Oleic acid glucose-independently stimulates glucagon secretion by increasing cytoplasmic Ca2+ via endoplasmic reticulum Ca2+ release and Ca2+ influx in the rat islet alpha-cells.
The effect of long-chain free fatty acids on glucagon secretion from islet alpha-cells has been a controversial issue. This study examined direct effects of oleic acid (OA) on glucagon release from rat pancreatic islets and on cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in single alpha-cells by fura-2 fluorescence imaging. OA at 30 microM increased glucagon release from isolated islets in the presence of low (2.8 mM) and elevated (8.3 mM) glucose concentrations. OA at 6-10 microm concentration-dependently increased [Ca(2+)](i) in alpha-cells, irrespective of glucose concentrations (1.4, 2.8, and 8.3 mM). OA at 10 mum increased [Ca(2+)](i) in 90% of alpha-cells. OA-induced [Ca(2+)](i) increases were strongly inhibited by the endoplasmic reticulum Ca(2+)-pump inhibitors cyclopiazonic acid and thapsigargin and by 2-aminoethoxydiphenyl borate, the blocker of both inositol 1,4,5-trisphosphate receptors and store-operated Ca(2+) channels. Furthermore, the amplitude, but not incidence, of OA-induced [Ca(2+)](i) increases was reduced substantially by Ca(2+)-free conditions and mildly by an L-type Ca(2+) channel blocker, nitrendipine, and an ATP-sensitive K(+) channel activator, diazoxide. OA-induced glucagon release was also inhibited mildly by nitrendipine and strongly by 2-aminoethoxydiphenyl borate. These results indicate that OA glucose-independently stimulates glucagon release by increasing [Ca(2+)](i) in rat pancreatic alpha-cells and that the [Ca(2+)](i) increase is triggered by Ca(2+) release from endoplasmic reticulum and amplified by Ca(2+) influx possibly via store-operated channels and via voltage-dependent L-type Ca(2+) channels. The glucose-independent action of OA to stimulate glucagon release from alpha-cells may operate under hypoglycemic conditions when plasma free fatty acids levels are elevated, possibly playing a role in maintaining glucose metabolism.
Fujiwara
Ken
K
Department of Integrative Physiology, Jichi Medical University, School of Medicine, Shimotsuke, Tochigi 329-0498, Japan.
Maekawa
Fumihiko
F
Dezaki
Katsuya
K
Nakata
Masanori
M
Yashiro
Takashi
T
Yada
Toshihiko
T
D000818:Animals; D001896:Boron Compounds / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2; D005934:Glucagon / Q000378:metabolism*; D050416:Glucagon-Secreting Cells / Q000187:drug effects* / Q000378:metabolism*; D005947:Glucose / Q000494:pharmacology*; D007211:Indoles / Q000494:pharmacology; D019301:Oleic Acid / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D019284:Thapsigargin / Q000494:pharmacology
D001896:Boron Compounds; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007211:Indoles; D019301:Oleic Acid; D019284:Thapsigargin; D005934:Glucagon; C109986:2-aminoethoxydiphenyl borate; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D005947:Glucose; D002118:Calcium; D016257:Fura-2; C000543:cyclopiazonic acid