β-adrenergic blockers increase cAMP and stimulate insulin secretion through a PKA/RYR2/TRPM5 pathway in pancreatic β-cells in vitro

Abstract β-adrenergic blockers (β-blockers) are extensively used to inhibit β-adrenoceptor activation and subsequent cAMP production in many cell types. In this study, we characterized the effects of β-blockers on mouse pancreatic β-cells. Unexpectedly, high doses (100 μM) of β- blockers (propranolol and bisoprolol) led to a 5–10 fold increase in cAMP levels, enhanced intracellular influx, and stimulated a 2–4 fold increase in glucose-and glimepiride-induced insulin secretion in MIN6-K8 clonal β-cells and isolated mouse pancreatic islets. These effects were observed despite minimal expression of β-adrenoceptors in these cells. Mechanistically, cAMP increase led to ryanodine receptor 2 (RYR2) phosphorylation via protein kinase A (PKA), triggering Ca 2+ -induced Ca 2+ release (CICR). CICR then activates transient receptor potential cation channel subfamily M member 5 (TRPM5), resulting in increased Ca 2+ influx via voltage-dependent Ca 2+ channels. These effects contradict the conventional understanding of the pharmacology of β-blockers, highlighting the variability in β-blocker actions depending on the experimental context. Graphical abstract At high concentrations (> 10 μM), the β-adrenergic blocker propranolol paradoxically increased intracellular cAMP levels in pancreatic β-cells. This leads to PKA-induced RYR2 phosphorylation and extracellular Ca 2+ influx, leading to CICR from the ER. CICR activated TRPM5, which augmented β-cell electrical activity, extracellular Ca 2+ influx, and insulin secretion.