Morphine release and displacement by naloxone in vivo in morphine naive and withdrawn rats

Male Long-Evans rats, implanted in the lateral cerebroventricle with chronic indwelling push-pull cannulae, were perfused (10 μl/min) for 120 min: 20 min with 1.5 × 10^?6M morphine in sterile isotonic saline containing 2.3 mM CaCl₂ (vehicle); 40 min with vehicle; 20 min with 1.5 × 10^?6M morphine; 10 min with vehicle and 30 min with 1 × 10^?6M naloxone in vehicle. These rats and drug-naive rats were implanted s.c. with 2 × 50 mg morphine pellets. After 72 hr the pellets were removed and 18–24 hr later the above perfusion procedure was repeated. The amount of morphine collected in the perfusate during the washout with naloxone was elevated, compared to the amount collected during the corresponding time of the washout with vehicle for both naive and withdrawn groups. The enhanced morphine release during the washout with naloxone did not differ significantly between the naive and withdrawn rats. However, significantly less morphine was recovered in the perfusate collected during the vehicle washout from the withdrawn rats, compared to that collected from the naive rats. The data suggest that $\text{in vivo}$ morphine is specifically bound to receptors and is sensitive to naloxone displacement. It is also concluded that morphine is differentially taken up or otherwise disposed of by brains of rats which are in opiate withdrawal.     

The effects of prostaglandins E2 and F2α on synaptosomal accumulation and release of 3H-norepinephrine

Prostaglandins (PG) of both the E and F series may serve as modulators of norepinephrine (NE) release from peripheral sympathetic neurons. We have studied the effects of PGE₂ and PGF_2α on the accumulation and release of ³H-NE in the CNS using synaptosomes isolated from rat hypothalami.The release of ³H-NE from synaptosomes superfused with Krebs-Ringer bicarbonate buffer was multiphasic with an initial fast release phase followed by a slower release. Raising KC1 concentration of the superfusion medium to 56mM during the slow release phase is known to stimulate ³H-NE release. PGE₂ (1 × 10^?6M) attenuated ³H-NE release during the fast phase and reduced the amount of ³H-NE released due to KC1 stimulation. At lower concentrations of PGE₂ there was no change in the release profile. PGF_2α was without effect on ³H-NE release at all concentrations tested.The accumulation of ³H-NE was significantly diminished by PGE₂ at a concentration of 1 × 10^?6M, while a lower concentration (1 × 10^?7M) was ineffective. PGF_2α had no effect on ³H-NE accumulation at all concentrations investigated.     

The effects of dopamine on prostaglandin E1-elicited electrophysiological changes in a neuronal somatic cell hybrid

PGE₁ elicited a slow, dose-dependent membrane depolarization with an increase in membrane conductance in the somatic cell hybrid TCX11. The ED ₅₀ was 1-2 × 10^?8 M with maximal responses at 1-5 × 10^?7 M. Dopamine (DA) reversed the effect of PGE₁ and caused the membrane potential and resistance to return to control levels. Chronic exposure of cells (measured in minutes) to DA alone would not cause this hyperpolarization. 5-HT was also tested and failed to consistently reverse the PGE₁ effects. Chlorpromazine antagonized the effects of DA on the PGE₁ response. The electrophysiological results reported here using TCX11 cells are discussed in light of previously reported biochemical results describing interactions of PGE₁ and DA, and the electrophysiological effects of DA alone.     

Effects of prostacyclin on the canine isolated basilar artery

Prostacyclin (PGI₂) produced a biphasic response in canine isolated basilar arteries. In low doses (1 × 10^?8M?1 × 10^?7M) PGI₂ caused a slight but consistent relaxation of resting muscle tone. In low concentrations (1 × 10^?8M?1 × 10^?6M) PGI₂ antagonized muscle contractions caused by serotonin or prostaglandin (PG) F_2α. This relaxant effect with low doses of PGI₂ on the isolated cerebral artery contrasts with findings obtained with other PGs and supports the hypothesis that PGI₂ is a mediator of vasodilatation. However, in 1 × 10^?5M concentrations PGI₂ contracted the arterial muscle and did not antagonize contractions induced by serotonin or PGF_2α.     

Cerebral endothelial cell culture II. Adenylate cyclase response to prostaglandins and their interaction with the adrenergic system

The response of endothelial adenylate cyclase (AC) to prostaglandins (PGE₁, PGE₂, PGF_1α, PGF_2α, PGD₂ and PGI₂) and the relationship of PGE₂ to adrenergic systems were investigated in cerebrovascular endothelial cultures. E-type prostaglandins and PGI₂ were more effective in stimulating endothelial AC (EC₅₀ = 3 × 10^?7M, and 3 × 10^?7M, respectively) than prostaglandins of the F-series and PGD₂ which activated AC at high doses only. A modulation of endothelial AC response to either PGE₂ or norepinephrine (NE) was observed in the presence of both agents in the system. It was manifested by a dose-dependent NE inhibition of the PGE₂-stimulated formation of cAMP, which was partially restored by phentolamine. Alpha and β-adrenergic agonists (α, clonidine and 6-fluoronorepinephrine; β, isoproterenol) also partly blocked while forskolin and PGE₂ synergistically stimulated the production of cAMP in the endothelial cultures. These findings strongly suggest that the interaction of prostaglandins and α- and β-adrenergic agonists with the AC system in cerebrovascular endothelium may play a role in the regulation of the cerebral microcirculation and/or blood pressure.     

Receptors for prostaglandins and gonadotropins in the cell membranes of bovine corpus luteum

The cell membranes exhibited specific binding to ³H-prostaglandin E₁ (³H-PGE₁) and ¹²⁵I-human chorionic gonadotropin (¹²⁵I-HCG). Unlabeled PGE₁,PGE₂ (1.4 × 10^?7M), PGF_1α and PGF_2α (1.4 × 10^?5M) decreased ³H-PGE₁ binding by more than 80%. The binding of ¹²⁵I-HCG was completely inhibited by 5 × 10^?8M unlabeled HCG. However, the unlabeled PGE₁ (1.15 × 10^?6M) and HCG (8.4 × 10^?7M) had no effect on ¹²⁵I-HCG and ³H-PGE₁ binding respectively. A PG antagonist, 7-oxa-13-prostynoic acid, inhibited only ³H-PGE₁ binding but not ¹²⁵I-HCG binding. These results suggest the presence of specific receptors for PGE₁ and HCG in the cell membranes and that the binding occurs either at two different sites on the same receptor or that each binds to a “different” receptor molecule.     

Opiate Receptor-Mediated Inhibition of Catecholamine Release in Primary Cultures of Bovine Adrenal Chromaffin Cells

Abstract: Primary cultures of chromaffin cells from bovine adrenal medulla were used to evaluate the ability of several opiates to reduce the release of catecholamines induced by stimulation of nicotinic receptors. Etorphine, β-endorphin, Met-enkephalin[Arg⁶,Phe⁷], and the synthetic peptide [d -Ala²,Me-Phe⁴,Met(O)^s-ol]enkephalin inhibited the acetylcholine-induced release of catecholamines with an IC₃₀ varying from 10^?7 to 1 × 10^?6M. The effect was stereospecific because levorphanol (IC₃₀= 7.5 × 10^?7M) was approximately two orders of magnitude more potent than dextrorphan. Morphine (μ-receptor agonist), [d -Ala², d -Leu⁵]enkephalin (δ-receptor agonist), ethylketazocine (k -receptor agonist), and N-allylnormetazocine (σ-receptor agonist) were at least 100–1000 times less potent than etorphine. Diprenorphine (IC₅₀= 5 × 10^?7M) and naloxone (IC₅₀= 10^?6M) antagonized the effect of etorphine. High-affinity, saturable, and stereospecific binding sites for [³H]etorphine, [³H]dihydromorphine, [³H-d -Ala²,d -Leu⁵]enkephalin, [³H]ethylketazocine, and for [³H]N-allylnormetazocine, [³H]diprenorphine, and [³H]naloxone were detected in chromaffin cell membranes and in membranes obtained from adrenal medulla homogenates. However, the number of binding sites for [³H]etorphine and [³H]diprenorphine was 10–70 times higher than the number of sites measured with the other ³H ligands. The rank order of potency of these compounds for the displacement of [³H]etorphine binding correlates (r = 0.90) with the rank order of potency of the same compounds for the inhibition of acetylcholine-induced catecholamine release. These data suggest that a stereoselective opiate receptor (different from the classic μ-, δ-, k -, or σ-receptor) with high affinity for etorphine, diprenorphine, β-endorphin, and Met-enkephalin[Arg⁶,Phe⁷] modulates the function of the nicotinic receptor in adrenal chromaffin cells.     

The Met-enkephalin analog D-Ala2-Met-enkephalinamide decreases the adrenocortical response to ACTH in dispersed rat adrenal cells

The effects of the Met-enkephalin analog D-Ala²-Met-enkephalinamide (DALA) on basal and ACTH-stimulated corticosterone secretion from dispersed adrenal cells were investigated. Low doses (10^?10 and 10^?12 M) of DALA resulted in no apparent alteration in the response to ACTH (8×10^?9, 3.2×10^?8 or 1.6×10^?7 M). High doses of DALA (10^?8 and 10^?6 M) produced a decline in the steroidogenic response to ACTH. The opiate receptor antagonist naloxone (10^?4–10^?10 M) did not influence the basal production of corticosterone or the stimulating action exerted by ACTH. However, the presence of naloxone reversed the blocking action on corticosterone production that was exerted by DALA. These findings indicate that enkephalins may decrease adrenocortical responsiveness to ACTH.     

Morphine diminishes the constancy of spontaneous uterine contractions, antagonizes the positive inotropic effects of prostaglandin E2, but not of prostaglandin F2α and inhibits prostaglandin E and F outputs from the uterus of ovariectomized rats

The effects of morphine on the constancy of spontaneous contractions (isometric developed TENSION = IDT and contractile FREQUENCY = CF), in uterine strips isolated from ovariectomized rats and the influence of naloxone, were explored. The inotropic responses to added prostaglandins (PGs) E₂ and F_2α and the influences of morphine and of morphine in the presence of naloxone on PG actions, were also determined. Moreover, the synthesis and outputs of PGs E and F from uteri and the effects of morphine alone and of morphine plus naloxone, were studied. Morphine (10⁻⁶ M) significantly depressed uterine constancy of IDT during the first hours following delivery, but its action on CF did not differ from controls. Naloxone, neither at 10⁻⁸ M nor at 10⁻⁶ M, altered the negative inotropoic influence of morphine on IDT. Exogenous PGs E₂ and F_2α, stimulated uterine inotropism in a concentration-dependent fashion. Morphine altered dose-response curves for exogenous PGE₂, evoking a parallel surmountable shift to the right, but did not affect the inotropic action of added PGF_2α. This antagonistic effect of the opioid was not altered by preincubation with naloxone. Basal synthesis and outputs of PGs E and F in uteri from ovariectomized rats were significantly depressed by morphine (10⁻⁶ M) but not altered by incubating tissues with morphine in presence of naloxone. Results are discussed in terms of a presumptive dual action of morphine on uterine motility, i.e., antagonizing PGE₂ receptors and inhibiting the synthesis of some PGs by the uterus. These influences of morphine do not appear to be subserved by the activation of μ opioid receptors. Moreover, the possibility that endogenous opioids could play a relevant role modulating uterine PG influences, is also discussed.     

Chronic Activation of Inhibitory δ-Opioid Receptors Cross-Regulates the Stimulatory Adenylate Cyclase-Coupled Prostaglandin E1 Receptor System in Neuroblastoma × Glioma (NG108-15) Hybrid Cells

Abstract: The present article investigates chronic opioid regulation of the stimulatory adenylate cyclase-coupled prostaglandin E₁ (PGE₁) receptor system in neuroblastoma × glioma (NG108-15) hybrid cells. Persistent activation of δ-opioid receptors by morphine (10 µmol/L; 3 days) substantially down-regulates the number of PGE₁ binding sites by ～30%, without affecting their affinity. Radioligand binding studies performed in the presence of GTPγS (100 µmol/L) further revealed that the remaining PGE₁ binding sites are still capable of interacting functionally with their associated stimulatory G proteins, G_s. On the postreceptor level, neither changes in the abundance nor in the intrinsic activity of the α subunit of G_s (G_sα) were found during the state of opioid dependence, as has been verified by western blot and S49 cyc^? reconstitution experiments, respectively. Evaluation of the functional interaction between PGE₁ receptors and G_s by means of receptor-stimulated, cholera toxin-catalyzed ADP-ribosylation of G_sα revealed a significant increase in the ability of PGE₁ receptors to activate G_sα (3.3-fold increase in EC₅₀; p < 0.05) in cells chronically exposed to morphine. This effect was completely blocked by coincubation of the cells together with the opiate antagonist naloxone (100 µmol/L; 3 days), whereas precipitation of morphine withdrawal by naloxone (100 µmol/L) had no further effect on sensitization in PGE₁ receptor/G_s coupling. These findings provide evidence that the stimulatory adenylate cyclase-coupled PGE₁ receptor system represents a potential target of chronic δ-opioid receptor activation in NG108-15 hybrid cells. They further suggest that sensitization in stimulatory signal transduction plays a critical role in the generation of opioid dependence.     

Distribution of prostaglandin E2-sensitive adenylate cyclase along the rat nephron

To define sites of prostaglandin action of renal tubules, the distribution of adenylate cyclase sensitive to prostaglandin E₂ (PGE₂) was examined in single nephron segments dissected from rat kidney. Further, the interaction between PGE₂ and vasopressin on adenylate cyclase activity in nephron segments sensitive to vasopressin was evaluated. Procedures involved in isolating nephron segments were without effects on adenylate cyclase stimulation by PGE₂. PGE₂ stimulated adenylate cyclase activity of the thin descending limb of Henle (tDL), cortical collecting tubules (CCT), and medullary collecting tubules (MCT) at concentrations of 1.4 × 10^?5 to 2.8 × 10^?5 M. PGE₂ was without effects in other nephron segments tested including proximal convoluated tubules, proximal pars recta, the thin and thick ascending limb of Henle's loop, and distal and connecting tubules. PGE₂, at both high (2.8 × 10^?5 M) and low (2.8 × 10^?8 M) concentrations, did not inhibit adenylate cyclase activity stimulated by submaximal doses of vasopressin in medullary thick ascending limb of Henle (MTAL), CCT, and MCT. These data define the distribution of PGE₂-sensitive adenylate cyclase in the rat nephron, i.e., tDL, CCT, and MCT, and show the lack of direct inhibitory actions of PGE₂ on vasopressin sensitive adenylate cyclase in MTAL, CCT, and MCT.     

The vasorelaxant effect of (±)-15-deoxy-16-hydroxy-16(α/β)-vinyl-prostaglandin E2 (CL 115,129) and its methyl ester (CL 115,347) on the isolated ductus arteriosus preparation

CL 115,129 and its methyl ester, CL 115,347, were studied for their vasorelaxant effects and compared to that of prostaglandin (PG) E₂ and its methyl ester on isolated ductus arteriosus (DA) from fetal lambs and rabbits. CL 115,129 and CL 115,347 potently relaxed the oxygen-indomethacin constricted ductus in a concentration dependent manner. The threshold concentration was 1×10^?13M and the estimated EC₅₀'s (M) were 6.9×10^?8 and 4.3×10^?8, respectively, for CL 115,129 and CL 115,347. Also confirmed was the vasorelaxant ability of PGE₂. These studies indicate that the CL compounds possess potent vasorelaxant effects on the DA although less potent than PGE₂ or its methyl ester.     

The presence of discrete receptors for prostaglandin F2 alpha in the cell membranes of bovine corpora lutea.

The cell membranes isolated from bovine corpora lutea bound ³H-prostaglandin (PG) F₂α with high affinity and specificity. The specific binding of ³H-PGF₂α was detectable at 10^?10M added ³H-PGF₂α and reached saturation at 10^?7M to 10^?6M. Unlabeled PGF₂α, as low as 10^?9M, inhibited the binding of ³H-PGF₂α with complete inhibition occurring at 10^?6M. The Scatchard analysis of equilibrium binding data revealed that the PGF₂α receptors are heterogeneous: Kd₁?5.1 × 10^?9M, n?289 fmoles/mg protein; Kd₂?1.8 × 10^?8M, n?780 fmoles/mg protein. The relative affinities of various other PGs for binding to PGF₂α receptors were (PGF₂α?100%): PGF₁α?17.5; PGE₁?0.8; PGE₂?22.4; PGA₁?0.007; PGB₁?0.01. The specificity and affinity of ³H-PGF₂α binding is consistent with the possibility that this receptor interaction may reflect an initial event in the action of PGF₂α as a luteolytic agent.     

Effects of papaverine on basal and on isoproterenol-stimulated renin secretion from rat kidney slices

Papaverine inhibited the basal renin secretion of rat kidney slices incubated in a physiological salt solution at 37°C. Inhibition was concentration-dependent; secretion was 99 ± 0.2 % inhibited by 5 × 10^?4 M papaverine, and 8 × 10^?5 M was the estimated ED₅₀. In contrast, 2 × 10^?4 M IBMx (3-isobutyl-1-methyl-xanthine) increased the rate of secretion from 215 ± 17 to 366 ± 30 ng hr^?1mg^?1/20 min (p < 0.001). Isoprotenol (4 × 10^?7, 8 × 10^?7, and 5 × 10^?6 M) stimulated renin secretion in a concentration-dependent manner; the stimulatory effects were antagonized by papaverine but unaffected by IBMx. Thus, two known inhibitors of phosphodiesterase--IBMx and papaverine--produce sharply contrasting effects on basal and on isoproterenol-stimulated renin secretion from rat kidney slices.     

Evidence that Inhibition of Nicotine-Mediated Catecholamine Secretion from Adrenal Chromaffin Cells by Enkephalin, β-Endorphin, Dynorphin (1-13), and Opiates is not Mediated via Specific Opiate Receptors

Abstract: The opioid peptides Met- and Leu-enkephalin, dynorphin (1-13), and β-endorphin and the narcotic analgesics, morphine, levorphanol, and dextrorphan all produced a dose-dependent inhibition of nicotine (5 × 10^?6m )-mediated release of [³H]norepinephrine ([³H]NE) from bovine adrenal chromaffin cells in culture. None of these agents affected [³H]NE release induced by high K⁺ (56 mm ). Although the above results suggest that the opioid peptides and narcotic analgesics inhibit catecholamine release from adrenal chromaffin cells in culture, we suggest that these effects are not mediated by specific opiate binding sites, since (1) the inhibition was only produced with high concentrations of the agents—the threshold concentrations were 10^?7 to 10^?5m and higher; (2) the inhibition produced by the narcotic analgesics did not display stereospecificity, because the (d-isomer, dextrorphan, was slightly more active than the l-isomer, levorphanol; (3) the narcotic antagonists naloxone, naltrexone, and levallorphan did not reverse the inhibition produced by either the narcotic analgesics (e.g., morphine) or the opioid peptides (e.g., dynorphin). These three antagonists themselves inhibited the nicotine-mediated release of [³H]NE from the adrenal chromaffin cells in culture. Finally (4), the I₂-Tyr¹ substituted analogues of β-endorphin and dynorphin that are biologically less active than the parent compounds produced an inhibition of the nicotine-mediated [³H]NE release similar to that of their parent compounds. These results do not support the idea that high-affinity stereospecific opiate binding sites are involved in the inhibitory modulation of nicotinic evoked catecholamine release from bovine adrenal chromaffin cells in culture.     

Stereospecific interaction of opiate narcotics in binding of 3H-dihydromorphine to membranes of rat brain

Membranes from homogenates of corpus striatum bound ³H-dihydromorphine in a saturable fashion with a Km value of 1 × 10^?9M. The binding of ³H-dihydromorphine to the membranes was reduced to about 10% by 10^?7M levorphanol but not by 10^?7M dextrorphan. The binding of ³H-dihydromorphine became less sensitive to 10^?7M levorphanol when the concentration of ³H-dihydromorphine was greater than 2 × 10^?9M. Other opiate narcotics, e.g. morphine and $\text{l}$-methadone, were as effective as levorphanol in competition for the binding ³H-dihydromorphine with ED₅₀ values of 2–4 × 10^?9M. $\text{d}$-Methadone and dextrorphan were about 1/50 and 1/2000 as effective as their respective levo-isomers. The opiate antagonist, naloxone, also competed effectively for the binding sites with an ED₅₀ value of 3.3 × 10^?9M. Substances like acetylcholine, choline, serotonin, norepinephrine and dopamine were ineffective. Only ionophores specific for divalent cations stimulated the binding of ³H-dihydromorphine suggesting that some endogenous divalent cations may be inhibitory to the binding of the opiate narcotic. The receptors of ³H-dihydromorphine probably exist in the membranes of nerve endings and have a density of 6 × 10¹² sites per g in corpus striatum. We conclude that the described technique can successfully detect the opiate narcotic receptors in the central nervous system without the usual method of displacement.     

SR 140333, a Novel, Selective, and Potent Nonpeptide Antagonist of the NK1 Tachykinin Receptor: Characterization on the U373MG Cell Line

The effects of a novel nonpeptide NK1 tachy-kinin receptor antagonist, SR 140333, on the functional consequences of NK1 receptor activation in a human astrocytoma cell line, U373MG, were investigated. Radioligand binding conducted with ¹²⁵l-Bolton-Hunter substance P revealed a competitive inhibition by SR 140333 and its R enantiomer SR 140603 with K_i values of 0.74 and 7.40 nM, respectively. The NK1-selective agonist, [Sar⁹,Met(O₂)¹¹]-substance P, stimulated the formation of inositol phosphates with an EC₅₀ of 3.8 × 10^?9M. SR 140333 blocked the stimulatory effect of this agonist (10^?7M) with an IC₅₀ of 1.6 × 10^?9M,whereas the effect of another NK1 agonist, septide (EC₅₀= 1.5 × 10^?8M)was antagonized with an IC₅₀ of 2.1 × 10^?10M.Enhancement of [³H]taurine release by [Sar⁹,Met(O₂)¹¹]-substance P (EC₅₀= 7.4 × 10^?9M) was also inhibited by SR 140333 with an IC₅₀ of 1.8 × 10^?9 M. SR 140603 was 10-fold less potent than SR 140333 in inhibiting inositol monophosphate formation and [³H]taurine release. The calcium mobilization induced by [Sar⁹,Met(O₂)¹¹]-substance P (10^?8M) was totally prevented by 10^?8MSR 140333. Patchclamp experiments showed that SR 140333 depressed the outward current evoked by 5 × 10^?8M [Sar⁹, Met(O₂)¹¹]-substance P with an IC₅₀ of 1.3 × 10^?9M. The expression of c-fos was stimulated by [Sar⁹,Met(O₂)¹¹]-substance P with an EC₅₀ of 2.5 × 10^?10M, an effect that was also inhibited by SR 140333 with an IC₅₀ of 1.1 × 10^?9M. The present results illustrate the sequential events of the response elicited by NK1 agonists, which were antagonized by SR 140333, demonstrating its powerful NK1 antagonist activity on a functional basis.     

Histamine alters prostaglandin output from diestrous rat uteri. Involvement of H2-receptors and 9-keto-reductase

The effects of exogenous histamine (H) on prostaglandin (PG) generation and release in uteri isolated from diestrous rats and the influences of H₂-receptors blockers (cimetidine and mitiamide) on the output of uterine PGs, were explored. Moreover, the action of H on the uterine 9-keto-reductase, was also studied. Histamine (10⁻⁴M) failed to alter the basal output of PGE₁ but reduced significantly the generation and release of PGE₂ and augmented the output of PGF_2α. On the other hand, cimetidine (10⁻⁵M) enhanced the basal release of PGE₂ but had no action on the outputs of PGs E₁ or F_2α. The enhancing effect of H on the production and release of PGF_2α was abolished in the presence of cimetidine. Also, the antagonist reversed the influence of H on the output of PGE₂. Metiamide, another H₂-receptor antagonist, did not alter the basal control generation and release of uterine PGs, but antagonized the augmenting influence of H on PGF_2α uterine output, as much as cimetidine did, and prevented the depressive action of H on the release of PGE₂ from uteri. Histamine (10⁻⁴M) significantly stimulated uterine formation of cyclic-adenosine monophosphate, an action which was antagonized by the presence of cimetidine (10⁻⁵M), a blocker of H₂ receptors. Also, histamine (10⁻⁵M) and dibutyril-cyclic-adenosine monophosphate (DB-cAMP) at 10⁻³M, enhanced significantly the formation ³H-PGF_2α from ³H-PGE₂. Results presented herein demonstrate that H is able to diminish the generation of PGE₂ in uteri from rats at diestrus augmenting the synthesis of PGF_2α, apparently via the activation of H₂-receptors, enhancing adenylate-cyclase. These effects appear to increase uterine 9-keto-reductase activity which transforms PGE₂ into PGF_2α. Relationships between the foregoing results and those evoked by estradiol, are also discussed.     

Relative contracting adn relaxing potencies of a series of prostaglandins on isolated canine mesenteric artery strips

The contracting and relaxing potencies of anf interactions between a number of prostaglandins (PGs) were studied $\text{in vitro}$ on spiral strips of small canine mesenteric arteries (outside diameter < mm). PGF₂α and PGE₂, the most potent contracting PGs, were nearly equal in potency (EC₅₀ 4 × 10^?7M) and did not cause relaxation under our experimental conditions. PGI₂ and PGE₁ were equal and the most potent relaxing PGs (EC₅₀ 3 × 10^?9M). PGE₁ also caused contraction, but this effect was not consistent. PGI₂ did not cause contraction in concentrations up to 3 × 10^?6M. In higher concentrations, however, it caused abrupt and near maximal contraction. PGD₂ was weak in both respect, causing incomplete relaxation and contraction or biphasic effects. Interaction studies showed that PGE₁ and PGI₂ mutually excluded the relaxing effects of each other. PGE₁ also reversed the relaxing effect of isoproterenol. However, pre-exposure to PGD₂ did not attenuate the relaxing effect of PGE₁ or PGI₂ nor was the relaxing effect of PGD₂ changed by pre-exposure to PGE₁. Two different orders of potency of PGs suggest two PG receptors subserving contraction and relaxation, respectively. Further, it appears that several PGs can act upon both receptors which may explain unusual interactions between the PGs and some of their atypical effects. Finally, the data also suggest that there may be subtypes of the PG receptors subserving contraction and relaxation.     

Blockade of central beta-adrenergic receptors by tazolol (1-isopropylamino-3-(2-thiazoloxy)-2-propanol).

Tazolol, a β₁-adrenergic agonist in heart, had no intrinsic β-adrenergic agonist activity with respect to cyclic AMP-generating systems in rat cerebral cortical slices or with respect to firing of rat cerebellar Purkinje cells. Instead, tazolol proved to be a relatively potent and specific β-adrenergic antagonist. The IC₅₀ for (±) tazolol in antagonizing (-) isoproterenol-elicited accumulation of cyclic AMP in rat cortical slices was 7 × 10^?7M. The IC₅₀ in antagonizing [³H] dihydroalprenolol-binding in rat cortical homogenates was 2.9 × 10^?7 M. Tazolol was about 10 fold more potent in both cases than the β-antagonist, (±) sotalol. Tazolol antagonized the inhibitory, β-adrenergically mediated effects of iontophoretically applied norepinephrine on firing of cerebellar Purkinje cells. The inhibitory effects of γ-aminobutyric acid on firing of Purkinje cells were not altered by tazolol. Tazolol appeared to lack significant local anesthetic activity as evidenced by its lack of effect on spike height in spontaneous firing Purkinje cells.