Thapsigargin defines roles of Ca2+ in initial, sustained, and potentiated stimulation of pepsinogen secretion.
The roles of Ca2+ in agonist-induced pepsinogen secretion from guinea pig chief cells remain unclear. We used cholecystokinin octapeptide (CCK-8) or secretin alone or with thapsigargin (TG) to clarify these roles. TG releases Ca2+ from intracellular stores by inhibiting microsomal Ca(2+)-adenosinetriphosphatase (ATPase), thereby depleting intracellular Ca2+ (Cai2+) stores. In most cells TG also causes Ca2+ influx. In the present study, with an extracellular Ca2+ concentration ([Ca2+]o) of 1.5 mM, CCK-8 (0.1 microM) caused a rapid increase in pepsinogen secretion; however, the rate decreased with time. With [Ca2+]o = 0, the initial increase was similar but later secretion was abolished, suggesting that Ca2+ influx was important for sustained secretion. With [Ca2+]o = 1.5 mM, TG (0.1 microM) caused a 2.7-fold sustained increase in in Cai2+ concentration ([Ca2+]i) and a ninefold sustained increase in pepsinogen secretion. With [Ca2+]o = 0, TG caused a transient 66% increase in [Ca2+]i and a 50% increase in pepsinogen secretion. The time course of TG-induced pepsinogen secretion correlated with the time course of TG-induced increases in [Ca2+]i. These data demonstrated that Ca2+ influx itself was a potent stimulant of pepsinogen secretion. We further focused on the roles of increasing [Ca2+]i from Cai2+ stores. With or without extracellular Ca2+ (Cao2+) present, addition of CCK-8 (0.1 microM) 10 min after TG caused no further increase in [Ca2+]i, demonstrating depletion of the inositol 1,4,5-trisphosphate-sensitive pool. The Ca(2+)-mobilizing agent CCK-8 caused no pepsinogen secretion 10 min after TG preincubation, demonstrating that mobilization of Ca2+ from intracellular stores was important in the rapid initial phase stimulation of pepsinogen secretion caused by CCK-8. In contrast, preincubation with TG had no effect on pepsinogen secretion by secretin, an agent that increases adenosine 3',5'-cyclic monophosphate. A 6-min preincubation with TG potentiated the subsequent stimulation of pepsinogen secretion caused by secretin in the presence of Cao2+ where [Ca2+]i remained elevated. However, TG-induced potentiations of secretin-stimulated pepsinogen secretion was abolished once [Ca2+]i had returned to the basal level in the absence of Cao2+.(ABSTRACT TRUNCATED AT 400 WORDS)
Kitsukawa
Y
Y
Digestive Diseases Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Felley
C
C
Metz
D C
DC
Jensen
R T
RT
D000818:Animals; D002118:Calcium / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005753:Gastric Mucosa / Q000166:cytology / Q000378:metabolism*; D006168:Guinea Pigs; D007700:Kinetics; D008297:Male; D010435:Pepsinogens / Q000378:metabolism*; D010936:Plant Extracts / Q000494:pharmacology; D012633:Secretin / Q000494:pharmacology; D012844:Sincalide / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D010435:Pepsinogens; D010936:Plant Extracts; D013729:Terpenes; D012633:Secretin; D019284:Thapsigargin; D012844:Sincalide; D002118:Calcium