Pancreatic islet cells are highly susceptible towards the cytotoxic effects of chemically generated nitric oxide.

To compare the sensitivity of different mammalian cell types towards the cytotoxic action of nitric oxide, freshly isolated rat pancreatic islet cells, hepatocytes, resident and activated macrophages, cultured aortic endothelial cells and two murine tumor cell lines were tested for susceptibility towards exogenous nitric oxide. As sources for nitric oxide nitroprusside, S-nitroso-N-acetyl-penicillamine and the sydnonimine-derivative SIN-1 were used. These generate nitric oxide by different mechanisms and kinetics. Among the cell types tested we found large differences in their susceptibility towards the three nitric oxide donors. Islet cells were by far the most sensitive of the investigated cells and were completely lysed by all three nitric oxide donors. Hepatocytes and endothelial cells were sensitive towards nitroprusside but relatively resistant towards toxicity of SIN-1 and S-nitroso-N-acetyl-penicillamine. Activated and resident macrophages were lysed by SIN-1, whereas high concentrations of nitroprusside and S-nitroso-N-acetyl-penicillamine led to partial cell lysis only. The tumor cell lines were both lysed by SIN-1 but showed differences in their sensitivity towards S-nitroso-N-acetyl-penicillamine. Nitric oxide, which is produced in large amounts during infection and inflammation, may play an important role in the destruction of islet cells during insulitis leading to insulin-dependent diabetes mellitus.

Correlation between nitric oxide formation during degradation of organic nitrates and activation of guanylate cyclase.

Organic nitrates develop their vasodilating potency by stimulating the enzyme guanylate cyclase. There are still several theories concerning the molecular mechanism of enzyme activation, the most likely of which sees nitric oxide (NO.) as the true modulator of the soluble guanylate cyclase. We therefore examined the release of nitric oxide from organic nitrates by means of a difference-spectrophotometric method and found that our results correlated well with the extent of enzyme activation. The more NO. was liberated from the compounds in question, the higher was the enzyme activation observed. When the examined nitrates were used in a concentration which caused a half-maximal enzyme stimulation, the result was a NO. liberation of striking uniformity. This correlation also applied to SIN-1 for which it has been assumed up to now that the intact molecule itself is able to stimulate the enzyme and not the nitric oxide released from it. We found the reaction between organic nitrates and cysteine to be highly dependent on temperature, while the extent of the observed enhancement increased with the number of nitrate groups per molecule. We also studied the potential effects of certain compounds on non-enzymatic NO. release and found that, in addition to methylene blue, thionine and brilliantcresyl blue, but not ferricyanide, were also effective inhibitors. So it seems likely that both an enzymatic and a non-enzymatic mode of inhibition of enzyme activity does exist. Since oxyhemoglobin is an effective scavenger of nitric oxide, its addition can inhibit enzyme activation by nitrovasodilators. Our results stress the important role of the non-enzymatic liberation of NO. from organic nitrates and related compounds as possible, perhaps even as the principal mode of activation of soluble guanylate cyclase by nitrovasodilators.

The effect of triamterene on myocardial phosphodiesterase and adenylate cyclase.

The study concerned the effect of triamterene (TA) and its phase-II metabolite p-hydroxytriamterene-sulphuric acid ester (OH-TA ester) on myocardial phosphodiesterase (PDE) and adenylate cyclase (AC): (1) TA and OH-TA ester exerted a concentration-dependent inhibition of PDE. The estimated IC50 values of 127 and 72 mumol/l were 16 and 9 times higher than that of papaverine with 8 mumol/l. At comparable concentrations, TA produced its inotropic effect (EC50 96 mumol/l) in isolated guinea-pig atria so that PDE inhibition may well account for its pharmacodynamic action. (2) In isolated guinea-pig atria, the cyclic AMP content increased significantly with 15-30 s after the addition of TA and remained increased over a period of 15 min. This increase preceded the positive inotropic action, which reached its maximum after 5-10 min. (3) TA did not affect the basal activity of myocardial AC preparations. AC activation by isoprenaline (30 mumol/l) was, however, strongly inhibited.