Amylase release from parotid glands of hypothyroid rats. II. Phosphorylation of microsomal proteins and effects of adenosine 3',5'-monophosphate and tolbutamide.

The present study was undertaken to examine the phosphorylation of the parotid microsomal fraction from normal and hypothyroid rats and also to compare the effects of cyclic AMP and tolbutamide on the protein phosphorylation in the two groups. After incubation in the presence of cyclic AMP, an increased rate of phosphorylation of three protein bands was revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. The apparent molecular weights of these proteins were 33,500, 26,000 and 19,000. The small protein band of 17,000 daltons decreased in 32P incorporation in the presence of cyclic AMP. Tolbutamide specifically inhibited 32P incorporation into the three proteins from normal rats, whereas the rate of phosphorylation of these proteins from hypothyroid rats remained essentially the same as that of the control, even in the presence of tolbutamide. These findings strongly suggest the possibility that stimulation of amylase release by the beta-adrenergic agonist or cyclic AMP was associated with phosphorylation of the three parotid microsomal proteins and that cyclic AMP-dependent, tolbutamide-resistant phosphorylation of these proteins from hypothyroid rats plays an important role in the increased responsiveness to beta-adrenergic stimulation.