Differential effects of triphenylphosphite and diisopropyl phosphorofluoridate on catecholamine secretion from bovine adrenomedullary chromaffin cells.

Types I and II organophosphorus compound-induced delayed neurotoxicity (OPIDN) is characterized by axonal degeneration. Type II compounds, however, uniquely cause cell body damage. Primary cultures of bovine adrenomedullary chromaffin cells were used to investigate and assess biochemically the cell body effects of the Type II compound triphenyl phosphite (TPP). Exocytotic secretion of neurotransmitter was measured to determine whether the cytotoxic action of TPP compromised synaptic events. TPP inhibited catecholamine secretion in both a time- and dose-dependent manner. By 4 h, TPP had inhibited nicotine-induced secretion by about 85%. TPP inhibited catecholamine secretion by about 35% as early as 15 min. The IC50 for TPP was about 45 microM. TPP inhibited secretion regardless of the secretagogue used, although nicotine-induced secretion was inhibited to the greatest extent. The Type I OPIDN diisopropyl phosphorofluoridate (DFP) and the nondelayed-type neurotoxic organophosphorus compound O,O-diethyl-O-4-nitrophenyl phosphate (paraoxon) did not inhibit catecholamine secretion from these cells. In contrast, when high potassium was used to induce secretion, significant stimulation was observed in the presence of DFP and paraoxon. Since Ca2+ homeostasis plays a key role in both exocytosis and neuronal necrosis, its uptake into the cells was measured radiometrically in the presence of TPP or DFP. Incubation with 100 microM TPP for 4 h resulted in the inhibition of 45Ca2+ uptake evoked either by nicotine or K+. No significant inhibition of 45Ca2+ uptake was observed in the presence of DFP. TPP and DFP produced 95% and 88% inhibition, respectively, of the activity of the neurotoxic esterase enzyme (NTE), a putative target for OPIDN. Results suggest that these changes in the secretory mechanisms of the cell may be involved in the TPP-induced pathological alterations in chromaffin cells.