Lipid signal transduction pathways in angiotensin II type 1 receptor-transfected fibroblasts

Physiological and metabolic control of diet selection

The fact that most farm animals have no dietary choice under commercial practices translates the dietary decisions to the carers. Thus, a lack of understanding of the principles of dietary choices is likely to result in a high toll for the feed industry. In healthy animals, diet selection and, ultimately, feed intake is the result of factoring together the preference for the feed available with the motivation to eat. Both are dynamic states and integrate transient stimulus derived from the nutritional status, environmental and social determinants of the animal with hard-wired genetic mechanisms. Peripheral senses are the primary inputs that determine feed preferences. Some of the sensory aspects of feed, such as taste, are innate and genetically driven, keeping the hedonic value of feed strictly associated with a nutritional frame. Sweet, umami and fat tastes are all highly appetitive. They stimulate reward responses from the brain and reinforce dietary choices related to essential nutrients. In contrast, aroma (smell) recognition is a plastic trait and preferences are driven mostly by learned experience. Maternal transfer through perinatal conditioning and the individual’s own innate behaviour to try or to avoid novel feed (often termed as neophobia) are known mechanisms where the learning process strongly affects preferences. In addtition, the motivation to eat responds to episodic events fluctuating in harmony with the eating patterns. These signals are driven mainly by gastrointestinal hormones (such as cholecystokinin [CCK] and glucagon-like peptide 1 [GLP-1]) and load. In addition, long-term events generate mechanisms for a sustainable nutritional homeostasis managed by tonic signals from tissue stores (i.e. leptin and insulin). Insulin and leptin are known to affect appetite by modulating peripheral sensory inputs. The study of chemosensory mechanisms related to the nutritional status of the animal offers novel tools to understand the dynamic states of feed choices so as to meet nutritional and hedonic needs. Finally, a significant body of literature exists regarding appetite driven by energy and amino acids in farm animals. However, it is surprising that there is scarcity of knowledge regarding what and how specific dietary nutrients may affect satiety. Thus, a better understanding on how bitter compounds and excess dietary nutrients (i.e. amino acids) play a role in no-choice animal feeding is an urgent topic to be addressed so that right choices can be made on the animal’s behalf.

Altered enterohepatic circulation of bile acids in Crohn's disease and their clinical significance: a new perspective

The role of bile acids (BA) extends far beyond lipid digestion and cholesterol metabolism. The transcriptional regulation of multiple genes within the liver and intestine are under their influence. BA exert these effects through binding and activating receptors in much the same way as endocrine hormones. The farnesoid X receptor (FXR) is the intracellular transcription factor for BA; TGR5 is the cell-surface receptor. The main target genes of FXR are those involved in BA and cholesterol metabolism. Yet more recently, FXR has also been shown to influence and promote certain protective pathways within the liver. These pathways are being harnessed by semisynthetic BAs in Phase II and III clinical trials. FXR activation within the intestine is also associated with similar protective pathways. This article examines the consequences of altered FXR activation in the context of BA malabsorption in Crohn's disease and the potential benefits of FXR agonists in Crohn's disease.

Enteral feeding without pancreatic stimulation

OBJECTIVE

All forms of commonly practiced enteral feeding techniques stimulate pancreatic secretion, and only intravenous feeding avoids it. In this study, we explored the possibility of more distal enteral infusions of tube feeds to see whether activation of the ileal brake mechanism can result in enteral feeding without pancreatic stimulation, with particular reference to trypsin, because the avoidance of trypsin stimulation may optimize enteral feeding in acute pancreatitis.

METHODS

The pancreatic secretory responses to feeding were studied in 36 healthy volunteers by standard double-lumen duodenal perfusion/aspiration techniques over 6 hours. Subjects were assigned to no feeding (n = 7), duodenal feeding with a polymeric diet (n = 7) or low-fat elemental diet (n = 6), mid-distal jejunal feeding (n = 11), or intravenous feeding (n = 5). All diets provided 40 kcal/kg ideal body weight/d and 1.5 g protein/kg ideal body weight/d. Plasma gut peptide responses were monitored in 15 subjects.

RESULTS

In comparison with basal fasting trypsin secretion rates (mean = 134 [standard error = 22] U/h), duodenal feeding with the polymeric and elemental formulae stimulated trypsin secretion (mean = 408 [standard error = 51] U/h; P < 0.001), whereas intravenous feeding (mean = 171 [standard error = 34] U/h) and mid-distal jejunal (mean = 119 [standard error = 16] U/h) did not. Stimulation was associated with an increase in plasma cholecystokinin, whereas distal jejunal feeding resulted in an increase in plasma glucagon-like peptide-1 and peptide YY concentrations.

CONCLUSIONS

Our results suggest that enteral feeding can be given without stimulating pancreatic trypsin secretion provided it is delivered into the mid-distal jejunum. The mechanism may involve activation of the ileal brake mechanism.

Tissue levels and post-prandial secretion of the intestinal growth factor, glucagon-like peptide-2, in controls and inflammatory bowel disease: comparison with peptide YY

BACKGROUND AND AIM

Glucagon-like peptide-2 (GLP-2) and peptide YY (PYY) are produced in endocrine L-cells of the intestine and secreted in response to food intake. GLP-2 has a trophic effect on the intestinal epithelium, whereas PYY has pro-absorptive effects. It can be speculated that, in inflammatory bowel disease (IBD), the production and secretion of GLP-2 and PYY could be affected as a part of a regulatory mechanism. Therefore, tissue levels and meal-stimulated secretion of GLP-2 and PYY were studied in IBD patients and compared to controls.

METHODS

Outpatients with IBD and control patients were included. Mucosal biopsies were taken from the ileum and colon and the content of GLP-2 and PYY was measured. After colonoscopy the patients took a mixed meal and plasma was collected for 90 min for plasma measurements of GLP-2 and PYY.

RESULTS

Tissue levels of GLP-2 in control patients were highest in the terminal ileum (407+/-82 pmol/g tissue, n=10), whereas PYY was highest in the rectum (919+/-249 pmol/g tissue, n=10). In IBD patients with acute inflammation, the content of GLP-2 was similar to controls, whereas PYY was decreased to 72.1+/-17.7% (P=0.03, n=13) of control values. Neither the fasting plasma levels nor the meal responses of GLP-2 and PYY differed between controls and IBD patients.

CONCLUSION

The similar responses of GLP-2 and PYY in patients and controls do not support the suggestion that L-cell secretion is altered in IBD. The decreased tissue PYY concentrations may contribute to the diarrhoea of some of these patients.

Changes in IP3 receptor are associated with altered calcium response to cholecystokinin in diabetic rat pancreatic acini

OBJECTIVES

Pancreatic acini of diabetic rats release amylase less than normal acini on cholecystokinin (CCK) stimulation. Pancreatic enzyme secretion by CCK is closely related to the second messenger inositol 1,4,5-trisphosphate (IP3), which mobilizes intracellular calcium stores via the endoplasmic reticulum-located receptor IP3 (IP3R). Recently, we observed altered intracellular calcium response on CCK-8 stimulation in streptozotocin (STZ)- treated diabetic rat acini.

METHODS

To determine whether IP3R is involved in altered calcium response, we measured inositol phosphate (IP) formation and the expression and phosphorylation of type III IP3R protein in diabetic acini. Also, CCK receptor mRNA expression was examined to determine whether the changes in IP formation and IP3R protein phosphorylation in diabetic acini might result from the defect at the postreceptor level.

RESULTS

CCK-8-induced IP formation at all concentrations used was significantly reduced in diabetic acini, though IP formation was increased in a concentration-dependent manner. The expression of type III IP3R protein was significantly reduced in diabetic acini. Additionally, CCK-8-stimulated phosphorylation of type III IP3R protein was not observed in diabetic acini. However, the reduction of CCK receptor mRNA expression was not detected in diabetic acini.

CONCLUSION

Our results indicate that altered calcium response to CCK-8 in diabetic acini might be associated with a post-CCK receptor defect including the changes in IP formation, type III IP3R protein expression, and phosphorylation of type III IP3R protein.

Elevated Rac1 activity changes the M3 muscarinic acetylcholine receptor-mediated inhibition of proliferation to induction of cell death

Although muscarinic acetylcholine receptors (mAChRs) regulate proliferation in many cell types, the signaling pathways involved are unclear. The participation of the small GTPases Rac1 and RhoA in M(3) mAChR-mediated inhibition of proliferation was investigated by activating M(3) mAChRs stably transfected in Chinese hamster ovary cells stably coexpressing hemagglutinin (HA)-tagged wild-type or mutant Rac1 or RhoA proteins. Activation of M(3) mAChRs activates both Rac1 and RhoA and inhibits cell proliferation in all cell lines tested. mAChR-mediated inhibition of proliferation is diminished in cells expressing dominant-negative HA-Rac1(Asn17) (m3DNRac) but is enhanced in cells expressing HA-Rac1 (m3WTRac) or constitutively active HA-Rac(Val12) (m3CARac). The activation of mAChRs in m3WTRac and m3CARac cells also induces apoptosis. Expression of wild-type or mutant RhoA proteins does not alter mAChR-mediated inhibition of proliferation. mAChR-induced inhibition of proliferation is abrogated in all cell lines when Galpha(q/11) signaling is terminated by transient expression of the COOH-terminal fragment of phospholipase C (PLC-beta1ct), the NH(2)-terminal fragment of G protein-coupled receptor kinase, or the regulator of G protein signaling 2. Pretreatment of all cells expressing wild-type or mutant Rac1 proteins with edelfosine, a phosphatidylinositol-specific PLC inhibitor, or Go 6976, which inhibits conventional protein kinase C (PKC) isoforms, diminishes the M(3) mAChR's ability to inhibit proliferation. Our results identify Galpha(q/11), PLC, and PKC as participants in the M(3) mAChR-mediated inhibition of cell proliferation. These findings indicate that in the context of high Rac1 activity, but not RhoA activity, M(3) mAChR-mediated activation of these participants triggers cell death.

Arachidonic acid modulates the spatiotemporal characteristics of agonist-evoked Ca2+ waves in mouse pancreatic acinar cells

In pancreatic acinar cells analysis of the propagation speed of secretagogue-evoked Ca2+ waves can be used to examine coupling of hormone receptors to intracellular signal cascades that cause activation of protein kinase C or production of arachidonic acid (AA). In the present study we have investigated the role of cytosolic phospholipase A2 (cPLA2) and AA in acetylcholine (ACh)- and bombesin-induced Ca2+ signaling. Inhibition of cPLA2 caused acceleration of ACh-induced Ca2+ waves, whereas bombesin-evoked Ca2+ waves were unaffected. When enzymatic metabolization of AA was prevented with the cyclooxygenase inhibitor indomethacin or the lipoxygenase inhibitor nordihydroguaiaretic acid, ACh-induced Ca2+ waves were slowed down. Agonist-induced activation of cPLA2 involves mitogen-activated protein kinase (MAPK) activation. An increase in phosphorylation of p38(MAPK) and p42/44(MAPK) within 10 s after stimulation could be demonstrated for ACh but was absent for bombesin. Rapid phosphorylation of p38(MAPK) and p42/44(MAPK) could also be observed in the presence of cholecystokinin (CCK), which also causes activation of cPLA2. ACh-and CCK-induced Ca2+ waves were slowed down when p38(MAPK) was inhibited with SB 203580, whereas inhibition of p42/44(MAPK) with PD 98059 caused acceleration of ACh- and CCK-induced Ca2+ waves. The spreading of bombesin-evoked Ca2+ waves was affected neither by PD 98059 nor by SB 203580. Our data indicate that in mouse pancreatic acinar cells both ACh and CCK receptors couple to the cPLA2 pathway. cPLA2 activation occurs within 1-2 s after hormone application and is promoted by p42/44(MAPK) and inhibited by p38(MAPK). Furthermore, the data demonstrate that secondary (Ca2+-induced) Ca2+ release, which supports Ca2+ wave spreading, is inhibited by AA itself and not by a metabolite of AA.

Effects of phospholipase A2 inhibitors on Ca2+ oscillations in pancreatic acinar cells

High-affinity cholecystokinin (CCK) receptors were reported to be coupled with phospholipase A2 (PLA2)-arachidonic acid (AA) pathways to mediate Ca2+ oscillations and amylase secretion in rat pancreatic acinar cells. To investigate which types of PLA2 were involved in PLA2-AA pathways, the effects of specific inhibitors for type II and type IV PLA2 on Ca2+ oscillations and amylase secretion were studied in isolated rat pancreatic acini. An inhibitor of type IV (cytosolic) PLA2, AACOCF3 inhibited Ca2+ oscillations elicited by CCK-8 (30 pM) and JMV-180 (100 nM). AACOCF3 inhibited amylase secretion stimulated by JMV-180 and low concentrations of CCK-8 (< or =30 pM). On the other hand, an inhibitor of type II (secretory, nonpancreatic) PLA2 had no effects on Ca2+ oscillations and amylase secretion stimulated by CCK-8 and JMV-180. These results suggest that high-affinity CCK receptors are coupled to cytosolic PLA2 to mediate Ca2+ oscillations and amylase secretion in rat pancreatic acinar cells.

Differential potentiation of arachidonic acid release by rat alpha2 adrenergic receptor subtypes

CHO transfectants expressing the three subtypes of rat alpha2 adrenergic receptors (alpha2AR): alpha2D, alpha2B, alpha2C were studied to compare the transduction pathways leading to the receptor-mediated stimulation of phospholipase A2 (PLA2) in the corresponding cell lines CHO-2D, CHO-2B, CHO-2C. The alpha2B subtype stimulated the arachidonic acid (AA) release after incubation of the cells with 1 microM epinephrine, whereas alpha2D and alpha2C gave no stimulation. Calcium ionophore A23187 (1 microM) increased the release by a factor of 2-4 in the three strains. When cells were incubated with both epinephrine and Ca2+ ionophore, the AA release differed greatly between cell lines with strong potentiation in CHO-2B (2-3 times greater than Ca2+ ionophore alone), moderate potentiation in CHO-2D, and no potentiation in CHO-2C. The three cell lines each inhibited adenylylcyclase with similar efficiencies when 1 microM epinephrine was used as the agonist. The potentiation depended on both alpha2AR and Gi proteins since yohimbine and pertussis toxin inhibited the process. Pretreatment of CHO-2B cells with MAFP which inhibits both cytosolic and Ca2+-independent PLA2, reduced the release of AA induced by epinephrine+Ca2+ ionophore to basal value, whereas bromoenol lactone, a specific Ca2+-independent PLA2 inhibitor, had no effect. Preincubation of the cells with the intracellular calcium chelator BAPTA gave a dose-dependent inhibition of the arachidonic acid (AA) release. In CHO cells expressing the angiotensin II type 1 receptor, coupled to a Gq protein, the agonist (10-7 M) produced maximal AA release: there was no extra increase when angiotensin and Ca2+ ionophore were added together. There was no increase in the amount of inositol 1,4, 5-triphosphate following stimulation of CHO-2B, -2C, -2D cells with 1 microM epinephrine. Epinephrine led to greater phosphorylation of cPLA2, resulting in an electrophoretic mobility shift for all three cell lines, so inadequate p42/44 MAPKs stimulation was not responsible for the weaker stimulation of cPLA2 in CHO-2C cells. Therefore, the stimulation of cPLA2 by Gi proteins presumably involves another unknown mechanism. The differential stimulation of cPLA2 in these transfectants will be of value to study the actual involvement of the transduction pathways leading to maximal cPLA2 stimulation.

High-resolution separation and quantification of neutral lipid and phospholipid species in mammalian cells and sera by multi-one-dimensional thin-layer chromatography

An improvement of current methods is needed for simple, rapid, and precise quantification of cellular lipids, including rare species of biologically active cellular lipids, such as phosphatidic acid (PA) and diradylglycerol (DG). In addition, further analysis of hydrolyzed acyl chains from these species by methods such as gas chromatography requires complete separations. Methods have been developed for the quantification of neutral lipids and several phospholipids extracted from mammalian cells and sera. Lipid masses were determined for the major classes of the neutral, nonpolar lipids, and of the phospholipids. The lipid classes were separated by a multistep thin-layer chromatography (TLC) procedure in different solvent systems, a method which we have designated as multi-one-dimensional thin-layer chromatography (MOD-TLC). Resolved lipid bands were visualized by the lipophilic dye primulin (direct yellow 59) and scanned by an automated laser-fluorescence detector. The mass of each band was determined by comparing band intensities of unknown samples to dilution curves of authentic standards. With modifications in solvent mixtures and length of separation times, the majority of biological lipids could be resolved and quantified with MOD-TLC methods. Since the detection method is nondestructive, purified lipids could then be recovered by scraping the visualized bands and extracting the lipids from the silica. The structural identities of the recovered lipids were confirmed by fast-atom bombardment and electrospray mass spectrometry. Extracted lipids were also hydrolyzed to release acyl chains and acyl chain species were determined in comparison to authentic standards by gas chromatography. PA and DG levels in ECV.304 cells were found to be 4. 6 and 3.3%, respectively, of PC levels, with a PA/DG ratio of 1.4, which is in accord with published experience using other methods and different cell types. PA in human serum was detected at 0.6% of PC, indicating the sensitivity of the technique. In contrast to two-dimensional thin-layer chromatography, which allows for good resolution of some lipid species, but cannot be used to analyze more than a single experimental point per plate, MOD-TLC allows for direct comparative analysis of multiple samples on a single TLC plate, while still providing good resolution for the quantification of most major classes of lipid species.

Evidence that angiotensin II and lipoxygenase products activate c-Jun NH2-terminal kinase

The effect of angiotensin II (Ang II) to activate c-Jun amino-terminal kinase (JNK) was studied in a Chinese hamster ovary fibroblast cell line overexpressing the rat vascular type-1a Ang II receptor (CHO-AT1a). Ang II treatment induced a time-dependent activation of JNK. Ang II (10(-7) mol/L) activated JNK activity, with a peak at 30 minutes (9.39 +/- 2.52-fold, n = 7, P < .02 versus control), which was maintained until 3 hours (2.7 +/- 0.65-fold, n = 3, P < .02 versus control). Ang II-induced JNK activation at 30 minutes was inhibited by a specific lipoxygenase (LO) pathway inhibitor, cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate (1 mumol/L) by 87.5% (n = 4, P < .01 versus Ang II-induced JNK activity). The direct addition of 12-HETE also induced a time-dependent JNK activation. 12-HETE (10(-7) mol/L) activated JNK activity, with a peak at 10 minutes (3.43 +/- 0.87-fold, n = 6, P < .02 versus control), which remained elevated until 1 hour. These results suggest that the LO pathway is a mediator of Ang II-induced JNK activation. 15-HETE can also activate JNK at 5 minutes, but this activity was reduced at 30 minutes and could not be seen at 1 hour, indicating that the time course was different from that seen with 12-HETE. N-Acetylcysteine (NAC), an antioxidant, was used to perturb intracellular reactive oxygen intermediate (ROI) levels to assess the role of endogenous ROIs in regulating JNK activity. Pretreatment of cells with 500 mumol/L NAC for 1 hour attenuated approximately 50% of Aug II-induced JNK activation, suggesting that ROIs, at least partially, mediate Ang II-induced JNK activation. Furthermore, 12-HETE-induced JNK activation was reduced by approximately 90% by NAC. Finally, pertussis toxin completely blocked 12-HETE-induced JNK activation, suggesting that Gi-protein signaling participates in 12-HETE-induced effects. These results suggest that LO activation plays a role in mediating Ang II-induced JNK activation in part by altering the redox tone and Gi-protein signaling of cells.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

Apical ANG II-stimulated PLA2 activity and Na+ flux: a potential role for Ca2+-independent PLA2

Type 1 angiotensin II (ANG II) receptors (AT1R), which mediate proximal tubule (PT) salt and water reabsorption, undergo endocytosis and recycling. Prior studies in a PT-like model (LLC-PKcl4 cells expressing rabbit AT1R) (LLC-PK-AT1R cells) determined that quinacrine, a nonspecific phospholipase A2 (PLA2) inhibitor, and the haloenol lactone suicide substrate (HELSS), a Ca2+-independent PLA2 inhibitor, attenuated apical (AP) AT1R recycling. Further studies were undertaken to examine the association between AT1R endocytotic movement and PLA2 activity in this model. AP ANG II (100 nM) increased [3H]arachidonic acid ([3H]AA) release 4.4 +/- 0.38-fold in LLC-PK-AT1R cells cultured on permeable supports. Basolateral (BL) ANG II had no significant effect. Reversed-phase high-performance liquid chromatography confirmed that AP ANG II stimulated free [3H]AA release. Quinacrine, HELSS, and palmitoyl trifluoromethyl ketone, another Ca2+-independent PLA2 inhibitor, inhibited AP ANG II-stimulated [3H]AA release, as did inhibiting AP AT1R internalization with phenylarsine oxide. The role of HELSS-inhibitable AA release in ANG II-mediated 22Na flux was examined, given the effects of AT1R-mediated PLA2 activity on salt and water reabsorption. AP ANG II (100 nM) stimulated 22Na flux (AP--> BL), a response inhibited by HELSS. Thus, in this model, AP AT1R activated PLA2 with concomitant 22Na flux (AP --> BL), suggesting a link between AP AT1R endocytotic movement, AT1R-stimulated PLA2 activity, and 22Na flux in this model. The effects of HELSS suggest that Ca2+-independent PLA2 activity may be involved in this AP ANG II response.

Visualization of G protein-coupled receptor trafficking with the aid of the green fluorescent protein. Endocytosis and recycling of cholecystokinin receptor type A

A chimeric protein consisting of the cholecystokinin receptor type A (CCKAR) and the green fluorescent protein (GFP) was used for studying receptor localization, internalization, and recycling in live cells in real time in four different cell lines. Fusion of the C terminus of the CCKAR to the N terminus of the GFP did not alter receptor ligand binding affinity, signal transduction, or the pattern of receptor surface expression and receptor-mediated cholecystokinin (CCK) internalization. The use of a new GFP mutant with increased fluorescence allowed the continuous observation of CCKAR-GFP in stably expressing cell lines. Newly obtained biologically active fluorescent derivatives of CCK were used for simultaneous observation of receptor and ligand trafficking in CHO, NIH/3T3, and HeLa cells stably expressing the fluorescent CCKAR and in transiently transfected COS-1 cells. Receptor internalization was predominantly ligand dependent in HeLa, COS-1, and CHO cells, but was mostly constitutive in NIH/3T3 cells, suggesting the existence of cell-specific regulation of receptor internalization. The CCKAR antagonists, L-364,718 and CCK 27-32 amide potently inhibited spontaneous internalization of the receptor. The average sorting time of CCK and the receptor in the endosomes was about 25 min. The receptor recycled back to the cell membrane with an average time of 60 min. While the ligands sorted to lysosomes, no receptor molecules could be detected there, and no receptor degradation was observed during recycling. These results demonstrate the usefulness of GFP tagging for real time imaging of G protein-coupled receptor trafficking in living cells and suggest that this technique may be successfully applied to the study of the regulation and trafficking mechanisms of other receptors.

Arachidonic acid activates the noncapacitative entry of Ca2+ during [Ca2+]i oscillations

Current models for agonist-activated Ca2+ entry in nonexcitable cells focus on the capacitative mechanism where entry is activated as a downstream result of the sustained depletion of agonist-sensitive stores without any direct requirement for inositol phosphates. This mechanism has been shown to be important for the sustained Ca2+ signals seen in a variety of nonexcitable cells under conditions of maximal stimulation. In contrast, relatively little attention has been given to Ca2+ entry under more physiological levels of agonist where, for example, oscillating Ca2+ responses are common. In recent studies using cells from the exocrine avian nasal gland, we have shown that agonist-activated Ca2+ entry under these conditions demonstrates properties that are inconsistent with current versions of the capacitative model. We now report that activation of this novel noncapacitative Ca2+ entry is via a distinct signaling pathway involving an agonist-induced, phospholipase A2-mediated generation of arachidonic acid.