Somatostatin biosynthesis by cerebral cortical cells in monolayer culture

We examined the ability of isolated rat cerebral cortical cells to synthesize and secrete immunoreactive somatostatin (IRS). Dispersed telencephalic cells (10(7] from rat embryos contain approximately 24 pg IRS. The IRS content of 10(7) cultured cells increased steadily to a level of 2800 pg by 20 days in vitro. The IRS content in the medium followed a parallel production curve and was, in general, 10% of the cellular content. The nature of this IRS was characterized by gel filtration. Bio-Gel P-10 chromatography resolved four discrete IRS peaks. The first peak had an apparent molecular weight of 11,500. The second peak, which comigrated with synthetic Somatostatin-28, had a molecular weight of 7,000. The third peak had a molecular weight of 1,800, and the fourth peak, which comigrated with Somatostatin-14, had a molecular weight less than 1,800. Incubation with [3H]phenylalanine (Phe) revealed that 3H was incorporated into each peak. Isolation of 3H-labeled peak 4 was carried out by double affinity column purification. Sequential automated Edman degradation of this material revealed [3H] Phe in the positions that Phe occupies in Somatostatin-14. These studies document the ability of isolated rat cerebral cortical tissue to synthesize several forms of IRS, establishing them as endogenous cortical peptides.