Critical roles of carbon monoxide and nitric oxide in Ca ²⁺ signaling for insulin secretion in pancreatic islets

Aims: Glucagon-like peptide-1 (GLP-1) increases intracellular Ca ²⁺ concentrations, resulting in insulin secretion from pancreatic β-cells through the sequential production of Ca ²⁺ mobilizing messengers nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR). We previously found that NAADP activates the neuronal type of nitric oxide (NO) synthase (nNOS), the product of which, NO, activates guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP), which, in turn, induces cADPR formation. Our aim was to explore the relationship between Ca ²⁺ signals and gasotransmitters formation in insulin secretion in β-cells upon GLP-1 stimulation. Results: We show that NAADP-induced cGMP production by nNOS activation is dependent on carbon monoxide (CO) formation by heme oxygenase-2 (HO-2). Treatment with exogenous NO and CO amplifies cGMP formation, Ca ²⁺ signal strength, and insulin secretion, whereas this signal is impeded when exposed to combined treatment with NO and CO. Furthermore, CO potentiates cGMP formation in a dose-dependent manner, but higher doses of CO inhibited cGMP formation. Our data with regard to zinc protoporphyrin, a HO inhibitor, and HO-2 knockdown, revealed that NO-induced cADPR formation and insulin secretion are dependent on HO-2. Consistent with this observation, the administration of NO or CO donors to type 2 diabetic mice improved glucose tolerance, but the same did not hold true when both were administered concurrently. Innovation: Our research reveals the role of two gas transmitters, CO and NO, for Ca ²⁺ second messengers formation in pancreatic β-cells. Conclusion: These results demonstrate that CO, the downstream regulator of NO, plays a role in bridging the gap between the Ca ²⁺ signaling messengers during insulin secretion in pancreatic β-cells.