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Effects of tetraiodothyronine and triiodothyronine on hamster cheek pouch microcirculation

¹Department of Neuroscience, "Federico II" University Medical School, Naples; ²Department of Physiology and Biochemistry, University of Pisa; ³Consiglio Nazionale delle Ricerche Institute of Clinical Physiology, Pisa, Italy

Submitted 7 September 2004 ; accepted in final form 29 November 2004

The aim of the present study was to assess the effects of topically applied triiodothyronine (T₃) and thyroxine (T₄) on the arterioles of hamster cheek pouch microcirculation in vivo. Microvessels were visualized using a fluorescent microscopy technique. Topical application of T₃ (3.08, 30.8, 61.5, 307, 615, and 6,150 nM/l) consistently induced dose-dependent dilation of arterioles within 2.0 ± 0.5 min of administration. The application of T₄ (150, 257, 514, and 5,140 nM/l) caused different dose-dependent effects: dilation at the three lower doses within 16 ± 2 min and rhythmic diameter changes at the highest dose. Aging of hamsters did not alter the arteriolar responses to T₃ and T₄. T₃-induced dilation was countered by the inhibition of nitric oxide synthase with N^G-nitro-L-arginine-methyl ester or N^G-nitro-L-arginine. Iopanoic acid (IPA), which inhibits types I and II 5'-deiodinase, abolished the dilation elicited by 514 nM T₄ but did not affect T₃-dependent dilation. 6-Propyl-2-thiouracil (PTU), which inhibits type I 5'-deiodinase only, did not affect the dilation induced by T₄. IPA and PTU did not impair arteriolar dilation induced by acetylcholine or sodium nitroprusside. These results indicate that T₃ induces arteriolar dilation, likely through nitric oxide release. The local conversion of T₄ to T₃ appears to be crucial for the dilation induced by T₄.

microcirculation; vasodilation; arterioles; nitric oxide; N^G-nitro-L-arginine methyl ester; N^G-nitro-L-arginine; iopanoic acid; 6-propyl-2-thiouracil; thyroid hormones