Stimulus-secretion coupling in the developing exocrine pancreas: secretory responsiveness to cholecystokinin.

Abstract

Abstract. We have studied the onset of secretory responsiveness to cholecystokinin (CCK) during development of the rat exocrine pancreas. Although acinar cells of the fetal pancreas (1 d before birth) are filled with zymogen granules containing the secretory protein, a-amylase, the rate of amylase secretion from pancreatic lobutes incubated in vitro was not increased in response to CCK. In contrast, the rate of CCKstimulated amylase discharge from the neonatal pancreas (1 d after birth) was increased four-to eightfold above that of the fetal gland. The postnatal amplification of secretory responsiveness was not associated with an increase in the number or cell surface expression of ~zsI-CCK binding sites. When 125I-CCK-33 binding proteins were analyzed by affinity crosslinking, two proteins of Mr 210,000 and 100,000-160,000 were labeled specifically in both fetal and neonatal pancreas. To determine if cell surface receptors for CCK in the fetal pancreas are functional and able to generate a rise in the cytosolic [Ca++], we measured 4sCa÷* et~ux from tracer-loaded lobules. 45Ca++ efllux from both fetal and neonatal pancreas was comparably increased by CCK, indicating CCKinduced Ca ÷+ mobilization and elevated cytosolic [Ca÷÷]. The Ca ++ ionophore A23187 also stimulated the rate of 45Ca++ extrusion from pancreas of both ages. Increased amylase secretion occurred concurrently with A23187-stimulated 45Ca++ efflux in neonatal pancreas, but not in the fetal gland. A23187 in combination with dibutyryl cAMP potentiated amylase release from the neonatal gland, but not from fetal pancreas. Similarly, the protein kinase C activator, phorbol dibutyrate, did not increase the rate of secretion from the fetal gland when added alone or in combination with A23187 or CCK. We suggest that CCK-receptor interaction in the fetal pancreas triggers intracellular Ca ++ mobilization. However, one or more signal transduction events distal to Ca +÷ mobilization have not yet matured. The onset of secretory response to CCK that occurs postnatally may depend on amplification of these transduction events.

Pharmacological and biochemical characterization of cholecystokinin/gastrin receptors in developing rat pancreas

“…In contrast to our present results, two proteins of 210 kDa and 100-160 kDa were previously identified as CCK receptors in both foetal and neonatal rat pancreas [7], where affinity labelling of the CCKA receptors was carried out with '251-BH-CCK-33, shown to cross-link not only the receptor but also a neighbouring protein [35]. The present molecular characterization used the cleavable photoactivable CCK ligand, '251-ASD-[Thr34,Ahx37]CCK(31 -39), well characterized as a powerful and specific probe for the pancreatic CCK receptor [17].…”

“…Additionally, no specific gastrin binding was demonstrated in pancreata of foetal rats (day 19 of gestation). In control adult rats, only by using very high amounts of membrane material and high ligand concentrations were specific '251-BH-de~-S03H-ga~trin (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) …”

“…[Thr34,Ahx37]-CCK(31-39) (Ahx, aminohexanoic acid), human cles-S03H-[AhxlS]gastrin, des-S03H-gastrin(2 -17) and gastrin-(2-17) were synthesized as described in [13, 141. (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) and gastrin (2 -17) were radiodinated by conjugation with lz5I-BH and purified by reverse-phase HPLC, as described in [16]. The specific activity was 66.6-74.0 GBq/pmol.…”

Carboxyl-Methylation of Rab3D in the Rat Pancreatic Acinar Tumor Cell Line AR42J

“…Third, we show here that dexamethasone induction of a regulated secretory pathway goes hand in hand with an obvious increase in unmethylated rab3D. This phenomenon is strikingly similar to that of the developmental change in rab3D methylation state which occurs during the perinatal period of pancreas development (12), and which also correlates with the acquisition of secretion-stimulus coupling (27). Although these data argue strongly for a role of carboxylmethylation of rab3D in stimulus-secretion coupling, the precise function of carboxyl-methylation is unknown.…”

The Effect of Cholecystokinin-Octapeptide, Insulin, Glucagon, Triiodothyronine, and Epidermal Growth Factor on Amylase Activity in Fetal Pancreas in Vitro

“…Previous studies have suggested that the unresponsiveness of the term fetal rat pancreas to secretagogue stimulation was related to the low binding capacity of the receptors, whereas the underlying machinery of the stimulus-secretion coupling was already mature (24,25). However, it was suggested in a recent report (26), that the fetal and the neonatal rat pancreas had the same binding capacity to CCK-8. The difference in responsiveness to CCK-8, in terms of amylase release, was viewed as related to an immaturity of the signal transduction events distal to calcium mobilization.…”