Influence of prostaglandin-synthesis inhibitors on carbachol-and histamine-induced vasodilatation in perfused rat hindquarters

In perfused rat hindquarters, in which vascular tone was maintained by norepinephrine, carbachol-induced dilatations were blocked by atropine (10^–7 M), while histamine dilatations were inhibited as well by mepyramine (10^–6 M) as by cimetidine (10^–5 M) indicating a histamine effect through both H₁- and H₂-receptors. This double-receptor histamine effect was confirmed by the observation that speccific H₁- and H₂-receptor agonists, respectively PEA (2-pyridyl-ethylaminedihydrochloride) and dimaprit also produced a vasodilation.Carbachol- and histamine-induced dilatations were also inhibited by ETYA (5,8, 11,14-eicosatetraynoic acid) and quinacrine but not by indomethacin. The inhibition of the histamine vasodilatation appeared to rest on an interference with the H₁-receptor mechanism. It is concluded that metabolites of arachidonic acid possibly mediate the dilating effect of carbachol, acting through muscarine receptors, and of histamine, acting through H₁-receptors.Preliminary communication held at the meeting of the Belgian Society for Fundamental and Clinical Physiology and Pharmacology, June 5, 1982 (Arch Int Pharmacodyn 1982, 260:312–313)     

Endothelium-dependent relaxing effect of histamine on the isolated guinea-pig main pulmonary artery strips

Contribution of endothelial cells (ECs) to the effects of histamine (HA) was investigated on the isolated guinea-pig main pulmonary artery (GPPA) strips precontracted with noradrenaline (NA). HA caused a dose-dependent relaxation at the concentrations ranged between 10^–8 to 10^–6 M but produced a contraction when a relatively higher concentration (10^–5 M) was used in unrubbed strips. Cimetidine partially inhibited the relaxing effect of HA without altering its constrictive action. In rubbed strips, however, HA produced a dose-dependent contraction. The constrictive effect of HA in rubbed strips enhanced after addition of cimetidine to the incubation medium. HA elicited a concentration-dependent relaxation in both unrubbed and rubbed strips in the presence of mepyramine. Impromidine produced a relaxation in the strips with and without endothelium.These data was taken as an evidence indicating that HA caused a relaxation in the isolated GPPA strips, first causing the release of endothelium derived relaxing factor (EDRF) which is triggered by H₁-receptors and secondly by the direct stimulation of H₂-receptors.     

Some characteristics of the inotropic effects of histamine H1- and H2-receptor agonists in comparison with those of alpha- and beta-adrenoceptor agonists

The positive inotropic effects of 2-pyridyl-ethylamine (PEA) and of 4-methylhistamine (4MeH) were studied in isolated guinea-pig ventricular strips electrically stimulated at a rate of 60 and 150/min. The increase in contractile tension induced by PEA (10^–7–3×10^–4 M) in the presence of cimetidine (10^–5 M) was associated with a slight increase in time to peak tension and with a lengthening of the relaxation phase; the positive inotropic effect of PEA was significantly higher at a frequency of 60/min than at 150/min. Conversely, the inotropic response to 4MeH (10^–8–3×10^–6 M)was not frequency dependent, and was associated with an evident decrease in relaxation time. Moreover, 4MeH consistently antagonized, in dosedependent manner, the negative inotropic effects induced by the calcium antagonistic drug D600 and by lowering calcium concentration in the medium, and was able to restore the contractility abolished by treatment of preparations with a high K⁺ medium. On the other hand PEA, in the presence of cimetidine, scarcely antagonized the negative inotropic effects induced either by D600 or by low calcium solution, and was unable to restore the contractility of K⁺-depolarized preparations. The characteristics of the inotropic response of the H₁-receptor agonist were very similar to those of the alpha-adrenoceptor agonist phenylephrine. This observation suggests that a common mechanism is probably involved in the inotropic effects mediated by H₁ and by alpha receptors, and that this mechanism does not include a stimulation of the calcium transmembrane influx.     

Histaminergic receptors modulate the coronary vascular response in isolated guinea pig hearts. Role of nitric oxide

Objective: The effects of histamine and of histamine receptor agonists and antagonists on the coronary outflow and on the generation of nitric oxide (NO) were evaluated on isolated guinea pig hearts.Methods: Isolated guinea pig hearts were perfused for 50 min in a Langendorff apparatus with histamine (10^–7– 10^–8 M), in the absence or in the presence of N^G-monome-thyl-L-arginine (L-NMMA, 10^–4 M), a NO synthase inhibitor and of triprolidine (310^–8 M) and cimetidine (10^–7 M), H₁ receptor and H₂ receptor antagonists, and with trifluoromethyl-phenylhistamine (TFMPH, 10^–7 M) and dimaprit (10^–7 M), H₁ and H₂ receptor agonists. The effects of (R)--methylhistamine (10^–7 M), a H₃ receptor agonist and of FUB 181 (10^–7 M), a H₃ receptor antagonist, were studied in the presence of bradykinin (10^–7 M).Results: Histamine increases the coronary outflow and the generation of NO in a concentration-dependent fashion. The effects were completely abolished by blocking NO-synthase (NOS) with L-NMMA (10 ^–4 M). The effects were also abolished by cimetidine (10 ^–7 M), H ₂ receptor antagonist, and only scarcely affected by triprolidine (310 ^–8 M), H ₁ receptor antagonist. The effects were reproduced by dimaprit (10 ^–7 M), H ₂ receptor agonist, and only scarcely by TFMPH (10 ^–7 M), a selective H ₁ receptor agonist. Bradykinin (10 ^–7 M) produces a sustained coronary dilation paralleled by a marked increase in the generation of NO; the effects were significantly reduced by L-NMMA. The stimulation of H ₃ receptors by (R)--methylhistamine (10 ^–7 M) significantly reduced both effects, which reverted to normal with FUB 181 (10 ^–7 M), an H ₃ receptor antagonist.Conclusion: These results suggest that, in isolated guinea pig hearts, histamine produces coronary dilation through an H ₂ /H ₃ -dependent mechanism involving the generation of nitric oxide.Received 3 December 2002; returned for revision 8 April 2003; accepted by M. Parnham 26 May 2003     

A radioimmunoassay for the histamine H2-antagonist,tiotidine

A specific and sensitive radioimmunoassay (RIA) for the histamine H₂-antagonist, tiotidine, has been developed. The assay is based upon competition of tiotidine with [³H]-tiotidine for antibody (Ab) obtained from immunized rabbits. The immunogen used was a glutaraldehyde coupled conjugate of the tiotidine derivative (ICI 147,655) and bovine serum albumin (BSA). Displacement of [³H]-tiotidine by unlabeled tiotidine was competitive over a concentration range of 10–1000 fmol. The Ab was 100-fold less sensitive to the pharmacologically inactive metabolite of tiotidine (ICI 129,585) and did not cross-react with histamine, cimetidine or phenylguanidine. In dogs given an i.v dose of tiotidine, plasma levels of the antagonist, as measured by the RIA, were correlated with inhibition of histamine-induced gastric acid secretion. Tiotidine (0.3 or 0.6 mol kg^–1) caused a dose-dependent and transient decrease in acid secretion at plasma concentrations of 10^–6 to 10^–8 M. Other potential analytical and research uses of H₂-antagonist radioimmunoassays are discussed.     

Properties of the luminal membrane of isolated perfused rat pancreatic ducts

The aim of the present study was to investigate by what transport mechanism does HCO ₃ ^– cross the luminal membrane of pancreatic duct cells, and how do the cells respond to stimulation with dibytyryl cyclic AMP (db-cAMP). For this purpose a newly developed preparation of isolated and perfused intra-and interlobular ducts of rat pancreas was used. Responses of the epithelium to inhibitors and agonists were monitored by electrophysiological techniques. Addition of HCO ₃ ^– /CO₂ to the bath side of nonstimulated ducts depolarized the PD across the basolateral membrane (PD_bl) by about 9mV, as also observed in a previous study [21]. This HCO ₃ ^– effect was abolished by Cl^– channel blockers or SITS infused into the lumen of the duct: i. e. 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 10^–5 M) hyperpolarized PD_bl by 8.2±1.6 mV (n=13); 3,5-dichlorodiphenylamine-2-carboxylic acid (DCl-DPC, 10^–5 M) hyperpolarized PD_bl by 10.3±1.7 mV (n=10); and SITS hyperpolarized PD_bl by 7.8±0.9 mV (n=4). Stimulation of the ducts with dbcAMP in the presence of bath HCO ₃ ^– /CO₂ resulted in depolarization of PD_bl, the ductal lumen became more negative and the fractional resistance of the luminal membrane decreased. Together with forskolin (10^–6 M), db-cAMP (10^–4 M) caused a fast depolarization of PD_bl by 33.8±2.5 mV (n=6). When db-cAMP (5×10^–4 M) was given alone in the presence of bath HCO ₃ ^– /CO₂, PD_bl depolarized by 25.3±4.2 mV (n=10). In the absence of exogenous HCO ₃ ^– , db-cAMP also depolarized PD_bl by 24.7±3.0 mV (n=10). The present data suggest that in the luminal membrane of pancreatic duct cells there is a Cl^– conductance in parallel with a Cl^–/HCO ₃ ^– antiport. Dibutyryl cyclic AMP increases the Cl^– conductance of the luminal membrane. Taking together our present results, and the recent data obtained for the basolateral membrane [21], a tentative model for pancreatic HCO ₃ ^– transport is proposed.Parts of this study have been presented at the Scandinavian Physiological Society Meeting in Copenhagen 1986 and 64th Meeting of the German Physiological Society in Homburg/Saar     

Interactions of morphine with PGE1, isoproterenol,dopamine and aminophylline in rat mast cells; their effect on IgE-mediated14C-serotonin release

The formaldehyde method was used to examine the interactions of morphine with PGE₁, isoproterenol, dopamine and aminophylline in rat mast cells by their effects on IgE-mediated¹⁴C-serotonin release. PGE₁ (2×10^–8–2×10^–5 M), isoproterenol (10^–10–10^–8 M), dopamine (4×10^–8–4×10^–6 M) and aminophylline (6×10^–6–6×10^–4 M) caused dose-related inhibition of the mediator release 1 min after an antigen challenge, and propranolol (10^–7 M) blocked the inhibition by isoproterenol (10^–8 M) but not that by dopamine (4×10^–6 M), while haloperidol (4×10^–6 M) blocked that by dopamine (4×10^–6 M) but not that by isoproterenol (10^–8 M). Morphine (3×10^–7–3×10^–5 M) reversed the inhibitory effects of PGE₁ (2×10^–6 M), isoproterenol (10^–8 M) and dopamine (4×10^–6 M) dose-dependently and stereospecifically; naloxone (2×10^–4 M) antagonized these reversing actions of morphine (3×10^–5 M). Morphine (10^–6–10^–4 M) did not reverse the inhibitory action of aminophylline (6×10^–4 M). These results suggest that the inhibitory responses of mast cells to PGE₁, isoproterenol and dopamine but not to aminophylline in immunological mediator release were reversed by morphine through opioid receptors, and that the inhibition of adenylate cyclase in mast cells is one of the biochemical actions of morphine.     

Pre- and postjunctional effects of histamine on the guinea pig urinary bladder: Evidence for heterogeneity in the H1-receptor population?

Histamine was tested on the guinea pig isolated urinary bladder to determine both the direct effect on the muscle and the influence on the contractions induced by field stimulation. Histamine (10^–7 –10^–3 M) caused contraction of the bladder and enhancement of the atropine-resistant response to field stimulation. These effects were sensitive to the H₁-receptor antagonist pyrilamine (pA₂=8.55 and 7.07, respectively). The H₂-antagonists cimetidine and famotidine were ineffective on both parameters. It is concluded that predominantly H₁-receptors are present in the guinea pig urinary bladder and that they are localized both on the muscle and on non-cholinergic nerve terminals. The significant difference between the pA₂ values for pyrilamine is likely to suggest a heterogeneity in the H₁-receptor population.     

Histamine and abnormal automaticity in barium- and strophanthidin-treated sheep Purkinje fibers

We used intracellular microelectrodes to study the effects of histamine on both normal and abnormal automaticity in sheep Purkinje fibers.Histamine, dimaprit and 4-methylhistamine caused a similar reduction of the action potential duration in driven Purkinje fibers. Histamine (10^–6 M) induced spontaneous activity in previously quiescent preparations more often than did equimolar concentrations of dimaprit and 4-methylhistamine. The effects of histamine on automaticity were enhanced in the presence of barium. In fact histamine, at concentrations which were unable to induce automaticity in normal preparations, induced it in the presence of barium. In Purkinje fibers manifesting barium-induced automatic activity, histamine (10^–7–10^–6 M) significantly increased the average number of spontaneous action potentials and shortened the time of their appearance. In the same range of concentrations, histamine dose-dependently increased the amplitude of the barium-induced oscillatory afterpotentials (OAP), which are considered an electrophysiological manifestation of calcium overload.Histamine (10^–6–10^–4 M) increased the OAP amplitude of strophanthidin-treated Purkinje fibers, eventually inducing triggered extrabeats.All these previously described effects were selectively blocked by cimetidine (10^–5 M). It is concluded that histamine may induce cardiac arrhythmias under conditions of calcium overload and that this effect may be due to induction or enhancement of oscillatory afterpotentials.     

Vasoconstriction in rat isolated mesentery and small intestine in response to various activators of protein kinase C

The vasculature of rat isolated mesentery and small intestine was perfused with a gelatin-containing physiological salt solution (GPSS). When 5-hydroxytryptamine (5HT, 1×10^–4 M), or the calcium ionophore A23187 (1×10^–4 M), or 12-deoxyphorbol 13-phenylacetate (DOPPA, 1×10^–6 M), or 12-deoxyphorbol 13-phenylacetate 20-acetate (DOPPAA, 1×10^–6 M) or thymeleatoxin (TMX, 1×10^–6 M) was added to the GPSS for 5 min there was a gradual rise in perfusion pressure, whereas resiniferatoxin (RFX, 1×10^–6 M) was without effect. Pre-treatment of the tissue with the protein kinase C (PKC) inhibitor Ro 31-8220 (1×10^–6 M) significantly reduced the rise in perfusion pressure in response to 5HT, DOPPA, DOPPAA and TMX, but not that to A23187. Platelet-activating factor (PAF, 5×10^–6 M) caused an almost immediate but transient rise in perfusion pressure, followed by a more gradual rise, neither response being blocked by Ro 31-8220. When blood vessels of the mesentery alone were perfused with gelatin-free PSS, PAF caused a transient rise in perfusion pressure, but with no subsequent gradual rise over 5 min. After Ca²⁺-depletion this transient response was also absent. In contrast, when blood vessels were perfused with gelatin-free PSS, DOPPA and TMX still caused gradual rises in perfusion pressure, which were totally abolished by Ro 31-8220. TMX had no effect at all when the tissue was depleted of Ca²⁺, whereas the response to DOPPA was only partially reduced. Phenylephrine (PE, 1×10^–6 M) produced a fairly rapid but non-sustained rise in perfusion pressure, which was reduced by Ro 31-8220, and more or less abolished by Ca²⁺-depletion. It is suggested that the pressor response to DOPPA involved activation of both Ca²⁺-dependent and Ca²⁺-independent PKC isoenzymes. TMX and PE, however, appear to activate only a Ca²⁺-dependent isoenzyme.     

Ionic conductances of cultured principal cell epithelium of renal collecting duct

The ionic conductive properties were studied of epithelia of collecting duct principal cells which had been grown in primary tissue culture from renal cortex/capsule explants. When pretreated with aldosterone (10^–6 mol/l) and bathed on either surface with isotonic HCO ₃ ^– -free Ringer's solution, the transepithelial voltage,V _te, varied between –21 and –72 mV (apical surface negative) while the transepithelial resistance,R _te, ranged from 0.4 to 1.5 kcm². By 10:1 step-changes in Na⁺ concentration the apical cell membrane was shown to have a high conductivity for sodium, inhibitable by amiloride, 10^–6 mol/l. However, contrary to observations in natural collecting duct under control conditions, amiloride never reversed the polarity ofV _te even at 10^–4 mol/l. Both the apical and the basolateral cell membranes were conductive for potassium and both conductivities were inhibitable by Ba²⁺ (5 mmol/l). 10:1 reduction of apical Cl^– concentration strongly hyperpolarizedV _te with a monophasic time course suggesting the presence of a paracellular shunt conductance for Cl^–. In addition there may be a small Cl^– conductance present in the apical cell membrane since apical application of the chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPAB) at 10^–7 mol/l produced a minute but significant hyperpolarization. On the other hand, 10:1 reduction of basolateral Cl^– concentration caused a biphasic change inV _te (initial depolarization, followed by repolarization) which indicates the presence of a large Cl^– conductance in the basolateral cell membrane. The latter was not inhibitable by 10^–7 mol/l NPPAB. Higher concentrations of this and of an other Cl^– channel blocker produced non-specific effects. In conclusion, our studies of a pure principal cell epithelium confirm findings described for the intact cortical collecting duct and add new information concerning chloride conductivity and related blocking agents.Dedicated to Prof. Dr. H. Sitte, Homburg, FRG, upon his 60th birthday.     

The mechanism of electrodiffusive K+ transport in leaky epithelia and some of its consequences for anion transport

The ventricular membrane of the epithelium from the choroid plexus ofNecturus maculosus was probed with double-barrelled ion-selective microelectrodes while the ventricular bathing solution was changed abruptly. The transient states induced by increasing the external concentration of K⁺ or by the application of ouabain showed that the passive movements of K⁺ across the ventricular membrane were electro-diffusive and could be described by the Goldmann equation and one constant permeability (P _K) of 24×10^–6 cm s^–1. The passive efflux was balanced by a ouabain-sensitive influx.P _K was different in different steady states; when the cell was acidified by half a pH unit by increasing CO₂ in the bathing solutions from 1 to 5%, thenP _K decreased to 13×10^–6 cm s^–1. Removal of Cl^– from the bathing solution increasedP _K by 50% and reduction of Cl^– transport by furosemide did not alterP _K, consequently major movements of K⁺ were independent of Cl^– movements. Depolarizations of the cell caused an increase in the cellular HCO ₃ ^– concentration due to an electrodiffusive permeability (P _HCO3), the value of which was estimated to 17×10^–6 cm s^–1.     

Effect of glucagon on gastric acid secretion by the isolated fundus from immature rats

The effect of glucagon was studied on the isolated gastric fundus from immature rats in comparison with histamine. Glucagon (10^–7–3×10^–6 M) caused a concentration-dependent increase in acid output, being approximately 25 fold more potent than histamine (ED₅₀ values were 6.38×10^–7 M and 2.42×10^–5 M for glucagon and histamine, respectively). These compounds, however, did not differ in regard to the maximum response. The stimulatory effect of glucagon was not enhanced by pretreatment with 3×10^–8 M forskolin or 10^–7 M ICI 63197, a phosphodiesterase (PD) inhibitor. Conversely, both forskolin and ICI 63197 shifted to the left the concentration-response curve to histamine. The increase in acid secretion by glucagon was reduced by PGE₁ (10^–5 M) and PGE₂ (10^–5 M) but only PGE₂ inhibited the response to histamine. From these data it can be concluded that glucagon stimulated acid production in the stomach from immature rats, and this effect does not seem to involve the same adenylate cyclase activated by histamine.     

Identification of histamine H1- and H2-receptors by means of mepyramine,cimetidine and theophylline in the cardiovascular system of the dog

The haemodynamic effects of histamine infusions (0.5 to 8 g kg^–1 min^–1) were studied in anaesthetized dogs previously instrumented for measurements of left ventricular pressure (LVP),dP/dt _max, aortic and femoral blood pressure (P _AO;P _AF) and femoral blood flow (F _AF). Blockage of H₁- or H₂-receptors alone with either mepyramine (1 mg kg^–1 min^–1) or cimetidine (2 mg kg^–1 min^–1) did not prevent the dose-dependent decrease in contractility and blood pressure responses to histamine. Since, however, both antihistamines administered in combination competitively antagonized responses to histamine, it is concluded that peripheral and cardiac effects of histamine involve interaction with both H₁- and H₂-receptors. The potentiation ofdP/dt _max,P _AO andF _AF responses to histamine as produced by theophylline (4 mg kg^–1 min^–1) was completely reversed by cimetidine, which thus may be taken as an indication that also under in vivo conditions cyclic AMP serves as a mediator only for histamine H₂-responses. However, since these results do not allow clearly to separate between primary cardiac and peripheral responses in a further set of experiments blood pressure was held constant during histamine infusions. When peripheral mechanisms were excluded as responsible for cardiac actions of histamine by this procedure histamine evoked small positive inotropic responses which were prevented by cimetidine. These findings suggest that on ventricular muscle of dogs there exists a small fraction of H₂-receptors mediating positive inotropic effects which, however, were masked on intact animals by negative inotropic responses due to a pronounced fall in blood pressure.     

Effect of amiloride on electrolyte transport parameters of the main duct of the rabbit mandibular salivary gland

We have studied the response of the rabbit mandibular main duct perfused in vitro to luminally administered amiloride. The half-maximal inhibitory concentrations (K_I) when the duct was bathed in Cl solutions were: for net Na⁺ transport, 3×10^–6 mol l^–1; for transepithelial potential difference, 6×10^–6 mol l^–1; and for transepithelial conductance, 3×10^–7 mol l^–1. Substitution of the impermeant SO ₄ ^2– anion for Cl^– changed the K_I for conductance to 3×10^–6 mol l^–1. Within Cl^–-containing media, the time course of the amiloride effect on potential difference showed an early rapid fall of 10 mV with a half-time 2 s, followed by a slower depolarization of 9 mV, and the conductance change followed the slower component of the potential change. In SO ₄ ^2– -containing media, the potential difference and conductance changes followed time courses similar to one another. Finally, experiments on the effect of serosal applications of ouabain revealed that, although, in general, ouabain reduced resistance, it caused an increase in resistance in those ducts where the initial resistance was low. We conclude that: i) luminal Na⁺ transport occurs via amiloride-sensitive, conductive Na⁺ channels; ii) the Cl^– conductance is the major determinant of transepithelial conductance; iii) the first phase of the potential response is due to blocking of the Na⁺ conductive channels, whilst the slow phase reflects secondary inhibition of an electrogenic Na⁺ pump; and iv) duct resistance changes are secondary to alterations in intracellular Cl^– concentration.     

Effects of H1- and H2-receptor antagonists on calcium-dependent action potentials produced by histamine in rabbit left auricles depolarized by potassium

After inactivation of the fast sodium channels by partial depolarization (25 mmol/l K⁺ Tyrode solution) histamine (10^–6–10^–5 mol/l) restored the electrical and mechanical activity in isolated electrically paced left auricle of rabbit. Transmembrane action potentials restored by histamine in this condition have a nearly normal height of overshoot with a low resting potential and with a very low rate of rise. The AP-restoring effect of histamine was affected neither by beta-adrenergic blocking agent, Pindolol (4×10^–7 mol/l) nor H₂-receptor antagonist, cimetidine (2×20^–6 mol/l) but was antagonized by H₁-receptor blocker, mepyramine (10^–5 mol/l) and by the selective Ca⁺²-channel blocker, D-600 (2×10^–6 mol/l). The results suggest that in this species histamine may stimulate the slow inward Ca²⁺ current mediated by H₁-receptors.     

Extra- and intracellular Ca2+ requirements for lysosomal enzyme secretion in human neutrophils

Ca²⁺-EGTA combinations were utilized in Hank's buffers to fix extracellular free Ca²⁺ concentrations [Ca²⁺ _f] for the study of human neutrophil lysosomal enzyme secretion. Ca²⁺-dependent neutrophil secretion initiated by formyl-methionyl-leucyl-phenylalanine (FMLP) and C5a required small amounts of [Ca²⁺ _f] (1–3×10^–6 M) while that caused by ionophore A23187 required 10^–5 M or greater [Ca²⁺ _f]. The inhibition of FMLP- and C5a-induced lysosomal enzyme secretion by the intracellular Ca²⁺ antagonist, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) was additive to lowering extracellular [Ca²⁺ _f] from 10^–4 M to 10^–6 M or blocking plasma membrane Ca²⁺ flux with verapamil. These results suggest that extracellular and intracellular Ca²⁺ flux may be coupled in the initiation of neutrophil secretion caused by FMLP and C5a.     

In vitro inhibition of phospholipase A2 by gabexate mesilate,camostate, and aprotinine

Summary Gabexate mesilate (FOY, ethyl-p-(6-guanidino-hexanoyl-oxy)-benzoate-methansulfonate), camostate (N,N-dimethyl-carb-amoylmethyl-4-(guanidinobenzoyloxy)-phenyl-acetate) methansulfonate and aprotinine (Trasylol) were tested for possible inhibition of phospholipase A₂. Gabexate mesilate at a concentration of 5×10^–4 mol/l and camostate at a concentration of 10^–3 mol/l caused a 50% reduction in enzyme activity. There was almost no inhibition by aprotinine at clinical doses; 40 million KIU/l were necessary to reduce phospholipase A₂ activity by 20%. From the therapeutic dose (4,000 mg/day per i.v. infusion) and the half-life of gabexate mesilate in blood circulation (1 min) it can be calculated that the in vitro concentration of gabexate mesilate is only 10^–6 to 10^–7 mol/l. Under these conditions gabexate mesilate cannot diminish the in vivo enzyme activity of phospholipase A₂.     

Electrophysiological effects of acetylcholine in sheep cardiac Purkinje fibers

The addition of acetylcholine (ACh) in concentrations of 10^–7 to 10^–5 mol·l^–1 to normal Tyrode solution results in the following changes of the electrical activity in sheep cardiac Purkinje fibers: prolongation of the action potential and shift of the plateau to more positive values, hyperpolarization of the maximum diastolic potential (E _max), and increase of the rate of diastolic depolarization.Prolongation of the action potential and shift of the plateau in the positive direction were more pronounced in distal Purkinje fibers, at low frequencies of stimulation, and in the presence of a reduced Ca (<3.6 mmol·l^–1) or K extracellular concentration (<5.4 mmol·l^–1); the effects persisted in Cl free media or after addition of Mn (2–10 mmol·l^–1) or verapamil (1–5 mg·l^–1).The effect of Ach on the maximum diastolic potential (stimulation frequency 60/min) was dependent on K₀. At 5.4 mmol·l^–1,E _max increased by 2 mV; at higher K₀ concentrations the change was less pronounced or absent; at low K₀ concentrations (membrane potential arrested at the plateau level) the addition of Ach invariably caused a depolarization. In unstimulated preparations the effect of Ach was unpredictable.The increase in rate of diastolic depolarization by Ach in 3.4 or 2.7 mmol·l^–1 K was large enough to result in spontaneous activity. When the membrane was depolarized to the plateau level (K₀<1.35 mmol·l^–1) Ach frequently reduced the frequency of oscillations.The effect of Ach was dose-dependent; desensitization was absent. Similar results were obtained with carbachol (10^–6 mol·l^–1) or choline (5·10^–3 mol·l^–1). The effect of Ach could selectively be abolished by atropine (10^–8 to 10^–6 mol·l^–1); it was not modified by succinylcholine (8·10^–5 mol·l^–1), phentolamine (10^–6 mol·l^–1) or propranolol (10^–6 mol·l^–1). The results indicate that the electrophysiological changes are due to stimulation of muscarinic receptors.Except for the hyperpolarization ofE _max the results in sheep Purkinje fibers are different and even opposite to those observed in the sino-atrial node and atrial muscle. Possible mechanisms and functional significance of the results are discussed.Supported by F.G.W.O. Belgium 3.0087.74     

Structural requirements of imidazole compounds to be inhibitors or activators of histamine methyltransferase: Investigation of histamine analogues and H2-receptor antagonists

The influence of imidazole compounds (histamine analogues and H₂-receptor antagonists) and of the specific histamine H₂-receptor agonist dimaprit on histamine methyltransferase (HMT) from pig gastric mucosa was investigated.By their effect on HMT two groups of substituted imidazole compounds could be differentiated. Some were pure inhibitors of the enzyme, whereas others were inhibitors only in high concentrations (>10^–3 M) and activators of the enzyme in low concentrations. The strongest inhibitor of all the histamine analogues tested was 2-methylhistamine (I.D.₅₀=0.6×10^–4 M), whereas the strongest activation was exerted by 4-[(2-amino-ethylmercapto)-methyl]-5-methylimidazole (57% increase of enzymic activity at 10^–4 M concentration). From the group of histamine H₂-receptor antagonists only burimamide was an inhibitor (I.D.₅₀=1.6×10^–4 M) whereas metiamide and cimetidine belonged to the strongest activators of the enzyme (179% enzymic activity at 10^–4 M concentration of metiamide).The strongest activator of all the substances tested in this series of compounds, however, was the non-imidazole compound dimaprit, which increased enzymic activity by 86% in as small a concentration as 10^–5 M.For substituted imidazole ring systems an attempt is made to evaluate the structural requirements of the single compound to classify it as a pure inhibitor or a concentration-dependent inhibitor/activator of HMT.Dedicated to Prof. Dr Dr E. Werle () on the occasion of his 75th birthday.Supported by grant Lo 199/7 from Deutsche Forschungsgemeinschaft.Presented at the Sixth Annual Meeting of the European Branch of the Histamine Club, held in London, April 20–22, 1977.