The mechanisms of enhancement and inhibition of field stimulation responses of guinea-pig vas deferens by prostacyclin analogues

Neurotoxicity of Tityus bahiensis (brown scorpion) venom in sympathetic vas deferens preparations and neuronal cells

Systemic scorpion envenomation is characterized by massive neurotransmitter release from peripheral nerves mediated primarily by scorpion venoms neurotoxins. Tityus bahiensis is one of the medically most important species in Brazil, but its venom pharmacology, especially regarding to peripheral nervous system, is poorly understood. Here, we evaluated the T. bahiensis venom activity on autonomic (sympathetic) neurotransmission by using a variety of approaches, including vas deferens twitch-tension recordings, electrophysiological measurements (resting membrane potentials, spontaneous excitatory junctional potentials and whole-cell patch-clamp), calcium imaging and histomorphological analysis. Low concentrations of venom (≤ 3 μg/mL) facilitated the electrically stimulated vas deferens contractions without affecting postsynaptic receptors or damaging the smooth muscle cells. Transient TTX-sensitive sustained contractions and resting membrane depolarization were mediated mainly by massive spontaneous ATP release. High venom concentrations (≥ 10 μg/mL) blocked the muscle contractions and induced membrane depolarization. In neuronal cells (ND7-23wt), the venom increased the peak sodium current, modified the current-voltage relationship by left-shifting the Nav-channel activation curve, thereby facilitating the opening of these channels. The venom also caused a time-dependent increase in neuronal calcium influx. These results indicate that the sympathetic hyperstimulation observed in systemic envenomation is presynaptically driven, probably through the interaction of α- and β-toxins with neuronal sodium channels.

EP3 receptor deficiency attenuates pulmonary hypertension through suppression of Rho/TGF-β1 signaling

Pulmonary arterial hypertension (PAH) is commonly associated with chronic hypoxemia in disorders such as chronic obstructive pulmonary disease (COPD). Prostacyclin analogs are widely used in the management of PAH patients; however, clinical efficacy and long-term tolerability of some prostacyclin analogs may be compromised by concomitant activation of the E-prostanoid 3 (EP3) receptor. Here, we found that EP3 expression is upregulated in pulmonary arterial smooth muscle cells (PASMCs) and human distal pulmonary arteries (PAs) in response to hypoxia. Either pharmacological inhibition of EP3 or Ep3 deletion attenuated both hypoxia and monocrotaline-induced pulmonary hypertension and restrained extracellular matrix accumulation in PAs in rodent models. In a murine PAH model, Ep3 deletion in SMCs, but not endothelial cells, retarded PA medial thickness. Knockdown of EP3α and EP3β, but not EP3γ, isoforms diminished hypoxia-induced TGF-β1 activation. Expression of either EP3α or EP3β in EP3-deficient PASMCs restored TGF-β1 activation in response to hypoxia. EP3α/β activation in PASMCs increased RhoA-dependent membrane type 1 extracellular matrix metalloproteinase (MMP) translocation to the cell surface, subsequently activating pro-MMP-2 and promoting TGF-β1 signaling. Activation or disruption of EP3 did not influence PASMC proliferation. Together, our results indicate that EP3 activation facilitates hypoxia-induced vascular remodeling and pulmonary hypertension in mice and suggest EP3 inhibition as a potential therapeutic strategy for pulmonary hypertension.

Investigation of the prostacyclin (IP) receptor antagonist RO1138452 on isolated blood vessel and platelet preparations

BACKGROUND AND PURPOSE

The current study examined the utility of the recently described prostacyclin (prostanoid IP) receptor antagonist RO1138452 (2-(4-(4-isopropoxybenzyl)-phenylamino) imidazoline) as a tool for classifying prostanoid receptors.

EXPERIMENTAL APPROACH

pA(2) values were determined on isolated smooth muscle and platelet preparations.

KEY RESULTS

RO1138452 antagonized relaxation of human pulmonary artery, guinea-pig aorta and rabbit mesenteric artery induced by the selective IP agonist cicaprost. Schild plots had slopes close to unity, generating pA(2) values of 8.20, 8.39 and 8.12 respectively. Non-surmountable antagonism was sometimes found with the higher concentrations of RO1138452, attributable to the EP(3) contractile action of cicaprost. RO1138452 did not block relaxation of guinea-pig trachea induced by the EP(2)-selective agonist butaprost. In contrast, there was a modest inhibition of butaprost-induced relaxation of human pulmonary artery by RO1138452, implying activation of both EP(2) and IP receptors by butaprost. RO1138452 did not affect relaxation induced by PGE(2) (EP(4) agonist) and substance P (NK(1)/endothelium-dependent agonist) in rabbit mesenteric artery. In human and rat platelet-rich plasmas, RO1138452 antagonized cicaprost-induced inhibition of platelet aggregation in a surmountable manner; pA(2) values may have been affected by binding of RO1138452 to plasma protein. RO1138452 did not affect the inhibitory actions of PGD(2) (DP(1) agonist) and NECA (adenosine A(2A) agonist) in human platelets.

CONCLUSIONS AND IMPLICATIONS

The data indicate that RO1138452 is a potent and selective IP receptor antagonist. RO1138452 represents an important addition to our armoury of prostanoid receptor antagonists and a potential clinical agent in situations where prostacyclin has a pathophysiological function.

Prostaglandins E1 and E2, but not F2alpha or latanoprost, inhibit monkey ciliary muscle contraction

PURPOSE

To investigate the effects of prostaglandin (PG) E1, E2, F2alpha, and latanoprost acid on the electrically evoked contractile response of isolated rhesus monkey ciliary muscle.

METHODS

Longitudinal ciliary muscle preparations from rhesus monkeys were mounted in an organ bath, and tension changes were recorded by an isometric transducer. Electrical field stimulation (100 Hz, 0.3 ms, 10 V) was applied through a pair of platinum plate electrodes.

RESULTS

The ciliary muscle produced atropine-sensitive excitatory contraction in response to field stimulation. PGE1 and PGE2 (1 microM) attenuated the contraction to levels that were 68% and 65.1%, respectively, of the normal amplitude. However, PGF2alpha and latanoprost acid (1 microM) did not significantly change the response amplitude.

CONCLUSIONS

Our results indicate that PGF2alpha and latanoprost acid do not interact with the prostanoid receptor involved at the pre- and/or postsynaptic site. Therefore, it is unlikely that the hypotensive action by these agents is due to relaxation of the ciliary muscle.

Coupling to protein kinases A and C of adenosine A2B receptors involved in the facilitation of noradrenaline release in the prostatic portion of rat vas deferens

In the prostatic portion of rat vas deferens, the non-selective adenosine receptor agonist NECA (0.1-30 microM), but not the A(2A) agonist CGS 21680 (0.001-10 microM), caused a facilitation of electrically evoked noradrenaline release (up to 43 +/- 4%), when inhibitory adenosine A(1) receptors were blocked. NECA-elicited facilitation of noradrenaline release was prevented by the A(2B) receptor-antagonist MRS 1754, enhanced by preventing cyclic-AMP degradation with rolipram, abolished by the protein kinase A inhibitors H-89, KT 5720 and cyclic-AMPS-Rp and attenuated by the protein kinase C inhibitors Ro 32-0432 and calphostin C. The adenosine uptake inhibitor NBTI also elicited a facilitation of noradrenaline release; an effect that was abolished by adenosine deaminase and attenuated by MRS 1754, by inhibitors of the extracellular nucleotide metabolism and by blockade of alpha(1)-adrenoceptors and P2X receptors with prazosin and NF023, respectively. It was concluded that adenosine A(2B) receptors are involved in a facilitation of noradrenaline release in the prostatic portion of rat vas deferens that can be activated by adenosine formed by extracellular catabolism of nucleotides. The receptors seem to be coupled to the adenylyl cyclase-protein kinase A pathway but activation of the protein kinase C by protein kinase A, may also contribute to the adenosine A(2B) receptor-mediated facilitation of noradrenaline release.

E-ring 8-isoprostanes inhibit ACh release from parasympathetic nerves innervating guinea-pig trachea through agonism of prostanoid receptors of the EP3-subtype

1. In the present study, we examined the effect of E-ring 8-isoprostanes on cholinergic neurotransmission in guinea-pig trachea and identified the receptor(s) involved. As isoprostanes are isomeric with prostaglandins, PGE(2) and sulprostone (a selective EP(3)-receptor agonist) were examined in parallel. 2. 8-Iso-PGE(1), 8-iso-PGE(2) (0.1 nm-1 microM), sulprostone (1 nm-1 microM) and PGE(2) (1 microM) suppressed EFS-evoked [(3)H]ACh release from guinea-pig trachea in a concentration-dependent manner, producing 39.5, 53.9, 61.2 and 59.9% inhibition, respectively, at 1 microM. It should be noted that an established maximum effective concentration was not determined. 3. Neither SQ 29,548 (1 microm; a TP-receptor antagonist) nor AH 6809 (10 microM; an EP(1)-/EP(2)-/DP-receptor antagonist) reversed the inhibitory effect of these compounds. 4. L-798,106, a novel and highly selective EP(3)-receptor antagonist, produced a parallel shift to the right of the concentration-response curves that described the inhibitory action of sulprostone on EFS-evoked contractile responses in guinea-pig vas deferens (an established EP(3)-receptor-expressing tissue), from which a mean pA(2) of 7.48 was derived. On guinea-pig trachea, L-798,106 also antagonised sulprostone-induced inhibition of EFS-induced twitch responses, with similar potency (mean pA(2)=7.82). 5. The inhibitory effects of 8-iso-PGE(1), 8-iso-PGE(2), sulprostone and PGE(2) on EFS-induced [(3)H]ACh release was blocked by L-798,106 at a concentration (10 microM) that binds only weakly to human recombinant EP(1)-, EP(2)- and EP(4)-receptor subtypes expressed in HEK 293 cells. 6. These data suggest that E-ring 8-isoprostanes, PGE(2) and sulprostone inhibit EFS-evoked [(3)H]ACh release from cholinergic nerves innervating guinea-pig trachea, by interacting with prejunctional prostanoid receptors of the EP(3)-subtype.

Prostanoid receptor subtypes

Prostanoids are a group of lipid mediators that include the prostaglandins (PG) and thromboxanes (TX). Upon cell stimulation, prostanoids are synthesized from arachidonic acid via the cyclooxygenase (COX) pathway and released outside the cells to exert various physiological and pathological actions in a variety of tissues and cells. The activities of prostanoids are mediated by specific G protein-coupled receptors, which have been classified on the basis of pharmacological experiments into eight types and subtypes according to their responsiveness to selective agonists and antagonists. These prostanoid receptors have been cloned from various species including human, and their distinct binding properties and signal transduction pathways have been characterized by analyses of cells expressing each receptor. Furthermore, the distribution patterns of prostanoid receptor mRNAs have been determined in tissues and cells for various species. This information is useful for understanding the molecular basis of the pathophysiological actions of prostanoids.

Prostanoid receptor assays

Prostanoids, which include the prostaglandins (PGs) and thromboxanes (TXs), interact with a specific family of G-protein coupled receptors, of which there are known to be five distinct types, DP, EP, FP, IP and TP, each particularly sensitive to one of the five natural prostanoids, PGD₂, PGE₂, PGF₂(, PGI₂ and TXA₂, respectively. Of these, it is known that the EP receptor comprises four well-characterized subtypes: EP₁, EP₂, EP₃ and EP₄. These receptor subtypes are widely distributed throughout mammals and other species, and show particularly high levels of expression in smooth muscle and blood platelets. Despite the fact that few of these preparations express a single receptor type/subtype in isolation, a range of useful smooth muscle and platelet assays for the various prostanoid receptors are available and are presented in this unit.

Factors affecting prostacyclin receptor agonist efficacy in different cell types

Octimibate and related nonprostanoid prostacyclin mimetics are partial agonists displaying highly tissue-specific responses. Octimibate demonstrated considerably greater efficacy for stimulation of adenylyl cyclase activity in Chinese hamster ovary cells transiently expressing mouse prostacyclin receptors (mIP-CHO cells) when compared to human SK-N-SH neuroblastoma cells, which endogenously express prostacyclin (IP) receptors. Pretreatment of both cell types with pertussis toxin (PTx) failed to influence IP agonist efficacy or potency, indicating a lack of involvement of an agonist-stimulated inhibitory G(i)-coupled pathway. Although stimulation of mIP-CHO cells with the full agonist cicaprost increased both [3H]cyclic AMP and [3H]inositol phosphate ([3H]IP) accumulation (pEC(50) values of 8.35 and 6.82, respectively), IP receptor signalling through G(q) in SK-N-SH cells was absent. Inhibition of protein kinase C (PKC) in mIP-CHO cells increased [3H]IP accumulation but had no effect on [3H]cyclic AMP accumulation. Therefore, the poor coupling of the IP receptor in SK-N-SH cells to G(q) is unlikely to explain the relatively low efficacy of octimibate for stimulating adenylyl cyclase in these cells. Furthermore, protein kinase A (PKA) inhibition appears to enhance IP receptor signalling through both G(s) and G(q) in mIP-CHO cells.

Distinction between relaxations induced via prostanoid EP(4) and IP(1) receptors in pig and rabbit blood vessels

1. Our study shows that the prostacyclin analogues AFP-07 and cicaprost are moderately potent agonists for prostanoid EP(4) receptors, in addition to being highly potent IP(1) receptor agonists. Both activities were demonstrated on piglet and rabbit saphenous veins, which are established EP(4) preparations. 2. On piglet saphenous vein, PGE(2) was 6.1, 24, 96, 138, 168 and 285 times respectively more potent than AFP-07, cicaprost, PGI(2), iloprost, carbacyclin and TEI-9063 in causing relaxation. Another prostacyclin analogue taprostene did not induce maximum relaxation (21 - 74%), and did not oppose the action of PGE(2). The EP(4) receptor antagonist AH 23848 (30 microM) blocked relaxant responses to PGE(2) (dose ratio=8.6+/-1.3, s.e.mean) to a greater extent than cicaprost (4.9+/-0.7) and AFP-07 (3.8+/-0.8), had variable effects on TEI-9063-induced relaxation (3.7+/-1.5), and had no effect on taprostene responses (<2.0). 3. On rabbit saphenous vein, AH 23848 blocked the relaxant actions of PGE(2), AFP-07, cicaprost, iloprost and carbacyclin to similar extents. 4. AFP-07, cicaprost and TEI-9063 showed high IP(1) relaxant potency on piglet carotid artery, rabbit mesenteric artery and guinea-pig aorta, with AFP-07 confirmed as the most potent IP(1) agonist reported to date. AH 23848 did not block cicaprost-induced relaxation of piglet carotid artery. EP(3) contractile systems in these preparations can confound IP(1) agonist potency estimations. 5. Caution is urged when using AFP-07 and cicaprost to characterize IP(1) receptors in the presence of EP(4) receptors. Taprostene may be a lead to a highly selective IP(1) receptor agonist.

Characterization of murine vasopressor and vasodepressor prostaglandin E(2) receptors

Four E-prostanoid (EP) receptors, designated EP(1), EP(2), EP(3), and EP(4), mediate the cellular effects of prostaglandin E(2) (PGE(2)). The present studies pharmacologically characterize the vasopressor and vasodepressor EP receptors in wild-type mice (EP(2)(+/+) mice) and mice with targeted disruption of the EP(2) receptor (EP(2)(-/-) mice). Mean arterial pressure (MAP) was measured via a carotid artery catheter in anesthetized male mice. Intravenous infusion of PGE(2) decreased MAP in EP(2)(+/+) mice but increased MAP in EP(2)(-/-) mice. Infusion of EP(3)-selective agonists, including MB28767, SC46275, and sulprostone, increased MAP in both EP(2)(+/+) and EP(2)(-/-) mice. Pretreatment with SC46275 desensitized mice to the subsequent pressor effect of sulprostone, but the vasodepressor effect of PGE(2) in EP(2)(+/+) mice remained intact. Although PGE(2) alone increased MAP in EP(2)(-/-) mice, prior desensitization of the pressor effect with SC46275 allowed a residual vasodepressor effect of PGE(2) to be seen in the EP(2)(-/-) mice. An EP(4)-selective agonist (prostaglandin E(1)-OH) functioned also as a vasodepressor in both EP(2)(-/-) and EP(2)(+/+) mice. High levels of EP(3) receptor mRNA were detected in mouse aortas and rabbit preglomerular arterioles by nuclease protection, with lower expressions of EP(1), EP(2), and EP(4) mRNA. The findings suggest that combined vasodepressor effects of EP(2) and EP(4) receptors normally dominate, accounting for the depressor effects of PGE(2). In contrast, in EP(2)(-/-) mice, EP(4) receptor activity alone is insufficient to overcome the EP(3) vasopressor effect. These findings suggest that a balance between pressor and depressor PGE(2) receptors determines its net effect on arterial pressure and that these receptors may be important therapeutic targets.

Prostaglandin E2 inhibits spontaneous inhibitory postsynaptic currents in rat supraoptic neurones via presynaptic EP receptors

Prostaglandin E2 (PGE2) has been implicated in the excitatory regulation of magnocellular neurones in the supraoptic nucleus (SON). We have recently reported that PGE2 excited SON neurones by directly activating postsynaptic PGE2 receptors (EP receptors) of a subclass other than EP1-3, but did not affect excitatory postsynaptic currents (EPSCs). In the present study, we examined presynaptic effects of PGE2 on rat SON neurones by measuring spontaneous inhibitory postsynaptic currents (IPSCs) by a slice patch-clamp technique. PGE2 inhibited spontaneous IPSCs in a dose-dependent and reversible manner. PGE2 selectively suppressed the frequency of IPSCs without affecting the amplitude of IPSCs in the presence of tetrodotoxin, a blocker of Na+ channels, indicating that the effects were presynaptic. The inhibitory effects of PGE2 on the frequency of IPSCs were mimicked by the EP1/EP3 agonists, 17PT-PGE2 and sulprostone, and the EP2/EP3 agonist, misoprostol, whereas the EP2 agonist, butaprost, or the FP agonist, fluprostenol, had little effect. The effects of PGE2 on IPSCs were unaffected by the selective EP1 antagonist, SC-51322. They were unaffected also by antagonists of GABAB and alpha2 adrenergic receptors, which are present at presynaptic terminals of GABA neurones in the SON and cause suppression of spontaneous IPSCs. The inhibitor of PG synthesis, indomethacin, had little effect on spontaneous IPSCs and on the inhibitory effects of PGE2 as well as of the GABAB agonist, baclofen, and noradrenaline. These results suggest that PGE2 inhibits release of GABA from the GABAergic terminals innervating SON neurones by activating presynaptic EP receptors, presumably of the EP3 subclass, and that such a presynaptic mechanism may play a role in the excitatory regulation of SON neurones by PGE2.

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Characterization of a prostanoid EP3-receptor in guinea-pig aorta: partial agonist action of the non-prostanoid ONO-AP-324

Contraction of guinea-pig isolated aorta induced by the prostaglandin E analogue sulprostone (1-400 nM) has a lower maximum response (40%) than that of phenylephrine or U-46619 (TP-receptor agonist). A prostanoid EP3-receptor subtype is involved based on agonist potency ranking: equi-effective molar ratios (EMR) are sulprostone (EC50 approximately equal to 23 nM) 1.0, SC-46275 0.11, misoprostol 2.2, gemeprost 3.3, PGE2 5.4, 17-phenyl PGE2 6.0, GR-63799 8.9. GR-63799, which contains a bulky ester group, is relatively more potent on neuronal EP3 preparations than on the aorta. ONO-AP-324, a relative of the non-prostanoid prostacyclin mimetic series, behaves as an EP3 partial agonist on the aorta, inhibiting sulprostone responses but acting synergistically (in a similar manner to sulprostone) with phenylephrine; it may be a useful pharmacological tool for studying EP3-receptors. Sulprostone contractions are markedly suppressed in zero-Ca2+ bathing fluid containing either 2 mM EDTA or 50 microM EGTA, and by Cd2+ (500 microM), but are usually unaffected by nifedipine (0.3 microM) and verapamil (4.44 microM). Influx of Ca2+, but not through L-type Ca2+-channels, appears to be the major contractile mechanism. The guinea-pig aorta is a valuable addition to the vascular EP3 preparations available and may increase our knowledge of the mechanisms whereby Gi-coupled receptors mediate vasoconstriction (c.f. 5-HT1B/D- and alpha2-receptors). The possibility of certain EP3 agonists distinguishing EP3-receptor isoforms is discussed.