Evidence for vascular and enzymatic events in the pathophysiology of acute laminitis: which pathway is responsible for initiation of this process in horses?

Abstract

To date, there is a substantial amount of data to support the hypotheses that vascular and enzymatic changes are ongoing in experimental laminitis. Furthermore, there is substantial in vitro evidence that the enzymatic changes weaken the dermo-epidermal attachments leading to mechanical failure of the hoof-bone interface of the equine digit. However, investigators of both the vascular and enzymatic theories have, to date, been unable to substantiate the effects of these pathophysiological changes in vivo on laminar tissues of horses afflicted with experimentally induced or naturally acquired laminitis. In addition, the effects of laminitis-inducing treatment have not been prevented or reversed by treatment with an MMP inhibitor or a vasoactive antagonist. It is possible that there is simultaneous activation of the vascular and enzymatic pathways and/or other inflammatory processes. Moreover, the third theory involving mechanical factors cannot be discounted simply because strong evidence for vascular and enzymatic changes exists. It is common for horses with severe musculoskeletal disease affecting weightbearing on a limb to develop laminitis in the contralateral limb. It remains to be determined what factors are responsible for initiation of laminitis in these individuals. Evidence has not been presented that precludes the possibility of coincident occurrence of vascular and enzymatic changes. In fact, many of the inflammatory mediators (e.g. interleukin-1beta) found in laminitic tissues can concurrently stimulate synthesis of vasoactive substances and activate MMPs. Because enzymatic action on proteins is largely dependent on the concentrations of proteins and enzyme, the enzymatic theory is not dependent upon increased delivery of enzymes via increased capillary flow. Likewise, because vascular changes can alter tissue function via increased capillary flow and oedema formation, the vascular theory is not dependent upon decreased capillary flow. It is true that naturally acquired laminitis is widely variable in severity and predisposing diseases. Therefore, most probably there are multiple mechanisms involved in the initiation and propagation of the pathophysiologic cascade(s) and, therefore, successful intervention will necessitate multiple treatment modalities.