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-homo-arginine and D-arginine on cytosolic calcium concentration were investigated to characterise the mechanisms of arg inine-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 respon se to typical beta-cell stimuli. Our data demonstrate that the arginin e-induced increase in cytosolic calcium concentration was completely d ependent on the influx of extracellular Ca2+ via verapamil-sensitive v oltage-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 thes e channels with diazoxide. L-arginine, as well as nitro-arginine and m ethyl-arginine which are not substrates for the production of nitric o xide, caused similar increases in cytosolic calcium concentration. Non -metabolisable arginine analogues homoarginine and D-arginine also cau sed increases in the cytosolic calcium concentration although nut to t he same extent. Insulin secretion was enhanced to the same extent by a ll analogues of arginine. It can be concluded that the arginine-induce d increase in cytosolic calcium concentration in NIT-1 cells is attrib utable to an electrogenic effect following the transport of arginine l eading to depolarisation of the plasma membrane potential, although me tabolism of the amino acid itself may also partially contribute to the response.