Effects of guanine nucleotides on bombesin-stimulated signal transduction in rat pancreatic acinar cells.
To study the role of guanine nucleotide binding proteins (G proteins) in bombesin receptor signal transduction, we investigated the effects of guanine nucleotide analogues and of the G protein activator NaF on bombesin-induced amylase release, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) production and release of intracellular Ca2+ in rat pancreatic acini. In digitonin-permeabilized acini, guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), a well-known activator of G proteins, potentiated bombesin-induced Ins(1,4,5)P3 production and increased amylase release at low bombesin concentrations (< 10 nM). By contrast, GTP gamma S decreased bombesin-stimulated amylase release at high bombesin concentrations (> 10 nM). Fluoride (10 mM), another G protein activator, had similar effects to GTP gamma S on amylase release. However, unlike GTP gamma S it had no effect on Ins(1,4,5)P3 production and release of intracellular Ca2+ induced by high bombesin concentrations. GDP and its analogues, such as 2'-desoxyguanosine 5'-diphosphate (dGDP) or guanosine 5'-[beta-thio]diphosphate (GDP beta S), inhibit activation of G proteins. GDP and dGDP both inhibited amylase release and Ins(1,4,5)P3 production at all bombesin concentrations tested. In contrast, GDP beta S mimicked the effects of GTP gamma S on bombesin-stimulated amylase release and Ins(1,4,5)P3 accumulation. In conclusion, we suggest that bombesin receptor-mediated signal transduction involves G proteins in pancreatic acini. The correlation between inhibition of maximum-stimulated enzyme secretion and further increase in Ins(1,4,5)P3 production in response to GTP gamma S at high bombesin concentrations suggests that overstimulation of phospholipase C inhibits amylase release. The discrepant effects of GDP and of GDP beta S on phospholipase C activity and amylase release might be due to the ability of GDP beta S, but not of GDP to activate G proteins persistently after phosphorylation by G protein-associated GDP kinases.
Piiper
A
A
Abteilung für Gastroenterologie, Universitätsklinikum, Frankfurt/Main, Germany.
Stryjek-Kaminska
D
D
Stein
J
J
Caspary
W F
WF
Zeuzem
S
S
D000681:Amylases / Q000378:metabolism; D000818:Animals; D001839:Bombesin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D002463:Cell Membrane Permeability; D016257:Fura-2 / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000096:biosynthesis; D010179:Pancreas / Q000378:metabolism* / Q000502:physiology; D051381:Rats; D018004:Receptors, Bombesin / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D012969:Sodium Fluoride / Q000494:pharmacology
D006150:Guanine Nucleotides; D018004:Receptors, Bombesin; D015544:Inositol 1,4,5-Trisphosphate; D012969:Sodium Fluoride; D000681:Amylases; D019204:GTP-Binding Proteins; D001839:Bombesin; D002118:Calcium; D016257:Fura-2