Effects of prostanoids on isolated feline cerebral arteries

The effects of various prostanoids on isolated feline basilar arteries (BA) were studied. Contractions were induced with the following order of potency: U 46619 ? U44069 > PGB₂ > PGF_2a? PGA, ? PGB, ≥ PGA, ? PGE₂= PGD₂ > PGF_1a? TXB₂? PGE₁? PGD₁. Distinct bimodal responses with a relaxation at low concentrations followed by a contraction at high concentrations, were induced by PGA₁, PGA₂, PGD, and PGE₂. None of the tested prostanoids relaxed K⁺-contracted arteries, and a sizable relaxant effect in PGF_2a-contracted arteries could be induced only by PGE₁. As judged by the relative order of potency, PG-induced contractions of the feline BA seem to be mediated by a thromboxane sensitive receptor. PGF_2a-induced contractions apparently do not involve the release of noradrenaline from perivascular nerves since phentolamine failed to affect contractions induced by this agent.     

Effects of prostanoids on isolated feline cerebral arteries

The roles of extra-and intracellular calcium for the contractile effects of PGF_2α in the feline basilar artery (BA) were investigated. Comparisons were made with contractions induced by K⁺ and noradrenaline (NA). Addition of nifedipine to PGF_2α-or K⁺ (124 mM)-contracted arteries resulted in an incomplete relaxation, whereas NA-contracted vessels were completely relaxed. Incubation of the preparations in a calcium-free medium containing 10^-5 M EGTA for 5–10 min almost abolished contractions induced by K⁺ and NA. In contrast, 63 % of the response to PGF_2α remained after pretreatment of the arteries in a calcium-free solution for 40 min; PGF_2α produced a biphasic contraction in 17 out of 20 preparations consisting of a rapidly developing initial phase followed by a second increase in tension after 1–6 min. The second phase was absent if the EGTA-concentration was increased to 10^-4 M, or if the arteries were pre-treated with nifedipine. After incubation of the arteries in a calcium-free medium for 40–120 min and K⁺-depolarization, re-addition of calcium elicited contractions at lower concentrations in the presence of PGF_2α than in controls. The results suggest that PGF_2α-induced contractions in the feline BA are considerably less dependent on extracellular calcium than contractions evoked by K⁺ or NA. PGF_2α appears to be able to release calcium from two cellular stores, and may also promote calcium influx through the cell membrane.     

Effects of prostanoids on isolated feline cerebral arteries. I. Characterization of the contraction-mediating receptor

The effects of various prostanoids on isolated feline basilar arteries (BA) were studied. Contractions were induced with the following order of potency: U 46619 approximately equal to U44069 greater than PGB2 greater than PGF2 alpha approximately equal to PGA2 approximately equal to PGB1 greater than or equal to PGA1 approximately equal to PGE2 = PGD2 greater than PGF1 alpha approximately equal to TXB2 approximately equal to PGE1 approximately equal to PGD1. Distinct bimodal responses with a relaxation at low concentrations followed by a contraction at high concentrations, were induced by PGA1, PGA2, PGD, and PGE2. None of the tested prostanoids relaxed K+-contracted arteries, and a sizable relaxant effect in PGF2 alpha-contracted arteries could be induced only by PGE1. As judged by the relative order of potency, PG-induced contractions of the feline BA seem to be mediated by a thromboxane sensitive receptor. PGF2 alpha-induced contractions apparently do not involve the release of noradrenaline from perivascular nerves since phentolamine failed to affect contractions induced by this agent.     

Potassium-induced relaxation in isolated cerebral arteries contracted with prostaglandin F2α

Summary In helically cut strips of canine cerebral arteries exposed to 5.4 mM [K⁺]_o and contracted with prostaglandin F₂, the addition of K⁺ in concentrations ranging from 0.5–5 mM caused a dose-related relaxation. The relaxing effect of K⁺ was potentiated at reduced [K⁺]_o and suppressed at reduced [Na⁺]_o. Reduction of Cl^– from bathing media failed to alter the effect of K⁺. Removal of external Ca²⁺ markedly attenuated the K⁺-induced relaxation and increase in [Ca²⁺]_o also attenuated the relaxation. Similar relaxation was induced by K⁺ in cerebral arteries from other species including humans, puppies, cats and rabbits. The addition of K⁺ also elicited a relaxation in peripheral arteries, including coronary, femoral, mesenteric and renal, contracted with prostaglandin, but this relaxation was markedly less than in cerebral arteries. The content of Na⁺ in freshly excised cerebral arteries was significantly greater than that of peripheral arteries, while the content of K⁺ in these arteries was not significantly different.The present study provides further evidence to support the hypothesis that an electrogenic Na⁺ pump is involved in the genesis of K⁺-induced relaxation. The Na⁺ pump does not appear to be fully activated at normal [K⁺]_o of 5.4 mM in cerebral arteries.     

Effects of ovariohysterectomy on prostaglandin F2alpha-induced nesting behaviour in pigs

Exogenously administered prostaglandin (PG) F2alpha induces behaviour similar to prepartum nest building in pregnant, pseudopregnant and nonpregnant female postpubescent pigs (Sus scrofa). These effects may be regulated by PGF2alpha-induced endocrine changes within the reproductive tract, such as those that initiate luteolysis. This study investigated the short-term effects of ovariohysterectomy on PGF2alpha-induced nesting behaviour in nonpregnant females. Cyclic 9-month-old virgin female pigs (gilts) received an oral dose (20 mg/day) of a synthetic progestogen (altrenogest; Regumate porcine, Hoechst, Milton Keynes, UK) for 18-21 days to synchronize oestrus. The gilts were then ovariohysterectomized (n=8) or sham-operated (n=7) on Days 3-8 after oestrus. They were housed individually and initially subjected to a series of control behavioural tests to establish the effect of ovariohysterectomy on their responses to the experimenters, novel objects, straw bedding and space restriction. Ovariohysterectomized gilts had a shorter latency to approach the experimenters than sham-operated animals, but there were no differences in their responses to a novel object, straw bedding or space restriction. Twelve to 16 days after oestrus, corresponding to the midluteal phase in sham-operated gilts, they were treated intramuscularly with 15 mg PGF2alpha (0.12 mg/kg, dinoprost; Lutalyse, Upjohn, Crawley, UK). PGF2alpha treatment induced a significant increase in straw gathering in ovariohysterectomized but not in sham-operated gilts. Other nesting behaviours, including rooting and pawing at straw, were induced in all animals. These results show that the uterus and ovaries are not required for the expression of PGF2alpha-induced nesting behaviour and the removal of the reproductive tract appears to have facilitated increased levels of gathering. This suggests that PGF2alpha induces luteolysis and nest building separately, and that PGF2alpha or a metabolite, may act centrally to mediate directly its effects on prepartum nest building in the pig.     

Some effects of the calcium promotor BAY K 8644 on feline cerebral arteries

The proposed calcium promotor BAY K 86(44) contracted feline basilar arteries partially depolarized by 10 mmol X 1(-1) potassium in a concentration-dependent manner (EC50 value: (2.1 +/- 1.2) X 10(-9) mol X 1(-1)). The concentration-response curve for prostaglandin (PG) F2 alpha was displaced to the left after pretreatment with BAY K 86(44). PGF2 alpha induced a biphasic contraction in calcium-free medium as has been described previously. The second PGF2 alpha-induced contraction phase in calcium-free medium was abolished by pretreatment with nifedipine or diltiazem. BAY K 86(44) restored the second phase in arteries pretreated with nifedipine, but not in vessels pretreated with diltiazem. The findings suggest that BAY K 86(44) acts as a promotor of calcium-influx, probably by interaction with the 'dihydropyridine receptor' in the cell membrane, and also provide support for the view that PGF2 alpha releases membrane-bound calcium.     

Effects of the thromboxane-receptor antagonist L-636,499 on feline cerebral arteries

The effects of the thromboxane-receptor antagonist L-636,499 on contractions induced by U46619 and prostaglandin (PG)F2 alpha were studied in the isolated feline basilar artery (BA). L-636,499 (10(-6)-3 X 10(-5) mol l-1) shifted concentration-response curves induced by U46619 to the right in a parallel manner. By contrast, concentration-response curves induced by PGF2 alpha were displaced only when L-636,499 concentrations of 10(-5) and 3 X 10(-5) mol l-1 were used. The Schild plot using U46619 as the agonist revealed a slope index (0.69) which was significantly less than unity. PA2-values calculated according to the method of van Rossum differed significantly between different concentration intervals of L-636,499. At 10(-6) to 3 X 10(-6) mol l-1 the PA2-value was 5.99, whereas at 10(-5) to 3 X 10(-5) mol l-1 it was 5.59. Using PGF2 alpha as the agonist the PA2-value at the L-636,499 concentration interval 10(-5) to 3 X 10(-5) mol l-1 was 5.69. It may be concluded that the feline BA contains two receptor sites mediating prostanoid-induced contraction, one of which may be characterized as a thromboxane-sensitive (TP) receptor. L-636,499 can interact with both receptor sites with highest affinity for the TP receptor site.     

Effects of prostaglandin E1, E2, and F2α on isolated pial arteries of cat

Responses to prostaglandin (PG), E₁, E₂, and F_2α were studied on isolated feline middle cerebral arteries. At resting state PGF_2α produced strong dose-dependent contractions. PGE₂ elicited weak relaxations at low concentrations, followed by powerful contractions at higher doses. PGE₁ had little effect on resting pial vessels. The relative constrictory potency was PGF_2α > PGE₂ > PGE₁. During active tone, induced by administration of either potassium, norepinephrine, or 5-hydroxytryptamine, relaxations induced by PGE₁ were enhanced, whereas PGE₂-induced relaxations were unaffected. PGE₁ induced relaxations were more pronounced when the active tension had been produced by administration of PGF_2α than with either of the vasoactive amines or potassium. This study demonstrates the importance of smooth muscle tone, and by what means this is achieved, when examining the responses of PG's on cerebral blood vessels.     

Pharmacological properties of prejunctional alpha-adrenoceptors in isolated feline middle cerebral arteries; comparison with the postjunctional alpha-adrenoceptors

In cat middle cerebral arteries (CMCA) preincubated with 3H-noradrenaline (NA), the outflow of tritium evoked by electrical stimulation was reduced to 32% by alpha-adrenoceptor (alpha-receptor) stimulation with oxymetazoline, and increased to 487% by alpha-receptor blockade with HEAT. The relative order of potency for alpha-receptor agonists on prejunctional receptors was: clonidine greater than or equal to oxymetazoline greater than phenylephrine, and the antagonist rauwolscine was more potent than prazosin. This indicates that the prejunctional alpha-receptors are mainly of alpha 2-type. Rauwolscine was more potent than prazosin in inhibiting the contractions induced by NA, indicating a predominance of alpha 2-receptors postjunctionally. Apart from clonidine having higher intrinsic activity pre- than postjunctionally, all drugs examined (oxymetazoline, phenylephrine, rauwolscine, HEAT (BE2254), and prazosin) had similar concentration-effect curves on the pre- and postjunctional receptors. Furthermore, the ratios of EC50-values pre- and postjunctionally of rauwolscine, oxymetazoline, and clonidine were all close to unity. These results indicate that pre- and postjunctional alpha 2-receptors in the CMCA have similar pharmacological characteristics and cannot be influenced separately by the presently used drugs.     

Effects of nifedipine on potassium-induced contraction and noradrenaline release in cerebral and extracranial arteries from rabbit

The present study was designed to evaluate the effects of the calcium antagonist nifedipine on potassium-evoked contractions and release of noradrenaline from sympathetic nerves in rabbit basilar and facial arteries. Contractions were measured isometrically in a small volume organ bath. While noradrenaline (NA) produced strong contractions in facial arteries, the majority of the basilar arteries responded only to the highest concentrations of NA employed (greater than 10 microM) with weak contraction. Prazosin (1 microM) and phentolamine (1-10 microM) effectively antagonized the responses to NA in both types of vessel. In contrast, contractions evoked by potassium (K+, 124 mM) were only slightly reduced by the alpha-adrenoceptor blocking agents, indicating that the participation of endogenous NA in maintaining the contractile response to K+ is either small or negligible in the vessel types studied. Nifedipine concentration-dependently inhibited K+-induced contractions in basilar and facial arteries, the former being significantly more affected as evidenced by the maximum inhibitions (approximately 80% compared to approximately 60%) and IC50 values (approximately 10 nM vs. approximately 30 nM). A combination of nifedipine (0.3 microM) and prazosin (1 microM) or phentolamine (1 microM) further suppressed the K+-evoked contractile response in facial arteries, but failed to do so in basilar arteries, when compared with the effect of nifedipine alone. The depressant effect of the alpha-adrenoceptor blockers was, however, still obtainable after reserpine treatment of the facial artery in vitro. Fluorescence histochemical demonstration of noradrenaline revealed a dense network of adrenergic nerve fibres in the walls of the basilar and facial artery. The vessels were also shown to accumulate 3H-NA and release it upon depolarization with K+. The uptake and subsequent release of 3H-NA were significantly reduced by desipramine (10 microM). Nifedipine (0.3-3.0 microM) failed to alter the K+-evoked 3H-NA efflux from sympathetic nerves in neither of the two vessel types. It may be concluded that nifedipine effectively inhibits K+-evoked contractions in isolated basilar and facial arteries from rabbit without interfering with nerve-mediated NA release. Possible explanations for this selective effect of nifedipine on muscle contraction are discussed.     

Pharmacological characterization of postjunctional α-adrenoceptors in isolated feline cerebral and peripheral arteries

The vascular α-adrenoceptors in isolated feline cerebral, lingual and mesenteric arteries were characterized and compared. In the middle cerebral artery the relative order of potency for agonists was: clonidine>oxymetazoline>noradrenaline>phenylephrine which indicates that the postjunctional α-receptor in this vessel is of α₂-type. This view is further supported by the finding that yohimbine, but not prazosin, had a potent, mainly competitive blocking action. In peripheral arteries, clonidine was without effect. In these vessels, the potency difference between phenylephrine and oxymetazoline was more than 40 times less than in cerebral vessels. The pA₂-values for prazosin correlated well with pA₂-values found for the interaction of this drug with α₁-receptors in a variety of other tissues, thus suggesting the occurrence of an α₁-receptor in these arteries. However, the pA₂-values for yohimbine and rauwolscine correlated well with an α₂-receptor, suggesting also the presence of α₂-receptors. Schild plots for prazosin and rauwolscine in lingual arteries displayed slopes significantly lower than unity, which also supports the view of a mixture of α₁-and α₂-receptors in these vessels. However, the Schild plots for the antagonists in mesenteric arteries did not differ significantly from unity, a finding possibly indicating the presence of an α-receptor unable to differentiate between substances that in other tissues act preferentially on α₁- or α₂-receptors.     

Further characterization of the contraction-mediating prostanoid receptors in feline cerebral arteries. Effects of the thromboxane-receptor antagonist AH 23848

The effects of the thromboxane-receptor antagonist AH 23848 were investigated on isolated feline basilar arteries (BA). AH 23848 (10(-6) mol l-1) had no effect on contractions induced by 5-hydroxytryptamine or potassium, whereas the drug (10(-8)-10(-6) mol l-1) induced a parallel shift to the right in contractions induced by the thromboxane A2 mimic U46619. There was no depression of the maximum contraction, indicating competitive antagonism. The Schild plot revealed a slope index of unity with a pA2 value of 8.46. In contrast, 10(-6) mol l-1 AH 23848 depressed the maximum PGF2 alpha-induced contraction significantly from 100% to 13%. U46619 was able to induce a contraction amounting to 98% if the drug was added on top of the PGF2 alpha-induced contraction in the presence of 10(-6) mol l-1 AH 23848. The results provide strong support for previous suggestions that prostanoid-induced contractions in the feline BA are mediated by two receptor subtypes, one of which can be classified as a thromboxane-sensitive (TP) receptor.     

Inhibition by derivatives of phloretin of anaphylactic histamine release from human lung tissue and of prostaglandin F2alpha-induced bronchoconstriction.

Derivatives of phloretin (25-1,000 mug/ml) among them polyphloretin phosphate (PPP), inhibited in a dose-dependent manner anaphylactic (birch pollen or horse dander) histamine release from human lung tissue passively sensitized with reaginic serum. Pretreatment with PPP of lung tissue sensitized both to birch pollen and horse dander counteracted to a similar extent the release of histamine induced by either allergen administered in sequence. The phloretin derivatives also antagonized the constrictor action of prostaglandin F2alpha on isolated human bronchi at concentrations which did not impair the responses to histamine. The low and high molecular weight derivatives of phloretin were comparably active on a weight basis in both experimental systems.     

Uterus and endometrium: Indomethacin reduces contraction of isolated non-pregnant human uterine artery induced by prostaglandin F2{alpha}

The purpose of this study was to explore whether cyclo oxygenaseproducts derived from endothelium or vascular smooth muscleparticipate In the response of human uterine artery to prostaglandinF² Experiments were performed using human uterine arterial rings.Prostaglandin F² (0.4 nM-1 µM) induced contraction ofhuman uterine arteries with both Intact and denuded endothellumwith similar potency and efficacy (pD² values: 7.93±0.01and 8.07±0.03 for vessels with and without endotheliuinrespectively; maximal response values: 89.1±4.7% and92.3±3.8% for vessels with and without endotheliuin respectively).Indomethacin (10 µM) significantly sup pressed the maximumeffects of prostaglandin F² and induced a shift towards theright of the prostaglandin F² concentration-response curves,regardless of the endo thelial condition. On the other hand,in both types of preparations, OKY-046 (10 µM), an inhibitorof throm boxane synthesis, did not affect prostaglandin F²-inducedcontraction of human uterine arteries. It is concluded thatin human uterine artery prostaglandin F²-induced contractionis mediated, at least in part, through constrictor prostanoid(s)of vascular smooth muscle origin that is not thromboxane A₂.     

Effects of calcium chelators on intracellular calcium and excitotoxicity

In an attempt to probe the relationship between excitotoxicity and increases in intracellular calcium ([Ca²⁺]_i), BAPTA-AM and its analogs were applied to cultured hippocampal neurons. Chelation of [Ca²⁺]_i depressed and prolonged transient responses to glutamate and did not effect elevation of [Ca²⁺]_i by prolonged exposure. This explains the inability of the chelators to prevent glutamate-induced toxicity.     

Simultaneous monitoring of vascular contractility, intracellular pH and intracellular calcium in isolated rat mesenteric arteries; effects of weak bases

We report the first simultaneous measurements of pHi, [Ca2+]i and tension, upon alteration of pHi, in isolated rat mesenteric arteries loaded with both carboxy SNARF and indo-1. In these vessels (pre-contracted by 30 mM KCl) alterations of pHi, by addition and subsequent washout of weak bases, produced complicated effects on tone. Although the changes in contractility did not mirror the changes in pHi they were, at all times, accompanied by parallel changes in [Ca2+]i.