Melatonin suppression of the colloid droplet formation in the thyroid is not related to the alteration of prostacyclin synthesis.
The goal of the present investigation has been to compare the effect of melatonin on the early secretory event in the thyroid follicular cells (thyroglobulin endocytosis; colloid droplet [CD] formation) and the effect of this pineal indoleamine on the release of a stable prostacyclin (PGI2) metabolite, 6-keto-PGF1 alpha from the thyroid. 6-keto-PGF1 alpha is believed to be a good index of PGI2 synthesis, since PGI2 is not stored within the tissue, but is continuously formed and released. We found that melatonin, administered to rats as daily subcutaneous injections at a dose of 25 micrograms and/or 250 micrograms for a total of 3 days, inhibited the CD formation. Moreover, melatonin partially suppressed the stimulatory effect of TSH on the CD formation. In turn, melatonin did not affect 6-keto-PGF1 alpha release in basal conditions, but blocked the stimulatory effect of TSH. It is concluded that melatonin decreases the thyroid secretion acting by a mechanism that is independent of the effect of this hormone on PGI2 synthesis.
D015121:6-Ketoprostaglandin F1 alpha / Q000378:metabolism; D000818:Animals; D004347:Drug Interactions; D004705:Endocytosis / Q000187:drug effects; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013954:Thyroglobulin / Q000378:metabolism; D013961:Thyroid Gland / Q000187:drug effects* / Q000378:metabolism; D013963:Thyroid Hormones / Q000378:metabolism*; D013972:Thyrotropin / Q000494:pharmacology
D013963:Thyroid Hormones; D015121:6-Ketoprostaglandin F1 alpha; D013972:Thyrotropin; D013954:Thyroglobulin; D008550:Melatonin