Effects of season and sample handling on measurement of plasma alpha-melanocyte-stimulating hormone concentrations in horses and ponies

OBJECTIVE: To investigate effects of sample handling, storage, and collection time and season on plasma alpha-melanocyte-stimulating hormone (alpha-MSH) concentration in healthy equids. ANIMALS: 11 healthy Standardbreds and 13 healthy semiferal ponies. PROCEDURE: Plasma alpha-MSH concentration was measured by use of radioimmunoassay. Effects of delayed processing were accessed by comparing alpha-MSH concentrations in plasma immediately separated with that of plasma obtained from blood samples that were stored at 4 degrees C for 8 or 48 hours before plasma was separated. Effects of suboptimal handling were accessed by comparing alpha-MSH concentrations in plasma immediately stored at -80 degrees C with plasma that was stored at 25 degrees C for 24 hours, 4 degrees C for 48 hours or 7 days, and -20 degrees C for 30 days prior to freezing at -80 degrees C. Plasma alpha-MSH concentrations were compared among blood samples collected at 8:00 AM, 12 noon, and 4:00 PM. Plasma alpha-MSH concentrations were compared among blood samples collected in January, March, April, June, September, and November from horses and in September and May from ponies. RESULTS: Storage of blood samples at 4 degrees C for 48 hours before plasma was separated and storage of plasma samples at 4 degrees C for 7 days prior to freezing at -80 degrees C resulted in significant decreases in plasma alpha-MSH concentrations. A significantly greater plasma alpha-MSH concentration was found in September in ponies (11-fold) and horses (2-fold), compared with plasma alpha-MSH concentrations in spring. CONCLUSIONS AND CLINICAL RELEVANCE: Handling and storage conditions minimally affected plasma alpha-MSH concentrations. Seasonal variation in plasma alpha-MSH concentrations must be considered when evaluating pituitary pars intermedia dysfunction in equids.