Secretagogue effects on intracellular calcium in pancreatic duct cells

Abstract

Regulation of intracellular free calcium ([Ca2+]i) in single epithelial duct cells of isolated rat and guinea pig pancreatic interlobular ducts by secretin, carbachol and cholecystokinin was studied by microspectrofluorometry using the Ca2(+)-sensitive, fluorescent probe Fura-2. Rat and guinea pig duct cells exhibited mean resting [Ca2+]i of 84 nM and 61 nM, respectively, which increased by 50%-100% in response to carbachol stimulation, thus demonstrating the presence of physiologically responsive cholinergic receptors in pancreatic ducts of both species. The carbachol-induced increase in [Ca2+]i involved both mobilization of Ca2+ from intracellular stores and stimulation of influx of extracellular Ca2+. In contrast, neither cholecystokinin nor secretin showed reproducible or sizeable increases in [Ca2+]i. Both rat and guinea pig duct cells showed considerable resting Ca2+ permeability. Lowering or raising the extracellular [Ca2+]i led, respectively, to a decrease or increase in the resting [Ca2+]i. Application of Mn2+ resulted in a quenching of the fluorescence signal indicating its entry into the cell. The resting Ca2+ and Mn2+ permeability could be blocked by La3+ suggesting that it is mediated by a Ca2+ channel.

Regulation of fluid secretion and intracellular messengers in isolated rat pancreatic ducts by acetylcholine.

“…In contrast, we found that ACh had no effect on the concentration of cyclic AMP in isolated ducts, confirming earlier results reported by Folsch et al (1980). However, as first shown by Stuenkel & Hootman (1990) using ducts that had been freshly isolated from the rat and guinea-pig pancreas, ACh does increase [Ca2"]1. We found that the [Ca2+]i in unstimulated duct cells was about 56 nm, and that this increased to about 103 and 82 nm during the peak and plateau phases respectively of the response to 1 /AM ACh.…”

“…Interestingly, ACh is much less effective at increasing [Ca2+], in rat duct as compared with acinar cells; a maximum dose causing a 2-fold [Ca2"], increase in ducts compared with a 5-fold increase in acini (Stuenkel & Hootman, 1990). Depletion of extracellular calcium inhibited the plateau phase of the [Ca2+]i response to ACh, suggesting that it results largely from calcium influx, whereas the initial peak is caused by calcium release from intracellular stores (see also Stuenkel & Hootman, 1990).…”

“…The fluid secretory response of isolated ducts to ACh was also reduced by extracellular calcium depletion, like ACh-evoked fluid secretion from the perfused rat pancreas (Habara, 1980 (Stuenkel & Hootman, 1990). The fact that increasing the extracellular potassium concentration to 25 mm (a manoeuvre which will depolarize the duct cell by about 16 mV, Novak & Greger (1988)) had no effect on [Ca2"], makes it very unlikely that voltage-dependent calcium channels underlie this high resting calcium permeability.…”

“…In fact, our data suggest that a substantial part of the fluid secreted from the intact rat pancreas in response to ACh is likely to originate from the ductal, rather than the acinar, epithelium. This effect of ACh on duct cells is associated with a rise in intracellular calcium concentration ([Ca2"]1) (see also Stuenkel & Hootman, 1990), and is mimicked by the calcium ionophore, ionomycin. Taken together, these results show that duct cells have a 'Ca2+ pathway' for triggering fluid secretion in addition to the well documented 'cyclic AMP pathway' that is utilized by secretin (see Case & Argent, 1989;Argent & Gray, 1990;Case & Argent, 1993).…”

Interactions between secretin and acetylcholine in the regulation of fluid secretion by isolated rat pancreatic ducts.

“…Indeed, the density of muscarinic receptors on the duct cell is about sevenfold higher than on pancreatic acinar cells (Hootman et al 1993). Acetylcholine and its analogue carbachol both increase the intracellular free calcium concentration ([Ca2P]) in duct cells (Stuenkel & Hootman, 1990;Ashton et al 1993;Palmer Smith, Yelamarty, Kramer & Cheung, 1993;Hug, Pahl & Novak, 1994;Zhao, Star & Muallem, 1994) and cause a marked depolarization of the duct cell basolateral membrane potential (Pahl & Novak, 1993;Hug et al 1994). (Ashton et al 1993).…”