Functional expression of brain cholecystokinin and bombesin receptors in Xenopus oocytes

Desensitization of inositol 1,4,5-trisphosphate/Ca2+-induced Cl- currents by prolonged activation of G proteins in Xenopus oocytes

Expression of G protein alpha subunits of the Gq family with various G protein-coupled receptors induces activation of an inositol 1,4, 5-trisphosphate (IP3)/Ca2+-mediated Cl- conductance in Xenopus oocytes. Our present data show that two members of this family, the human Galpha16 subunit and the murine homologue Galpha15, can induce both activation and inhibition of these agonist-induced currents. Although extremely low amounts (10-50 pg) of injected Galpha16 subunit cRNA cause modest ( approximately 2-fold) enhancement of ligand-induced Cl- currents in oocytes co-injected with thyrotropin-releasing hormone (TRH) receptor cRNA 48 h postinjection, larger Galpha16 and Galpha15 cRNA injections cause >10-fold inhibition of TRH or 5HT2c receptor responses. The inhibition is analyzed in this study. The inhibited currents are recovered if various Gbetagamma subunit combinations are also expressed with the Galpha subunits. The constitutively active mutant, Galpha16Q212L, also causes a strong attenuation of the ligand-induced Cl- currents, but this inhibition is not recovered by co-expression of Gbetagamma subunits. These results indicate that the free Galpha subunit is responsible for the inhibitory signal. Although expression of TRH receptor alone produces maximum responses approximately 48 h after injection, co-expression of TRH receptor with Galpha16 results in enhanced responses 6-12 h postinjection, followed by complete attenuation at 36 h. Furthermore, injection of Galpha16 cRNA alone at comparable levels gives rise to spontaneous Cl- currents within 6-12 h postinjection, suggesting that the early spontaneous activation underlies the later suppression. Expression of other G protein alpha subunits of the Gq family, at cRNA levels considerably higher than effective for Galpha16, produces both analogous spontaneous Cl- currents and, later, inhibition of ligand-induced Cl- currents. Experiments with direct injection of IP3 and of Ca2+ suggest that this inhibition is consistent with the down-regulation of IP3 receptors. These data indicate that both enhancement and inhibition of signaling through G protein-coupled receptors can be mediated by the expression level and/or activity of an individual G protein.

Cholecystokinin increases intracellular Ca2+ concentration in the Human JURKAT T Lymphocyte cell line

We have recently demonstrated the presence of specific high-affinity cholecystokinin binding sites of the central type on the Human JURKAT T Lymphocyte Cell line. In this paper, changes in intracellular Ca2+ concentration ([Ca2+]i) in response to CCK (cholecystokinin) peptides were measured in the Human JURKAT T Lymphocyte Cell line by fura-2 fluorometry. CCK-8 (the C-terminal octapeptide of CCK), the potent CCK analog Boc-[Nle28,31]CCK-7, stimulated ([Ca2+]i) mobilization in a dose-dependent manner in cells preloaded with fura-2 AM with an EC50 of 2.4 +/- 1 nM and 8 +/- 2 nM, respectively. The selective CCKB receptor agonists, namely Boc-Trp-Nle-Asp-Phe-NH2 and the cyclic analog JMV320, [formula: see text], were also potent in stimulating mobilization of [Ca2+]i with an EC50 of 32 +/- 10 nM and 25 +/- 10 nM, respectively. Compound JMV180, Boc-Tyr(SO3H)-Nle-Trp-Nle-Asp-2-phenylethyl ester, did not stimulate [Ca2+]i but inhibited the mobilization of [Ca2+]i elicited by 10 nM CCK-8 in a dose-dependent manner with an IC50 of 10 +/- 2 nM. The selective non-peptide CCKB receptor antagonist L-365,260 was more potent than the selective CCKA receptor antagonist MK-329 in inhibiting the [Ca2+]i mobilization elicited by 10 nM CCK-8 with IC50 values of 20 +/- 8 nM and 400 +/- 100 nM, respectively. These data indicated that CCK-8 and potent CCK analogs induced [Ca2+]i mobilization in the Human JURKAT T cell line through the CCKB/gastrin receptor type.

Molecular cloning and characterization of the angiotensin receptor subtype in porcine aortic smooth muscle

A cDNA encoding porcine aortic smooth muscle angiotensin II (AII) receptor has been isolated using the homology screening approach and sequenced. Specific binding of [125I]AII was found in COS-7 cells transfected with the cDNA (Kd = 0.37 nM, Bmax = 1 approximately 3 x 10(4)/cell) and was displaced with unlabeled AII-related peptides and DUP753 in the order of [Sar1,Ile8]AII = AII > des-Asp1-[Ile8]AII = DUP753 > angiotensin I = angiotensin III. EXP655 had no effect on [125I]AII binding. COS-7 cells transfected with the cDNA responded to AII by only a small increase in the concentration of intracellular free Ca2+. However, electrophysiological study of the receptor expressed in Xenopus laevis oocytes provided strong evidence that it could functionally couple to a second messenger system leading to the mobilization of intracellular stores of Ca2+. Northern blot analysis in cultured porcine aortic smooth muscle cells demonstrated that the expression of this gene varies with the culture media. These results indicate that the cDNA encodes the functional and regulated AT1 subtype of AII receptors.

Pharmacological studies on CCKB receptors in guinea pig synaptoneurosomes

Preliminary studies on CCK receptors in the central nervous system were carried out on guinea pig cerebral cortical synaptoneurosome preparations. In binding assays, the range of affinity of CCK-8, Boc-[Nle28,Nle31]CCK-7, a potent CCK analog, Boc-[Leu31]CCK-4 and of the two benzodiazepine CCK receptor antagonists L-365,260 and MK-329, is in agreement with the presence of CCKB receptors on this model. The effects of Boc-[Nle28,Nle31]CCK-7 on inositol phosphates, cAMP accumulation and 45Ca2+ efflux were investigated. Neither inositol phosphate nor cAMP accumulations could be observed. On the other hand, evidence of Boc-[Nle28,Nle31]CCK-7-, CCK-8- and Boc-[Leu31]CCK-4-induced 45Ca2+ efflux was found in a dose-dependent manner. The CCKB-selective receptor antagonist L-365,260 and, with a weaker efficiency, the CCKA-selective receptor antagonist MK-329, are able to block a maximal effect of Boc-[Nle28,Nle31]CCK-7-induced 45Ca2+ efflux, suggesting that CCKB receptors may regulate calcium ion mobilization.

Distinguishing bombesin receptor subtypes using the oocyte assay

Physiological responses to mammalian bombesin-like peptides were studied in Xenopus oocytes injected with mRNA isolated from Swiss 3T3 cells and rat esophagus in order to identify and characterize bombesin receptor subtypes. Both groups respond similarly to either gastrin releasing peptide or neuromedin B, but only the response to neuromedin B in oocytes expressing the esophagus mRNA is not blocked by a specific gastrin releasing peptide receptor antagonist, des-Met-[D-Phe6]Bn(6-13) ethyl ester. Complete desensitization of gastrin releasing peptide-evoked responses in oocytes expressing esophagus mRNA does not abolish neuromedin B-evoked responses. A single application of neuromedin B abolishes responses to subsequently applied gastrin releasing peptide in oocytes expressing esophagus, but not Swiss 3T3, mRNA. RNA blot hybridization studies using a Swiss 3T3 gastrin releasing peptide receptor cDNA probe show no detectable hybridization in esophagus mRNA samples. These data suggest that a gastrin releasing peptide receptor is expressed in the esophagus and that it is distinct from that expressed in Swiss 3T3 cells and may represent a third subtype of mammalian bombesin receptor.

Cholecystokinin increases intracellular Ca2+ concentration in cultured striatal neurons

Although it has been established that pancreatic cholecystokinin (CCK) receptors are coupled to phosphatidylinositol turnover, the events which follow activation of CCK receptors in the central nervous system have not received much attention. In this paper, changes in intracellular Ca2+ concentration ([Ca2+]1) in response to CCK peptides were measured in single cultured rat striatal neuron by fura-2 fluorometry. CCK peptides dose-dependently increased [Ca2+]i in a monophasic manner. The order of the potencies of CCK peptides to increase [Ca2+]i was as follows: caerulein greater than CCK-8 greater than desulfated CCK-8 greater than CCK-4. The effect of caerulein was completely blocked in a Ca2(+)-depleted medium. In addition, omega-conotoxin GVIA completely inhibited the effect of caerulein, while neither nifedipine nor verapamil affected it. Our results indicate that CCK receptors couple to N-type voltage-sensitive Ca2+ channels in cultured rats striatal neurons.

Expression of Histamine HReceptors in Xenopus Oocytes Injected with Messenger Ribonucleic Acid from Bovine Adrenal Medulla: Pertussis Toxin Insensitive Activation of Membrane Chloride Currents

Abstract Histamine H(2)-receptors have been identified in Xenopus oocytes previously microinjected with poly(A) + ribonucleic acid from bovine adrenal glands. Bath application of histamine to ribonucleic acid-primed oocytes evoked concentration-dependent, oscillating membrane currents under voltage-clamp conditions. H(1)-receptor specific antagonists clemastine, doxepin, pyrilamine, promethacine, diphenylhydramine, dephenylpyraline and chlorpheniramine, but not H(2)-receptor antagonists, cimetidine and ranitidine, inhibited histamine-induced responses. Membrane currents evoked by bath-applied histamine were insensitive to pertussis toxin, carried by chloride ions and dependent on intracellular but not extracellular calcium.

Expression of functional pituitary somatostatin receptors in Xenopus oocytes

Functional somatostatin (SRIF, somatotropin release-inhibiting factor) receptors were expressed in Xenopus oocytes after injection of RNA isolated from the anterior pituitary tumor cell line AtT20. SRIF receptors were detected by measuring the ability of SRIF to inhibit cAMP formation stimulated by beta 2-adrenergic agonists in individual oocytes. beta 2-Adrenergic receptors (beta 2ARs) were expressed in oocytes by coinjecting RNA prepared by in vitro transcription of a beta 2AR cDNA clone with the pituitary cell RNA. Uninjected oocytes do not express detectable levels of either beta 2ARs or SRIF receptors. In oocytes coinjected with AtT20 and beta 2AR RNA, on the other hand, isoproterenol treatment led to a 2- to 3-fold increase in cAMP levels, whereas cotreatment with SRIF reduced this accumulation by 50-60%. The SRIF precursor somatostatin-28 and the cyclohexapeptide agonist MK678 also inhibited cAMP formation, whereas the biologically inactive N-terminal 14-amino acid fragment of somatostatin-28 was ineffective. The ability to detect changes in cAMP levels in individual oocytes may provide a simple procedure for the expression cloning of SRIF receptor cDNAs and other receptors functionally coupled to stimulation or inhibition of adenylate cyclase.

Modulation of Neuropeptide-lnduced Membrane Currents by Protein Kinase C in Xenopus Oocytes Injected with GH Pituitary Cell Poly(A) RNA

Abstract Protein kinase C was activated in Xenopus laevis oocytes by phorbol ester treatment and its effects on the inositol trisphosphate/Ca(2+) transmembrane signalling pathway analysed. Induction of the pathway was achieved by ligand stimulation of TRH receptors translated from GH(3) pituitary cell mRNA. In voltage-clamped oocytes bath application of peptide, injection of guanosine 5'-(3-O-thio) triphosphate (GTPgammaS), inositol trisphosphate or Ca(2+) all elicited inward membrane currents. Treatment of oocytes with tumour-promoting phorbol esters for 35 min almost completely abolished the ligand and GTPgammaS-induced responses. In contrast, phorbol ester treatment enhanced inositol trisphosphate-generated membrane currents. Ca(2+)-mediated responses remained unaffected by tumour promoters. The data indicate a dual role for protein kinase C in the modulation of transmembrane signalling: a feedback mechanism prevents phosphoinositide turnover whereas a feedforward reaction triggers the effect of intracellular inositol trisphosphate on the Ca(2+) release.