Actions of various muscarinic agonists on membrane potential, potassium efflux, and contraction of longitudinal muscle of guinea-pig intestine

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

K+ channel openers relax longitudinal muscle of guinea pig ileum

The plexus-free longitudinal muscle was used to investigate the muscle-relaxing effects of the known K+ channel openers, cromakalim, pinacidil and nicorandil, and compared with other known muscle relaxants, calcitonin gene-related peptide (CGRP) and isoprenaline. The three K+ channel openers all decreased basal tension and inhibited the tonic tension evoked by 30 mM KCl, 0.5 microM histamine or 0.1 microM oxotremorine in a dose-dependent manner. The order of potency is cromakalim > pinacidil > nicorandil in KCl or oxotremorine-precontracted muscle strip and nicorandil > cromakalim > pinacidil in histamine-precontracted muscle strip. Inhibition by cromakalim was completely reversed by glibenclamide, a blocker of ATP-sensitive K+ channels, while inhibition by nicorandil or pinacidil was only partially antagonized. The tonic tension evoked by KCl, histamine or oxotremorine was relaxed by CGRP or isoprenaline. Inhibition by neither of these compounds was relieved by glibenclamide. These results suggest that while ATP-sensitive K+ channels may be present in the longitudinal muscle cells, they may not be involved in the actions of CGRP or isoprenaline on the longitudinal muscle.

Subtypes and excitation-contraction coupling mechanisms for neurokinin receptors in smooth muscle of the guinea-pig Taenia caeci

This study investigated the subtype and coupling mechanisms mediating the direct contractile response to tachykinins in the guinea-pig Taenia caeci preparation in vitro. Coupling of neurokinin receptors was compared throughout with coupling of muscarinic receptors. The smooth muscle neurokinin receptors seem to be predominantly of the NK-1 subtype. Thus, the relative activities of the common naturally-occurring tachykinins fell within one order of magnitude, and the selective NK-1 receptor agonist substance P methyl ester was high in activity (0.38 relative to substance P). Some contribution from NK-3 receptors is, however, possible in view of the appreciable activity of the selective NK-3 agonist succ-[Asp6, N-MePhe8]-SP(6-11) (senktide; activity 0.004 relative to substance P), and NK-2 or NK-3 receptors in view of the higher activity of the D-isomer of [Glp6, *Pro9]-SP(6-11) as compared to its NK-1 selective L-isomer (D/L-activity ratio 1.53). Contractile actions of tachykinins were compared with carbachol for reliance on membrane-potential dependent (electromechanical) and membrane-potential independent (pharmacomechanical) coupling mechanisms. Log concentration-response curves to carbachol and substance P in normal Krebs' medium were compared with curves obtained in a high-K+ solution where processes dependent on changes in membrane potential could play no part in excitation. In the high-K+ depolarizing solution, a concentration-related relationship was maintained, though with some diminution in the maximal additional tension generated: the maximum tension with carbachol was under both conditions greater than that with substance P. The relative effects of several tachykinins and carbachol in producing receptor-mediated changes in membrane permeability through presumed receptor-operated ion channel opening, was estimated in terms of the ability to increase 86Rb-efflux, as a marker for K+, in a high-K+ depolarizing solution. Carbachol (10 microM) consistently increased 86Rb-efflux. In contrast, no permeability increase could be detected with any tachykinin tested (substance P, eledoisin, substance P methyl ester, neurokinin A, neurokinin B, 1 or 10 microM). Tachykinins and carbachol were compared in terms of ability to increase phosphatidylinositol hydrolysis. Both substance P and carbachol showed a concentration-related increase in accumulation of total inositol phosphates; though the maximal response to carbachol was considerably greater than that to any tachykinin (substance P, eledoisin, substance P methyl ester, senktide, neurokinin A, neurokinin B), or combination of two tachykinins (substance P and eledoisin, senktide and substance P methyl ester).(ABSTRACT TRUNCATED AT 400 WORDS)

Tetramine: occurrence in marine organisms and pharmacology

The occurrence of tetramethylammonium ion (TMA) in marine organisms is reviewed. The pharmacological action of TMA is also discussed, with special emphasis on the sign and symptoms experienced by oral poisonings. It is concluded that the major manifestations of TMA poisonings may be attributed to interactions with the autonomous nervous system. Fatal intoxications are due to neuromuscular blockade.

Similarity of relaxations evoked by BRL 34915, pinacidil and field-stimulation in rat oesophageal tunica muscularis mucosae

1. In the rat oesophageal tunica muscularis mucosae (TMM) the potassium channel openers, BRL 34915 and pinacidil, raised the threshold for concentration-dependent K+ contractions, suppressed contractions evoked by field stimulation of the TMM in the presence of tetrodotoxin (TTX) and tetraethylammonium (TEA), and relaxed tonic contractions resulting from muscarinic cholinoceptor stimulation. 2. BRL 34915 and both (+)- and (--)-pinacidil increased 86Rb efflux from tracer-loaded tissues; nifedipine abolished this effect. 3. Relaxations produced by potassium channel openers were inhibited by a temperature drop from 37 degrees C to 26.5 degrees C, an increase in extracellular K+ concentration to 64 mM, and treatment with the calcium channel antagonist, nifedipine. The same treatments also blocked field stimulation-evoked TTX-insensitive relaxations. 4. It is concluded that field stimulation of rat oesophageal smooth muscle in the presence of cholinoceptor-induced tone results in an increase in K+ permeability that is directly or indirectly coupled to Ca2+ influx through potential-operated channels.

Comparison of the effluxes of 42K+ and 86Rb+ elicited by cromakalim (BRL 34915) in tonic and phasic vascular tissue

The cromakalim-induced effluxes of 42K+ and 86Rb+ were compared in rat aortic segments and in guinea-pig portal vein. In both vessels, low concentrations of cromakalim (0.1 microM) increased the permeability to 86Rb+ 3-4 times less than that to 42K+; at 10 microM the difference was about a factor of 1.3-2. In rat aorta, the threshold concentration of cromakalim for 42K+ efflux was greater than or equal to 0.03 microM; with 86Rb+ as the tracer ion it was 0.1 microM. At similar concentrations, cromakalim relaxed the tension of aortic segments precontracted with 23 mM KCl (IC50 = 0.06 +/- 0.01 microM). However, no concomitant increase in 42K+ or 86Rb+ efflux could be detected from this stimulated preparation at these concentrations. In guinea-pig portal vein, 42K+ efflux measurements were performed in the presence and absence of the dihydropyridine Ca2+ entry blocker PN 200-110 (isradipine) yielding comparable results. In the presence of PN 200-110, where spontaneous activity and the K+ efflux associated with it were abolished, the threshold concentration of cromakalim for 42K+ efflux was 0.02 microM as compared to 0.06 microM for 86Rb+ efflux. In the absence of PN 200-110, spontaneous activity of the portal vein was inhibited by 70% and 90% at these concentrations. In double isotope experiments, the K+ channel inhibitor tetraethylammonium did not discriminate between the effluxes of 42K+ and 86Rb+ stimulated by cromakalim. It is concluded that in the two vascular tissues examined, cromakalim increased the permeability to 42K+ more than to 86Rb+, the difference being more marked at low cromakalim concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Leiurus quinquestriatus venom inhibits BRL 34915-induced 86Rb+ efflux from the rat portal vein

The effect of the crude venom of the Israeli scorpion Leiurus quinquestriatus hebraeus on the 86Rb+ efflux stimulated by the K+ channel opener BRL 34915 in the rat portal vein was examined. Applied alone, the venom greatly increased the spontaneous mechanical activity of and the concomitant 86Rb+ efflux from the vessel. When the excitability of the vein was suppressed by the dihydropyridine calcium antagonist, PN 200-110, the 86Rb+ efflux stimulated by BRL 34915 could be shown to be inhibited by the venom. From the concentration dependence of this inhibition an IC50 value of 0.17 +/- 0.01 mg/ml was estimated. This venom is thus the most potent blocker of BRL 34915-evoked 86Rb+ efflux reported so far.

Effect of the K+ efflux stimulating vasodilator BRL 34915 on 86Rb+ efflux and spontaneous activity in guinea-pig portal vein

The effect of BRL 34915 on 86Rb+ efflux and myogenic activity was studied simultaneously in guinea-pig portal vein. 86Rb+ was used as a tracer ion for K+. BRL 34915 inhibited myogenic activity with an IC50 value of 12 +/- 2 nM by reducing primarily the frequency of spontaneous contractions. Washout of the substance was followed by hyperreactivity of the vessel. 86Rb+ efflux was slightly reduced by concentrations of BRL 34915 below 100 nM; above 300 nM efflux was increased in a concentration-dependent manner. Above 10 microM BRL 34915, a slow desensitization of the effect on flux was observed during the 10 min application period of the agonist. The Ca2+ entry blocker, isradipine (PN 200-110, 200-500 nM) did not modify BRL 34915-stimulated 86Rb+ efflux at any BRL 34915 concentration tested, indicating that the influx of extracellular Ca2+ through dihydropyridine-sensitive Ca2+ channels is not necessary for this effect. However, by abolishing spontaneous activity, it allowed the 86Rb+ efflux promoting effect of BRL 34915 to be observed at a concentration of 60 nM. The K+ channel blockers tetraethylammonium and 3,4 diaminopyridine inhibited the BRL 34915-induced 86Rb+ efflux with IC50 values of 13 and 3 mM, respectively. Cell permeable derivatives of cyclic AMP and cyclic GMP had no major effect on BRL 34915-induced 86Rb+ flux, indicating that cyclic nucleotide-induced phosphorylation does not play an important modulatory role here. In conclusion, there is an at least 5 fold difference between the concentrations of BRL 34915 necessary to inhibit myogenic activity and those needed to stimulate 86Rb+ efflux. This may be explained by a primary effect of BRL 34915 on the pacemaker cells of the portal vein. explained by a primary effect of BRL 34915 on the pacemaker cells of the portal efflux. This may be

Histamine-operated calcium channels in intestinal smooth muscle of the guinea-pig

The effects of Bay K 8644 and of nifedipine on histamine-induced mechanical and electrical responses were studied in the longitudinal smooth muscle of the ileum and in the taenia coli isolated from the guinea-pig. Nifedipine (10(-9)-10(-7) M) depressed the tonic and phasic components of histamine contraction. Phasic tension was less sensitive to nifedipine inhibition than was tonic tension (I50: 27 +/- 6 and 2.6 +/- 0.4 nM respectively). Bay K 8644 (10(-8)-10(-7) M) increased tension and rhythmic activity of intestinal smooth muscle and potentiated the histamine responses. The phasic tension evoked by histamine 10(-5) M and the phasic tension evoked by the KCl depolarizing solution showed the same sensitivity to nifedipine inhibition (I50: 28 +/- 5 nM) and were similarly potentiated by Bay K 8644. The tonic tension in response to the KCl-depolarizing solution was more sensitive to nifedipine inhibition than was the tonic tension in response to histamine and was not potentiated by Bay K 8644. These results indicate that different Ca entry pathways, dependent or not on modification of the membrane potential, are involved in the contractile response evoked by histamine in intestinal smooth muscle.

Unsuitability of the 86Rb+ uptake method for estimation of (Na+ + K+)-ATPase activity in innervated tissues

(Na+ + K+)-ATPase activity was estimated by 86Rb+ uptake in dog saphenous vein to determine the validity of the technique in tissues that have a sympathetic innervation. When saphenous vein rings were incubated at 37 degrees C in Krebs' solution containing 86Rb+, the cardenolide acetylstrophanthidin caused a concentration-dependent inhibition of Rb+ uptake. The threshold for inhibition was approx. 10 nM acetylstrophanthidin and the maximum effect was obtained at 9 microM. In the upper part of this concentration range (greater than 1 microM) acetylstrophanthidin released noradrenaline from the sympathetic nerve terminals associated with the tissue. In this upper part of the acetylstrophanthidin concentration range the alpha-adrenoceptor antagonist phentolamine (8 microM) reduced, by up to 25%, the degree of 86Rb+ uptake inhibition caused by the cardenolide. In other experiments, saphenous vein strips were loaded with 86Rb+ and perifused with Krebs' solution containing acetylstrophanthidin. At concentrations which release noradrenaline, acetylstrophanthidin increased the efflux of 86Rb+. Phentolamine (8 microM) prevented the acetylstrophanthidin-evoked efflux of the isotope as did prior in vitro denervation of 86Rb+ loaded strips with 6-hydroxydopamine. Exogenous noradrenaline (1-100 microM) added to the perifusing fluid also caused an efflux of 86Rb+ that was attenuated by phentolamine. The data indicate for dog saphenous vein that with low concentrations of acetylstrophanthidin the extent of 86Rb+ accumulation might accurately reflect prevailing (Na+ + K+)-ATPase activity. At higher concentrations of acetylstrophanthidin, however, noradrenaline is released from the nerve endings and causes 86Rb+ efflux from the smooth muscle cells consequent upon alpha-adrenoceptor activation. Since this efflux reduces the extent of Rb+ accumulation, measurement of the latter does not adequately reflect uptake mediated by the activity of (Na+ + K+)-ATPase. This is significant because in most applications of the 86Rb+ uptake method it is the estimate of Rb+ accumulation made in the presence of a high concentration of cardenolide that forms the basis of all subsequent calculations with respect to (Na+ + K+)-ATPase activity.

A study of the contractile response to acetylcholine in human ileal and detrusor muscle: origin of the low efficacy of acetylcholine

In Krebs solution (3.36 mM Ca2+), the maximal contractile response of human ileal and urinary bladder detrusor muscle to acetylcholine (ACh) was 40-60% that to carbachol (CCh). The maximum response to ACh was reached at a bath concentration of about 1 microM and was maintained throughout a range extending to 100 microM. In the presence of neostigmine (0.1 microM), the maximum response to ACh reached the level of that of CCh. However, bioassay of bath concentrations of ACh at various points of the maximal response in the absence of neostigmine revealed only slight to insignificant diminution of the applied concentration of ACh. Joint application of ACh and CCh generated a dose-response profile consistent with a model of competitive antagonism between a full agonist (CCh) and a partial one (ACh). Also, choline (100 microM) reduced the maximum response to ACh in the presence of neostigmine and that to CCh to 60-80% of control. These observations are consistent with a mechanism whereby intact cholinesterase together with its substrate ACh and possibly a breakdown product of ACh constitute a filter or diffusional barrier regulating the flow of agonist from the enzyme compartment to the receptor compartment.

Relationship between stimulated inositol lipid hydrolysis and contractility in guinea-pig visceral longitudinal smooth muscle

Carbamylcholine caused a marked, concentration-dependent stimulation of [3H]Ins P, [3H] InsP2 and to a lesser extent [3H]InsP3 production in guinea-pig longitudinal smooth muscle prelabelled with myo-[3H]inositol. Accumulation of these three inositol phosphates showed differential sensitivity to LiCl. Muscle contraction was apparent at lower concentrations of carbamylcholine. Both responses were mediated via muscarinic-type receptors. An association of inositol phosphate production and contractility was also observed in response to substance P, histamine and noradrenaline, the latter via an alpha-adrenergic mechanism. The Ca2+-channel agonist CGP 28392 failed to stimulate inositol phosphate production despite inducing a contractile response. Carbamylcholine -induced inositol phosphate production persisted in the presence of D600 or Mn2+ despite loss of contractile activity. However, both responses showed a similar, marked dependence on the presence of Ca2+ in the extracellular medium. Mn2+ could restore basal and stimulated inositol phosphate production in low Ca2+ solutions but could not substitute for Ca2+ in restoring contractility. The results suggest that stimulated inositol lipid hydrolysis in longitudinal smooth muscle does not result from Ca2+ entry into the tissue, although the response does depend on the concentration of divalent cations in the extracellular medium. This dependency may be related to the maintenance of membrane potential and possibly phospholipid conformation.

The effect of gallamine, gallopamil and nifedipine on responses to acetylcholine and carbachol in the taenia of the guinea-pig caecum

The effects of gallamine, gallopamil and nifedipine on isotonic contractions of the isolated taenia of the guinea-pig caecum produced by acetylcholine (ACh) or carbachol (CCh) were investigated. Gallamine (0.1 to 0.3 mM) inhibited contractions produced by CCh more than those produced by ACh. The difference was still present after pretreatment of the tissue with paraoxon (10 microM for 20 min) to inhibit cholinesterases or in experiments carried out in the presence of tetrodotoxin (0.3 microM) to exclude possible ganglionic stimulation by the agonists. Gallopamil or nifedipine selectively inhibited the tonic response to ACh in the absence or presence of paraoxon. The phasic response to ACh or the tonic response to CCh (0.1 or 1 microM) was much less affected. Reduction of the Ca2+ content of the bath medium reduced phasic and tonic responses to ACh more than the tonic response to CCh. These results suggest that there are differences in the interaction of ACh and CCh with muscarinic receptors in this muscle.

Comparison of the excitatory actions of substance P, carbachol, histamine and prostaglandin F2 alpha on the smooth muscle of the taenia of the guinea-pig caecum

A comparison was made of the actions of substance P, prostaglandin F2 alpha (PGF2 alpha), histamine and carbachol on the membrane potential and conductance of the longitudinal smooth muscle of the taenia of the guinea-pig caecum using the double sucrose gap apparatus. The increases in conductance produced by the four drugs during matched depolarizations in the sucrose gap were not significantly different and they were substantially larger than the increase in conductance brought about during the same depolarization produced by passing outward current. In sucrose-substituted, Na- and Cl-deficient solution the increases in conductance and depolarization due to carbachol, substance P, and PGF2 alpha were attenuated to a similar extent. The depolarization due to histamine under these conditions was reduced to a significantly greater extent than that due to carbachol. In Tris-benzene-sulphonate substituted Na- and Cl-deficient solution the responses due to carbachol and histamine were attenuated to a similar extent. This suggests that sucrose addition may have a specific effect on the histamine response. In Tris-substituted Na-deficient solution the increases in conductance and depolarization produced by substance P, histamine and carbachol were attenuated to a similar extent. The depolarization due to PGF2 alpha was reduced by a significantly greater amount which may be due to unmasking an inhibitory effect that was sometimes apparent in normal solution. In benzenesulphonate-substituted, Cl-deficient solution the increases in conductance and depolarizations produced by moderate concentrations of PGF2 alpha, histamine and carbachol were attenuated to a similar extent. The response to substance P was little affected. In glucuronate-substituted, Cl-deficient solution the increases in conductance and depolarizations due to substance P and carbachol were attenuated to a similar extent. This result, and the observation that the depolarization to large concentrations of carbachol was not reduced in benzenesulphonate-substituted, Cl-deficient solution, suggest that benzenesulphonate interferes with the reactions of the non-peptide stimulants with their respective receptors. The similarities in the effects of activating the four types of receptor under some conditions could be explained if they all acted on the same population of receptor-operated channels. In addition it seems that PGF2 alpha acts also on a population of inhibitory receptors, sucrose interferes with histamine's action, and benzenesulphonate interferes with the reactions of non-peptide stimulants with their respective receptors.

Differential calcium dependence of contractile responses and 86Rb efflux from the rabbit aorta induced by vasoactive stimuli

86Rb was used to monitor potassium movements in strips of rabbit aorta simultaneously with measurements of tension. Histamine, noradrenaline, the prostaglandin endoperoxide analogue U46619, angiotensin II, and 144 mM K+ each induced an increase in 86Rb efflux concomitantly with contraction. For the first four agonists there was a rank-order correlation between the contractile response and 86Rb efflux, but 144 mM K+ induced a massive increase in 86Rb efflux although it was the weakest contractile stimulus. Contraction and increase in 86Rb efflux-induced K+ were both reduced by verapamil, which blocks voltage-sensitive calcium channels, implying that both effects of K+ were mediated mainly by a depolarisation-induced influx of calcium. Noradrenaline increased both tension and 86Rb efflux through an action on alpha-adrenoceptors, but its effect on efflux, unlike its effect on tension, was apparently totally dependent on the presence of extracellular calcium. Experiments performed in the presence of lanthanum, which blocks calcium influx, showed that the intracellular store of calcium released by noradrenaline apparently played no role in inducing 86Rb efflux, although it could trigger contraction. Lanthanum also blocked contraction induced by K+ but had less effect on the increase in 86Rb efflux induced by K+. Thus, agonist-induced vascular contraction and 86Rb efflux can be dissociated, but under normal conditions all the contractile stimuli tested induced 86Rb efflux.

Mechanism of action of vasoactive intestinal polypeptide on myometrial smooth muscle of rabbit and guinea-pig

1. The action of vasoactive intestinal peptide (VIP) on the electrical and mechanical activity of strips of longitudinal myometrial smooth muscle from rabbits and guinea-pigs treated with oestradiol was studied in the sucrose-gap apparatus. 2. In myometrial strips which spontaneously exhibited regular contractions, or which were induced to contract rhythmically to the application of oxytocin, VIP reduced both the frequency and the force of contraction. 3. Contractions were associated with bursts of action potential discharge. In guinea-pig, the membrane potential reached its most negative value shortly following a burst and a slow decay of negativity followed ("generator potential'). VIP inhibited the decay of this negativity and increased the duration of the period between bursts. In rabbit myometrical strips, electrical discharges occurred less regularly but VIP also had an inhibitory action. The inhibitory action of VIP was not affected by the beta-adrenoreceptor blocker propranolol, by tetrodotoxin, or by apamin. 4. Using the double sucrose-gap apparatus, bursts of action potentials and contractions were elicited with depolarizing electrical pulses in the absence of oxytocin. Changes in membrane resistance were also estimated by eliciting hyperpolarizing electrotronic potentials. VIP hyperpolarized the membrane and inhibited contractions as depolarizing pulses now failed to reach threshold for action potential discharge or fewer action potentials were discharged. A small (about 10%) reduction in membrane resistance was freqeuently observed during the hyperpolarization. 5. If a single action potential was elicited in the presence of VIP, the tension generated by the muscle was less than in its absence. 6. In a calcium-free high-potassium (126 mM) solution, readmitting calcium produced contraction; VIP inhibited this contraction. Activation of beta-receptors by means of isoprenaline had a similar effect but unlike isoprenaline the action of VIP was not blocked by propranolol. 7. It is suggested that the primary action of VIP is on the calcium economy of the myometrial smooth muscle cell, possibly to accelerate sequestration and/or extrusion of calcium from the cell. In some way this is associated with inhibition of the generator potential, hyperpolarization, and with a small increase in permeability of the membrane to potassium.

Effects of histamine, high potassium and carbachol on 42K efflux from longitudinal muscle of guinea-pig intestine

1. Shortening and rate of loss of (42)K were studied in strips of longitudinal muscle taken from guinea-pig ileum.2. Carbachol, histamine and raising the external potassium concentration, [K(+)](o), to 120 mm in the presence of atropine caused equal maximal shortenings of the muscle, but unequal maximal increases in (42)K efflux: maximal (42)K effluxes obtainable in response to raised [K(+)](o) and histamine were about (2/3) and (1/3) respectively of the maximal efflux in response to carbachol. In the absence of atropine the increase in (42)K efflux produced by 120 mm-[K(+)](o) was about 50% larger, probably because of the release of acetylcholine from nerve endings in the tissue.3. If inhibitors of histamine metabolism were applied, or a H(2)-receptor blocker (cimetidine), the maximum (42)K efflux produced by histamine was not increased. An analogue of histamine reputed to resist metabolic degradation did not produce a larger increase in (42)K efflux than histamine. The smaller maximal effect of histamine on (42)K efflux than carbachol may be because it can open fewer ion channels in the smooth muscle membrane.4. The ratio of the concentrations producing 50% maximal shortening and 50% maximal (42)K efflux was about 1:1.3 for raised [K(+)](o) but about 1:20 for histamine and carbachol. Depolarization by raising [K(+)](o) appears to be less effective in causing tension development than similar depolarizations produced by carbachol or histamine.5. The relative effects of carbachol, histamine and raised [K(+)](o) were discussed in the light of their similar depolarizing actions. Increases in (42)K efflux did not appear to be caused primarily either by contraction or by depolarization of the muscle. Access of the stimulant to cells and receptors other than those which are superficially situated was suggested as being an important factor in deciding the smaller increase in (42)K efflux seen with some stimulants. Histamine receptors may be fewer in number than muscarinic receptors and less able in their activated form to open channels through which potassium ions can escape.

Evidence that histamine and carbachol may open the same ion channels in longitudinal smooth muscle of guinea-pig ileum

1. Membrane potential was recorded intracellularly by micro-electrode in separated longitudinal muscle of guinea-pig ileum. Electrotonic potentials were evoked in longitudinal strips by passing current between large external electrodes in the partition chamber.2. Histamine increased the frequency of action potential discharge at low concentrations and depolarized the membrane. At higher concentrations it caused substantial depolarization and action potential discharge was abolished. Carbachol had similar actions but the maximal depolarization by carbachol (using 10(-4)m) was some 4-5 mV greater than maximal depolarization by histamine (using 10(-4)m).3. The change in size of evoked electrotonic potentials was used to estimate the effects of carbachol and histamine on the conductance of the smooth muscle membrane. The equilibrium potentials for histamine and carbachol depolarizations were estimated from their relative effects on potential and conductance and were found to be not significantly different; measurements of the effects on conductance showed that 10(-4)m-histamine increased conductance about 8-fold whilst 10(-4)m-carbachol had a much greater effect on conductance. This difference could explain the differing maximal depolarizing effects of these agents if both were assumed to open channels having the same ionic selectivity (i.e. equilibrium potential).4. The efflux of (42)K was studied in separated strips of longitudinal ileal muscle from guinea-pig. In the presence of a concentration of carbachol (2 x 10(-5)m or 10(-4)m) having a maximal effect on (42)K efflux rate, histamine (10(-4)m) did not increase efflux further although 120 mm-potassium did so. Experiments with the irreversible muscarinic receptor blocker, propylbenzilylcholine mustard, indicated that the number of muscarinic receptors did not limit the (42)K efflux response to carbachol and it was suggested that the response was limited by the availability of ion channels which could be opened by activated muscarinic receptors.5. Contractions to histamine and carbachol in 120 mm-potassium depolarizing solution were followed upon washing by a relaxation below basal tension. Carbachol, but not histamine, showed a pronounced and long lasting secondary contraction following this relaxation.6. These results are consistent with the idea that activated histamine and activated muscarinic receptors open the same ion channels in the smooth muscle membrane to produce depolarization, increased action potential discharge and contraction, although muscarinic receptors can open more of these. However, there was evidence that the opening of these channels is not the only pathway between receptor activation and contraction.