The action of magnesium upon stimulus-secretion coupling in the exocrine pancreas

Abstract

I. Investigations were carried out to establish (a) the regulation of magnesium homeostasis in the exocrine pancreas under agonist stimulation (b) that secretagogueevoked cellular processes in the exocrine pancreas are sensitive to physiological
changes in Mg2+ and (c) the intracellular roles of Mg2+ in the secretagogue-evoked exocytotic processes in the exocrine pancreas.
H. CCK8 and ACh can induce a dose-dependent transmembrane efflux of Mg2+ which results in a subsequent lowering of [M g2+] . The routes for movement of Mg2+ do not appear to be associated with those sites of Ca2+ mobilisation from either the
internal Ca2+ stores or across cell membranes but appears to be associated with those of Na+ and is ATP-dependent. Stimulation with secretin results in an elevation of [Mg2+] compared to resting values. The secretin-evoked elevation of [Mg2+] may be coupled to G protein activation of adenylate cyclase. Digital imaging technology further demonstrates that mobilisation of Mg2+ follows a different route from that of Ca2 . The CCK8-evoked Ca2 signal is initiated in an area towards the luminal pole of the pancreatic acini and rapidly spreads throughout the entire cell. Stimulation of single pancreatic acini with CCK8 results in an initial transient increase in [Mg 2 ] throughout the entire cell which is followed by a sustained decrease in [Mg2+] which is more prominent around the basolateral membrane.
III. Pertubations in extracellular and intracellular Mg2+ had marked effects on the CCK5- and secretin-evoked pancreatic juice flow and protein output in (a) the isolated perfused intact pancreas, and (b) amylase release in pancreatic segments and permeabilised pancreatic acini. A nominally Mg2+ deficient saline augments the CCK8-induced responses, whereas elevated Mg2+ attenuated secretion compared to normal (1.0 mM) Mg2 conditions. Parallel effects were found on Ca 2 mobilisation in Fura 2 loaded acini suspensions and on oscillations of Ca2+ in single acini stimulated with CCK8. These effects of Mg 2 appear to be mediated by direct ffil modification of Ca2+ release, propagation and re-uptake into cytoplasmic stores. The effects of Mg2+ on secretin-evoked secretion followed a bell shape characteristics curve for Mg2+dependent enzyme activity, i.e. elevating and reducing the intracellular free magnesium levels attenuate the secretin-induced response.
IV. Infusion of CCK8 and secretin evoked time course increases in both pancreatic juice flow and total protein output in anaesthetised rats and in the isolated perfused intact pancreas. Simultaneous application of CCK5 and secretin caused some degree of attenuation in secretory responses. This reduction in secretion may not involve protein kinase C or cyclic AMP but may occur at some point prior to the activation of protein kinase A. Secretin may influence mobilisation of Ca2+ which is required to initiate secretion, possibly by stimulating influx of Mg 2 .
V. Electrical stimulation of vagus nerves led to elevated pancreatic juice flow and protein output in anaesthetised rats. Secretin attenuated these secretory parameters. Activation of protein kinase C with TPA, had no significant affect on these nervemediated responses. Electrical field stimulation (EPS, 50 V, 20 Hz, 1 msec) andlor ACh stimulation of pancreatic segments results in marked amylase release; 45Ca2+ influx and elevations in [Ca 2+]i. Secretin had little effect on these parameters but attenuated the EPS and ACh-induced responses. The secretin-evoked inhibition of EFS and ACh -induced secretion was abolished when extracellular Mg2+ was absent. Secretin may control nerve-mediated secretory responses by stimulating an influx of Mg2+ to interact with Ca2+.
VI. The present investigations have demonstrated that transport mechanisms exist in the exocrine pancreas to regulate cytosolic free Mg2+ levels and that exocytotic processes exist in the exocrine rat pancreas which are sensitive to Mg2+ within a physiological range. Mg2 may act as an intracellular modulator, primarily by controlling mobilisation of Ca2+.