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

Effects of Particle Size and Physical Form of Diets on Mast Cell Numbers, Histamine, and Stem Cell Factor Concentration in the Small Intestine of Broiler Chickens

The aim of this study was to investigate the hypothesis that particle size and diet form may affect the growth of mast cells and histamine release from the small intestine of broiler chickens. A total of 288, day-old male broiler chicks were randomly allocated to 1 of 4 corn-soy diets in a 2 x 2 factorial design. The factors included particle size (coarse vs. fine) and physical form (mash vs. pellet). The birds were housed in 90 x 60 cm pens containing 12 birds, and each treatment contained 6 replicate pens of birds from d 1 to 22. On d 22, 6 broilers from each treatment were slaughtered. Tissues from the small intestine (duodenum, jejunum, and ileum) were obtained to quantify mast cells using the toluidine blue staining technique. The results showed that mast cells in the jejunum were concentrated in the upper part of the villus in birds fed the coarsely ground mash diet, whereas mast cells were evenly distributed throughout the intestine in birds fed the other 3 diets. The number of mast cells was significantly lower in the duodenum (P = 0.04), jejunum (P < 0.01), and ileum (P = 0.01) of birds fed coarsely ground diets compared with finely ground diets, and there was no difference in mast cell numbers between birds fed mashed or pelleted diets at any site in the intestine. The histamine content (P = 0.02) and stem cell factor concentration (P = 0.03) were markedly lower in the jejunum of birds that were fed coarsely ground diets compared with finely ground diets. The stem cell factor concentration in the duodenum (P < 0.01) and jejunum (P = 0.05) was higher in birds fed pelleted compared with mash diets. The overall results of this experiment suggest that particle size and diet form affect mast cell number and histamine content in the small intestine by regulation of stem cell factor concentration.

Downregulation of Galphaq-11 protein expression in guinea pig antral and colonic circular muscle during pregnancy

Pregnancy has an inhibitory effect on motility of the gastrointestinal tract. The present study was designed to examine the mechanisms responsible for antral and colonic hypomotility in pregnant guinea pigs. Circular smooth muscle cells from the antrum and left colon were isolated by enzymatic digestion with collagenase from pregnant and nonpregnant guinea pigs. Contractile responses to agonists were expressed as percent shortening from resting cell length. The function of G proteins in antral and colonic circular smooth muscle was assessed by [35S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding induced by CCK-8 and G protein quantitation. The contraction of antral and colonic circular smooth muscle from pregnant guinea pigs was reduced in response to CCK-8 and to GTPgammaS but was normal in response to KCl and D-myo-inositol 1,4,5-trisphosphate compared with nonpregnant animals. The stimulation of [35S]GTPgammaS binding to Galphaq-11 induced by 1 microM CCK-8 was significantly lower in antral and colonic circular smooth muscle from pregnant guinea pigs than that in controls. Furthermore, Western blot analysis showed a decreased Galphaq-11 and an increased Gsalpha protein content in both tissues during pregnancy. It is concluded that pregnancy appears to impair gastrointestinal circular smooth muscle contractility by downregulating G proteins such as Galphaq-11 protein, which mediates muscle contraction, and upregulating Gsalpha protein, which mediates muscle relaxation.

The effect of L-type calcium channel modulators on the mobilization of intracellular calcium stores in guinea-pig intestinal smooth muscle

1. The action of Ca2+ channel modulators has been examined on the intracellular Ca2+ signal in the longitudinal smooth muscle cells of the guinea-pig intestine after exposure to histamine and to agents known to affect intracellular Ca2+ stores. Isometric contraction has been measured simultaneously with front-surface fluorometry of fura 2-loaded preparations. 2. Histamine (10 microM) evoked a phasic and tonic increase in [Ca2+]i and contraction which were both sensitive to the Ca2+ channel blockers, nimodipine and D600. 3. Caffeine (10 mM) evoked in rapid increase in [Ca2+]i which was sustained as long as the preparation was exposed to the drug, whereas the contractile response was only phasic. In the presence of nimodipine 1 microM, the phasic contraction was absent although the fura 2-Ca2+ signal amounted to 32% of the control. 4. Ryanodine (10 microM) evoked a slow increase in [Ca2+]i and a contraction, both of which were reversed after exposure to nimodipine (1 microM) or D600 (10 microM). In the presence of diazoxide (500 microM), a hyperpolarizing agent, the ryanodine-evoked increase in [Ca2+]i and in muscle tone were inhibited. 5. Thapsigargin (1 microM) also produced an increase in [Ca2+]i and a contraction both of which were blocked by nimodipine (1 microM). 6. In Ca2+-free solution, histamine 10 microM evoked non-reproducible phasic Ca2+ signal and contraction. This response was recovered after refilling in Ca2+ containing solution. The recovery was blocked by nimodipine, D600 or diazoxide and was facilitated by the Ca2+ channel activator, Bay K 8644. When the refilling medium was supplemented with thapsigargin, the recovered response was significantly reduced, but Bay K 8644 still had some action. 7. The present results show that blockage of L-type Ca2+ channels inhibited changes in [Ca2+]i evoked by histamine, caffeine and ryanodine which are generally attributed to Ca2+ mobilization from intracellular stores. They also show that when the tissue was exposed to nimodipine, D600 and diazoxide during the procedure of refilling after depletion of intracellular stores, the action of histamine on [Ca2+]i and contraction was blocked. Bay K 8644 had an opposite effect even when the Ca2+ pumping activity of the sarcoplasmic reticulum was reduced by thapsigargin. This indicates that refilling of intracellular Ca2+ stores depleted by histamine in guinea-pig intestine mainly occurred through L-type Ca2+ channels.

Mechanism of carbachol-evoked contractions of guinea-pig ileal smooth muscle close to freezing point

1. The effect of lowering the temperature to near freezing-point upon the contractions and [3H]-inositol phosphate responses to carbachol were investigated in longitudinal smooth muscle from the guinea-pig ileum. 2. The peak amplitude of the contraction to a single application of 100 microM carbachol was the same at 37 degrees C and temperatures near freezing-point. However, the sensitivity to carbachol was reduced upon lowering the temperature and the time to peak contraction was increased from 5-10 s to 2-10 min. Even when the temperature was maintained near freezing-point, washing off carbachol produced a relaxation and eventual return of tension to basal levels. 3. Incubating the tissue in 140 mM K+, calcium-free solution or in calcium channel antagonists significantly reduced the carbachol-induced contraction to 10-30% of the control at 37 degrees C and also at 3 degrees C. Thus the majority of the activator calcium required for contraction entered the tissue via voltage-dependent calcium channels (VDCs) at both 37 degrees C and 3 degrees C. 4. The contractions produced by high potassium solutions were less at temperatures close to freezing-point than those at 37 degrees C suggesting that voltage-dependent calcium entry was inhibited as the temperature was lowered. 5. A small part of the contractile response to 100 microM carbachol was resistant to the removal of extracellular calcium at both 37 degrees C and 3 degrees C and this component was increased under depolarizing conditions. This suggests that the release of stored calcium contributes to a minor degree to contraction at both 37 degrees C and 3 degrees C.6. Although 100 microM carbachol produced a statistically significant rise in several [3H]-inositol phosphate isomers at both 37 degrees C and 3 degrees C, the production of [3H]-inositol phosphates was less at 3 degrees C than at 37 degrees C and the increase in their production caused by carbachol was much slower.7. These results suggest that the carbachol-induced contraction at 3 degrees C utilizes both calcium entry through VDCs and calcium release from intracellular stores, as at 37 degrees C. The components of the responses dependent upon intracellular calcium release at 37 degrees C and at temperatures near freezing-point were similar. However, the production of [3H]-inositol phosphates, including the calcium-mobilizing second messenger inositol (1,4,5) trisphosphate (Ins(1,4,5)P3), is reduced at such low temperatures.

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)

Inositol trisphosphate releases stored calcium to block voltage-dependent calcium channels in single smooth muscle cells

In single cells obtained by enzymic treatment of rabbit small-intestinal smooth muscle, and held under voltage clamp by patch pipette in the whole-cell recording mode, release of inositol trisphosphate (InsP3) from its caged precursor by flash photolysis caused complete inhibition of the voltage-dependent calcium current. No inhibition was seen in control experiments where the cage (2-nitrosoacetophenone) was released by flash photolysis from caged ATP. The inhibition by InsP3 of the calcium current was prevented if 10 mM EGTA or 2 mg/ml heparin was included in the pipette solution. Heparin is known to block InsP3 receptors. These results suggest that release of calcium stores by InsP3 raises Cai and that calcium ions inhibit the calcium current by acting either directly or otherwise on the internal mouth of the calcium channel.

Gallbladder contraction in patients with pigment and cholesterol stones

Thirty gallbladders were studied in vitro; 5 had black pigment stones and 25 contained manifestations of excess cholesterol in bile. Of the 25, 14 had cholesterol stones, 7 had macroscopic cholesterolosis, and 4 had cholesterol crystals. There were no differences in basal active tension among these groups, but the force of spontaneous phasic contractions was reduced in gallbladders with cholesterol stones, cholesterolosis, and cholesterol crystals compared with specimens with pigment stones (p less than 0.001). The forces developed in response to cholecystokinin-8 (10(-10)-10(-6) M), acetylcholine (10(-7)-10(-3) M), and potassium chloride (20-60 mM) were greater in strips from specimens with pigment stones than in strips from specimens with cholesterol stones or cholesterolosis (p less than 0.001). In cholesterol stones and cholesterolosis specimens, relatively strong muscle strips had similar responses to 10(-6) M cholecystokinin-8 in normal calcium (2.5 mM) and in the absence of extracellular calcium. Weaker muscle strips had a reduced response to cholecystokinin-8 in the absence of extracellular calcium (p less than 0.01). It is concluded that muscle strips exposed to bile with excess cholesterol have a reduced contractility compared with muscle strips from specimens with pigment stones; this impaired contractility precedes gallstone formation, and results from muscle dysfunction.

Effect of leukotriene C4 on electromechanical activity and Ca2+ uptake in taenia coli

The actions of leukotriene C4 (LTC4) on electromechanical activity and 45Ca2+ uptake in guinea pig taenia coli were investigated. The contractile action of LTC4 was abolished by the removal of extracellular Ca2+. LTC4 concentrations eliciting a maximal contraction in normal medium produced no response in preparations depolarized with KCl. In single sucrose gap studies, LTC4 increased both the frequency of electrical spiking and tension. These effects were blocked by the dihydropyridine Ca2+-channel antagonist PY 108-068 and by the leukotriene receptor antagonist FPL 55712. In double sucrose gap experiments, LTC4 caused a small depolarization without measurable change in membrane conductivity; increased spontaneous electrical activity was again accompanied by an increase in tension. LTC4 caused a detectable increase in 45Ca2+ uptake only at extracellular Ca2+ concentrations less than 1 mM, and this was again inhibited by PY 108-068 or FPL 55712. It is concluded that the contractile effects of LTC4 in guinea pig taenia coli occur as a consequence of its ability to open voltage-sensitive Ca2+ channels, an effect that may occur independently of membrane depolarization.

Dependence on calcium of potassium- and agonist-induced changes in potassium permeability of rabbit ear artery

The effect of K+ depolarization and agonists on the 86Rb+ efflux from rabbit ear artery has been investigated. K+ depolarization with 59 mM-K+ induces an increase of the 86Rb+ efflux rate, which is dependent on [Ca2+]o and is correlated with the concomitant force development. This effect is largely reduced by Ca2+ antagonists, such as D-600 and Mn2+. The residual increase of the 86Rb+ efflux rate is much smaller than that predicted by the constant-field equations. Stimulation with 10(-5) M-noradrenaline or 10(-4) M-histamine induces a biphasic increase of the efflux rate. The initial transient effect is reduced in low [Ca2+]o solutions, whereas the maintained component is largely independent of [Ca2+]o. Stimulation with noradrenaline during depolarization of the tissues with K+ induces, after a transient increase of the efflux rate, an inhibition of the K+-induced increase of the efflux rate. Both phases of the noradrenaline action are due to activation of alpha-adrenoreceptors. Exposure to Ca2+-free medium induces a progressive increase of the 86Rb+ efflux rate, which reaches a new steady-state value after about 60 min. Stimulation with noradrenaline after this 60 min exposure to Ca2+-free solution no longer induces a significant effect. Stimulation with noradrenaline after shorter exposures to Ca2+-free solution immediately increases the 86Rb+ efflux to a value close to the steady-state value obtained after prolonged exposure to Ca2+-free medium. Washing out the agonist has no effect on the rate constant. It will only return to its control value after exposure to solutions containing Ca2+. This recovery of the rate constant by external Ca2+ also occurs in the presence of 1 mM-Mn2+ in the perfusion fluid. On re-exposure of the tissues in the presence of 1 mM-Mn2+ to Ca2+-free solution the rate constant of the 86Rb+ efflux increases at once to the steady-state value observed in Ca2+-free solution. This increase proceeds gradually if the tissues have been re-exposed in the absence of Mn2+. It is concluded that K+ permeability might be regulated by [Ca2+]i and that this relationship can be affected by agonists. In order to explain the effects of Ca2+-free medium on the 86Rb+ efflux we have to assume that at very low values of [Ca2+]o and [Ca2+]i the membrane permeability for K+ is modified by a different mechanism.

Histamine-induced inositol phospholipid breakdown in the longitudinal smooth muscle of guinea-pig ileum

The characteristics of histamine-stimulated inositol phospholipid breakdown in slices of guinea-pig ileal smooth muscle and cerebellum have been investigated. In cerebellar slices the inhibition of the inositol phospholipid response to histamine by mepyramine was consistent with competitive antagonism of histamine H1-receptors. In slices of the longitudinal smooth muscle of guinea-pig ileum, mepyramine produced only a weak inhibition of the response to histamine, at concentrations up to 1 microM. This was in striking contrast to the potent competitive antagonism of the H1-mediated contractile responses obtained with mepyramine in this tissue. The H1-receptor antagonists (+)-chlorpheniramine and promethazine similarly had no effect on the EC50 value for histamine in guinea-pig ileum, while promethazine competitively antagonized the muscarinic receptor-mediated inositol phospholipid response in this tissue (Ka 3.6 X 10(7)M-1). Cimetidine, on its own, did not significantly inhibit the inositol phosphate accumulation elicited by histamine in ileum. In the presence of 0.2 microM mepyramine, cimetidine (0.1 mM) produced a small parallel shift of the histamine concentration-response curve (Ka 3 X 10(4) M-1). This inhibition, however, was not consistent with antagonism of an H2-receptor-mediated response. The effect of a range of histamine analogues on inositol phospholipid breakdown was determined. Dose-response curves were constructed and characterized in terms of the EC50, slope and maximal response attainable relative to histamine. The H1-agonists, N alpha,N alpha-dimethylhistamine, N alpha-methylhistamine, 2-pyridylethylamine and 2-thiazolyethylamine produced the largest accumulations of [3H]-inositol-1-phosphate. A very weak response was produced by the H2-selective agonist impromidine, while dimaprit (also H2-selective) was without significant effect. Mepyramine appeared to antagonize competitively the response to the H1-selective agonist 2-pyridylethylamine. This was in contrast to the data obtained with other H1-agonists, where mepyramine produced only a small dextral shift of the agonist curves at low agonist concentrations and an increase in the Hill coefficient. This was particularly striking in the case of 2-methylhistamine. The results suggest that an H1-receptor component in guinea-pig ileum, may coexist with a larger inositol phospholipid response to histamine which is independent of the activation of H1- or H2-receptors.

Maintained contractions of rat uterine smooth muscle incubated in a Ca2+-free solution

The effects of acetylcholine (10(-4) M), prostaglandin E2 (10(-6) M), vanadate (5 X 10(-4) M) and fluoride (10(-2) M) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. All four substances produced maintained contractions which could be initiated repeatedly after exposure to Ca2+-free solution for more than 1 h, without a significant decrease. The largest contractions were obtained with vanadate and the smallest ones with acetylcholine. The tension was usually 7-30% of the control contraction triggered by an action potential in Ca2+ containing solution. Maintained contractions induced by fluoride were unaffected by isoprenaline while those induced by acetylcholine, prostaglandin E2 and vanadate were completely relaxed. Prostaglandin E2- and vanadate-induced contractions were slightly reduced by Na+ removal or by adding Ca2+ antagonists. In contrast, contractions induced by acetylcholine were suppressed in Na+-free solution and largely inhibited in the presence of Ca2+ antagonists. The depolarization induced by acetylcholine in Ca2+-free solution was strongly dependent on the external Na+ concentration. The relationship between the size of the acetylcholine-induced depolarization and the membrane potential (shifted by constant currents) was linear, giving an apparent reversal potential for acetylcholine close to zero potential. In Ca-free solutions and in the presence of atropine, Na+ action potentials of long duration can be evoked which produced contractions of the same order of magnitude as those initiated by acetylcholine-induced depolarizations. 7 These results are consistent with the hypothesis that the maintained contractions in Ca2+-free solutions induced by several stimulants could be related to Ca2+-independent mechanisms (fluoride) or Ca2+ release from an intracellular store. This latter mechanism would include both pharmacomechanical (prostaglandin E2, vanadate) and electromechanical (acetylcholine) coupling.

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.

Patch-clamp studies of slow potential-sensitive potassium channels in longitudinal smooth muscle cells of rabbit jejunum

1. The patch-clamp technique was used to study single channel currents in membrane patches of longitudinal smooth muscle cells of rabbit jejunum dispersed by collagenase treatment. Recordings were made from both cell-attached and isolated patches.2. The predominant unit currents observed were outward at membrane potentials positive to the potassium equilibrium potential (E(K)) and they were rapidly and reversibly blocked by tetraethylammonium (TEA). Their size varied as E(K) was changed but was not noticeably affected by changing E(Na), E(Cl) or E(Ca); it was little altered in calcium-free EGTA solution. Thus, these currents apparently result mainly, if not exclusively, from the movements of potassium ions through channels insensitive to the calcium ion concentration. The present study describes the properties of these potassium channels.3. The unit conductance varied slightly with potential in most experiments; around zero potential it was about 50 pS. The conductance was dependent upon the potassium, but not the calcium, gradient. Sub levels of conductance of about two-thirds and, less commonly, one-third of the fully conducting channel state were sometimes seen.4. Membrane patches were studied which showed one to about twelve levels of outward current which were presumed to result from the opening of up to twelve channels having the same characteristics. The probability of channel open state varied with membrane potential, increasing in the potential range -40 to +40 mV. Channel openings were rare negative to -40 mV. No inward currents through these potassium channels were observed as openings were not seen at membrane potentials negative to E(K).5. When the probability of channel opening was low, channel openings occurred in bursts which could be separated by several seconds. Analysis of the openings of a single channel revealed that open times and short closed times were exponentially distributed with mean durations of 15-45 ms and about 6 ms at zero potential. In some patches regular cyclical openings of several channels occurred. In other patches openings of individual channels appeared to be independent events as they were reasonably fitted by a binomial distribution.6. Following a step change from negative potentials, where channels were closed, to more positive potentials, channel openings increased during a period of 10 s to reach a steady state. No evidence of inactivation was observed.7. These results suggest the existence of a population of potential-sensitive potassium-selective ion channels in the smooth muscle cell membrane which are closed at the resting membrane potential and which open upon depolarization with slow (seconds) kinetics; these may be involved in the slow potential (wave) activity of this muscle.

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.

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.