A technique is described using radioactiveiodine (I131) and the goitrogen, Tapazole, in mature dairy cows, to compare the relative biological activity of l-thyroxine (T4) and l-triiodothyronine (T3) in individual animals to inhibit pituitary thyrotropin (TSH) release and, in turn, release of thyroidal-I131. The minimal amount of either hormone required is considered the equivalent T4 or T3 secretion rate. The individuals of the Holstein breed showed a mean molar relation of 1:2.5, which was significantly higher than the means of the other breeds. The mean of 22 animals showed a ratio of 1:2.14, indicating that T3 is slightly over twice as effective as T4 in blocking TSH and thyroidal-I131 release. While T3 is more potent in blocking TSH discharge, it was observed that upon withdrawal of T4 and T3 the resumption of thyroidal-I131 release was more rapid after T3 administration.
Droperidol is used in the anaesthetic management of pheochromocytoma because of its sedative, anti-dysrhythmic and alpha-adrenoreceptor blocking properties. However, droperidol when used in pheochromocytoma, has been reported to produce a paradoxical hypertensive response.In vitro experiments with perfused rabbit ear arteries using a histochemical fluorescence technique, showed droperidol to be an inhibitor of noradrenaline uptake into sympathetic nerve endings, and this /Iptake inhibition was dose related. The uptake inhibition effect did not, however, produce pressor changes in experiments simulating pheochromocytoma in cats. The hypertensive response to droperidol may be due to blockade of presynaptic alpha-adrenoreceptors and this possible mechanism of action is discussed.
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