Abstract
We have previously reported that KI inhibits TSH-induced release of radioiodine (McKenzie TSH bioassay method) by 60%. In this investigation the action of KI was studied on c-AMP induced iodine release. A 55% inhibition was found. The results indicate that KI inhibits thyroidal iodine release at a step further than c-AMP formation. When imidazole was injected 20 min before the intravenous administration of TSH, a 50 % inhibition of the response in the McKenzie assay was observed. This drug failed to affect TSH-induced release of radioiodine when given 20 min after the iv injection of hormone, indicating that the triggering effect of the hormone is rapid. Although imidazole is known to increase phosphodiesterase activity and block c-AMP action in some tissues, we could not demonstrate this action in mouse thyroid homogenates. Similar inhibition of TSH action was observed when methylmercaptoimidazole, thiouracil, or thiourea was administered 20 min before TSH. The mechanism of the shortterm effect of these ...
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