Golden hamster pancreatic islets: a tissue rich in monoamine oxidase

We compared the steady state monoamine oxidase (MAO) activity of collagenase-isolated pancreatic islets of the golden hamster with the steady state MAO activity of liver, kidney, median eminence, pituitary, acinar pancreas, and cerebral cortex. The MAO activity of the islets (5384 +/- 412 pmol/mg protein . min) was 3-fold greater than the activity of the next highest tissue (liver) and 12.5 times greater than the activity of the acinar pancreas. This high MAO activity was not due to collagenase exposure. The islet MAO was mainly of the so-called type B, although there was also some type A activity in this tissue. By assaying the formation of new islet MAO after the irreversible activation of MAO by the MAO inhibitor pargyline, we found that the t 1/2 of islet MAO (5.9 days with 95% confidence limits of 4.1-10.5 days) did not differ from the t 1/2 of MAO in tissues with lower steady state MAO activities. This suggested that the high steady state MAO activity in golden hamster islets is due to a high rate of synthesis rather than to a lower rate of degradation. Although rats, Chinese hamsters, guinea pigs, mice, and rabbits had substantial MAO activity in their pancreatic islets, their levels, at most, were only 16% that of the golden hamster. When the MAO activity of golden hamster islets was inhibited by the administration of pargyline plus clorgyline, there was a 3-fold increase in islet serotonin concentration, with no increase in islet norepinephrine concentration, suggesting that islet MAO activity plays a role in regulating islet serotonin concentration.