Basal and Dibutyryl Cyclic AMP-Stimulated Release of Newly Synthesized and Stored Growth Hormone from Perifused Rat Pituitaries

Abstract

The basal rate of pre-labeled stored 14C-labeled rat growth hormone ([14C]rGH) release from perfused rat pituitary explants is a constant fraction of pituitary GH content, suggesting random release from the storage pool. Basal release of newly synthesized [3H]rGH occurs in 2 phases: immediate and associated with [3H]rGH synthesis, and late (delayed by 60 min) and independent of concurrent [3H]rGH synthesis. Dibutyryl cyclic AMP (dbcAMP, 10-2 M)-stimulated release of stored [14C]rGH is characterized by an initial acute rise followed by a second phase of continuous rapid release. Immediate and late release of new [3H]rGH is increased by dbcAMP, and the late phase of [3H]rGH is less delayed. Simultaneous exposure of pituitary explants to [3H]alanine and [14C]leucine resulted in the release of immunoprecipitable rGH whose ratio of incorporated 3H and 14C varied with time. The observed changes suggest that after it is synthesized, a GH molecule may either be released directly or be processed into the somatotroph''s storage compartment. Stored GH is composed of 2 pools, one of which is immediately releasable. The differential incorporation of [3H]alanine and [14C]leucine into big and small rGH, together with the ability to differentially displace 3H-labeled big and 14C-labeled small rGH from the GH antibody suggest that big rGH is a heterogenous molecule including small rGH and another peptide rather than simply a dimer of small rGH.