Lactate activation of α-cell KATP channels inhibits glucagon secretion by hyperpolarizing the membrane potential and reducing Ca2+ entry

Abstract

Objective Elevations in pancreatic α-cell intracellular Ca 2+ ([Ca 2+ ] i ) lead to glucagon (GCG) secretion. Although glucose inhibits GCG secretion, how lactate and pyruvate control α-cell Ca 2+ handling is unknown. Lactate enters cells through monocarboxylate transporters (MCTs) and is also produced during glycolysis by lactate dehydrogenase A (LDHA), an enzyme expressed in α-cells. As lactate activates ATP-sensitive K + (K ATP ) channels in cardiomyocytes, lactate may also modulate α-cell K ATP . Therefore, this study investigated how lactate signaling controls α-cell Ca 2+ handling and GCG secretion. Methods Mouse and human islets were used in combination with confocal microscopy, electrophysiology, GCG immunoassays, and fluorescent thallium flux assays to assess α-cell Ca 2+ handling, V m , K ATP currents, and GCG secretion. Results Lactate-inhibited mouse (75 ± 25%) and human (47 ± 9%) α-cell [Ca 2+ ] i fluctuations only under low-glucose conditions (1 mM) but had no effect on β- or δ-cells [Ca 2+ ] i . Glyburide inhibition of K ATP channels restored α-cell [Ca 2+ ] i fluctuations in the presence of lactate. Lactate transport into α-cells via MCTs hyperpolarized mouse (14 ± 1 mV) and human (12 ± 1 mV) α-cell V m and activated K ATP channels. Interestingly, pyruvate showed a similar K ATP activation profile and α-cell [Ca 2+ ] i inhibition as lactate. Lactate-induced inhibition of α-cell [Ca 2+ ] i influx resulted in reduced GCG secretion in mouse (62 ± 6%) and human (43 ± 13%) islets. Conclusions These data demonstrate for the first time that lactate entry into α-cells through MCTs results in K ATP activation, V m hyperpolarization, reduced [Ca 2+ ] i , and inhibition of GCG secretion. Thus, taken together, these data indicate that lactate either within α-cells and/or elevated in serum could serve as important modulators of α-cell function.

A plasma membrane-associated glycolytic metabolon is functionally coupled to KATPchannels in pancreatic α and β cells from humans and mice

“…In addition to glycolysis, LDH activity was found to be present in the K ATP microcompartment and efficiently drive NAD + production to support glycolysis at the GAPDH step in both human α and β cell membranes. In support of previously published work 32,35–37 , we found that lactate opens K ATP channels and opposes PK-mediated K ATP closures in mouse α cells. Since lactate can reach millimolar levels in the blood, and α-cells express a high level of MCT pyruvate/lactate transporters ( Slc16a1 ) 32 , lactate regulation of K ATP channels is expected to occur in vivo.…”

“…In addition to glycolysis, LDH activity was found to be present in the K ATP micro-compartment by driving its NAD + production to support glycolysis at the GAPDH step in both human α and β cells. In support of previously published work 27,30–32 , we found that lactate opens K ATP channels and opposes PK-mediated K ATP closures in mouse α cells, which express a high level of MCT1 ( Slc16a1 ) 27 . However, lactate derived from glycolytic pyruvate had no impact on K ATP channels in our experiments.…”

“…Given the ability of lactate to open K ATP channels and hyperpolarize mouse α cells that express MCT1 pyruvate/lactate transporters 27 , we next tested whether exogenous lactate at concentrations mimicking circulatory levels is potent enough to oppose PK-mediated K ATP channel closure in mouse α cell membranes. The application of 2 mM lactate was able to reopen K ATP channels inhibited by 2 mM PEP and 0.5 mM ADP (Supplemental Figure S3A, B).…”

Combined proteomic and lipidomic studies in Pompe disease allow a better disease mechanism understanding

“…Pathway enrichment analysis shows that LDHB and PKM participate in glucagon signaling pathway, connected with nutrient deprivation and the breakdown of cell glycogen. Interestingly, the substrate of LDHB, lactate, and the product of PKM, pyruvate, reduce the secretion of glucagon in pancreatic cells, 54 while glucagon deactivates the PKM enzyme, inhibiting glycolysis and regulating glucose homeostasis 55 . Therefore, the up‐regulation of these two glycolytic enzymes, may reflect the impairment of glucose/glycogen metabolism in PD 56 …”

Liraglutide increases islet Ca2+ oscillation frequency and insulin secretion by activating hyperpolarization‐activated cyclic nucleotide‐gated channels

“…Widefield or confocal microscopy was used to measure βand δ-cell intracellular Ca 2+ ([Ca 2+ ] i ) and cAMP levels using fluorescent dyes or genetically encoded indicators as previously described. 33,34 A detailed description can be found in Appendix S1.…”