G-protein tonic inhibition of calcium channels in pancreatic β-cells.

Voltage-gated calcium channels (Ca) conduct Ca influx promoting neurotransmitters and hormone release. Ca are finely regulated by voltage-dependent and independent pathways either by G-protein-coupled receptors (GPCRs) or intramembrane lipids, respectively, in neurons and glands. Interestingly, pancreatic β-cells are abundantly innervated by both sympathetic and parasympathetic neurons, while a variety of high-voltage activated (HVA) Ca channels are present in these cells. Thus, autonomic system seems to exert a tonic inhibition on HVA Ca channels throughout GPCRs, constitutively preventing hormone secretion. Therefore, this work aimed to investigate noradrenergic and cholinergic inhibition of HVA Ca channels in pancreatic β-cells. Experiments were conducted in pancreatic β-cells of rat by using patch-clamping methods, immunocytochemistry, pharmacological probes, and biochemical reagents. A voltage-clamp protocol with a strong depolarizing prepulse was used to unmask tonic inhibition. Herein, we consistently find a basal tonic inhibition of HVA Ca channels according to a GPCRs regulation. Facilitation ratio is enhanced by noradrenaline (NA) according to a voltage-dependent regulation and a membrane-delimited mechanism, while no facilitation changes are observed with carbachol or phosphatidylinositol 4,5-bisphosphate (PIP). Furthermore, carbachol or intramembrane lipids, such as PIP, do not change facilitation ratio according to a voltage-independent regulation. Together, HVA Ca channels of pancreatic β-cells are constitutively inhibited by GPCRs, suggesting a natural brake preventing cells from exhaustive insulin secretion. Our results support the hypothesis that GPCRs tonically inhibit HVA Ca channels in pancreatic β-cells. A voltage-clamp protocol with a strong depolarizing prepulse was used to unmask voltage-dependent inhibition of Ca channels. The novelty of these results strengthens the critical role of Gβγ's in Ca channel regulation, highlighting kinetic slowing and increased facilitation ratio. Together, HVA Ca channels of pancreatic β-cells are constitutively inhibited by GPCRs underlying fine-tuning modulation of insulin secretion.