Effects of tetraiodothyronine and triiodothyronine on hamster cheek pouch microcirculation

The aim of the present study was to assess the effects of topically applied triiodothyronine (T-3) and thyroxine (T-4) on the arterioles of hamster cheek pouch microcirculation in vivo. Microvessels were visualized using a fluorescent microscopy technique. Topical application of T-3 (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 T-3 and T-4. T-3-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 T-4 but did not affect T-3-dependent dilation. 6-Propyl-2-thiouracil (PTU), which inhibits type I 5 '-deiodinase only, did not affect the dilation induced by T-4. IPA and PTU did not impair arteriolar dilation induced by acetylcholine or sodium nitroprusside. These results indicate that T-3 induces arteriolar dilation, likely through nitric oxide release. The local conversion of T-4 to T-3 appears to be crucial for the dilation induced by T-4.