A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic β-cells

Abstract

Jung SR, Reed BJ, Sweet IR. A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic ␤-cells. Am J Physiol Endocrinol Metab 297: E717-E727, 2009. First published July 7, 2009 doi:10.1152/ajpendo.00282.2009 ) influx is required for the sustained secretion of insulin and is accompanied by a large rate of energy usage. We hypothesize that the energy usage reflects a process [Ca 2ϩ /metabolic coupling process (CMCP)] that couples Ca 2ϩ to insulin secretion by pancreatic islets. The aim of the study was to test this hypothesis by testing the effect of inhibiting candidate Ca 2ϩ -sensitive proteins proposed to play a critical role in the CMCP. The effects of the inhibitors on oxygen consumption rate (OCR), a reflection of ATP usage, and insulin secretion rate (ISR) were compared with those seen when L-type Ca 2ϩ channels were blocked with nimodipine. We reasoned that if a downstream Ca 2ϩ -regulated site was responsible for the OCR associated with the CMCP, then its inhibition should mimic the effect of nimodipine. Consistent with previous findings, nimodipine decreased glucose-stimulated OCR by 36% and cytosolic Ca 2ϩ by 46% and completely suppressed ISR in rat pancreatic islets. Inhibitors of three calmodulin-sensitive proteins (myosin light-chain kinase, calcineurin, and Ca 2ϩ /calmodulin-dependent protein kinase II) did not meet the criteria. In contrast, KN-62 severed the connection between Ca 2ϩ influx, OCR, and ISR without interfering with Ca 2ϩ influx. In the presence of nimodipine or KN-62, potentiators of ISR, acetylcholine, GLP-1, and arginine had little effect on insulin secretion, suggesting that the CMCP is also essential for the amplification of ISR. In conclusion, a KN-62-sensitive process directly mediates the effects of Ca 2ϩ influx via L-type Ca 2ϩ

Control of Insulin Secretion by Cytochrome c and Calcium Signaling in Islets with Impaired Metabolism

“…-Cytosolic Ca 2ϩ was measured by fluorescent imaging of islets after loading them with Fura-2 AM (Invitrogen) as described previously (28). Dyed islets were pipetted into a temperature-controlled perifusion dish as a monolayer (Bioptechs, Butler, PA) that was mounted onto the stage of a Nikon Eclipse TE-200 inverted microscope, and fluorescent emission was detected at 510 nm during alternating excitation at either 340 nm or 380 nm.…”

“…We have previously established that Ca 2ϩ , along with cytochrome c signaling, co-stimulate a highly energetic process, the CMCP, whose activation is essential for stimulating ISR. This is based on observations that the glucose dose response of the CMCP is superimposable to that of ISR (12), the CMCP is blocked by an inhibitor of Ca 2ϩ -dependent kinases (15), and like ISR, the CMCP is only activated when both the reductive state of cytochrome c, and the activity of L-type Ca 2ϩ channels, are increased (11). In the present study, Ca 2ϩ -sensitive OCR was completely absent in the islets precultured in low glucose and quantitatively restored with the GKa.…”

“…The fact that both Ca 2ϩ and a metabolic factor need to be increased suggested to us a model where these two signals interact downstream, presumably via a regulatory protein complex. We have previously delineated this scenario by defining a process (the Ca 2ϩ /metabolic coupling process (CMCP)) that is activated only when both Ca 2ϩ influx is stimulated and cytochrome c is reduced and perhaps translocated (11,15 the work carried out by the regulatory process. Functionally, cytochrome c reduction and Ca 2ϩ influx both have to increase to activate the CMCP and ISR.…”

Reduced Cytochrome c Is an Essential Regulator of Sustained Insulin Secretion by Pancreatic Islets

“…These findings suggested that Ca 2ϩ -sensitive OCR reflects a highly energetic process that couples Ca 2ϩ to the secretion of insulin (6). In lieu of the identification of the proteins involved with this process, we termed this process the Ca 2ϩ /metabolic coupling process (CMCP) (24). Further studies revealed that potentiation of the insulin secretion rate (ISR) by protein kinase C (PKC) and cAMP-dependent pathways occur with only very small changes in OCR, demonstrating that activation of insulin secretion by these mechanisms is mediated by steps that are downstream of the CMCP (6).…”

Impact of Extracellular Calcium Along With Ascorbic Acid on Cellular Reactivation and Insulin Secretion in Diabetic Min6 Cells

“…The potent effect of Ca 2+ influx on oxygen consumption rate reflect an essential downstream and highly energetic process that couples Ca 2+ influx through L-type Ca 2+ channel with insulin secretion rate with the process Ca 2+ /metabolic coupling process (CMCP) 13 . Ascorbic acid (vitamin C) is a natural antioxidant, it can play a very important role in cell proliferation by scavenging free radicals.…”