Thapsigargin defines roles of Ca2+ in initial, sustained, and potentiated stimulation of pepsinogen secretion
Kitsukawa, Y.; Felley, C.; Metz, D.C.; Jensen, R.T.
American Journal of Physiology 266(4 Pt 1): G613-G623
1994
ISSN/ISBN: 0002-9513 PMID: 8179000 Document Number: 427637
The roles of Ca-2+ 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 Ca-2+ from intracellular stores by inhibiting microsomal Ca-2+-adenosinetriphosphatase (ATPase). thereby depleting intracellular Ca-2+ (Ca-i-2+) stores. In most cells TG also causes Ca-2+ influx. In the present study, with an extracellular Ca-2+ concentration ((Ca-2+)-o) of 1.5 mM, CCK-8 (0.1 mu-M) caused a rapid increase in pepsinogen secretion; however, the rate decreased with time. With (Ca-2+)-o = 0, the initial increase was similar but later secretion was abolished, suggesting that Ca-2+ influx was important for sustained secretion. With (Ca-2+)-o = 1.5 mM, TG (0.1 mu-M) caused a 2.7-fold sustained increase in Ca-i-2+ concentration ((Ca-2+)-i and a ninefold sustained increase in pepsinogen secretion. With (Ca-2+)-o = 0, TG caused a transient 66% increase in (Ca-2+)-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 (Ca-2+)-i. These data demonstrated that Ca-2+ influx itself was a potent stimulant of pepsinogen secretion. We further focused on the roles of increasing (Ca-2+), from Ca-2+ stores. With or without extracellular Ca-2+ (Ca-o-2+) present, addition of CCK-8 (0.1 mu-M) 10 min after TG caused no further increase in (Ca-2+)-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 Ca-2+, 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 Ca-o-2+ where (Ca-2+)-o, remained elevated. However, TG-induced potentiation of secretin-stimulated pepsinogen secretion was abolished once (Ca-2+)-i had returned to the basal level in the absence of Ca-2+. These results suggest that 1) increases in (Ca-2+)-i due to Ca-2+ influx caused by TG or CCK-8 can result in pepsinogen secretion, 2) mobilization of Ca-2+ from intracellular stores is important in the rapid initial phase stimulation of pepsinogen secretion caused by CCK-8, and 3) the increase in (Ca-2+)-i per se, whether caused by mobilization from Ca-i-2+ stores or Ca-2+ influx, is responsible for CCK-8 or TG potentiating secretin-stimulated pepsinogen secretion.