Monoamine oxidase (MAO) in pancreatic islets of the mouse: some characteristics and the effect of chemical sympathectomy.

We have previously observed that chemical sympathectomy in the guinea pig induced an increase in the intensity of serotonin fluorescence in the insulin cells of the pancreatic islets. In the present study we have studied some basal characteristics of mouse islet monoamine oxidase (MAO) and the effect of chemical sympathectomy induced by 6-hydroxydopamine on the activity of this enzyme. Estimated Km-values for MAO activity with different substrates were as follows: serotonin (5-HT; 2 x 10(-4) M), dopamine (DA; 4 x 10(-4) M), and 2-phenylethylamine (PEA; 4 x 10(-6) M). The highest specific activity was displayed with PEA and DA as substrates, whereas it was about 10 times lower with 5-HT as substrate. Two days after the destruction of islet adrenergic nerves by 6-hydroxydopamine we observed a decrease of approximately 30% of the MAO activity with serotonin as substrate and after seven days the MAO activity with PEA as substrate had decreased by 18%. By contrast the MAO activity with DA as substrate was slightly increased (+ 12%) at two days after sympathectomy. There is a need for a further characterization of islet MAO activity(ies) and for further studies on its putative role in insulin secretory processes through its regulation of the islet monoamine stores.