Occurrence and putative hormone regulatory function of a constitutive heme oxygenase in rat pancreatic islets

Recent observations suggest that carbon monoxide (CO) may serve as a neuroendocrine modulator in hypothalamus. Here we provide evidence, for the first time, that the islets of Langerhans contain the constitutive isoform of the CO-producing enzyme heme oxygenase (HO-2), the activity of which was found to modulate islet hormone release. Most insulin and glucagon cells in the rat endocrine pancreas expressed strong immunoreactivity for HO-2. In the exocrine parenchyma, scattered HO-2-positive ganglionic cell bodies were occasionally observed. Furthermore, Western blot analysis revealed the presence of HO-2 in isolated islets but not in acinar cells. Islet homogenates displayed a comparatively high HO-2 enzymatic activity measured as CO formation (approximately 600 pmol CO.min-1.mg islet protein-1). This HO-2 enzymatic activity was greatly suppressed by zincprotoporphyrin-IX (ZnPP-IX), a recognized inhibitor of HO activity. Neither ZnPP-IX nor the HO activator, hemin, influenced basal insulin release from isolated rat islets at low (1 mM) glucose. However, glucagon release at 1 mM glucose was increased by hemin and inhibited by ZnPP-IX. The hemin-induced increase in glucagon secretion was abolished by ZnPP-IX. Furthermore, a series of experiments at high glucose (16.7 mM) revealed that hemin induced a dose-dependent potentiation of glucose-stimulated insulin release. Moreover, glucose-induced insulin release was dose-dependently suppressed by ZnPP-IX but unaffected by protoporphyrin-IX, a compound known not to influence HO-2 activity in other tissues. Similarly, glucagon release at high glucose was dose-dependently increased by hemin and suppressed by ZnPP-IX. Finally, the hemin-induced increase in islet hormone release at high glucose was totally abolished by ZnPP-IX. The data strongly suggest that CO production positively modulates both glucagon and insulin secretion. We propose that CO may serve as a novel messenger molecule within the islets of Langerhans.