Adenylate Cyclase Activity of Normal and Goitrous Rat Thyroids 1

Abstract

Summary

The influence of Mg²⁺, Mn²⁺, and Ca²⁺ as well as the effects of guanosine triphosphate (GTP) and prostaglandin E₁ (PGE₁) on adenylate cyclase activity in ho-mogenates of thyroid glands from rats treated with propylthiouracil (PTU) and from normal control rats were compared. Increasing concentrations of Mg²⁺ in the medium stimulated adenylate cyclase activity: at 8 and 12 mM Mg²⁺, cyclase activity was significantly higher in the PTU-treated group. The effect of Mn²⁺ on adenylate cyclase activity was essentially the same in goitrous and normal thyroids. Calcium inhibited adenylate cyclase activity; the K₁ 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 fluoride-stimulated adenylate cyclase activity determined at various Mg²⁺ concentrations. Addition of GTP to the assay mixture stimulated adenylate cyclase activity in both groups. GTP also made tissue from both groups of rats responsive to stimulation of adenylate cyclase 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 adenylate cyclase of normal and goitrous thyroids with regard to (a) activity in the presence of excess Mg²⁺ and (b) responsiveness to PGE₁ in the presence of GTP.

We thank Mrs. Shirley Dohrman and Mr. Paul Hebl for their valuable assistance. Some of the preliminary experiments which led us to the present investigations were carried out by Mr. Dan Francisco. Dr.