I. Kennedy

COMPARISON OF THE ACTIONS OF U‐46619, A PROSTAGLANDIN H2‐ANALOGUE, WITH THOSE OF PROSTAGLANDIN H2 AND THROMBOXANE A2 ON SOME ISOLATED SMOOTH MUSCLE PREPARATIONS

1 The actions of the prostaglandin H2 (PGH2) analogue, U‐46619, have been compared with those of PGH2 and thromboxane A2 (TxA2) on a range of isolated smooth muscle preparations in a superfusion cascade system. 2 U‐46619 was a potent agonist on guinea‐pig lung strip, dog saphenous vein and rat and rabbit aortae. In contrast, U‐46619 was weak or inactive on guinea‐pig ileum and fundic strip, cat trachea and dog and cat iris sphincter muscles, preparations on which either PGE2 or PGF2α was the most potent agonist studied. 3 PGH2 was active on all of the preparations and displayed little selectivity. On some of the preparations, the actions of PGH2 may have been mediated indirectly by conversion to other prostanoids. 4 In contrast, TxA2 displayed the same pattern of selectivity as U‐46619, being a potent agonist on the lung strip and vascular preparations but weak or inactive on the others. 5 It is suggested that U‐46619 is a selective TxA2‐mimetic and that it should therefore be a valuable tool in the study of the actions of TxA2

Studies of the characterisation of prostanoid receptors: A proposed classification

Comparison of rank orders of agonist potency of the naturally occurring prostanoids, PGD2, PGE2, PGF2 alpha and PGI2 as well as the stable TxA2 mimetic, U-46619, on a range of smooth muscle preparations provides evidence for the existence of distinct receptors for PGE2, PGF2 alpha and TxA2. Since others have provided evidence for the existence of distinct receptors for PGD2 and PGI2, we suggest that receptors exist for each of these naturally occurring 2-series prostanoids. Results obtained with two specific prostanoid receptor blocking drugs, SC-19220 and AH 19437, support and extend these conclusions. SC-19220 selectively blocks some but not all PGE-sensitive receptors, while AH 19437 selectively blocks all U-46619/TxA2-sensitive receptors. A nomenclature for prostanoid receptors is proposed, in which each receptor is designated the letter P preceded by a letter signifying the most potent natural prostanoid agonist at that receptor, such that receptors sensitive to PGs D2, E2, F2 alpha, I2 and TxA2 become DP-, EP-, FP-, IP and TP- receptors respectively. Where some sub-division is required within a receptor group, e.g. EP-receptors (SC-19220-sensitive and SC-19220-insensitive), subscript numbers may be used such that these are EP1 and EP2 subtypes. The resulting scheme is a working hypothesis and its confirmation requires the development of potent selective prostanoid receptor blocking drugs for each postulated type.

Pharmacology of AH 5158; a drug which blocks both α‐ and β‐adrenoceptors

1 AH 5158 differs from conventional adrenoceptor blocking drugs in producing competitive blockade of both α‐ and β‐adrenoceptors. 2 AH 5158 is 5–18 times less potent than propranolol in blocking β‐adrenoceptors. It resembles propranolol in its non‐selective blockade of β1‐cardiac and β2‐vascular and tracheal adrenoceptors and in its lack of intrinsic sympathomimetic activity. 3 AH 5158 is 2–7 times less potent than phentolamine in blocking α‐adrenoceptors. AH 5158 itself is more active on β‐ than α‐adrenoceptors. 4 Blockade of noradrenaline vasopressor responses by AH 5158 in anaesthetized dogs was dose‐dependent up to 1 mg/kg but no further blockade was obtained with larger doses of AH 5158. ‘Self‐limiting’ blockade was not observed in dogs pretreated with cocaine, or in untreated dogs if the vasopressor agent was oxymetazoline instead of noradrenaline. A possible cause of ‘self‐limiting’ blockade is discussed. 5 In doses higher than those required for either α‐ or β‐adrenoceptor blockade, AH 5158 produced effects on cardiac muscle that are attributable to membrane‐stabilizing activity. This was manifested as a negative inotropic action in spinal dogs and in guinea‐pig left atrial strips, as a negative chronotropic action in syrosingopine pre‐treated dogs, and as an increase in the effective refractory period of guinea‐pig left atrial strips. AH 5158 was 3–11 times less potent than propranolol in these tests. 6 In open chest dogs AH 5158 resembled propranolol in reducing cardiac output, rate and contractility, effects which are attributable to β‐adrenoceptor blockade. The drug differed from propranolol in decreasing rather than increasing total peripheral resistance and in causing larger decreases in arterial blood pressure at equipotent β‐adrenoceptor blocking doses. These differences are attributable to the α‐adrenoceptor blocking actions of AH 5158. 7 In anaesthetized dogs, intravenously administered AH 5158 antagonized both catecholamine and ouabain‐induced arrhythmias. Orally administered AH 5158 lowered systolic arterial pressure in conscious renal hypertensive dogs. 8 These results show AH 5158 to possess a novel profile of activity. Possible uses of the drug in cardiovascular disorders such as hypertension, angina pectoris and cardiac arrhythmias are discussed.

The effects of epithelium removal on the actions of cholinomimetic drugs in opened segments and perfused tubular preparations of guinea‐pig trachea

1 Isolated segments of guinea‐pig trachea or perfused tracheal tubes were arranged for the recording of trachealis tension changes in Krebs solution containing indomethacin (2.8 μm). 2 In opened tracheal segments, epithelium removal caused modest (2–3 fold) potentiation of the effects of acetylcholine (ACh) and methacholine (MeCh) but failed to potentiate carbachol (CCh), bethanechol (BeCh), oxotremorine or KCl. 3 Pretreatment with ecothiopate potentiated effects of ACh and MeCh but not of CCh or BeCh. Removal of epithelium in ecothiopate‐treated tissue potentiated effects of ACh and MeCh but not of CCh or BeCh. 4 Guinea‐pig ileum challenged with ACh was used as a bioassay system for cholinesterase activity. Scrapings of tracheal epithelium did not hydrolyse ACh. 5 Histochemical staining revealed no fibres positive for acetylcholinesterase or pseudocholinesterase in the tracheal epithelium. However, the underlying tissues contained acetylcholinesterase‐positive nerve fibres and the trachealis muscle itself stained positively for pseudocholinesterase activity. 6 Neither tetrodotoxin (3 μm) nor hexamethonium (500 μm) modified the ability of epithelium removal to potentiate ACh. 7 In perfused tracheal tubes where spasmogens were added to the luminal perfusate, epithelium removal potentiated effects of ACh (31 fold), CCh (10 fold), oxotremorine (2 fold) and KCl. 8 In perfused tracheal tubes where spasmogens were added to the Krebs solution superfusing the adventitial surface of the tissue, epithelium removal significantly reduced the potency of CCh, oxotremorine and KCl. 9 It is concluded that the selectivity and magnitude of the potentiation of cholinomimetics caused by epithelium removal depends on the route by which the cholinomimetic agent gains access to the trachealis muscle. The potentiation of acetic acid esters of choline seen in opened tracheal segments does not reflect the loss of epithelial cholinesterase activity and does not depend on the activity of nervous reflex arcs in the tracheal wall. The reduced potency of adventitially‐applied cholinomimetics and KCl seen in epithelium‐denuded tissue strongly suggests that the epithelium can moderate trachealis sensitivity to cholinomimetic agents not only by releasing epithelium‐derived relaxing factor but also by acting as a barrier to drug diffusion.

CONTRACTILE AND RELAXANT ACTIONS OF PROSTAGLANDINS ON GUINEA-PIG ISOLATED TRACHEA

1 The effects of 12 prostaglandins on guinea‐pig isolated trachea have been examined in the presence of indomethacin. Two series of experiments were carried out, the first on preparations without tone (‘zero tone’), and the second on preparations with tone induced with acetylcholine (‘high tone’). 2 The compounds tested fell into two groups. The first, comprising prostaglandins F1α, F2α F2α acetal, I2 and Wy 17186, contracted both zero and high tone preparations. The second, comprising prostaglandins A1, A2, B1, B2, E1 E2 and F2β, contracted zero, but relaxed high tone preparations. Responses to the second group of compounds are probably the resultant of their contractile and relaxant actions. 3 The order of potency for contracting zero tone preparations was prostaglandin E (PGE) > F = I = Wy 17186 > B > A, 2‐series compounds being 5 to 18 times more potent than 1‐series compounds. 4 The order of potency for relaxing high tone preparations was PGE > Fβ > B > A > Wy 17186 > Fα = I = 0. There was little difference between the potency of 1‐ and 2‐series compounds. 5 The possible relevance of these results to the interpretation of the effects of prostaglandins on human airways is discussed.

Spasmogen action in guinea‐pig isolated trachealis: involvement of membrane K+‐channels and the consequences of K+‐channel blockade

1 Acetylcholine (ACh), histamine, prostaglandin E2 and potassium chloride (KCl) each evoked concentration‐dependent spasm of guinea‐pig isolated trachealis treated with indomethacin (2.8 μm). 2 Neither tetraethylammonium (TEA; 0.1–10 mm) nor procaine (0.1–10 mm) potentiated these spasmogens. Indeed, procaine (10 mm) depressed the log concentration‐effect curves of all the spasmogens while TEA (1–10 mm) caused some depression of the log concentration‐effect curve of prostaglandin E2. 3 Intracellular electrophysiological recording was performed in trachealis bathed by normal Krebs solution or by Krebs solution containing 2.8 μm indomethacin. In either medium the majority of trachealis cells exhibited spontaneous electrical slow waves while some cells were electrically quiescent. In either medium the spasmogenic effects of ACh (1 mm) and histamine (0.2 mm) were accompanied by depolarization and abolition of slow wave discharge. In many cases the record of membrane potential subsequently exhibited noise which incorporated fast, hyperpolarizing transients. 4 In the absence and presence of indomethacin, TEA (10 mm) and procaine (5 mm) markedly reduced the membrane noise and hyperpolarizing transients evoked by ACh or histamine without augmenting the evoked tension. 5 It is concluded that slow wave discharge does not depend on prostaglandin synthesis. The membrane noise and hyperpolarizing transients evoked by ACh and histamine represent the opening of membrane K+‐channels. While such K+‐channel opening may offset spasmogen‐induced depolarization it does not moderate the evoked tension.