Relationship between hormonal, GTP and Ins(1,4,5)P3-stimulated Ca2+ uptake and release in pancreatic acinar cells

Characteristics and mechanism of enzyme secretion and increase in [Ca2+]i in Saikosaponin(I) stimulated rat pancreatic acinar cells

AIM

This investigation was to reveal the characteristics and mechanism of enzyme secretion and increase in [Ca2+]i stimulated by saikosaponin(I) (SA(I)) in rat pancreatic acini.

METHODS

Pancreatic acini were prepared from male Wistar rats. Isolated acinar cells were suspended in Eagle's MEM solution. After adding drugs, the incubation was performed at 37 degrees for a set period of time. Amylase of supernatant was assayed using starch-iodide reaction. Isolated acinar single cell was incubated with Fura-2/AM at 37 degrees, then cells were washed and resuspended in fresh solution and attached to the chamber. Cytoplasm [Ca2+]i of a single cell was expressed by fluorescence ratio F340/F380 recorded in a Nikon PI Ca2+ measurement system.

RESULTS

Rate course of amylase secretion stimulated by SA(I) in rat pancreatic acini appeared in bell-like shape. The peak amplitude increased depended on SA(I) concentration. The maximum rate responded to 1 x 10(-5)mol/L SA(I) was 13.1-fold of basal and the rate decreased to basal level at 30 min. CCK-8 receptor antagonist Bt(2)-cGMP markedly inhibited amylase secretion stimulated by SA(I) and the dose-effect relationship was similar to that by CCK-8. [Ca2+]i in a single acinar cell rose to the peak at 5 min after adding 5 x 10(-6)mol/L SA(I) and was 5.1-fold of basal level. In addition, there was a secondary increase after the initial peak. GDP could inhibit both the rate of amylase secretion and rising of [Ca2+]i stimulated by SA(I) in a single pancreatic acinar cell.

CONCLUSION

SA(I) is highly efficient in promoting the secretion of enzymes synthesized in rat pancreatic acini and raising intracellular [Ca2+]i. Signaling transduction pathway of SA(I) involves activating special membrane receptor and increase in cytoplasm [Ca2+]i sequentially.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Agonist-sensitive and -insensitive intracellular Ca2+ pools. Separate Ca(2+)-releasing mechanisms revealed by manoalide and benzohydroquinone

The mechanism of action of a novel compound, 2,5-di-(t-butyl)-1,4-benzohydroquinone (BHQ), used to modulate cell free cytosolic Ca2+ concentration ([Ca2+]i) was studied in AR42J cells and pancreatic acini by using single-cell fluorescence techniques applied to Fura-2-loaded cells. In the presence of extracellular Ca2+ (Ca(2+)out), BHQ induced a biphasic [Ca2+]i increase, an initial and rapid transient followed by a sustained increase. The initial increase was due to Ca2+ release from intracellular stores, being independent of Ca(2+)out. The sustained response was due to Ca2+ entry, being dependent on Ca(2+)out, blocked by La3+ and correlated with an increased rate of Mn2+ entry, all indicative of increased plasma-membrane permeability to Ca2+. Treatment of AR42J cells with BHQ for about 5 min reversibly blocked agonist-dependent Ca2+ release and oscillations, whereas agonist pretreatment decreased, but did not prevent, the effects of BHQ on [Ca2+]i. Accordingly, depletion of the Ins(1,4,5)P3-mobilizable pool in permeabilized AR42J cells by BHQ required 5 min of incubation, although inhibition of the internal Ca2+ pump by BHQ was rapid. These observations suggest that BHQ mobilized an additional intracellular Ca2+ pool that did not respond to changes in Ins(1,4,5)P3. Manoalide, an inhibitor of Ca2+ channels, inhibited agonist-evoked [Ca2+]i oscillation and [Ca2+]i increase in a dose- and time-dependent manner without significant effect on internal Ca2+ pumps and Ca2+ content of the internal stores. Manoalide also inhibited the BHQ-evoked [Ca2+]i increase in the absence and presence of Ca(2+)out. Neither BHQ nor manoalide affected Ins(1,4,5)P3 levels in resting or stimulated cells. Therefore, the effect of BHQ appears to involve unmasking of passive Ca(2+)-permeation pathways in the plasma and intracellular membranes that do not respond to cholecystokinin octapeptide, following its described inhibition of the internal-store Ca2+ pumps responsible for accumulating Ca2+ in these pools.

Heterogeneity of guanine nucleotide binding proteins in human red blood cell membranes

Membranes from human erythrocytes bind radioactive GTP and GTP analogs according to apparently homogeneous patterns. In spite of this uniform type of association, multiple guanine nucleotide binding proteins have been identified both by SDS-PAGE analysis of native and of variously ADP-ribosylated membrane preparations and by FPLC chromatography of solubilised erythrocyte membranes preliminarily incubated with [alpha-32P] GTP in the presence of 5 mM MgCl2. From eight to nine peak fractions of pronase-digestible GTP-binding activity were separated on a MA7Q anion exchange column, this pattern being highly reproducible with different membrane preparations. Prior incubation of membranes with [alpha-32P] GTP in the presence of excess unlabeled GDP resulted in displacement of bound labeled nucleotide from all FPLC fractions. The patterns of GTP binding were also markedly modified by preliminary treatment of membranes with N-ethylmaleimide. Detectable GTPase activity was present in each of the FPLC peak fractions. This wide heterogeneity of guanine nucleotide binding proteins raises so far unanswered questions as to their physiological significance in the mature erythrocyte.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.