MUSCLE BLOOD-FLOW DURING EXERCISE IN SEDENTARY AND TRAINED HYPOTHYROID RATS

Hypothyroidism is characterized by exercise intolerance. We hypothesiz ed that active muscle blood flow during in vivo exercise is inadequate in the hypothyroid state. Additionally, we hypothesized that enduranc e exercise training would restore normal blood flow during acute exerc ise. To test these hypotheses, rats were made hypothyroid (Hypo) over 3-4 mo with propylthiouracil. A subset of Hypo rats was trained (THypo ) on a treadmill at 30 m/min (15% grade) for 60 min/day 5 days/wk over 10-15 wk. Hypothyroidism was evidenced by similar to 80% reductions i n plasma triiodothyronine levels in Hypo and THypo and by 40-50% reduc tions in citrate synthase activities in high oxidative muscles in Hypo compared with euthyroid (Eut) rats. Training efficacy was indicated b y increased (25-100%) citrate synthase activities in muscles of THypo vs. Hypo. Regional blood flows were determined by the radiolabeled mic rosphere method before exercise and at 1-2 min of treadmill running at 15 m/min (0% grade). Preexercise muscle blood flows were generally si milar among groups. During exercise, however, flows were lower in Hypo than in Eut for high oxidative muscles such as the red section of vas tus lateralis [277 +/- 24 and 153 +/- 13 (SE) ml . min(-1) . 100 g(-1) for Eut and Hypo, respectively; P < 0.01] and vastus intermedius (317 +/- 32 and 187 +/- 20 ml . min(-1) . 100 g(-1) for Eut and Hypo, resp ectively; P < 0.01) muscles. Training (THypo) did not normalize these flows (168 +/- 24 and 181 +/- 24 ml . min(-1) 100 g(-1) for red sectio n of vastus lateralis and vastus intermedius muscles, respectively). B lood flows to low oxidative muscle, such as the white section of vastu s lateralis muscle, were similar among groups (21 +/- 5, 25 +/- 4, and 34 +/- 7 ml . min(-1) . 100 g(-1) for Eut, Hypo, and THypo, respectiv ely; P = NS). These findings indicate that hypothyroidism is associate d with reduced blood flow to skeletal muscle during exercise, suggesti ng that impaired delivery of nutrients to and/or removal of metabolite s from skeletal muscle contributes to the poor exercise tolerance char acteristic of hypothyroidism.