Cyclic AMP/cAMP-GEF pathway amplifies insulin exocytosis induced by Ca2+ and ATP in rat islet beta-cells.
BACKGROUND
Cyclic AMP (cAMP) plays a pivotal role in insulin secretion induced by incretins. The effects of the second messenger extend to many sites and there has been much controversy on the mechanisms. The aim of this study was to examine how cAMP amplified insulin exocytosis.


METHODS
Rat islets were permeabilized with alpha-toxin to measure insulin exocytosis in the fixed conditions of Ca(2+) and ATP. The effects of several agents on insulin exocytosis were observed in perifusion experiments.


RESULTS
Cyclic AMP enhanced the Ca(2+)-induced insulin release by around 30%, independent of Ca(2+) concentrations between 10 and 3000 nmol/L. A cAMP-GEF selective cAMP analogue, 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate, also amplified insulin release. The effect disappeared in the absence of ATP. Conversely, cAMP-independent gradual increase in insulin release was observed with ATP. These results suggested that the site of action of cAMP-GEF existed proximal to that of ATP. An analogue selective to PKA, N(6)-Benzoyladenosine-3',5'-cyclic monophosphate, had little effect. Also, a PKA-selective inhibitor, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide, reduced insulin releases induced by 1000 nmol/L Ca(2+), but did not influence the relative increase produced by Ca(2+) and cAMP.


CONCLUSIONS
Cyclic AMP potentiated Ca(2+) and ATP-induced exocytosis to a similar relative extent independent of Ca(2+) concentrations. The process appeared to be mainly mediated by cAMP-GEF. In addition, the cAMP/cAMP-GEF pathway may enhance insulin release by replenishing the readily releasable pool.
Hashiguchi
Hiroshi
H
Department of Cardiovascular, Respiratory and Metabolic Medicine, Kagoshima University, Kagoshima, Japan.
Nakazaki
Mitsuhiro
M
Koriyama
Nobuyuki
N
Fukudome
Michiyo
M
Aso
Katsumi
K
Tei
Chuwa
C
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D000242:Cyclic AMP / Q000494:pharmacology*; D005089:Exocytosis / Q000187:drug effects*; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction
D020662:Guanine Nucleotide Exchange Factors; D007328:Insulin; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D002118:Calcium