Activation of tyrosine hydroxylase by pituitary adenylate cyclase-activating polypeptide (PACAP-27) in bovine adrenal chromaffin cells

The effect of pituitary adenylate cyclase-activating polypeptide (PACAP-27) on tyrosine hydroxylase activity has been studied in intact, cultured, bovine adrenal chromaffin cells. Tyrosine hydroxylase activity was determined in situ by measuring the production of ¹⁴CO₂ following the hydroxylation and rapid decarboxylation of [¹⁴C]tyr offered to the cells. PACAP-27 increased tyrosine hydroxylase activity 3-fold over 10 min, with an EC₅₀ of 10-20 nM. PACAP-38 was approximately 2-fold less potent. Removing extracellular Ca²⁺ reduced basal tyrosine hydroxylase activity and the activation produced by both PACAP-27 and forskolin by about 20%. In the absence of extracellular Ca²⁺, chelation of intracellular Ca²⁺ by treating cells with BAPTA-AM (50 μM) caused a consistent 40-50% reduction in basal tyrosine hydroxylase activity and in the responses to forskolin and PACAP-27. The tyrosine hydroxylase activation produced by PACAP-27 was unaffected by the protein kinase C inhibitor Ro 31-8220 (3 μM), but was reduced by 85% by the protein kinase A inhibitor 1189 (10 μM). PACAP-27 increased cellular cyclic AMP levels 3-fold at 100 nM. The results suggest that PACAP-27 activates tyrosine hydroxylase in bovine chromaffin cells through cyclic AMP formation and protein kinase A activation, and that both extracellular and intracellular Ca²⁺ modulate the effect of the adenylate cyclase/cyclic AMP/protein kinase A signalling pathway on tyrosine hydroxylase activity.