The endocrine system and the challenge of exercise Kenneth Harrington McKeever, PhD * Department of Animal Science, Cook College, Rutgers—The State University of New Jersey, 84 Lipman Drive, New Brunswick, NJ 08901-8525, USA Forthemostpartthedomestichorsespendsagoodpartofitsdayeither eating or seeking food. In feral horses the latter can involve treks over a wide range; however, most horses spend the majority of their day eating, standing, and occasionally exercising. Exercise can range from running up and down the fence line in anticipation of the feed truck to athletic training for a variety of competitive endeavors. Under resting conditions, the horse has a relatively easy job of maintaining its internal environment. Whatever the activity, however, the performance of work or exercise is a major phys- iologic challenge, a disturbance to homeostasis, that invokes an integrative response from multiple organ systems. On a gross level there is a pairing of those systems associated with convective transport and those associated with the transduction of potential energy into kinetic energy: the response toexerciserequiresthetransportofoxygenfromtheatmospheretothecells in the working muscles where it is used in metabolic pathways generating ATP for fuel utilization. In reality though, the adjustments to acute exercise require the coordination of several systems including the respiratory, cardi- ovascular, muscular, integumentary, renal, hepatic, and the various organs of the digestive tract [1–4]. Each tissue or organ called upon to facilitate movement must function in coordination with others in a variety of classic feedback loops. Multiple layers of control exist in the body to facilitate work. The first muscle contractions associated with work alter mechanisms ofautoregulation,causingchangesinlocalcontrolofthelocalenvironment, which are sensed peripherally. Longer work causes system wide alterations thatrequireintegratedwholebodyresponsesrequiringneuralandendocrine mediation. Vet Clin Equine 18 (2002) 321–353 * E-mail address: mckeever@aesop.rutgers.edu (K.H. McKeever). This work was supported by funds from New Jersey Agricultural Experiment Station project 99466. 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII:S0749-0739(02)00005-6