1 Recent studies have suggested that the generation of nitric oxide (NO) and hydrogen peroxide (H202) by islet NO synthase and monoamine oxidase, respectively, may have a regulatory influence on insulin secretory processes. We have investigated the pattern of insulin release from isolated islets of Langerhans in the presence of various pharmacological agents known to perturb the intracellular levels of NO and the oxidation state of SH-groups.2 The NO synthase inhibitor, N0-nitro-L-arginine methyl ester (L-NAME) dose-dependently increased L-arginine-induced insulin release. D-Arginine did not influence L-arginine-induced insulin secretion.However, D-NAME which reportedly has no inhibitory action on NO synthase, modestly increased L-arginine-induced insulin release, but was less effective than L-NAME. High concentrations (10 mM) of D-arginine as well as L-NAME and D-NAME could enhance basal insulin release. 3 The intracellular NO donor, hydroxylamine, dose-dependently inhibited insulin secretion induced by L-arginine and L-arginine + L-NAME.4 Glucose-induced insulin release was increased by NO synthase inhibition (L-NAME) and inhibited by the intracellular NO donor, hydroxylamine. Sydnonimine-I (SIN-1), an extracellular donor of NO and superoxide, induced a modest suppression of glucose-stimulated insulin release. SIN-1 did not influence insulin secretion induced by L-arginine or the adenylate cyclase activator, forskolin. L-arginine or tert-butylhydroperoxide at 3 or 300 gM but was markedly increased (14 fold) by a high hydroxylamine concentration (300 gM). In contrast, islet adenosine 3':5'-cyclic monophosphate (cyclic AMP) content was increased (3 fold) by L-arginine (10 mM) and (2 fold) by tert-butylhydroperoxide (300 gM).7 Our results strongly suggest that NO is a negative modulator of insulin release induced by the nutrient secretagogues L-arginine and glucose. This effect is probably not mediated to any major extent by the guanylate cyclase-cyclic GMP system but may rather be exerted by the S-nitrosylation of critical thiol groups involved in the secretory process. Similarly the inhibitory effect of tert-butylhydroperoxide is likely to be elicited through affecting critical thiol groups. The mechanism underlying the secretionpromoting action of tert-butylhydroperoxide on IBMX-induced insulin release is probably linked to intracellular Ca2'-perturbations affecting exocytosis.8 Taken together with previous data the present results suggest that islet production of low physiological levels of free radicals such as NO and H202 may serve as important modulators of insulin secretory processes.

1 The nitric oxide (NO)-releasing properties of two new mesoionic 3-aryl substituted oxatriazole-5-imine derivatives (GEA 3162 and GEA 3175) were characterized and compared with the known NOdonors 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP). 2 GEA 3162, GEA 3175, SIN-i and SNAP inhibited adenosine 5'-diphosphate-induced platelet aggregation (IC50 values 0.18, 0.39, 3.73 and 2.12 gM, respectively). All four compounds induced a dosedependent and more than 4 fold increase in cyclic GMP in platelets. The increase in cyclic GMP concentration was potentiated more than 1.5 fold by a phosphodiesterase inhibitor, zaprinast (10 gM) and inhibited 38-97% by oxyhaemoglobin (10-45 gM).3 All of the four compounds studied converted oxyhaemoglobin to methaemoglobin and formed a paramagnetic NO-haemoglobin complex. All but GEA 3175 formed nitrite and nitrate in phosphate buffer. During a 40 min incubation, GEA 3162, SIN-I and SNAP (100 pM) produced 50-70 gM NO2-+ NO3-as determined by high performance liquid chromatography. The release of NO and NO2 by GEA 3175 was increased 140 fold in the presence of human plasma (0.14 and 19.7 ppb in the absence and presence of 1% human plasma, respectively) as analyzed by ozone chemiluminescence. 4 The results suggest that the mesoionic 3-aryl substituted oxatriazole-5-imine derivatives GEA 3162 and GEA 3175 as well as SIN-l and SNAP release nitric oxide.

Cyclic nucleotides (cAMP, cGMP) are suggested to participate in the regulation of cell growth and differentiation. Guanylate cyclase is the enzyme which catalyzes the synthesis ofcGMP. The basal guanylate cyclase activity was slightly higher in well-differentiated squamous cell carcinomas than in normal mucosa, but was two-fold higher in papillomas. Poorly differentiated squamous cell carcinoma did not show any increased basal activity. Stimulation with nitroprusside (NP) resulted in a 20% increased activity for normal mucosa and a 30% increase for poorly differentiated carcinomas, whereas enzyme prepared from well-differentiated squamous carcinomas and papillomas showed a two-fold increase.

In mice, injected subcutaneously with nitroglycerin (GTN) for 12 days, adrenaline exhibited an increased toxicity. The LD50 value for adrenaline in control animals was 11.1 mg/kg b.wt. In GTN‐treated animals the LD50 value for adrenaline, measured 3 days after the last injection of GTN, was 9.1 mg/kg b.wt. (P = 0.05). In the animals sensitized with GTN, the adrenergic α‐receptor blocker phentolamine (1 or 10 mg/kg b.wt.) protected from the lethal action of adrenaline (P = 0.06 and P = 0.001, respectively). A low dose (1 mg/kg b.wt.) of the adrenergic β receptor blocker propranolol, was without effect while a higher dose (10 mg/kg b.wt.) potentiated the toxicity of adrenaline (P = 0.007). The α1 adrenoreceptor antagonist, prazosin, (1 or 10 mg/kg b.wt.) was found to be highly effective in protecting the GTN‐sensitized mice towards adrenaline (P = 0.003 and P = 0.001, respectively). By contrast, the α2 adrenoreceptor antagonist, yohimbine, (1 or 10 mg/kg b.wt.) was much less effective (P = 0.988 and P = 0.111, respectively). It is concluded that the lethal action of adrenaline was caused by stimulation of α1 adrenoreceptors, and that long term treatment with GTN caused a sensitization of these receptors in mice. The possible relevance of this finding for the reported wihdrawal symptoms and sudden death phenomenon in nitroglycerin‐exposed industrial workers is discussed.