Mechanisms of arginine-induced increase in cytosolic calcium concentration in the beta-cell line NIT-1

The effects of L-arginine and its analogues N(G)-nitro-L-arginine, N(G)-methyl-L-arginine, L-homoarginine and D-arginine on cytosolic calcium concentration were investigated to characterise the mechanisms of arginine-induced stimulation and to determine if nitric oxide production played a role in this stimulation. NIT-1 cells, a transgenic beta-cell line, were used for this purpose since they release insulin in response to typical beta-cell stimuli. Our data demonstrate that the arginine-induced increase in cytosolic calcium concentration was completely dependent on the influx of extracellular Ca²⁺ via verapamil-sensitive voltage-activated Ca²⁺channels and that arginine stimulation requires the presence of a nutrient in order to cause an increase in cytosolic calcium concentration. The nutrient likely acted by closing the K⁺ (ATP) channels, since its effect could be inhibited by activation of these channels with diazoxide. L-arginine, as well as nitro-arginine and methyl-arginine which are not substrates for the production of nitric oxide, caused similar increases in cytosolic calcium concentration. Non-metabolisable arginine analogues homoarginine and D-arginine also caused increases in the cytosolic calcium concentration although not to the same extent. Insulin secretion was enhanced to the same extent by all analogues of arginine. It can be concluded that the arginine-induced increase in cytosolic calcium concentration in NIT-1 cells is attributable to an electrogenic effect following the transport of arginine leading to depolarisation of the plasma membrane potential, although metabolism of the amino acid itself may also partially contribute to the response.