319-OR: Na+/K+ ATPase Activation Is a Conserved Mechanism for Gi/o Protein-Coupled Receptor Control of Pancreatic ß-Cell Electrical Excitability and Insulin Secretion

G_i/o protein-coupled receptor (G_i/o-GPCR) inhibition of pancreatic β-cell Ca²⁺ entry limits islet insulin secretion; however, the mechanism has not been identified. As G_i/o-GPCRs activate G protein-gated inwardly rectifying K⁺ (GIRK) channels, we examined their contribution to β-cell G_i/o signaling. However, G_i/o-GPCR-induced β-cell GIRK currents were not observed; this was demonstrated with a pharmacological approach and by genetic knockout of the predominant β-cell GIRK channel subunit. Thus, we increased the chemical driving force for K⁺ movement through GIRK channels by removing extracellular K⁺. Interestingly, under these conditions G_i/o-GPCR-induced β-cell membrane potential (V_m) hyperpolarization was lost even though K⁺ channel activation could still hyperpolarize V_m, which indicated that G_i/o-GPCRs do not hyperpolarize β-cell V_m by activating K⁺ channels. As extracellular K⁺ is required for Na⁺/K⁺ ATPase (NKA) activity, we next tested and found that mouse and human β-cell NKAs were activated by G_i/o-GPCRs. Stimulation of G_i/o-GPCRs initiated cyclical β-cell NKA activity, resulting in islet Ca²⁺ oscillations. Moreover, paracrine activation of β-cell NKAs by intraislet δ-cell somatostatin (SST) secretion decelerated islet Ca²⁺ oscillations and diminished insulin secretion, which was mediated by direct activation of β-cell G_i/o-GPCRs. G_i/o signaling induced islet Ca²⁺ oscillations through Src tyrosine kinase-dependent NKA phosphorylation and activation; an effect that was recapitulated by stimulating insulin receptor tyrosine kinases. Whereas, islet NKA function was completely inhibited by cAMP-dependent PKA activation. Therefore, these findings reveal that NKA-mediated hyperpolarization of β-cell V_m is the primary and conserved mechanism for G_i/o-GPCR control of electrical excitability, Ca²⁺ handling, and insulin secretion.