Incubation of Bovine Thyroid Slices with Thyrotropin is Associated with a Decrease in the Ability of Pertussis Toxin to Adenosine Diphosphate-Ribosylate Guanine Nucleotide Regulatory Component(s)

Pretreatment of bovine thyroid slices with TSH resulted in desensitization of TSH-sensitive adenylyl cyclase activity but no change in stimulatory nucleotide binding regulatory component of adenylyl cyclase (Gs) activity assessed by reconstitution of the Gs-defective cyc-S49 adenylyl cyclase system. Possible changes in substrates for pertussis toxin (PT)-induced ADP ribosylation due to TSH treatment and/or in endogenous ADP ribosylation of membrane proteins were explored. Using 10 microM 32PNAD+ as substrate, endogenous ADP ribosylation was not observed in membranes from control or TSH-treated slices. ADP ribosylation of alpha-subunits of Gs by cholera toxin was also unaffected by incubation of thyroid slices with TSH. In contrast, ADP ribosylation of 40 kilodalton (kDa) substrates for PT was decreased between 40% and 60% by TSH treatment. This effect of TSH was dependent on its concentration and the time of incubation of the slices and was specific for labeling of the 40 kDa PT substrate. Prostaglandin E1 treatment of thyroid slices, which results in a much smaller homologous desensitizing effect, did not result in changes in ADP ribosylation by PT. The effect of incubation of slices with TSH was abolished by pretreatment of the membranes with 0.3-1.0% Lubrol PX, which increased the labeling of the 40 kDa polypeptides. The data suggests that TSH induces in thyroid tissue a redistribution of 40 kDa polypeptides changing their availability to PT.