Characterization of muscarinic receptor subtype mediating contraction and relaxation in equine coronary artery in vitro

In coronary arterial rings isolated from horses, 10-^-8-10^-6 mol/l acetylcholine (ACh) induced concentration-dependent contractions which were potentiated by the removal of endothelium and by pretreatment with I,-nitro-arginine (LNAG) or methylene blue (MB). Relatively lower concentrations of Ach 10-¹⁴-10-⁸ mol/l) induced relaxation when the coronary rings were contracted by phenylephrine (PE). ACh-induced contractions in the coronary rings without endothelium were competitively inhibited by each muscarinic subtype selective antagonist in the following order of potency: 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > pirenzepine ≥ parafluoro-hexahydrosiladiphenidol (pFHHSiD) > methoctramine. ACh-induced relaxation in the rings with endothelium was inhibited by LNAG or MB, and by each selective antagonist in the following order of potency: 4-DAMP < pFHHSiD ≥ pirenzepine ≥ methoctramine. These results suggest that the ACh-induced contraction and relaxation in equine coronary arteries are mediated mainly by an M₃-receptor located on the smooth muscle cells and endothelial cells, respectively, and that the stimulation of the M₃-receptor on the endothelial cells liberates nitric oxide.     

Functional characterization of the muscarinic receptors involved in endothelium-dependent relaxation in isolated canine uterine artery

Acetylcholine interacts with endothelial muscarinic receptors releasing nitric oxide and causing vasodilatation. To identify the receptor subtype responsible for acetylcholine-induced relaxation in canine uterine artery, the usual organ bath method for in vitro investigation on isolated blood vessels was applied. Using a range of muscarinic receptor antagonists such as atropine (nonselective), pirenzepine (M₁-selective), methoctramine (M₂-selective) and p -fluoro-hexahydro-sila-difenidol ( p -FHHSiD) (M₁/M₃) and determining pA2 value of those antagonists through Shild analysis, we aimed at establishing a precise receptor mechanism underlying acetylcholine-induced relaxation in isolated canine uterine artery. The relaxation of uterine arterial rings in response to acetylcholine in the presence or absence of selective muscarinic receptors antagonists was calculated using concentration response curves. Acetylcholine induced concentration-dependent and endothelium-dependent relaxation of arterial rings precontracted with phenylephrine (pEC₅₀ = 6.90 ± 0.02). Muscarinic receptors antagonists atropine, pirenzepine, methoctramine and p -FHHSiD competitively antagonized the response to acetylcholine and obtained pA₂ values were 9.91 ± 0.06, 6.60 ± 0.04, 6.21 ± 0.08 and 8.05 ± 0.1, respectively. This study showed that acetylcholine induced endothelium-dependent relaxation of canine uterine artery by stimulation of muscarinic receptors localized on the endothelial cells. On the basis of differential antagonist affinity, we suggest that the muscarinic receptors involved in the acetylcholine-induced relaxation of canine uterine artery are predominantly of M₃ subtype.     

Endothelium-dependent relaxation of canine uterine artery in response to acetylcholine: the possible involvement of alternative pathways

The effect of acetylcholine on the isolated, pre-contracted, uterine artery of non-pregnant dog was investigated. Acetylcholine-induced concentration-dependent relaxation of isolated canine uterine artery with endothelium (pEC50 = 6.48 +/-0.01, n = 37) and was without effect on arterial segments denuded of endothelium. Indomethacin, 4-aminopyridine (10-5 m) and pre-contraction with K+-rich Krebs-Ringer bicarbonate solution had no effect on acetylcholine-induced relaxation. NG-nitro-l-arginine (l-NOARG) (10-5 m) inhibited relaxation evoked by acetylcholine. Indomethacin applied with l-NOARG led to further inhibition of acetylcholine-induced relaxation. In the presence of both l-NOARG and indomethacin, 4-aminopiridine did not provoke further inhibition of acetylcholine-induced relaxation of canine uterine artery. It is concluded that the acetylcholine-induced relaxation of canine uterine artery is probably mediated by endothelial production of nitric oxide (NO). However, if NO-synthase is inhibited, acetylcholine-induced vasorelaxation may be, in part, mediated through activation of cyclooxygenase pathway.     

Mechanisms of acetylcholine-induced vasorelaxation in high K+-stimulated rabbit renal arteries

To characterize the mechanisms of acetylcholine (ACh)-induced vasorelaxation in rabbit renal arteries precontracted with high K+ (100 mM), muscle tension and cytosolic free Ca2+ concentration ([Ca2+]i) were measured simultaneously in the fura-2-loaded arterial strips. In the artery with endothelium, high K+ increased both [Ca2+]i and muscle tension. Addition of ACh (10 microM) during high-K+ induced contraction significantly relaxed the muscle and induced additional increase in [Ca2+]i. In the presence of NG-nitro-L-arginine (L-NAME, 0.1 mM). ACh increased [Ca2+]i without relaxing the muscle. In the artery without endothelium, high K+ increased both [Ca2+]i and muscle tension although ACh was ineffective, suggesting that ACh acts selectively on endothelium to increase [Ca2+]i. 4-DAMP (10 nM) or atropine (0.1 microM) abolished the ACh-induced increase in [Ca2+]i and relaxation. However, pirenzepine (0.1 microM), AF-DX 116 (1 microM) and tropicamide (1 microM) were ineffective. The ACh-induced increase of [Ca2+li and vasorelaxation was significantly reduced by 3 microM gadolinium, 10 microM lanthanum or 10 microM SKF 96365. These results suggest that, in rabbit renal artery, ACh-evoked relaxation of 100 mM K+-induced contractions is mediated by the release of endothelial NO. ACh may stimulates the M3 subtype of muscarinic receptor in the endothelial cells, resulting in the opening of the nonselective cation channels followed by an increase of [Ca2+]i and stimulation of NO synthase.     

Endothelin receptor mediating contraction of isolated bovine coronary artery

We studied endothelin (ET) receptors and their subtypes on isolated bovine coronary arteries. Endothelin receptors that mediated contraction of isolated bovine coronary artery were characterized by the use of antagonists and agonists. Contractions induced by the nonselective agonist ET-1 (10-10-10-7 M) were not affected by the removal of the endothelium (pEC50: 8.52, maximal contraction: 105% of that induced by 60 mM KCl). BQ-123 (3 x 10-7 M) antagonized contractions of endothelium-denuded coronary rings induced by low concentrations of ET-1 (10-10 or 10-9 M), but potentiated the contractions induced by higher concentrations of ET-1 (3 x 10-8 and 10-7 M). BQ-788 (10-6 M) potentiated contractions induced by ET-1 (3 x 10-10 and 10-7 M). In the presence of BQ-788 (10-6 M), BQ-123 (3 x 10-8-3 x 10-6 M) concentration - dependently inhibited contractions induced by ET-1 (3 x 10-10 and 10-7 M) (pA2: 6.61). Sarafotoxin S6b (10-9-3 x 10-7 M) evoked contractions in the denuded coronary artery (pEC50: 8.49, maximal contraction: 139% of 60 mM KCl). The BQ-123 caused a concentration-dependent rightward shift of contractions induced by sarafotoxin S6b (pA2: 7.89). The present study indicates that ET-1 and sarafotoxin S6b contract the isolated bovine coronary artery by stimulating ETA receptors on smooth muscle cells, and that ETB receptors might suppress the ET-1-induced contractions.     

In vitro effects of bethanechol on smooth muscle preparations from abomasal fundus, corpus, and antrum of dairy cows

Abomasal displacement has been associated with gastric hypomotility. The supply of prokinetic drugs available to address this problem is insufficient. The goal of the study was to investigate the effect of the muscarinic agonist bethanechol (BeCh) on contractility parameters of smooth muscle preparations from several regions of the bovine abomasum (fundus, corpus, and antrum). Cumulative concentration-response curves were constructed using BeCh in vitro with and without pre-incubation with antagonists targeted at M(2) and M(3) muscarinic acetylcholine receptor (mAChR) subtypes. In all preparations investigated, BeCh induced a significant and concentration-dependent increase in all contractility parameters investigated. The maximal attainable effect (V(max)) was more pronounced in circular specimens, and V(max) of antral specimens in circular orientation were significantly lower when compared to the other preparations. Both antagonists caused a rightward shift of the concentration-response curve, suggesting that the effect of BeCh is mediated at least partly by M(2) and M(3) AChRs.     

Characterization of bradykinin-induced endothelium-independent contraction in equine basilar artery

We investigated the effect of bradykinin (BK) on isolated equine basilar arterial rings with and without endothelium. BK induced concentration-dependent contraction of resting arterial rings and no relaxation when the rings were precontracted by prostaglandin F_2α. The maximal response and pD₂ value were 161.2 ± 28.1% (to 60 m m KCl-induced contraction) and 8.24 ± 0.25 respectively. The cumulative concentration–response curve for BK was not shifted to the right by des-Arg⁹-[Leu⁸]-BK (a B₁-receptor antagonist), HOE140 (a B₂-receptor antagonist) or NPC567 (another B₂-receptor antagonist). In four of six basilar arteries, NPC567 induced concentration-dependent contraction. Indomethacin (a cyclooxygenase inhibitor), nordihydroguaiaretic acid (a lipoxygenase inhibitor), quinacrine (a phospholipase A₂ inhibitor), tetrodotoxin (a selective blocker of Na⁺ channels), guanethidine (a nor-adrenergic neuron blocking drug), phentolamine (an α-adrenoceptor antagonist), Nω-nitro- l -arginine ( l -NNA, a nitric oxide (NO) synthase inhibitor) and endothelial denudation did not affect the BK-induced contraction. l -NNA and indomethacin induced contraction and relaxation under resting vascular tone respectively. These results suggest that endothelial cells are not involved in BK-induced contraction and that the contraction is not mediated via activation of known B₁ and B₂ receptors. Arachidonic acid metabolites and neurotransmitters like norepinephrine and NO might not play a role in BK-induced contraction in equine basilar artery.     

Comparative responses of bronchial rings to mediators of airway hyperreactivity in healthy horses and those affected with summer pasture-associated obstructive pulmonary disease

OBJECTIVE: To compare responses of bronchial rings obtained from healthy horses and horses affected with summer pasture-associated obstructive pulmonary disease (SPAOPD) to selected mediators of airway hyperreactivity in vitro. SAMPLE POPULATION: Bronchial rings from 6 healthy horses and 6 horses affected with SPAOPD. PROCEDURE: Bronchial rings obtained from each group of horses were mounted in organ baths and attached to force transducers interfaced with a polygraph. After applying 2g of tension, each ring was allowed to equilibrate for 45 minutes in Tyrode's solution at 37 C. Cumulative concentration-response relationships to graded concentrations of selected mediators (10(-8) to 10(-4) M) were determined and analyzed for significance at each concentration. RESULTS: Acetylcholine, histamine, 5-hydroxytryptamine, and leukotriene D4 induced concentration-dependent contractile responses in bronchial rings. Prostaglandin F2alpha induced weak and inconsistent contractile responses. The other 2 agents, norepinephrine and substance P, did not induce concentration-dependent responses. Considering the overall group-drug effect, acetylcholine, histamine, 5-hydroxytryptamine, and leukotriene D4 were effective in inducing consistent concentration-dependent contractile responses in both groups. Only 5-hydroxytryptamine and histamine induced significant responses in contractility between groups. The response of bronchial rings from horses with SPAOPD to 5-hydroxytryptamine was significantly greater than those from control horses, whereas the response to histamine was significantly lower. Significant responses were evident at concentrations ranging from 10(-6) to 10(-4) M for both drugs. CONCLUSIONS AND CLINICAL RELEVANCE: Because the airways of horses with SPAOPD had increased responsiveness to 5-hydroxytryptamine in vitro, treatment modalities using 5-hydroxytryptamine antagonists should be investigated to address this phenomenon.     

Participation of H1-receptors in histamine-induced contraction and relaxation of horse coronary artery in vitro.

The mechanisms of histamine-induced contraction and relaxation were investigated in rings isolated from a middle part of the left descending coronary arteries of horses. Intact and endothelium-denuded preparations were compared. Rings of horse coronary arteries contracted in response to histamine in a concentration dependent manner, but some of them relaxed with lower concentrations and contracted with higher concentrations. Removal of the endothelium abolished the relaxation and potentiated the contraction. The pD2 values were 4.70 +/- 0.08 in the rings with intact endothelium and 4.95 +/- 0.08 in endothelium-denuded rings. Histamine-induced contractions in intact and denuded preparations were not affected by an H2-antagonist, cimetidine, but were inhibited by an H1-antagonist, diphenhydramine in non-competitive manner in the rings with endothelium and in competitive manner in denuded rings. After precontraction with PGF2 alpha or norepinephrine, histamine relaxed preparations with intact endothelium (pD2 value, 7.80 +/- 0.11), although histamine-induced relaxations were not observed in denuded preparations. The relaxation was competitively inhibited by diphenhydramine. Relaxing response was significantly attenuated by methylene blue, quinacrine, L-nitro-arginine, gossypol and AA861 but not by indomethacin. These results suggest that the histamine-induced contraction and relaxation in horse coronary arteries are mediated mainly by H1-receptors in the smooth muscle and endothelium, respectively, and H1-receptor activation of endothelial cells may liberate vasodilator substances.     

Estrous cycle-dependent differences in responsiveness to prostaglandins and contractile agents in sheep (Ovis aries) cervical smooth muscle

We investigated the influence of the phase of the estrous cycle on mechanical responses elicited in sheep cervix by potassium chloride (KCl), acetylcholine chloride (ACh), prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E1 (PGE1). The cervix of adult ewes (n = 48) were classified according to the presence or absence of corpora lutea (luteal or follicular phase, respectively). Muscle strips of the circular and longitudinal layers were prepared in an organ bath and coupled to an isometric force transducer. Concentration-response curves were obtained noncumulatively. KCl and ACh produced concentration-dependent contractions in all preparations in both phases of the estrous cycle. However, maximum effect, EC50 and slope values of KCl and ACh were not significantly different between muscle layers, as well as between the phases of the estrous cycle. The prostanoid, PGF2 alpha, produced a significant reduction in the amplitude of spontaneous contractions for all preparations. The depressant effect of PGF2 alpha on spontaneous contractions of circular smooth muscle was significantly greater during the follicular than the luteal phase, whilst the depressant effect of PGF2 alpha on the longitudinal layer did not differ between phases of the estrous cycle. PGE1 significantly reduced the amplitude of spontaneous contractions on circular but not on longitudinal preparations. In conclusion, we have characterized with in vitro preparations of circular and longitudinal muscle layers of ewes during the follicular and luteal phases of the estrous cycle, the parameters of the K- and ACh-induced contractions on cervix and the efficacy of PGF2 alpha and PGE1 on inhibition spontaneous contractile activity.     

Role of endothelium and nitric oxide in modulating in vitro responses of colonic arterial and venous rings to vasodilatory neuropeptides in horses

The objective of this study was to determine and compare the in vitro responses of equine large colon arterial and venous rings to vasodilatory neuropeptides; calcitonin gene-related peptide (CGRP); substance P (SP); vasoactive intestinal polypeptide (VIP); and acetylcholine (ACh), a standard nonpeptide endothelium-dependent vasodilator. Responses of vessel rings to graded concentrations (10(-11) M to 10(-5) M) of each drug were determined in endothelium-intact, denuded, and Nomega-nitro-L-arginine methyl ester (L-NAME, 10(-5) M)-treated rings that were pre-contracted with norepinephrine. Percentage maximal relaxation (PMR), defined as the % decrease from the contracted state, was determined. Because all rings did not relax at least 50%, EC50 values could not be consistently calculated. Arterial rings with intact endothelium were more sensitive to CGRP, compared with VIP and SP, and venous rings of all conditions were more sensitive to VIP than CGRP or SP. Overall, arteries had a greater PMR for ACh compared with SP and VIP. Intact and L-NAME treated arteries had a greater PMR than denuded arteries; there were no differences in PMR of intact and L-NAME treated arteries. Veins had a greater PMR for VIP than CGRP, SP, or ACh. Calcitonin gene-related peptide caused greater relaxation in intact arteries, whereas VIP causes greater relaxation in veins. Arterial relaxation was dependent upon the presence of intact endothelium. The response of veins to VIP among the conditions tested was not different, suggesting VIP has direct actions on venous smooth muscle. These neuropeptides modulate vasomotor tone via vasorelaxation in colonic arteries and veins.     

Effects of muscarinic receptor antagonists on acetylcholine-induced contractions of jejunal smooth muscle in horses

This study investigated the effects of a muscarinic type 1 (M(1)), 2 (M(2)), and 3 (M(3)) antagonists (4-DAMP, pirenzepine, and methoctramine, respectively) on acetylcholine (Ach)-induced contractions of longitudinal jejunal muscle strips of horses. Strips were irrigated with Krebs-Henseleit solution gassed with 95% O(2) and 5% CO(2), and the developed tension in response to Ach was recorded before and after incubation with increasing concentrations of 4-DAMP (10(-8)-10(-6) M), pirenzepine (10(-6)-10(-4) M), and methoctramine (10(-5)-10(-3) M). When competitive antagonism was characterized, the affinity constant (pA(2)) was calculated by Schild plots. A parallel rightward shift in the concentration-response curves was observed after 4-DAMP and pirenzepine. Methoctramine presented a dual effect on the concentration-response curves: lower concentrations induced a parallel rightward shift without altering the maximum intensity of contraction (E(max)), while the highest concentration increased slope of the concentration-response curve and increased E(max). The pA(2) for 4-DAMP and pirenzepine was 9.18 and 7.13, respectively. Acetylcholine-induced contractions of longitudinal jejunal smooth muscle are mediated mainly via M(3) receptors. The complex role of M(2) receptors in jejunal smooth muscle contractions was evident because methoctramine potentiated the contractile response to higher doses of Ach.     

Leukocyte endothelial cell interactions in pig to human organ xenograft rejection

In cases where antibody- and complement-mediated hyperacute rejection (HAR) of vascularized organ xenografts has been prevented, acute vascular rejection (AVR) and acute T cell-mediated rejection (ACR) cause graft destruction. Infiltration of leukocytes (innate and graft-primed T cells) into the graft execute the latter two rejection modalities. The leukocyte extravasation process, which is a prerequisite for graft infiltration, is governed by adhesion molecules, including the selectin, integrin and immunoglobulin protein families, and the chemokine protein family. The compatibility between porcine endothelial cell and human leukocyte adhesion molecules was investigated in dynamic adhesion and static transendothelial migration assays. The effect of human anti-pig antibodies on human leukocyte adhesion to, and transendothelial migration across, porcine endothelium was assessed under dynamic and static conditions, respectively. In contrast to previously published results, no difference in the ability of neutrophils to adhere to pig and human endothelium was observed. Furthermore, no evident quantitative or qualitative differences in the capacity of human and porcine endothelium to support transendothelial migration of human leukocytes (T, B and natural killer (NK) cells, monocytes, and neutrophils) could be detected. The presence of human anti-pig antibodies (Abs) modulated the migration of leukocytes across porcine endothelium, as well as neutrophil adhesion to porcine endothelium under conditions of flow. Antibodies specific for pig endothelial adhesion molecules can potentially be used as species (graft)-specific immunosuppressive reagents in order to prevent cellular organ xenograft rejection.     

猪乳腺细胞分离培养及EGFP基因转化

本研究旨在从猪乳腺组织中分离得到上皮细胞和成纤维细胞,并将EGFP基因导入这些细胞.利用乳腺细胞堵养体系从成年猪乳腺组织中分离培养上皮细胞和成纤维细胞,并利用脂质体介导转染技术将EGFP基因导入这些细胞.结果,从成年猪乳腺组织中成功分离培养出上皮细胞和成纤维细胞,获得转EGFP基因上皮细胞和成纤维细胞.上皮细胞呈短梭形或多角形,细胞之间紧密相靠,互相衔接,连接成片;细胞核呈圆形或椭圆形,核仁2～4枚,比较明显.成纤维细胞呈长梭形.结果表明,可以从猪乳腺组织中分离上皮细胞和成纤维细胞,EGFP可以在这些细胞中表达.     

In vitro pharmacologic effect of two endothelin-1 antagonists on equine colonic arteries and veins

OBJECTIVE: To evaluate the effectiveness of 2 potential endothelin (ET)-1 antagonists in blocking the contractile responses of equine colonic vessels to increasing concentrations of ET-1. SAMPLE POPULATION: Mesenteric vessels from 6 clinically healthy horses. PROCEDURE: Colonic vessels (arterial and venous rings) were placed in organ baths with oxygenated Tyrode solution at 37 C. Each was attached to a force transducer interfaced with a polygraph, and 2 g of tension was applied and equilibrated for 45 minutes. Then, B-1 (PD 142893) and B-2 (PD 145065) ET-1 antagonists were tested. One ring from each vessel type was used as a control for determining concentration-response relationships of ET-1 (10(-10) to 10(-6)M). Three rings of each vessel type were incubated with 3 concentrations of each antagonist (10(-7), 10(-6), and 10(-5) M) for 30 minutes before ET induced contractions were determined. The maximum contractile response and pA2 values were determined. RESULTS: Vessels contracted in a concentration-dependent manner to ET-1. Arteries responded slowly but reached greater contractions. Veins responded immediately with sustained contractions. Both antagonists inhibited contractions in a concentration-dependent manner with significant differences at 10(-6) and 10(-5)M for arteries and 10(-5) M for veins. Complete blockade of contractions was observed with B-2 (10(-5)M). The pA2 values for B-1 were 8.26 and 6.82 for arteries and veins, respectively, whereas they were 8.25 and 7.21 for B-2. CONCLUSION AND CLINICAL RELEVANCE: Both antagonists effectively blocked ET-1-induced contractions of equine colonic vessels. Because B-2 is water soluble and caused complete blockade at 10(-5) M, it appears to be the preferred antagonist.     

Responses of pulmonary vasculature of cattle to dopamine and apomorphine

Both dopamine and apomorphine caused concentration-dependent contractions of the bovine pulmonary artery from rest. Both of these compounds caused relaxation of histamine-precontracted arterial and venous strips after α-adrenoceptor blockade. Arterial contraction elicited by dopamine was inhibited either by phentolamine (α-blocker) or by the dopamine-selective antagonists, spiperone and butaclamol. Apomorphine in the highest concentration (<10^-5 M) inhibited dopamine-induced contractions. Dopamine- and apomorphine-induced vascular relaxations were attenuated by propranolol but not by spiperone or butaclamol. These data suggest that dopamine- and apomorphine-induced relaxation in these preparations is most likely mediated through β-adrenergic mechanisms, whereas dopamine-induced contractions seem to involve both α-adrenergic and dopa-minergic receptors.     

Palmar digital vessel relaxation in healthy horses and in horses given carbohydrate

OBJECTIVE: To compare in vitro smooth muscle relaxation of palmar digital vessels from healthy horses with those from horses in the prodromal stage of experimentally (carbohydrate) induced laminitis. ANIMALS: 16 adult horses. PROCEDURE: Segments of palmar digital vessels were obtained from 5 healthy horses and 6 horses given carbohydrate. Vascular rings from the palmar digital artery and vein were suspended in individual organ baths containing buffer solution and indomethacin; isometric tension was recorded, and contraction and relaxation were compared. Smooth muscle contraction in response to cumulative addition of phenylephrine was recorded in the absence and presence of 1 microM NG-nitro-L-arginine methyl ester (L -NAME). After wash out, vascular rings were preconstricted with phenylephrine (0.3 microM), and cumulative endothelium-dependent (acetylcholine-induced) and independent (nitroprusside-induced) smooth muscle relaxations were recorded in the absence or presence of L -NAME. RESULTS: Phenylephrine increased vascular smooth muscle tone in ring preparations of palmar digital arteries and veins. Addition of acetylcholine or nitroprusside induced relaxation of palmar digital artery and vein ring preparations. Use of L-NAME (1 microM) significantly reduced maximal relaxation induced by acetylcholine, but not by nitroprusside. Maximal relaxation induced by acetylcholine, but not by nitroprusside, was reduced in vascular rings prepared from carbohydrate-overloaded horses. CONCLUSION AND CLINICAL RELEVANCE: Reduced endothelium-dependent relaxation of palmar digital vessels may have a role in the pathophysiology of acute laminitis after carbohydrate overload in horses.     

Anticholinergic effects of artemisinin, an antimalarial drug, in isolated guinea pig heart preparations

Concern has been growing about the cardiac toxicity of antimalarial drugs. Artemisinin, a unique type of antimalarial drug originating from a Chinese medicinal plant, has minimal adverse effects, but it has been reported to inhibit delayed rectifier potassium current, a voltage-gated potassium current. However, no studies have been published concerning the effect of artemisinin on ligand-gated potassium currents. Therefore, in the present study, we examined the influence of artemisinin on the acetylcholine receptor-operated potassium current (IK.ACh), a ligand-gated potassium current, in guinea pig atrial myocytes using a patch clamp technique. Artemisinin (1 to 300 microM) inhibited I(K.ACh) induced by extracellular application of both carbachol (1 microM) and adenosine (10 microM) and that induced by intracellular loading of GTPgammaS (100 microM) in a concentration-dependent manner. Artemisinin inhibited carbachol-induced, adenosine-induced, and GTPgammaS-activated IK.ACh within almost the same concentration range. In left atria, artemisinin (1 to 100 microM) partially reversed the shortening of action potential duration induced by carbachol in a concentration-dependent manner. Carbachol-induced negative inotropic action in left atria was also inhibited by artemisinin (10 to 300 microM). In conclusion, we suggest that the anticholinergic action of artemisinin is mediated through inhibition of IK.ACh via inhibition of the muscarinic potassium channel and/or associated GTP-binding proteins.     

Tremorgenic mycotoxins increase gastric smooth muscle activity of sheep reticulum and rumen in vitro

Reticulum and rumen strips (consisting of both muscle layers and the myenteric plexus) were superfused with Tyrode Ringer and their contractions recorded isometrically. The strips were subjected to exogenous acetylcholine and electrical field stimulation (EFS) resulting in contractions that could be blocked by atropine. Responses to the tremorgenic mycotoxin penitrem A and others thought to be involved in ryegrass staggers, paxilline and lolitrem B (10(-10)-10(-6)M), were compared with those of control vehicle (0.1% DMSO). The tremorgens were without effect on quiescent preparations. Penitrem A and paxilline enhanced spontaneously active preparations and the amplitude of contractions in response to EFS. Responses to paxilline had a shorter latency than to penitrem A. Responses of spontaneously active preparations were resistant to atropine. Penitrem A, but not paxilline, increased responses to exogenous acetylcholine. Lolitrem B (10(-6)M) increased responses to EFS, but many responses were equivocal, possibly due to the lower solubility of lolitrem B in aqueous solutions compared to the other tremorgens. The results show that these mycotoxins have peripheral excitatory effects on the reticulorumen and it is suggested that such activity in vivo may reflexly affect centrally derived cyclical contractions.