ALTERATIONS OF INSULIN-RESPONSE TO DIFFERENT BETA-CELL SECRETAGOGUES AND PANCREATIC VASCULAR-RESISTANCE INDUCED BY N-OMEGA-NITRO-L-ARGININE METHYL-ESTER

1 We studied a possible interplay of pancreatic NO synthase activity on insulin secretion induced by different beta cell secretagogues and also on pancreatic vascular bed resistance. 2 This study was performed in the isolated perfused pancreas of the rat. Blockade of NO synthase was achieved with N-omega-nitro-L-arginine methyl ester (L-NAME); the specificity of the antagonist was checked by using its D-enantiomer as well as by substitutive treatments with sodium nitroprusside (SNP) as a NO donor in studies of glucose-induced insulin secretion. 3 Arginine (5 mM) induced a monophasic insulin response which was, in the presence of L-NAME at equimolar concentration, very strongly potentiated and converted into a 13 times higher biphasic one. D-NAME (5 mM) was only able to induce a 3 times higher response, but provoked a similar vasoconstrictor effect. 4 The small biphasic insulin secretion induced by L-leucine (5 mM) was also strongly enhanced, by 8 times, in the presence of L-NAME (5 mM) vs 2 times in the presence of D-NAME (5 mM). 5 beta cell responses to KCl (5 mM) and tolbutamide (0.185 mM) were only slightly increased by L-NAME (5 mM) to values not far from the sum of the effects of L-NAME and of the two drugs alone. D-NAME (5 mM) was totally ineffective on the actions of both secretagogues. 6 L-NAME, infused 15 min before and during a rise in glucose concentration from 5 to 11 mM, was able in the low millimolar range (0.1-0.5 mM) to blunt the classical biphasic pattern of beta cell response to glucose and, at 5 mM, to convert it into a significantly greater monophasic one. In contrast, D-NAME (5 mM) was unable to induce similar effects. 7 SNP alone at 3 mu M was ineffective but at 30 mu M substantially reduced the second phase of insulin response to glucose; however, at both concentrations the NO donor partly reversed alterations in insulin secretion caused by L-NAME (5 mM) and restored a biphasic pattern of insulin response. At a high (300 mu M) concentration, SNP drastically reduced the second phase of beta cell response, but in the presence of L-NAME, provoked a significantly greater biphasic response. 8 When L-NAME was infused only for the 15 min before high glucose, an exaggerated first phase of beta cell response was followed by an abortive second one. SNP, at a low concentration (30 nM), given simultaneously with L-NAME, restored a biphasic pattern and prevented the vasoconstrictor effect induced by the inhibitor. 9 L-NAME, when infused only during high glucose, markedly enhanced the second phase of insulin response which could be significantly reduced by SNP (3 mu M). The NO donor induced a dilator effect significantly greater in L-NAME-treated pancreata than in non-treated ones. 10 In conclusion our data bring evidence that NO synthase activity exerts an inhibitory control on pancreatic beta cell response to various nutrient secretagogues and may, at least partly, be implicated in the generation of the biphasic pattern of insulin response to glucose.