Endogenous substrates for cAMP-dependent phosphorylation in dispersed bovine parathyroid cells.

Agonists that increase intracellular cAMP in bovine parathyroid cells ((l)-isoproterenol (ISO), dopamine, prostaglandin E2) as well as cAMP analogs (dibutyryl cAMP and 8-bromo-cAMP) stimulated the phosphorylation of 2 endogenous proteins with apparent molecular weights of 19K and 15K. The time course and concentration-dependence of ISO-stimulated phosphorylation in these cells correlated well with known effects of ISO on intracellular cAMP and PTH release. ISO-stimulated phosphorylation of these two proteins was rapidly reversed by (l)-propranolol. Although 2 mM extracellular calcium inhibited ISO-stimulated PTH secretion, it did not significantly affect the phosphorylation of the 19K and 15K bands. These results are consistent with a physiologic role for the phosphorylation of these proteins in agonist-stimulated PTH secretion in bovine parathyroid cells.