Evidence of two steps in the homologous desensitization of vasopressin-sensitive phospholipase C in WRK1 cells. Uncoupling and loss of vasopressin receptors

Vasopressin-induced morphological changes in polarized rat hepatocyte multiplets: dual calcium-dependent effects

Calcium-mobilizing hormones and neurotransmitters are known to affect cell morphology and function including cell differentiation or division. In this study, we examined vasopressin (AVP)-induced morphological changes in a polarized system of rat hepatocytes. Light and electron microscope observations showed that AVP induced microvilli formation and a remodeling of the isolated hepatocyte F-actin submembrane cytoskeleton, these two events being correlated. We showed that these effects were rapid, reversible, observed at nanomolar AVP concentration and mediated by the V(1a) receptor. On polarized multicellular systems of hepatocytes, we observed a rapid reduction of the bile canaliculi lumen at the apical pole and micovilli formation at the basolateral domain with an enlarged F-actin cytoskeleton. Neither activation of protein kinase C nor A via phorbol ester or dibutyryl cAMP induced such rapid morphological changes, at variance with ionomycin, suggesting that AVP-induced intracellular calcium rise plays a crucial role in those effects. By using spectrofluorimetry and cytochemistry, we showed that calcium release from intracellular stores was involved in bile canaliculus contraction, while calcium entry from the extracellular space controlled microvilli formation. Taken together, AVP and calcium-mobilizing agonists differentially regulate physiological hepatocyte plasma membrane events at the basal and the apical domains via topographically specialized calcium-dependent mechanisms.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Vasopressin V1a receptor signaling in a rat choroid plexus cell line

A new cell line was derived from primary culture of rat choroid plexus (RCP) by immortalization with the TSOri minus adenovirus. The selected clone expressed vasopressin V1a receptors at a density of 64,000 sites per cell, and a K(d) of 7.2 nM. Addition of vasopressin to the RCP cells induced a transient calcium peak comparable to V1a receptor signalling in different expression systems. This [Ca(2+)](i) increase was dose-dependent with an EC(50) of 22 nM vasopressin. Similar [Ca(2+)](i) increase was elicited by addition of serotonin, angiotensin II, endothelin-1, and bradykinin. Heterologous desensitization of V1a receptor was observed in RCP cells exposed to the phorbol ester PMA or following stimulation of other receptors coupled to the phosphoinositide pathway. Positive immunolabelling with Factor VIII, Flt1 and CD 34 antibodies suggests that this new RCP cell line originated from endothelial cells of rat choroid plexus.

Homologous and heterologous phosphorylation of the vasopressin V1a receptor

The vasopressin V1a receptor undergoes homologous and heterologous desensitizations which can be mimicked by activation of protein kinase C. This suggests that phosphorylation of the V1a receptor may be involved in the desensitization mechanisms. Such a phosphorylation was presently investigated in HEK 293 cells stably transfected with rat vasopressin V1a receptor. Metabolic labelling and immunoprecipitation of epitope-tagged V1a receptor evidenced a 52-kDa band and a 92-kDa band. Glycosidase treatments and immunoblotting experiments suggest that the 52-kDa band corresponds to an immature unprocessed receptor protein, whereas the 92-kDa band would correspond to a highly glycosylated form of the mature V1a receptor. Exposure of the cells to vasopressin induced a selective 32P phosphate incorporation in the 92-kDa form of the receptor. This homologous ligand-induced phosphorylation was dose dependent with maximal phosphate incorporation corresponding to four times the basal level. Stimulation of the endogenous phospholipase C-coupled m3 muscarinic receptor by carbachol-induced heterologous phosphorylation of the V1a receptor whose amplitude was half that of the homologous phosphorylation. This heterologous phosphorylation was associated with a reduced vasopressin-dependent increase in intracellular calcium.

Visualization of cell surface vasopressin V1a receptors in rat hepatocytes with a fluorescent linear antagonist

To visualize cell surface V1a vasopressin receptors in rat hepatocytes in the absence of receptor-mediated endocytosis, we used a high-affinity fluorescent linear antagonist, Rhm8-PVA. Epifluorescence microscopy (3CCD camera) and fluorescence spectroscopy were used. Rhm8-PVA alone did not stimulate Ca2+ signals and competitively blocked Ca2+ signals (Kinact of 3.0 nM) evoked by arginine vasopressin (vasopressin). When rat hepatocytes were incubated with 10 nM of Rhm8-PVA for 30 min at 4C, the fluorescent antagonist bound to the surface of cells, presumably the plasma membrane. The V1a receptor specificity of Rhm8-PVA binding was confirmed by its displacement by the nonfluorescent antagonist V4253 and by the natural hormone vasopressin at 4C. Prior vasopressin-mediated endocytosis of V1a receptors at 37C abolished binding of the labeled antagonist, whereas in non-preincubated cells, Rhm8-PVA labeled the cell surface of rat hepatocytes. When cells labeled with Rhm8-PVA at 4C were warmed to 37C to initiate receptor-mediated internalization of the fluorescent complex, Rhm8-PVA remained at the cell surface. Incubation temperature at 4C or 37C had little effect on binding of Rhm8-PVA. We conclude that Rhm8-PVA is unable to evoke receptor-mediated endocytosis and can readily be used to visualize cell surface receptors in living cells.

The thyrotropin-releasing hormone-receptor complex and G11alpha are both internalised into clathrin-coated vesicles

It has been proposed that, after agonist binding, the thyrotropin-releasing hormone receptor (TRHR) becomes internalised associated with Gq, as part of a TRH-TRHR-Gq ternary complex [13]. We tested this hypothesis directly by examining the intracellular distribution of the TRHR and Gq/11 after agonist binding. The localisation of the TRH-TRHR complex and Gq/11alpha was studied by the biochemical isolation of clathrin-coated vesicles (CCVs). The internalised TRH-TRHR complex was localised in CCVs. The CCVs, which had internalised [3H]MeTRH, contained 4-fold higher levels of radiolabelled ligand than did CCVs from cells incubated with [3H]MeTRH at 4 degrees C. Like the receptor-ligand (RL) complex, G11alpha also translocated to these endocytic vesicles. For example, CCVs from cells with internalised TRH-TRHR complexes contained G11alpha, whereas CCVs from cells without internalised RL complexes lacked G11alpha. We conclude that, after agonist-induced TRHR-G11alpha coupling, both the TRH-TRHR complex and G11alpha are internalised in CCVs.

Homologous and heterologous acute desensitization of vasopressin V1a receptor in Xenopus oocytes

The mechanism of short-term desensitisation of the V1a vasopressin receptor, a phospholipase-C beta linked receptor, was investigated in albino Xenopus oocytes. V1a receptors showed rapid agonist-dependent mobilisation of intracellular calcium, as detected by aequorin photon emission. Agonist-induced homologous short-term desensitisation was evidenced within minutes after stimulation. Injection of the second messengers calcium or inositol triphosphate inside the cell did not desensitise the receptors. In contrast, protein kinase C (PKC) activators 1-oleoyl-2-acetyl-sn-glycerol (OAG) (50 microM) and 1,2-dioctanoyl-glycerol (DIC8) (10 microM), as well as phorbol -12-myristate-13-acetate (1 microM) and phorbol -12,13-dibutyrate (1 microM) blunted the calcium responsiveness of the V1a receptors. The specific PKC inhibitor bisindolylmaleimide (GF109203X) (1 microM) prevented the effect of DIC8 and OAG on V1a receptor desensitisation. Heterologous desensitisation induced by agonist occurred in oocytes that co-expressed the V1a receptor and the PKC-activating M5 muscarinic receptor. It was concluded that PKC activation has a role in short-term desensitisation of the V1a receptor.

Role of protein kinase C and carboxyl-terminal region in acute desensitization of vasopressin V1a receptor

The role of protein kinase C activation and carboxyl-terminal region in rapid desensitization of the vasopressin V1a receptor was investigated in Xenopus oocytes. Preincubation of the oocytes with vasopressin or with the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG), or direct injection of active protein kinase C, all blunted the calcium response of the V1a receptor. Truncation of the 51 terminal amino acids (S374STOP) modified neither the intracellular calcium response to vasopressin nor its desensitization by vasopressin or OAG. These data suggest that desensitization of the V1a receptor is mediated by PKC activation and that its carboxyl-terminal domain is not required for signal transduction and rapid desensitization.

Endothelium-dependent and NO-mediated desensitization to vasopressin in rat aorta

1. The present study was performed to characterize the tachyphylaxis of rat aortae to vasopressin. Isometric tension generated by rat thoracic aorta sliced in 4 mm rings, was recorded. 2. Tension generated by intact rings increased with cumulative additions of vasopressin up to 10 nM (1.51 +/- 0.15 g). After this concentration, most rings lost their tension and relaxed to 1.09 +/- 0.17 g (P < 0.001) despite further addition of vasopressin. This tachyphylaxis was not observed in endothelium-denuded rings (from 2.87 +/- 0.12 g to 2.68 +/- 0.17 g). 3. Repeated administrations of supramaximal concentration (100 nM) of vasopressin confirmed an enhanced desensitization in intact rings, compared to endothelium-denuded rings. No desensitization to phenylephrine was observed in intact or in endothelium-denuded rings. 4. Dose-response curves to a V1 receptor agonist, [Phe2, Ile3, Orn8]-vasopressin, and to a V2 receptor agonist, [deamino-Cys1,D-Arg8]-vasopressin, were performed in intact preparations. An increase in tension, followed by a desensitization was observed with the V1 receptor agonist. In contrast, the V2 receptor agonist did not induce any response. 5. Pretreatment of intact aortic rings with the cyclo-oxygenase inhibitor, diclofenac (1 microM), did not prevent the desensitization to vasopressin. In contrast, NO synthase inhibition with NG-nitro-L-arginine (30 microM) resulted in an attenuated desensitization to vasopressin in intact rings (from 2.46 +/- 0.17 to 2.25 +/- 0.22 g, NS). 6. To confirm the involvement of NO, endothelium-denuded rings were pretreated with sodium nitroprusside (SNP). At a concentration of 10 nM, SNP induced a desensitization to vasopressin comparable with that observed in intact rings. 7. Pretreatment of endothelium-denuded rings with 8-bromo-cyclic GMP (100 microM) reduced maximum contraction to vasopressin without producing any desensitization. In contrast, guanylate cyclase inhibition with either LY 83,583 (10 microM) or methylene blue (10 microM) blocked completely the desensitization of intact rings to vasopressin. 8. The results suggest that the endothelium-dependent tachyphylaxis to vasopressin is due to rapid desensitization and is mediated by NO. However, it is unclear whether this effect of NO involves cyclic GMP.

Mechanism of desensitization of the cloned vasopressin V1a receptor expressed in Xenopus oocytes

The vasopressin V1a receptor exerts its effects by G protein-mediated increases in cytosolic Ca2+ (Cai2+) and activation of protein kinase C. The V1a receptor also undergoes autologous desensitization. To clarify the mechanism of this desensitization, we expressed the cloned receptor in Xenopus oocytes, and vasopressin-induced Cai2+ waves were examined as an index of V1a activation using confocal microscopy. Pretreatment of oocytes with a minimal concentration of vasopressin inhibited further generation of Cai2+ waves upon maximal stimulation. Such pretreatment did not abolish Cai2+ waves induced by subsequent microinjection of inositol trisphosphate, suggesting that this phenomenon represents receptor desensitization rather than depletion of inositol trisphosphate-sensitive Cai2+ stores. Pretreatment with phorbol dibutyrate, ionomycin, or 8-bromoadenosine 3',5'-cyclic monophosphate had no effect on vasopressin-induced Cai2+ waves. Oocytes recovered from desensitization within 1 h, but the microtubule inhibitor methyl-5-[2-thienylcarbonyl]-1H-benzimiidazol-2-yl)-carbamate (nocodazole) inhibited this recovery. Receptor binding sites were reduced by over 50% within 10 min of exposure to vasopressin, with no associated change in the Kd for the V1a receptor. These findings indicate that 1) expression of the cloned V1a receptor in Xenopus oocytes, coupled with subcellular Cai2+ imaging, provides a useful system to examine mechanisms of V1a desensitization, 2) the V1a receptor undergoes autologous desensitization in this experimental system, and 3) protein kinase C, Cai2+, and adenosine 3',5'-cyclic monophosphate do not appear responsible for this desensitization, but 4) microtubule-dependent recycling of the receptor is preserved in this system and may be important for receptor desensitization.

The role of protein kinase A and protein kinase C in prostanoid IP receptor desensitization in NG108-15 cells

Pretreatment of NG108-15 cells for 1 h with 1 microM phorbol 12-myristate,13-acetate produced no significant effect on the subsequent stimulation of adenylate cyclase activity by the IP receptor agonist, iloprost, the adenosine A2 receptor agonist, N-ethylcarboxamidoadenosine (NECA), or sodium fluoride, suggesting that protein kinase C activation does not produce desensitization in this system. Pretreatment of cells with 10 microM iloprost or forskolin for 17 h produced a decrease in the specific binding of [3H]iloprost, consistent with a decrease in IP receptor number. Iloprost pretreatment produced a decrease in responses to iloprost, NECA and sodium fluoride, whereas forskolin pretreatment produced a decrease in subsequent responsiveness to iloprost and NECA, but the response to sodium fluoride remained unaffected. The desensitization produced by forskolin could be completely inhibited by the inhibitor of protein kinase A and protein kinase C, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), but H7 had no effect on the desensitization produced by iloprost.

Different mechanisms of homologous and heterologous desensitization of thrombin-induced endothelial prostacyclin production

Several workers have described desensitization of endothelial prostacyclin production but conflicting evidence has been published regarding the mechanism of desensitization, whether it is homologous (agonist specific) or heterologous, and whether inactivation of cyclooxygenase is involved. The purpose of the present study was to determine the relation between the intensity of a first thrombin stimulus and the subsequent response to a repeat thrombin, histamine, ionophore A23187 or aluminium fluoride (AlF4) stimulation and to determine possible targets of desensitization. Following thrombin stimulation of confluent cultured human umbilical vein endothelial cells (HUVEC) only homologous desensitization of inositol phosphate production was observed. Both homologous and heterologous desensitization of arachidonic acid release and prostacyclin production occurred, the latter towards both histamine and the ionophore A23187. For any given dose of the first stimulant there was a much greater effect on the homologous response than on the heterologous response. These differences suggest different mechanisms. The homologous desensitization probably involves the receptor whereas the present results suggest that the target of heterologous desensitization is distal to calcium mobilization in the signal transduction pathway. The possibilities include decreased activity of phospholipase A2 or a decreased pool of accessible arachidonic acid.

A selective biotinylated probe for V1a vasopressin receptors

We have designed and synthesized a biotinylated vasopressin antagonist which is a selective probe for studying the V1a subtype of vasopressin receptor. Initially we synthesized the novel vasopressin analogue d(CH2)5Tyr(Me)2LysNH2(9)AVP (ALVP). Biotinamidocaproate was subsequently coupled to the epsilon-amino group of ALVP to generate the novel biotinylated probe d(CH2)5Tyr(Me)2Lys(N epsilon-biotinamido-caproate)NH2(9)AVP (ALBtnVP). Pharmacological characterization of ALVP and ALBtnVP established that both ligands were high affinity antagonists at V1a receptors, and that both displayed marked V1a/V2 selectivity. The observation that receptor-bound ALBtnVP was bi-functional, and thereby able to bind conjugated derivatives of avidin or streptavidin, allowed ALBtnVP to be utilized as a selective probe for V1a receptors. This strategy allowed the visualization of V1a receptors on the surface of WRK-1 cells and hippocampal neurons, by using streptavidin-gold with electron microscopy and fluorescein-avidin with light microscopy. We conclude that ALBtnVP is a useful probe for V1a receptors.