Adenylate Cyclase Activity of Normal and Goitrous Rat Thyroids

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Abstract
The influence of Mg2+, Mn2+, and Ca2+ and the effects of GTP and prostaglandin E1 (PGE1) on adenylate cyclase [AC] activity in homogenates of thyroid glands from rats treated with propylthiouracil (PTU) and from normal control rats were compared. Increasing concentrations of Mg2+ in the medium stimulated AC activity: at 8 and 12 mM Mg2+, AC activity was significantly higher in the PTU-treated group. The effect of Mn2+ on AC activity was essentially the same in goitrous and normal thyroids. Ca inhibited AC activity; the Ki for Ca2+ (about 0.4 mM) was approximately equal for normal and goitrous glands. There was also no significant difference between goitrous and control thyroid tissue in F--stimulated AC activity determined at various Mg2+ concentrations. Addition of GTP to the assay mixture stimulated AC activity in both groups. GTP also made tissue from both groups of rats responsive to stimulation of AC by PGE1; the effect occurred at lower PGE1 concentrations and was greater in the goitrous thyroids. These results suggest a qualitative difference between the basal AC of normal and goitrous thyroids with regard to activity in the presence of excess Mg2+ and responsiveness to PGE1 in the presence of GTP.

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