sensitive, with respect to the concentration of agonist required to initiate a contractile response and to achieve 50% of the maximum response (EC50), for all agonists tested. EC50 values, for veins and arteries respectively, were 84±7 nM vs. 688±42 nM for PE, 35±6 nM vs. 224±13 nM for 5-HT, 496±43 nM vs. 3.0±0.6 µM for PGF2 and 467±38 pM vs. 70.6±6.4 nM for ET-1 . Moreover, when expressed as a percentage of the response to a depolarizing stimulus (80mM potassium), the maximal contractile response of laminar veins exceeded that for the laminar arteries for each agonist. These results indicate that there may be a pre-disposition for venoconstriction within the vasculature of the equine digit. While this physiological pre-disposition for venoconstriction may be important in the regulation of blood flow during exercise, it also may help to explain why laminitis can result from a variety of pathological systemic conditions.
