NITRIC-OXIDE IMPLICATION IN THE CONTROL OF NEUROSECRETION BY CHROMAFFIN CELLS

In this work, we have studied the effects of pure nitric oxide (NO) on the regulation of catecholamine (CA) secretion by chromaffin cells, a s well as the possible presence of its synthesizing enzyme L-arginine: NO synthase (NOS) in these cells. Our results show that NO produces a large stimulation of basal CA secretion. This effect was calcium- and concentration-dependent (EC(50) = 64 +/- 8 mu M) and was not due to no nspecific damage of the tissue by NO. NO also modulates the CA secreti on evoked by nicotine in a dose-dependent manner. Although it has a st imulatory effect on the CA secretion evoked by low doses of nicotine ( <3 mu M; EC(50) = 16 +/- 3 mu M), it produces a dose-dependent inhibit ion of the CA secretion induced by high doses of nicotine (greater tha n or equal to 30 mu M; IC50 = 52 +/- 6 mu M). The mechanism by which N O modulates CA secretion seems to be through the increase in the cycli c GMP levels, because there was a close correlation between the CA sec retion and the cyclic GMP levels. The presence of a specific activity of NOS in chromaffin cells has been demonstrated by two independent me thods: release of [C-14]citrulline from [C-14]arginine and formation o f an NO-hemoglobin complex. NOS activity was about 0.5 pmol/min/mg of protein. It was calcium- and mainly calmodulin-dependent and could be specifically blocked by the NOS inhibitor N-methyl-L-arginine. These r esults suggest that NO could be an important intracellular messenger i n the regulation of neurosecretion in chromaffin cells.