Alterations of insulin response to different β cell secretagogues and pancreatic vascular resistance induced by Nω-nitro-L-arginine methyl ester

1 We studied a possible interplay of pancreatic NO synthase activity on insulin secretion induced by different β 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 Nω-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 β cell responses to KC1 (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 β 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 μm was ineffective but at 30 μm 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 μm) concentration, SNP drastically reduced the second phase of β 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 β 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 μm). 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 β 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.