Thermoregulatory-induced compromise of muscle blood flow in ponies during intense exercise in the heat: a contributor to the onset of fatigue?

Cardiac output (Q) and tissue blood flows (BF) were measured in four ponies at rest, after 30 min of moderate exercise (ME) (approximately 30% VO2max) and following moderately intense exercise (M-IE) (approximately 65%) until fatigue, in a mildly hot (MH) environment (dry bulb temperature = 41 degrees C, wet bulb temperature = 27.5 degrees C). Exercise at both intensities resulted in increases in Q (15.6 +/- 1.7 [rest] vs. 35.8 +/- 2.6 [ME] vs. 48.6 +/- 3.9 [M-IE] l/min) and VO2 (4.7 +/- 0.9 vs. 24.5 +/- 1.2 vs. 55.8 +/- 5.8 ml/kg/min). When compared to ME, M-IE resulted in decreases in BF to the skin (approximately 15%), nonworking muscle (46%), fat (93%), gut (56%), kidneys (57%), CNS (31-66%) and an increase in BF to respiratory (68%), limb (98.5%) and cardiac (50%) muscles. In contrast, BF to working muscle during ME in thermoneutral (TN) conditions (McConaghy et al. 2002) was higher than found during ME in MH. We calculated that limb BF was approximately 25% lower for ME in MH vs. ME in TN. Similarly, we estimated that limb BF during M-IE in MH was approximately 25% lower than for M-IE in TN. Attenuation of the increases in muscle BF during exercise in the heat may be a determinant of the onset of fatigue.