EXERCISE TRAINING ALTERS AORTIC VASCULAR REACTIVITY IN HYPOTHYROID RATS

Hypothyroidism induces a number of cardiovascular adaptations in rats, including decreases in blood flow to high-oxidative skeletal muscle a nd increases in total peripheral resistance. Conversely, exercise trai ning results in elevations in blood flow to high-oxidative skeletal mu scle and decreases in vascular resistance. The purpose of this study w as to determine whether hypothyroidism induces changes in the vasomoto r responses of arterial vessels and whether exercise training modifies these responses. Rats were divided into three groups, sedentary euthy roid (S-Eut), sedentary hypothyroid (S-Hypo), and exercise-trained hyp othyroid (ET-Hypo). Responses to vasoactive compounds were examined in vitro using abdominal aortic rings. Maximal isometric contractile ten sion (g/mm(2)) evoked by KCl and norepinephrine (NE) were not differen t among groups. However, sensitivity to KCl [agonist concentration pro ducing 50% of maximal vasoconstrictor response (EC(50); in mM): S-Eut, 21.1 +/- 1.1; S-Hypo, 35.7 +/- 2.7; ET-Hypo, 43.8 +/- 2.0] and to NE [EC(50) (in M): S-Eut, 4.0 x 10(-8) +/- 2.3 x 10(-8); S-Hypo, 8.3 x 10 (-8) +/- 3.4 x 10(-8); ET-Hypo, 3.6 x 10(-7) +/- 1.1 x 10(-7)] was dif ferent among groups, and in the order S-Eut > S-Hypo > ET-Hypo. Maxima l vasodilator responses induced by acetylcholine (10(-7) M NE preconst riction) were lower in rings from S-Hypo animals than those from S-Eut and ET-Hypo rats. Dilatory responses induced by sodium nitroprusside (SNP) with the same NE preconstriction were not different among groups . However, with a 10(-4) M NE preconstriction, maximal dilatory respon ses induced by SNP were lower in vessels from hypothyroid animals. Dil atory responses to forskolin (10(-4) M NE preconstriction) were not di fferent among groups. These results indicate that hypothyroidism and e xercise training alter vasomotor function in rat abdominal aortas thro ugh adaptations of both endothelial and smooth muscle cells.