Gi/o protein-coupled receptor (Gi/o-GPCR) inhibition of pancreatic β-cell Ca2+ entry limits islet insulin secretion; however, the mechanism has not been identified. As Gi/o-GPCRs activate G protein-gated inwardly rectifying K+ (GIRK) channels, we examined their contribution to β-cell Gi/o signaling. However, Gi/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 Gi/o-GPCR-induced β-cell membrane potential (Vm) hyperpolarization was lost even though K+ channel activation could still hyperpolarize Vm, which indicated that Gi/o-GPCRs do not hyperpolarize β-cell Vm 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 Gi/o-GPCRs. Stimulation of Gi/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 Gi/o-GPCRs. Gi/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 Vm is the primary and conserved mechanism for Gi/o-GPCR control of electrical excitability, Ca2+ handling, and insulin secretion.
M.Dickerson: None. D.Jacobson: None. P.Dadi: None. K.E.Zaborska: None. A.Y.Nakhe: None. C.Schaub: None. J.Dobson: None. N.M.Wright: None. J.Lynch: None. C.Scott: None.
American Diabetes Association (1-17-IBS-024) ; Vanderbilt Integrated Training in Engineering and Diabetes Grant (T32DK101003) , National Institutes of Health Grants (DK-097392 and DK-115620) , Juvenile Diabetes Research Foundation Grant (2-SRA-2019-701-S-B) , and a Pilot and Feasibility grant through the Vanderbilt Diabetes Research and Training Center Grant (P60-DK-20593) .