Characterization of adherens junction protein expression and localization in pituitary cell networks

Our view of anterior pituitary organization has been altered with the recognition that folliculo-stellate (FS) and somatotroph cell populations form large-scale three-dimensional homotypic networks. This morphological cellular organization may optimize communication within the pituitary gland promoting coordinated pulsatile secretion adapted to physiological needs. The aim of this study was to identify the molecules involved in the formation and potential functional organization and/or signaling within these cell-cell networks. Here, we have focused on one class of cell adhesion molecules, the cadherins, since beta-catenin has been detected in the GH cell network. We have characterized, by qPCR and immunohistochemistry, their cellular expression and distribution. We have also examined whether their expression could be modulated during pituitary tissue remodeling. The mouse anterior pituitary has a restricted and cell-type specific repertoire of cadherin expression: cadherin-11 is exclusively expressed in TSH cells; N-cadherin displays a ubiquitous expression pattern but with different levels of expression between endocrine cell types; E-cadherin is restricted to homotypic contacts between FS cells; while cadherin-18 is expressed both in somatotrophs and FS cells. Thus, each cell type presents a defined combinatorial expression of different subsets of cadherins. This cell-type specific cadherin expression profile emerges early during development and undergoes major changes during postnatal development. These results suggest the existence within the anterior pituitary of cell-cell contact signaling based on a defined pattern of cadherin expression, which may play a crucial role in cellular recognition during the formation and fate of pituitary cell homotypic networks.

Functional remodeling of gap junction-mediated electrical communication between adrenal chromaffin cells in stressed rats

An increase in circulating catecholamine levels represents one of the mechanisms whereby organisms cope with stress. In the periphery, catecholamines mainly originate from the sympathoadrenal system. As we reported, in addition to the central control through cholinergic innervation, a local gap junction-delineated route between adrenal chromaffin cells contributes to catecholamine exocytosis. Here, we investigated whether this intercellular communication is modified when the hormonal demand is increased as observed during cold stress. Our results show that in cold exposed rats, gap-junctional communication undergoes a functional plasticity, as evidenced by an increased number of dye-coupled cells. Of a physiological interest is that this upregulation of gap-junctional coupling results in the appearance of a robust electrical coupling between chromaffin cells that allows the transmission of action potentials between coupled cells. This enhancement of gap-junctional communication parallels an increase in expression levels of connexin36 (Cx36) and connexin43 (Cx43) proteins. Both transcriptional and posttranslational mechanisms are involved because Cx36 transcripts are increased in stressed rats and the expression of the scaffolding protein zonula occludens-1, known to interact with both Cx36 and Cx43, is also upregulated. Consistent with an upregulated coupling extent in stressed rats, the cytosolic Ca(2+) concentration rises triggered in a single cell by an iontophoretic application of nicotine occur simultaneously in several neighboring cells. These results describe for the first time a functional plasticity of junctional coupling between adult chromaffin cells that should be crucial for adaptation to stress or sensitization to subsequent stressors.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Synthesis, conformational studies and biological activity of Να-mono-biotinylated rat relaxin*

Biotin-avidin immobilization can be a useful tool in structure-function studies of hormone receptors. A crucial step is the preparation of a specifically biotinylated hormone that is able to bind to its receptor while leaving the biotin group free for interaction with avidin. The receptor for relaxin, an ovarian peptidic hormone produced during pregnancy, has not yet been isolated. We therefore undertook to prepare a specifically monobiotinylated rat relaxin for use in ligand-searching strategies. Rat relaxin is a convenient analogue because reliable bioassays exist, thus allowing assessment of the effect of N-biotinylation on bioactivity. To help improve the yield of the two-chain, three-disulfide bond rat relaxin, 2-hydroxy-4-methoxybenzyl (Hmb) backbone protection was used during the solid-phase assembly of the B-chain to help prevent any possible chain aggregation. As a final step, while the protected peptide was still on the resin, the biotin label was introduced at the N-terminus of the B-chain using standard coupling protocols. The chain combination with the A-chain was accomplished in reasonable yield. Secondary structural measurements demonstrated that the biotin caused the starting B-chain to adopt a more ordered conformation. The labelled synthetic relaxin exhibited similar circular dichroism spectra to native and synthetic single B-chain peptides. In addition, the biotinylated relaxin showed no significant difference in its chronotropic activity in the rat isolated heart assay compared with the native peptide. Biosensor studies showed that antibody recognition was retained upon attachment of the synthetic relaxin to the streptavidin-derivatized surface.

Gap junctions mediate electrical signaling and ensuing cytosolic Ca2+ increases between chromaffin cells in adrenal slices: A role in catecholamine release

In adrenal chromaffin cells, a rise in cytosolic calcium concentration ([Ca(2+)]i) is a key event in the triggering of catecholamine exocytosis after splanchnic nerve activation. Action potential- or nicotine-induced [Ca(2+)]i transients are well described in individual chromaffin cells, but whether they remain spatially confined to the stimulated cell or propagate to adjacent cells is not yet known. To address this issue, the spatiotemporal organization of electrical and associated Ca(2+) events between chromaffin cells was investigated using the patch-clamp technique and real-time confocal imaging in rat acute adrenal slices. Spontaneous or electrically evoked action potential-driven [Ca(2+)]i transients were simultaneously detected in neighboring cells. This was likely attributable to gap junction-mediated electrotonic communication, as shown by (1) the bidirectional reflection of voltage changes monitored between cell pairs, (2) Lucifer yellow (LY) diffusion between cells exhibiting spontaneous synchronized [Ca(2+)]i transients, and (3) the reduction of LY diffusion using the uncoupling agent carbenoxolone. Furthermore, transcripts encoding two connexins (Cx36 and Cx43) were found in single chromaffin cells. This gap junctional coupling was activated after a synaptic-like application of nicotine that mediated synchronous multicellular [Ca(2+)]i increases. In addition, nicotinic stimulation of a single cell triggered catecholamine release in coupled cells, as shown by amperometric detection of secretory events. Functional coupling between chromaffin cells in situ may represent an efficient complement to synaptic transmission to amplify catecholamine release after synaptic stimulation of a single excited chromaffin cell.

The human neuroblastoma SK-SY5Y cell line bears functional endothelin-A-receptors and endothelin

The study reported here characterizes the presence both of endothelin (ET) receptors and of a synthesizing ET apparatus in the human neuroblastoma SK-SY5Y cell line. We demonstrated, using reverse transcriptase polymerase chain reaction (RT-PCR), that these cells bound [125I]ET-1. The potency order of ET analogs to inhibit [125I]ET-1 binding was consistent with the presence of ET(A)-receptors. [Ca2+]i was increased by both ET-1 and ET-3 (potency order: ET-1 > ET-3. The mRNAs of preproendothelin-1 and of endothelin converting enzyme (ECE) were expressed by cells, as shown by RT-PCR studies. These mRNAs were translated into functional proteins as the cells were able to release mature (1-21) ET-like immunoreactivity into the culture medium. That secretion was time-dependent and was enhanced by treatment of the cells by the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate. These results show that the human SK-SY5Y neuroblastoma cell line produces mature ET which could act as an autocrine/paracrine factor these cells.

Novel strategy for the synthesis of template-assembled analogues of rat relaxin

The 'template-assembled synthetic protein' (TASP) concept provides a simple and elegant approach for the preparation of analogues that retain key structural elements. We have synthesized TASP molecules containing the putative active site of relaxin, a peptide that has similar structural features to insulin but a markedly different biological role. Two types of chemoselective thiol ligation strategies (thioether and thiazolidine) were used and compared. The synthetic pendant peptides contain an essential region for bioactivity that is located in the alpha-helical region of the relaxin B-chain. Depending on whether the thioether or the thiazolidine chemistry was used to attach the peptides to the template, the reacting amino acid was placed either at the C-terminus or N-terminus, respectively, thus allowing the choice of orientation relative to the carrier molecule. The template molecule consists of a decapeptide with two proline-glycine turns and four evenly spaced lysine residues that were functionalized with the appropriate chemical moiety. This allowed reaction with the appropriately derivatized peptides in solution. To improve the template ligation step using the thioether approach, a pendant peptide C-terminal cysteamine residue was used to reduce potential steric hindrance during conjugation. The design of the peptides as well as the synthetic strategy resulted in the acquisition of mimetics showing weak non-competitive and weak competitive antagonist properties.

Antiproliferative, differentiating and apoptotic effects elicited by imidazo[1,2-a]pyrazine derivatives

1. The activity of two series of imidazo[1,2-a]pyrazine derivatives on cell proliferation and differentiation and on apoptosis was examined in relation to their effects on phosphodiesterase (PDE) activity and on purinoceptors. 2. In the first series SC-18 and SC-51 inhibited mitogen-induced 3H-thymidine incorporation in human lymphocytes. 3. The compounds of the new series PAB13, PAB23 and SCA40 inhibited the proliferation of the HEL cell line. 4. Nine imidazo[1,2-a]pyrazine derivatives of the new series have been studied on the Dami cell proliferation. SCA41 and SCA44 inhibited cell growth, SCA40 and PAB40 were moderately effective, whereas PAB12 and PAB30 were devoid of effect. The antiproliferative effects of these six non-cytotoxic compounds could not be related to their action on PDE or on purinoceptors, but rather to their lipophilicity. Conversely, for PAB13, PAB15, and PAB23, the decrease in cell number was related to their cytotoxic and apoptotic effects through their cAMP-increasing and PDE-inhibitory potency, but unrelated to an effect on purinoceptors. 5. Imidazo[1,2-a]pyrazine derivatives decreased the expression of Glycoprotein (GP)Ib in Dami cells while some of them enhanced that of GPIIb/IIIa. These effects appeared to involve inhibition of both cAMP- and cGMP-PDE. 6. These studies demonstrate the potential interest of imidazo[1,2-a]pyrazine derivatives in the query of novel anticancer drugs.

Adrenal steroid-secreting cells express endothelin-1

Endothelin-1 (ET-1) is involved in adrenal steroid secretion but its cell origin remains unclear. We showed, using RT-PCR the expression of the mRNAs for preproET-1 and ECE-1 in primary cultures of human adrenal cells enriched in glomerulosa cells. Since these expressions could be due to contamination of steroid secreting cells by other cells, we also used the human adrenocortical cell line H295R, which was shown to produce steroids. This cell line also expressed preproET-1-RNA and released mature ET. Functional ET receptors were shown on H295R and cultured human adrenocortical cells. These findings indicate that adrenal steroid-secreting cells synthesize and release ET-1, raising the possibility for an autocrine-paracrine effect of ET-1 on adrenocortical functions.

Human bone marrow platelet progenitors express and release endothelin-1

Because it has been shown that human platelet precursors from normal bone marrow express preproendothelin-1 (ET-1) mRNA, this investigation was designed to find out whether these cells could synthesize and release mature ET-1 and express ET receptors. Therefore, we examined the expression of endothelin-converting enzyme (ECE) mRNA and of mRNAs for ET receptors in cells purified from normal bone marrow and measured immunoreactive (ir)ET in their culture medium. RT-PCR applied to RNAs from platelet precursors yielded amplified fragments of the expected sizes of 567, 428, and 299 bp for ECE, ETB and ETA receptors, respectively. These cells released ir-ET into the culture medium in a time-dependent fashion. These results raise the possibility of autocrine actions of the intrinsic ET system in bone marrow platelet precursor cells.

Signaling mechanisms induced by endothelin agonists in human adrenal glomerulosa cells

Endothelin-1 (ET-1) exhibits secretagogue and trophic actions on the adrenal zona glomerulosa (ZG). Little information is available on the intracellular signaling events that follow stimulation of ET receptors on ZG cells. This study examined the expression of ET receptor subtypes and their involvement in transduction mechanisms induced by ET agonists on human ZG cells in primary culture. RT-PCR allowed the detection of both ETA and ETB receptor mRNAs in these cells. ET-1 induced a concentration-dependent increase in inositol phosphate (IP) accumulation in the presence of LiCl, whereas ETB agonists were inactive. The ET-1-induced increase in IP accumulation was prevented by BQ-123. ET-1 evoked an increase in [Ca2+]i, which was partially prevented by BQ-788. IRL 1620 also delayed the rise in [Ca2+]i. These results show that in human adrenal ZG cells, ET-1 induces an increase in IP accumulation through ETA receptor activation and evokes a rise in [Ca2+]i via stimulation of both ETA and ETB receptors.

Phosphoramidon-sensitive and -insensitive endothelin-converting enzyme in human megakaryoblastic cell lines

The human megakaryoblastic cell lines HEL, MEG-01, and DAMI express preproendothelin-1 mRNA. This investigation was designed to find out whether they could also express endothelin-converting enzyme (ECE) and release mature endothelin (ET). RT-PCR applied to RNA isolated from the cell lines amplified fragments of the expected size. The amplified cDNA of MEG-01 was submitted to restriction enzymes, which generated the expected subfragments. Membrane ECE activity was phosphoramidon-sensitive, in contrast to the cytosolic activity capable of producing ET-1 from big ET-1. The three cell lines produced ir-ET in a time-dependent manner. These results show that human megakaryoblastic cell lines express functional, phosphoramidon-sensitive and insensitive ECE activity and produce mature ET.

Change of endothelin receptor subtype in the MEG-01 human megakaryoblastic cell line

The aim of this investigation was to determine whether the endothelin receptor subtype of a megakaryoblastic cell line (MEG-01) changes during culture passages as cells undergo maturation and differentiation. On early-passage cells, binding of [125I]endothelin-1 was completely inhibited by 1 microM BQ 123 (cyclo-[D-tryptophanyl-D-aspartyl-prolyl-D-valyl-leucyl]), but not by sarafotoxin 6C. Also the endothelin-1-enhancing effect on [Ca2+]i was prevented by BQ 123, whereas sarafotoxin 6C had no effect on [Ca2+]i. In late-passage cells, endothelin ET(B) analogs, unlike endothelin ET(A) analogs, competed with binding of [125I]endothelin-1. Endothelin ET(B) receptor agonists increased [Ca2+]i while the endothelin-1-induced response was inhibited by BQ 788 ([N-[(2R,6S)-2,6-dimethyl-piperidinocarbonyl]-4-methyl-D-leucyl]-[ N(omega)-(methoxycarbonyl)-D-tryptophanyl]-D-norleucine), but not by BQ 123, although both endothelin ET(A) and ET(B) receptor mRNAs were expressed, as shown by reverse transcriptase-polymerase chain reaction. These results demonstrate that in MEG-01 cells switch from expression of endothelin ET(A) to expression of ET(B) receptors during culture. The data also suggest that late-passage MEG-01 cells look like platelets, in terms of endothelin receptor subtype.

Apoptotic effects of imidazo[1,2-a]pyrazine derivatives in the human Dami cell line

cAMP-elevating agents like phosphodiesterase inhibitors and purines have been shown to induce apoptosis. In the present work we have studied the effects of imidazo[1,2-a]pyrazine derivatives with a purine-like structure: PAB13 (6-bromo-8-(methylamino)imidazo[1,2-a] pyrazine), PAB15 (6-bromo-8-(ethylamino)imidazo[1,2-a]pyrazine), PAB23 (3-bromo-8-(methylamino)imidazo[1,2-a]pyrazine) on the growth of the Dami cell line in comparison to that of adenosine. The growth effect of PAB13, PAB15 and PAB23 was investigated in relation to their phosphodiesterase-inhibitory action and their activity on purinoceptors. Inhibition in cell growth was up to 71.0%, 76.3% and 89.7% for PAB23, PAB13 and PAB15, respectively and 100% for adenosine. Cell viability was affected in a concentration-dependent manner by PAB13, PAB15 and adenosine, with a correlation between growth inhibition and cytotoxicity. These effects of imidazo[1,2-a]pyrazine derivatives were found to be unrelated to an action on purinoceptors, but rather appear quantitatively linked to their ability in inducing apoptosis through their cAMP-increasing and phosphodiesterase-inhibitory potency.

Endothelin expression in human megakaryoblastic leukemia cell lines and normal platelet precursors

The aim of the present investigation was to determine whether endothelin (ET) could be expressed in and released from the human leukemia megakaryoblastic cell lines HEL, MEG-01, DAMI and the normal human platelet progenitors. Using the reverse transcriptase-polymerase chain reaction (RT-PCR) on total RNA isolated from the cells, we amplified a cDNA of the expected size (453 bp). Southern-blotting hybridization revealed that RT-PCR products from the cell lines were specific of ET-1 mRNA. Immunocytochemical analyses highlighted immunoreactive ET-1 in the cytoplasm of these cells which also released the mature peptide. ET-1 release from the three cell lines was increased by thrombin exposure. Although MEG-01 cells express ET receptors, ET-1, the selective ETB agonist sarafotoxin 6C and the non-selective ET-receptor antagonist PD 142893 showed no proliferative or antiproliferative action in basal or stimulating medium. This indicated a lack of autocrine ET-mediated effect on growth. These results demonstrate for the first time that human megakaryoblastic leukemia cell lines and normal bone marrow platelet precursors express ET-1 mRNA and release the mature peptide.

Effect of chronic treatment with trandolapril or enalapril on brain ACE activity in spontaneously hypertensive rats

The aim of the present study was to determine whether the new ACE inhibitor trandolapril was able to inhibit brain ACE activity in spontaneously hypertensive rats (SHRs). Therefore, we have measured ex vivo ACE activity in discrete brain areas of SHRs after a 2-week oral treatment with trandolapril (0.001, 0.01, 0.1 and 1 mg/kg/day). The effects of trandolapril were compared to those of enalapril (10 mg/kg/day), used as a reference compound. Enalapril induced a decrease in ACE activity in brain areas not protected by the blood brain barrier (subfornical organ and median eminence) and in cerebral cortex. Conversely, trandolapril at a dose of 0.01 mg/kg/day and above induced a dose-dependent inhibition of ACE activity in all brain areas assayed, including the supraoptic and paraventricular hypothalamic nuclei, septum, amygdala, hippocampus, cerebellar and cerebral cortex, nucleus of the tractus solitary and caudate nucleus. The inhibition was roughly similar in all brain areas studied. These data suggest that after chronic oral administration in SHRs, trandolapril or its metabolite, in contrast to enalapril or enalaprilat, was able to reach all brain areas of SHRs, including those protected by the blood brain barrier.

Effects of endothelins on the human megakaryoblastic cell line MEG-01

Some effects of endothelin-1 (ET-1) were studied on the megakaryoblastic cell line MEG-01. ET-1 induced an elevation of the intracellular levels of Ca2+([Ca2+]i) as measured with the fluorescent indicator indo-1, which consists of an initial transient increase and an ensuing sustained plateau. The plateau phase was abolished by removal of extracellular Ca2+. In addition, ET-1 induced a rapid (within 5 s) increase in the accumulation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and more delayed increases in Ins(1,3,4)P3 and Ins(1,3,4,5)P4, but did not modify cAMP levels. ET-1 homologues (ET-2, ET-3, sarafotoxin 6b and vasointestinal constrictor) also induced biphasic effects on [Ca2+]i. The Ca2+ elevation was concentration dependent, the order of potency being sarafotoxin 6b > ET-1 > ET-2 = vasointestinal constrictor > ET-3. The actions of ET analogs in raising [Ca2+]i were mutually exclusive, suggesting that they act through the same mechanism. These results suggest that cells of the megakaryoblast/megakaryocyte lineage are targets for endothelins.

Platelet receptor expression on three human megakaryoblast-like cell lines

Platelets, the progeny of bone marrow megakaryocytes, are nonnucleated cells; many platelet proteins, including platelet membrane receptors, are believed to be derived from megakaryocytes. Several hematopoietic cell lines that exhibit megakaryocytic characteristics have been established as models for the study of megakaryocyte biology. We report here the screening of platelet receptor expression, in terms of functional coupling with the formation of two second messengers, calcium and cAMP, in three cell lines exhibiting megakaryoblastic properties: HEL, MEG-01, and DAMI. We show that all these cell lines respond to thrombin, ADP, epinephrine, and prostaglandin E1 (PGE1). However, transmembrane signaling pathways appear partly different from those present in mature platelets, because the action of thrombin was found to be positively coupled with the cAMP pathway, in addition to that of calcium, and because PGE1, which interacts with the cAMP pathway, also raises intracellular calcium levels in the three cell lines studied. Furthermore, an endothelin-1-induced increase in intracellular calcium level was observed in MEG-01 cells, strongly suggesting the expression of endothelin receptors on platelet precursors cells, whereas the presence of such receptors is controversial on platelets. These cell lines should prove useful in further studies of the expression and molecular pharmacology of platelet receptors on platelet precursor cells, as well as for the investigation of functional roles for platelet receptors on megakaryoblastic cells.

Thrombin inhibits proliferation of the human megakaryoblastic MEG-01 cell line: a possible involvement of a cyclic-AMP dependent mechanism

Thrombin, a potent platelet activating agent, has previously been found to increase intracellular calcium levels and/or thromboxane A2 synthesis in leukemic cell lines exhibiting specific markers of the megakaryocyte/platelet lineage. However, its functional role on these cells has not been defined. As thrombin is implicated in the regulation of cellular proliferation or differentiation in various other cell types, we investigated the functional effects of thrombin on the megakaryoblastic MEG-01 cell line, and further explored its receptor coupling mechanisms on these cells. We observed that thrombin caused in 1% serum containing culture medium, a reduction in the proliferation of MEG-01 cells, without affecting their differentiation stage as determined by the expression of platelet glycoproteins GPIIb/IIIa and GPIb, FVIII-related-antigen and cell-size measurement, which are specific markers for megakaryocyte maturation. In addition, incubation of MEG-01 cells with thrombin resulted in dose-dependent increases in cAMP levels, and in inositol-trisphosphate formation and intracellular Ca2+ levels. All these responses required thrombin proteolytic activity. The lipoxygenase inhibitor, nordihydroguaiaretic acid, blunted thrombin-induced calcium increase without affecting thrombin-induced increase in cAMP levels, suggesting different thrombin coupling mechanisms with these two second messenger pathways. In addition, the inhibitory effect of thrombin on MEG-01 cell growth was mimicked by cAMP level enhancing agents such as forskolin, prostaglandin E1 and Bt2cAMP. These results suggest the involvement of a cAMP-dependent mechanism in the thrombin-induced reduction in MEG-01 cell growth.

Cardiovascular actions and tissue-converting enzyme inhibitory effects of chronic enalapril and trandolapril treatment of spontaneously hypertensive rats

The angiotensin I-converting enzyme (CE) inhibitors, trandolapril (RU 44570) and enalapril were administered for 2 weeks to SHR at doses (3 and 10 mg/kg/day, p.o., respectively) that produced important and comparable inhibitions of plasma (84 and 88%), aorta (97 and 88%), and atrium (89 and 82%) CE activities. At these doses, the inhibitory effects of trandolapril and enalapril were nonetheless different on CE in heart ventricle (58 and 72%) and kidney (45 and 85%). In addition, although both drugs reduced blood pressure (BP) and heart hypertrophy, trandolapril was more potent despite a lower dose-ratio. All these parameters were reexamined 1, 3, and 8 days after drug withdrawal: BP returned to control levels within 3 days in enalapril-treated rats, whereas it remained low for at least 8 days in trandolapril-treated animals. The reduction of heart hypertrophy owing to trandolapril was still present 8 days after drug discontinuation. On cessation of treatment, plasma CE increased above controls, ventricle CE returned to control levels within 3 days, whereas atrial and aortic CE activities remained inhibited for 8 days in the enalapril group. In contrast, in trandolapril-treated rats, CE activities in serum and tissues were still inhibited after 8 days. These results demonstrate that at the doses used trandolapril is more potent and longer acting than enalapril.

Properties of vasopressin-activated human platelet high affinity GTPase

Effect of 8-arginine vasopressin (AVP) was examined on human platelet membrane GTPase activity as an index of a G-protein involvement. AVP stimulated a high-affinity GTPase activity in a dose-dependent manner (Kact = 1.1 +/- 0.2 nM). This stimulation was blocked by a V1a antagonist, thus confirming the V1a nature of the platelet AVP receptor. There were important variations among individuals in the AVP-induced stimulation of GTPase activity, that were in relation with the AVP-maximal binding capacity. These data suggest a causal relationship between the binding of AVP to its receptor and transduction elicited by a G-protein, without amplification. In addition, in view of the variable AVP responsiveness observed among individuals, platelet AVP-receptor appears to be subject to regulation.

Vasopressin inhibition of human platelet adenylate cyclase: variable responsiveness between donors and involvement of a G-protein different from Gi

There is controversy concerning the inhibitory effect of arginine-vasopressin (AVP) on human platelet adenylate cyclase activity, which putatively involves Gi as the G-protein. To clarify this point, the effects of AVP on human platelet membranes were studied by measuring the activities of the high-affinity GTPase, as an index of G-protein involvement, and of adenylate cyclase. AVP stimulated GTPase activity in a dose-dependent fashion (KAct = 1.1 +/- 0.2 nM) and caused a parallel adenylate cyclase inhibition (KAct = 1.3 +/- 0.7 nM). The extent of these AVP-induced responses varied considerably from one subject to another but they were linearly related, suggesting a causal relationship between the two activities. Moreover, a difference in responsiveness to the inhibitory effects to epinephrine on adenylate cyclase was also observed between donors. Since the AVP- and epinephrine-stimulated GTPase activities were additive at their respective maximal effect, and in view of the lack of linear relationship between AVP- and epinephrine-induced adenylate cyclase inhibition, our results suggest, that in spite of the AVP inhibitory action on platelet adenylate cyclase, the G-protein involved in this effect is different from Gi.

Differential effects of oral trandolapril and enalapril on rat tissue angiotensin-converting enzyme

Trandolapril (3-100 micrograms/kg) and enalapril (10-300 micrograms/kg) were administered orally to conscious rats. Angiotensin-converting enzyme (CE) activity was inhibited in serum, heart ventricle, renal inner cortex, lung, aorta, adrenal cortex and adrenal medulla, but not in the striatum. Inhibition was maximal at 2 h and with trandolapril was maintained for 24 h. Blood pressure and heart rate were not affected by either compound. Trandolapril was 6-10-fold more potent than enalapril. Differences between trandolapril and enalapril in CE inhibition observed in heart ventricle, adrenal cortex and medulla could be due to the presence of more than one type of CE or CE-like activity.

V1a-vasopressin specific receptors on human platelets: potentiation by ADP and epinephrine and evidence for homologous down-regulation

Using very specific vasopressin (VP) analogues, the human platelet VP receptor was characterized as a V1a rather than a V1b receptor, on the basis of the effect of the analogues on shape-change and aggregation. The platelet VP binding sites appeared to be subject to homologous down-regulation by plasma VP, in view of the inverse correlation found between the maximal capacity of binding of tritiated VP to platelets and the immunoreactive VP concentration in poor platelet plasma from the same individual. Aggregating effect of VP on human platelets was potentiated by both ADP and epinephrine. In addition, VP was able to release serotonin from human platelets, but only at high concentration.

Influence of Ca2+e on 5-HT2- and alpha 1-induced arterial contraction and phosphoinositide metabolism

Serotonin and phenylephrine were found to induce contractile responses and inositol phosphate (IP) formation in isolated rat tail artery. Both processes displayed similar concentration dependence in the presence of 2.5 mM external calcium (Ca2+e) and both were respectively inhibited by either ketanserine or prazosin, depending on the agonist used. In the absence of Ca2+e the amines no longer produced a contractile effect. In addition, lack of Ca2+ caused a shift to the right in the dose-response curve for phenylephrine-induced IP formation whereas serotonin-induced IP formation was not affected by changes in Ca2+e. The results suggest that alpha 1- and 5-HT2-induced contractions are quantitatively related to phosphatidylinositol metabolism. Contraction, but not IP formation requires the presence of Ca2+e. Different effects of Ca2+e on phenylephrine- and serotonin-induced IP formation could be related to a differential Ca2+ effect on binding of alpha 1- or 5-HT2 agonists.

V1a vasopressin-induced accumulation of inositol trisphosphate in cultured rat aortic myocytes; modulation by protein kinase C

Arginine vasopressin stimulated the accumulation of labeled inositol phosphate in cultured rat aortic myocytes prelabeled with tritiated myo-inositol. This accumulation was prevented by pretreating the myocytes with the phorbol ester PMA. The time-course and concentration-effect curves were similar for inositol phosphate formation in myocytes and contractile effects on isolated aorta. Vasopressin agonists also stimulated inositol phosphate formation, whereas vasopressin-induced response could be inhibited by V1a-specific antagonists. These results suggest that stimulation of inositol phosphate formation in myocytes is due to V1a receptor activation and could be modulated by protein-kinase-C-mediated mechanisms.

Reduction in hepatic but not in renal and vascular vasopressin receptor number in hyperinsulinemic mice and rats

Hepatic plasma membranes of female obese mice C57 BL-6 orl ob/ob (ob/ob mice) completely lack vasopressin (VP) receptors of the V1 type whereas kidney VP receptors are normally expressed and functionally coupled to adenylate cyclase. To discover if these alterations are linked to a genetic defect of the V1 receptor, we have studied the binding of VP on liver and kidney membranes of two other models, female diabetic mice C57 BL-6 orl db/db (db/db mice) and female Zucker rats Fatty/orl fa/fa (fa/fa rats), which exhibit different temporal pattern of obesity, hyperinsulinemia and insulin resistance. In addition, since VP is known to exert its vascular response through stimulation of V1 receptors, we have studied the reactivity of VP of isolated tail artery in the three different models, ob/ob and db/db mice and fa/fa rats, and in their respective controls. In all cases, VP kidney receptors and VP vascular reactivity are normal. db/db mice exhibit a marked decrease in hepatic VP receptors whereas a 50% decrease was observed in 32 week fa/fa rats. Angiotensin II and prazosin binding sites are still present as well as the adenylate cyclase response to glucagon. These results suggest that the specific alteration in liver VP receptors is not related to a defect in V1 receptor genetic expression but is specific for liver and appears to parallel the level of hyperinsulinemia and/or insulin resistance.

Hydralazine: effect on the outflow of noradrenaline and mechanical responses evoked by sympathetic nerve stimulation of the rat tail artery

1 The effects of hydralazine on the vasoconstrictor responses to field stimulation of sympathetic nerves were studied in the isolated proximal segments of the rat tail artery. Vasoconstrictor responses to transmural stimulation were depressed by superfusion of hydrazine (0.3, 3 and 30 muM) in a concentration-dependent manner. The inhibition appeared slowly and was not easily reversed by washing. 2 Hydralazine (30 nM, 0.3 and 3 muM) reduced the stimulation-induced overflow of tritium from proximal and distal segments of the tail artery labelled with [3H]-noradrenaline in a concentration-dependent manner. This phenomenon appeared rapidly and was easily reversed by washing. 3 Theophylline (0.5 mM) did not affect the inhibitory effect of hydralazine on the stimulation-induced tritium efflux from the distal segment of the rat tail artery. 4 The present results indicate that hydralazine has, in addition to its action on vascular smooth muscle, a very marked effect on sympathetic nerve terminals. The mechanism of this presynaptic inhibition appears to be different from the postsynaptic effect, in view of the much shorter delay, the shape of the dose-effect curve, and the lack of interaction with theophylline.

Prejunctional actions of piribedil on the isolated kidney of the rabbit: comparison with apomorphine

1 The effects of piribedil on contractile responses and noradrenaline release evoked by sympathetic nerve stimulation have been studied in the isolated kidney of the rabbit. These effects were compared to those of apomorphine.2 Electrical stimulation (2, 5 and 10 Hz) of sympathetic renal nerves produced frequency-dependent increases in perfusion pressure and noradrenaline release. Piribedil did not affect (0.1 mug/min) or diminished (1 and 10 mug/min) the stimulation-evoked increase in perfusion pressure, and increased noradrenaline release in a dose-dependent manner.3 Increases in renal perfusion pressure and noradrenaline release induced by electrical stimulation were decreased by apomorphine (0.1 and 1 mug/min). These inhibitory effects were more marked at low frequencies of stimulation and were prevented by haloperidol (0.2 mumol/1).4 Piribedil (0.1 and 1 mug/min) and apomorphine (0.1, 1 and 10 mug/min) did not affect the increases in renal perfusion pressure elicited by exogenously administered noradrenaline, but piribedil (10 mug/min) diminished them.5 In the presence of desipramine (0.5 mumol/l), piribedil (0.1, 1 and 10 mug/min) produced a dosedependent inhibition of the increases in renal perfusion pressure and noradrenaline release evoked by sympathetic nerve stimulation; the inhibitory effect of piribedil was more marked at low frequencies of stimulation and was prevented by haloperidol.6 Piribedil increased the resting release of noradrenaline from the rabbit kidney, in contrast to apomorphine, which was without effect.7 It is suggested that piribedil has a complex effect on sympathetic transmission. This drug exhibits an ;amphetamine-like' action, causing noradrenaline release from its postganglionic stores. This releasing effect masks an action on prejunctional inhibitory dopamine receptors. In addition, at high doses, piribedil exhibits a marked action on postjunctional sites, since it reduces the vasoconstrictor effect of exogenous noradrenaline.

Mechanism of the depressor response to dopamine in the rat treated with phenoxybenzamine

Alterations of hypotensive responses to dopamine by antagonists were characterized in alpha-blocked, anaesthetized rats. Responses were not affected by d-propranolol (0.1 mg/kg) whereas d,1-propranolol (0.1 mg/kg) or haloperidol (1.0 mg/kg) attenuated them; higher doses of inhibitors (1.0 mg/kg; 5.0 mg/kg, respectively) failed to produce a higher inhibition, but combinations of low doses abolished the depressor responses. In adrenalectomized rats, hypotensive responses decreased; haloperidol always attenuated the responses while d,1-propranolol became ineffective. Dopamine produced an enhancement of plasma adrenaline and noradrenaline levels, which was decreased by d,1-propranolol and increased by haloperidol. The data suggest that in rats the depressor component of dopamine is due to activation of both dopaminergic and beta-adrenoceptors. The beta component appears to be due to the release of adrenaline. The results also support the concept of the existence in sympathetic nerve endings and adrenal glands of stimulatory beta-adrenergic and inhibitory dopaminergic prejunctional receptors.