Phorbol diesters stimulate somatostatin secretion from cultured brain cells

Phorbol diester tumor promoters are potent cocarcinogens which also possess activity in a variety of biological assays. We have examined the effect of phorbol diesters on secretion of somatostatin-like immunoactivity (SRIF-LI) by dispersed cells of fetal rat brain maintained in long term culture. Phorbol-12-myristate-13-acetate (PMA) and phorbol 12, 13-dibutyrate stimulate SRIF-LI secretion in a dose-dependent fashion. 4-O-Methyl-PMA is approximately 100 times less potent than phorbol 12, 13-dibutyrate. 4-beta-Phorbol was inactive. Treatment with a nonphorbol irritant, teleocidin, also was associated with significantly augmented release of SRIF-LI. Significant stimulation is seen within 7.5 min of treatment and response is linear over 1 h. Administration of phorbol diesters in low calcium buffer (0.1 mM) with or without cobalt or pretreatment with verapamil are associated with significantly diminished secretion. Substitution for sodium ion by choline or pretreatment with tetrodotoxin (10(-7) M) also inhibits response to PMA. gamma-Aminobutyric acid (50 microM) or the gamma-aminobutyric acid agonist muscimol (5 microM) decrease response to PMA as does sodium pentobarbital (IC50 approximately 30 microM). Phenobarbital is less potent as an inhibitor; significant suppression is not seen until approximately 300 microM. The data are consistent with an action for phorbol diesters mediated at least in part by voltage dependent sodium channels and calcium influx into excitable cells. Inhibition by hyperpolarizing agents is compatible with this mechanism. Phorbol diesters may thus mimic endogenous modulator substances active at the nerve cell membrane.