Lack of effects of carbachol on the Na-Ca exchange mechanism in frog atrial muscle treated with goniopora toxin

Carbachol (CCh) at a concentration of 10(-7) M completely inhibited the twitch contraction of frog atrial muscle. However, when the preparation was treated with goniopora toxin (GPT), a selective inhibitor of Na channel inactivation, and the twitch contraction was augmented through the activation of Na-Ca exchange by this toxin, the reduction in contractile amplitude by CCh was only about 30% even at higher concentrations. The maximum rate of rise of the twitch contraction was not affected by CCh in GPT-treated preparations. The time course of configuration change of the twitch contraction after application of CCh in GPT-treated muscle indicated that the attenuation of amplitude preceded the shortening of duration. The residual contraction under the combined treatments with GPT and CCh was abolished by removal of external Ca++ or addition of TTX. CCh only moderately shortened the action potential which was prolonged by GPT, and further addition of TTX abolished the action potential. From these results, we suggest that CCh does not influence the twitch contraction which is augmented via the activation of the Na-Ca exchange mechanism in frog atrial muscle.     

Some pharmacological properties of piperazine

1. The action of piperazine on mammalian smooth, cardiac and skeletal muscles has been studied.2. Piperazine increased tone and produced a dose dependent contraction of isolated smooth muscle, which was antagonized by atropine.3. With cardiac muscle, piperazine depressed both the rate and force of contraction and was antagonized by atropine. At higher concentrations, a non-specific depression of cardiac muscle was found. The intravenous injection of piperazine produced a transient decrease in both heart rate and blood pressure and this was followed by an increase.4. In about half of the mammalian skeletal muscle preparations, piperazine potentiated the twitch of the muscle evoked by electrical stimulation.5. On the frog neuromuscular preparation, piperazine produced potentiation of the twitch but this was followed first by blockade of the effects of nerve stimulation and then by depression of the effects of direct muscle stimulation.     

The action of dantrolene sodium on rat fast and slow muscle in vivo.

1. Rats, anaesthetized with urethane, were injected intravenously with dantrolene sodium in a carrier solution of 5% mannitol taken to pH 10 with NaOH. This carrier solution itself was without effect on extrafusal muscle contraction. 2. Dantrolene sodium (5 mg/kg) had a greater depressant action on the twitch contraction of the fast extensor digitorum longus (EDL) muscle than on the slow soleus (SOL) muscle. The EDL twitch was depressed to 25.9% +/- 1.2% (mean + s.e. mean, n = 7) of control whereas the SOL twitch was depressed to 31.3% +/- 0.4% (n = 9). These values were significantly different at the P less than 0.001 level. 3. The twitch contraction time to peak was reduced by approximately 35% in both EDL and SOL by dantrolene sodium. However, the drug reduced the half relaxation time of SOL by approximately 30% but that of EDL was hardly affected. 4. The effect of dantrolene sodium on contractions elicited by repetitive stimulation was dependent upon the stimulation frequency. For the SOL muscle the greater depression was produced at a stimulation frequency of 25 Hz and for EDL at 75 Hz. The minimum of depression was produced for a full fused tetanus for both muscles. 5. The significance of these findings is discussed in terms of the action of dantrolene sodium on motor control in the intact animal.     

Effects of 3,4-diaminopyridine on mechanical and electrical responses of frog single muscle fibres

The effects of 3,4-diaminopyridine (3,4-DAP) were studied on isolated muscle fibres of the frog in concentrations ranging between 0.025 and 5.0 mM. Isometric twitch and tetanus responses were recorded at temperatures between 2.5 and 3.9 degrees. 3,4-DAP caused a concentration-dependent increase in twitch amplitude, maximum effects being obtained at a concentration of 3 mM with a mean increase in tension of 70 +/- 12% of control (n = 7). 3,4-DAP in 3 mM concentration had only a slight increase in initial rate of rise of twitch tension (mean increase 8 +/- 4%) but increased the time to half peak tension by 59 +/- 9% and the time from peak tension to half relaxation by 78 +/- 10%. No significant effect of 3,4-DAP was observed on the initial rate of rise and total amplitude of the isometric tetanus. The twitch potentiating effect of 3,4-DAP developed gradually with the number of times the fibre was stimulated and reached a maximum level after 40-50 stimulations. A gradual increase in the duration of the action potential was also observed. It is suggested that 3,4-DAP, like 4-aminopyridine, potentiates the twitch by means of prolonging the duration of the action potential.     

The action of dantrolene sodium on individual fast and slow motor units of the rat anaesthetized with urethane

Rats, anaesthetized with urethane, were injected intravenously with dantrolene sodium in a vehicle of 5% mannitol taken to pH 10 with NaOH. The muscle relaxant action of dantrolene sodium was measured from the contractions of individual motor units of the extensor digitorum longus (EDL), soleus (SOL) and segmental tail (ST) muscles. Data were also collected from their parent whole muscle preparations. The depressant action of dantrolene sodium on the percentage-normalized amplitude of contraction of the individual motor units was greater than its effect on the whole muscle twitch amplitude, in all three muscles. The twitch amplitude of fast contracting motor units was significantly more reduced (P less than 0.001) by dantrolene sodium than was that of slow contracting motor units. Dantrolene sodium reduced the contraction time of all motor units. The effect of the drug on half-relaxation time varied within and between groups of motor units studied. The drug was confirmed to have a greater depressant action on the twitch contraction than on the fused tetanus of whole muscle. This was true also for single motor units. With tetanic stimulation the effect of dantrolene sodium was also dependent upon the motor unit type, fast or slow. A maximum depression of contractile tension occurred at a stimulation frequency of 64 Hz for fast EDL motor units whereas the maximum depression for ST slow units, the slowest units tested, was at a stimulation frequency of 14 Hz.     

The acute effect of trimetazidine on the high frequency fatigue in the isolated Rat diaphragm muscle

The objective of this study was to determine the acute effect of trimetazidine (TMZ) on the pre-fatigue, fatigue and post-fatigue contractile characteristics and tension-frequency relationships of isolated rat diaphragm muscle. Muscle strips were taken from the ventral-costal aspects of the diaphragm muscle of rats killed by decapitation. The muscle strips were suspended in organ baths containing Krebs solution, with a gas mixture of 95% O2 and 5% CO2 at 37 degrees C and pH 7.35-7.45. After determining the thermoregulation and optimum muscle length the muscles were subjected to direct supramaximal stimulation with 0.05 Hz frequency square pulses for periods of 0.5 msec to obtain control values. After adding 5 x 10(-6) and 5 x 10(-5) M trimetazidine solution to the respective bath media, the contractile parameters of the muscles were recorded. The contractile parameters were also recorded for both the trimetazidine and trimetazidine-free media after application of the high frequency fatigue protocols. Later, the tension-frequency relationship was determined by applying stimulating pulses of 10, 20, 50 and 100 Hz to the muscle strips. Whilst the twitch tension obtained from the 5 x 10(-6) and 5 x 10(-5) M trimetazidine media showed numerical increases compared to that of the controls, these were not statistically significant (p>0.05). The contraction time exhibited a dose dependent increase (p<0.001), whilst the contraction and relaxation rates did not differ significantly. The isometric contraction forces obtained with the different stimulating frequencies showed a significant increase in the tetanic contraction only at 100 Hz (p<0.05). A comparison of the pre- and post-fatigue twitch tensions in the trimetazidine media showed the post- fatigue twitch tensions to be significantly higher than those of the pre-fatigue contraction forces (p<0.05). In the 5 x 10(-6) and 5 x 10(-5) M trimetazidine media the increases in the post-fatigue contraction force were 22 and 30%, respectively. These results demonstrated that in isolated rat diaphragm muscle, TMZ significantly limited the mechanical performance decrease during fatigue. It is our opinion that trimetazidine contributed to the observed fatigue tolerance by eliminating the factors of fatigue, due to preservation of intracellular calcium homeostasis, provision of the ATP energy levels needed by ATPase dependent pumps and especially by keeping the intracellular pH within certain limits.     

Reduced effect of caffeine on twitch contraction of oesophageal striated muscle from stroke-prone spontaneously hypertensive rats

1. There are known differences in the sensitivity to caffeine between skeletal muscle (soleus) of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The present study was performed in order to examine differences in the effects of caffeine on twitch contraction between visceral striated muscle using the outer layer of the oesophagus from WKY rats and stroke-prone SHR (SHRSP). 2. Caffeine, at concentrations ranging from 0.3 to 10 mmol/L, exhibited potentiating effects on twitch contraction in preparations from both WKY rats and SHRSP. The potentiating effect of caffeine was markedly less prominent in preparations from SHRSP compared with preparations from WKY rats. 3. The rate of contraction and relaxation, the time to peak tension and 80% relaxation time were not significantly altered by caffeine at concentrations lower than 3 mmol/L in preparations from either strain. 4. With 10 mmol/L caffeine, the rate of relaxation was markedly reduced and the 80% relaxation time was prolonged, with no significant changes in the rate of contraction, in preparations from WKY rats. These changes were significantly smaller in preparations from SHRSP. 5. The duration of the action potential was greater in preparations from SHRSP than in preparations from WKY rats, although the membrane potential and the amplitude of the action potential were not significantly different between preparations from WKY rats and SHRSP. 6. Caffeine, at 10 mmol/L, prolonged the duration of the action potential in preparations from both strains. The effect of caffeine was not different between preparations from WKY rats and SHRSP. 7. The results of the present study suggest that caffeine augments release of Ca2+ from the sarcoplasmic reticulum (SR) at low concentrations and attenuates Ca2+ re-uptake at 10 mmol/L. Decreased reactivity of SR to caffeine may be a cause of the lesser potentiation of twitch contraction by caffeine in preparations from SHRSP.     

Effects of toxin II from the sea anemone Anemonia sulcata on contractile and electrical responses of frog skeletal muscle fibres

The effects of Anemonia sulcata toxin II (ATX-II) were studied on mechanical and electrical activities of frog muscle fibres isolated from semitendinosus or tibialis anterior muscles of Rana temporaria (2.8-7.7 degrees C). In concentrations ranging between 7.7 and 100 microM, ATX-II greatly potentiated the isometric twitch of single muscle fibres in a time-dependent manner. Increase in twitch amplitude by ATX-II was associated with an increase in time to peak tension and time from peak tension to half relaxation. ATX-II caused no change in maximum force production during fused tetanus, but the tension was maintained for several seconds after the cessation of stimulation. Such long tetanic contractions were also obtained in low-Na Ringer solution, but their duration was somewhat shorter. No specific action of ATX-II was detected on relaxation kinetics during a tetanus. The twitch potentiating effect of ATX-II was markedly increased by 3,4-diaminopyridine. Action potentials recorded from single muscle fibres in the presence of ATX-II showed a delayed onset of repolarization with a reduced rate of fall. In addition, ATX-II caused repetitive spontaneous firing of action potentials after the cessation of tetanic stimulation. ATX-II (3.3 - 10 microM) also increased action potential duration by reducing the rate of repolarization in surface fibres of formamide-treated sartorius or cutaneous pectoris muscles (20 degrees C) stimulated indirectly or directly. The potentiation of twitch force and the prolongation of contractions caused by tetanic stimulation can be attributed to the membrane action of ATX-II, which leads to prolongation of action potentials, to repetitive muscle firing and to the appearance of plateau potentials.     

Distribution and types of adrenoceptors in the guinea-pig ileum: the action of α-and β-adrenoceptor agonists

1 Segments of guinea-pig ileum and the myenteric plexus-longitudinal smooth muscle preparation were used for a study of the actions of adrenaline, noradrenaline, isoprenaline, ephedrine and phenylephrine on the responses of coaxially stimulated ileum at different distances from the ileocaecal valve.2 The responses of the ileum to electrical stimulation were suppressed by adrenaline, nonadrenaline and ephedrine, while phenylephrine and isoprenaline inhibited them only partially.3 The twitch inhibition elicited by these adrenoceptor agonists was the same at all distances from the ileocaecal valve. There was no significant difference between their cumulative and non-cumulative concentration-response curves.4 Smooth muscle relaxation was induced only by isoprenaline and contraction only by phenylephrine at all distances from the ileocaecal junction. Adrenaline and noradrenaline evoked smooth muscle contraction in the terminal (0 to 20 cm), a concentration-dependent, biphasic response in the intermediate part (21 to 50 cm) and a relaxation in the proximal ileum (> 50 cm from the ilecocaecal valve). Ephedrine did not change significantly the smooth muscle tension in the terminal and the intermediate segments and induced smooth muscle relaxation in the proximal ones.5 Ouabain and a potassium-free solution did not appear to influence the prejunctional action of noradrenaline nor the amplitude of smooth muscle relaxation in the proximal ileum, whereas the concentration-contractor response curves were significantly depressed and shifted to the right by ouabain and in a potassium-free solution.6 The brief initial (phasic) contraction induced by acetylcholine was not influenced during the sustained increase or decrease in tension induced by catecholamines. On the contrary, the stimulatory catecholamine actions disappeared or were changed to smooth muscle relaxation by acetylcholine pretreatment. Potassium chloride pretreatment did not change the character of the adrenoceptor agonist action of the agonists studied.7 Since there is a similar prejunctional action at all distances from the ileocaecal valve and a different postjunctional effect of the adrenoceptor agonists at different distances from the ileocaecal junction, it could be suggested that in the guinea-pig ileum there are at least two alpha-adrenoceptors (inhibitory prejunctional-alpha(2), stimulatory postjunctional-alpha(1)), an inhibitory postjunctional beta-adrenoceptor and an as yet uncharacterized inhibitory postjunctional receptor.8 Based on the specific postjunctional action of phenylephrine and the prejunctional action of ephedrine in the guinea-pig ileum, these drugs could be used with success as ;specific' alpha(1)- and alpha(2)-adrenoceptor stimulants.     

Cholinergic agonist and antagonist interactions on motor nerve endings of the rat--evidence for the involvement of presynaptic receptors in the regulation of acetylcholine release

The effects of atropine and oxotremorine on the amplitude of contraction and on the release of Ach from rat isolated diaphragm were examined. Atropine (14-112 microM) induced a dose-related increase in the amplitude of contractions, the effect being potentiated by neostigmine (10 and 100 nM) or by increasing the rate of nerve stimulation and was accompanied by no change in the twitch evoked by retrograde injection of Ach. Atropine (112 microM) depressed the post-tetanic twitch response in muscles incubated in nutrient solution containing or not non-paralyzing concentration of d-tubocurarine (1 nM). Atropine (28 and 56 microM) enhanced and d-tubocurarine (1 nM) reduced the carbachol-induced neuromuscular facilitation. Atropine (28 and 56 microM) enhanced the evoked release of Ach, the effect being potentiated by increasing the rate of nerve stimulation. Oxotremorine (5-20 microM) inhibited the muscle contraction and depressed the evoked release of Ach. The effects were both prevented by atropine. The oxotremorine-induced blockade was potentiated by d-tubocurarine (1 nM), the effect being accompanied by no change in the twitch induced by retrograde injection of Ach. These results suggest the presence of muscarinic and nicotinic presynaptic receptors participating in a mechanism which might regulate the release of Ach.     

甘草水提剂与阿托品对离体兔肠管运动的影响

甘草水提剂(0.165g)对离体兔肠管平滑肌运动有明显的抑制作用,能显著地降低肠管蠕动的频率和幅度(p<0.01;p<0.05),作用时间比较对照组阿托品长,给药后5min降低肠蠕动幅度为阿托品的1.71倍,且能加强阿托品对肠管运动的抑制作用,是单用阿托品的2.87倍。结果表明:甘草水提剂对离体肠管运动有明显的抑制作用,且能加强阿托品的作用。     

An anti-curare effect of hexamethonium at the mammalian neuromuscular junction

1. Experiments were performed on the isolated phrenic nerve and diaphragm preparation of the rat.2. In preparations partly blocked with (+)-tubocurarine, the twitch amplitude increased after hexamethonium. This enhancement was not seen in preparations partly blocked with Mg(++) or with gallamine. High concentrations of hexamethonium produced failure of contraction.3. Extracellular endplate potentials were recorded from blocked preparations. The administration of hexamethonium resulted in an increased amplitude of these potentials only in curarized muscle.4. Hexamethonium had no anticholinesterase activity nor did it depolarize muscle cells or increase the quantal release of transmitter.5. It is concluded that hexamethonium exerts a specific anti-curare action. Experiments on the recovery of the twitch after washing out antagonists indicate that this process is limited by diffusion. The difference in rates of diffusion of hexamethonium and (+)-tubocurarine does not account for their interaction. The basis of the anti-curare action of hexamethonium is discussed.     

Effect of some benzodiazepines on peripheral neuromuscular function in the rat in-vitro hemidiaphragm preparation

The effect of equimolar cumulative concentrations of 11 different benzodiazepines on the indirectly evoked twitch contraction was investigated in the rat in-vitro phrenic nerve-hemidiaphragm preparation. Depending on the pattern of the concentration-response curves two groups of benzodiazepines can be distinguished: (i) a first group with a biphasic action, e.g. potentiation of twitch tension in low concentrations and depression of twitch tension in high concentrations, and (ii) a second group with primary depression of twitch tension with increasing concentrations. All of the tested compounds ultimately caused a 100% depression of twitch tension at concentrations ranging from 0.175 to 0.35 mmol litre-1. Although this peripheral effect of benzodiazepines on neuromuscular function is not the main site of action of these compounds, there are enough arguments to state that it is not a simple toxic effect. There is some evidence from this study that the peripheral component of the benzodiazepine effect on muscle relaxation may involve a multi- rather than one single receptor system.     

Abnormal response to ryanodine in oesophageal striated muscle of spontaneously hypertensive rats

The effects of ryanodine on twitch contraction and basal tension of oesophageal striated muscle were compared between preparations from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY). Ryanodine (3 x 10(-7) M) augmented the twitch contraction in WKY preparations, butt attenuated it in SHRSP preparations. Rates of contraction and relaxation of twitch contraction, normalized to developed tension, were slightly decreased by ryanodine in both preparations. The effect of ryanodine was not different between WKY and SHRSP preparations. Ryanodine elevated the basal tension in WKY preparations but not in SHRSP preparations. Ryanodine elevated the intracellular Ca(2+) level in both preparations, but the response was significantly less in SHRSP preparations. Resting and action potentials were not significantly different between WKY and SHRSP preparations, while the duration of the action potential was significantly longer in SHRSP preparations. Ryanodine did not alter the resting and action potentials of either preparation. These results suggest that the Ca(2+) handling properties, including the ryanodine receptor, of the sarcoplasmic reticulum are genetically altered in oesophageal striated muscle of SHRSP.     

Effect of capsaicin on gastric corpus smooth muscle of the rat in vitro

Circular strips of the rat gastric corpus muscle were mounted in Krebs solution for isometric tension recording. Addition of capsaicin usually led to either relaxation or contraction, but in some strips a biphasic response was observed. Although no clear-cut concentration-response relationship could be established, capsaicin predominantly induced contraction at 500 nM, whereas at 5 microM it mainly induced relaxation. In Krebs solution containing atropine plus guanethidine, the contraction induced by 500 nM capsaicin was significantly reduced. The contraction induced by capsaicin was abolished by spantide, a tachykinin antagonist, or by tachyphylaxis to substance P. Calcitonin gene-related peptide relaxed gastric smooth muscle, however, a dose-response relationship could not be established. This peptide contracted the muscle strips only at 1 microM. Tachyphylaxis to calcitonin gene-related peptide did not significantly influence the action of capsaicin. Vasoactive intestinal polypeptide dose dependently relaxed gastric corpus strips; however, these responses were qualitatively different from those to capsaicin. It is concluded that capsaicin contracts rat gastric smooth muscle via the release of tachykinins; cholinergic interneurones are involved. The mediator of the capsaicin-induced relaxation has yet to be determined.     

Mechanical and current responses to neurotensin in the smooth muscle of guinea-pig intestine

1 The effects of neurotensin (NT) on the mechanical activity of the smooth muscle and membrane currents of enzymatically-dispersed single smooth muscle cells of guinea-pig intestine were investigated by the isometric tension recording method and the whole-cell patch clamp technique, respectively. 2 NT (0.3–670 nm ) produced an initial relaxation followed by a contraction in segment preparations of the jejunum and ileum. Atropine (0.2 μm ) abolished the contraction. In 30% of ileal segments, NT produced a slowly-developed contraction in the presence of atropine. The atropine-resistant contraction was insensitive to tetrodotoxin (TTX, 0.2 μm ). The relaxant effect of NT was unaffected by TTX (0.2 μm ) and guanethidine (2 μm ), but markedly reduced or abolished by apamin (67 nm ). 3 In segment preparations of the colon and rectum, NT produced a biphasic response, similar to that in the small intestine, or a triphasic one (relaxation, contraction and relaxation). The contractile effect of NT was affected neither by atropine (0.2 μm ) nor by TTX (0.2 μm ). The first relaxation in response to NT was similar in pharmacological properties to that in the small intestine. 4 Responses to NT in longitudinal muscle strips were similar to those in segment preparations. The taenia caecum responded to NT with a contraction alone and the effect was unaffected by atropine (0.2 μm ) and TTX (0.2 μm ). 5 NT had little or no effect on the mechanical activity in the circular muscle of the small intestine. In the circular muscle of the large intestine, NT had a weak inhibitory effect on the spontaneous activity which was followed by a small rise in muscle tone. Apamin (67 nm ) converted the biphasic response to a contraction. 6 In cells dialysed with a KCl-based solution under voltage clamp at 0 mV, NT (5 μm ) produced a brief, outward current in a very small fraction of cells (4 out of 40 cells). When cells were dialysed with a CsCl-based solution under voltage clamp at – 50 or – 60 mV, no current response to NT (5 μm ) was observed in all 37 cells, but NT increased inward Ca²⁺ currents evoked upon depolarization. 7 From these results, it appears that there is a regional difference in the mode of action of NT to contract and relax the smooth muscle of guinea-pig intestine, and NT acts on its receptors on the smooth muscle to enhance activation of voltage-dependent Ca²⁺ channels, which underlies the slow contraction in the longitudinal muscle of the intestine.     

A comparison between the effects of a tetanus and the effects of sympathomimetic amines on fast- and slow-contracting mammalian muscles

The effects of adrenaline and isoprenaline on the tension and time-course of the contractions of the tibialis anterior and soleus muscles of cats and rabbits have been compared with the effects of previous high-frequency stimulation. Like a tetanus, adrenaline possessed a facilitating action on neuromuscular transmission and an action exerted directly on the muscle fibres. Isoprenaline possessed only the second of these two actions. The effect of adrenaline and isoprenaline on the muscle fibres was blocked by dichloroisoprenaline and by 1-(2 naphthyl)-2-isopropylaminoethanol, whereas the effect of adrenaline on neuromuscular transmission was blocked by phentolamine and by phenoxybenzamine. In the soleus muscle, both the catechol amines and a previous tetanus caused similar decreases in maximal twitch tension and in the times to peak tension and to half-relaxation. The muscle action potentials were unaltered or slightly increased in amplitude. In the tibialis anterior muscle, a previous tetanus and the catechol amines caused an increase in twitch tension and an increase in the overall duration of the twitch. The muscle action potentials were either unchanged or were slightly decreased in amplitude. In this muscle the effect of a tetanus differed from that of the catechol amines in that the large post-tetanic change was associated with a marked increase in the rate of rise of twitch tension.     

Effects of droperidol on neuromuscular transmission and muscle membrane

The effect of droperidol on miniature endplate potential (mepp) frequency and amplitude, acetylcholine sensitivity, action potential generation, twitch tension, input resistance and alteration of acetylcholine release were studied in frog muscle. Interaction with d-tubocurarine was studied in the cat sciatic gastrocnemius preparation. Between 10^?11 and 10^?7 g/l droperidol did not change mepp frequency or amplitude. Between 7.5 × 10^?6 and 2.5 × 10^?5 g/l mepp amplitude, endplate sensitivity to acetylcholine and twitch tension were depressed. At 5 × 10^?5 g/l the threshold for action potential generation increased while amplitude and rate of rise decreased. Input resistance was unaltered. With indirect interrupted tetanic stimulation of the muscle for 30 min droperidol 10^?6 to 10^?4 g/l reduced the contraction height while acetylcholine release was concomitantly depressed. Droperidol 10 mg/kg in cats did not block muscle contraction, but combination with d-tubocurarine prolonged recovery time.     

Mechanism of the inhibitory actions of some spasmogens on the bradykinin relaxation of the guinea-pig ileum

The bradykinin relaxation of the acetylcholine-contracted guinea-pig ileum is decreased by increasing the concentration of acetylcholine. This finding prompted an investigation to determine whether other spasmogens can reduce the bradykinin relaxation, and the mechanisms involved.Linear inverse relationships were found between both the percentages of maximum contraction and concentrations of histamine and eledoisin, and the bradykinin relaxation. Eledoisin and histamine showed smaller regression equation slopes than acetylcholine. No relationship existed for angiotensin, though low non-spasmogenic doses in the presence of contraction by acetylcholine reduced the bradykinin relaxation. Morphine and atropine reduced the angiotensin contraction, and potentiated the bradykinin relaxation in its presence. Only atropine reduced the acetylcholine contraction and potentiated the bradykinin relaxation in its presence. Neither morphine nor atropine had any effect on the bradykinin reduction of its own responses.The magnitude of the bradykinin relaxation on the guinea-pig ileum is dependent on the manner of stimulation, rather than on the state of contraction; and, also is sensitive to regulation by agents that have their actions on the parasympathetic system. Angiotensin reduces the bradykinin relaxation by a mechanism involving its indirect parasympathomimetic effects. In contrast, the bradykinin reduction of its own relaxation is not via a parasympathetic mechanism, but probably is a direct action.     

Membrane and contractile properties of the dog ciliary muscle

Membrane properties and excitation-contraction coupling mechanisms of the dog ciliary muscle were investigated by use of microelectrode and isometric tension recording methods. The mean resting membrane potential of the smooth muscle cell was -59.6 +/- 1.6 mV and the membrane was electrically quiescent. With applications of inward and outward current pulses, by the partition stimulating method (pulse duration, 1.0 s), electrotonic potentials, but not action potentials (spike) were evoked even in the presence of tetraethylammonium (TEA, 10 mM). The space and time constants calculated from the current-voltage relationship observed at various distances from the stimulating electrode were 0.43 +/- 0.07 mm (+/- s.d. n = 5) and 82.5 +/- 10.6 ms (+/- s.d. n = 4), respectively. Electrical field stimulation of short duration (50-300 microseconds) evoked excitatory junction potentials (e.j.ps) followed by twitch contraction, both of which were potentiated by neostigmine (10(-7) M) and abolished by tetrodotoxin (TTX, 10(-7) M) or atropine (10(-6) M). However, e.j.ps of amplitude over 3-5 mV did not evoke action potentials. Excess [K]o solution dose-dependently depolarized the membrane (the maximum slope of the depolarizations produced by a tenfold increase in [K]o plotted on a log scale was 52.5 mV) and evoked contractions. Atropine (10(-6) M) had no effect on the [K]o-induced membrane depolarization, however, it greatly reduced the amplitude of [K]o-induced contraction. Nisoldipine (5 X 10(-7) M) had no effect on the contraction evoked by field stimulation, however, this agent suppressed the [K]o-induced contraction to 78.3 +/- 6.8% of the control value. Combined application of nisoldipine and atropine completely inhibited the [K]o-induced contraction. Although the membrane depolarization evoked by carbachol (greater than 10(-7) M) was small (10 mV depolarization at 10(-5) M), the contraction evoked by 10(-5) M carbachol was larger than that evoked by 118 mM [K]o (185 +/- 50%, n = 9). Ca-free 3 mM EGTA-containing solution did not alter the resting tension, but greatly reduced the carbachol-induced contraction (to 8.6 +/- 2.3% of the control value). The present results indicate that the membrane of dog ciliary muscle behaves as an electrical syncytium, and that the contractile responses occur mainly through activation of the receptor-operated Ca channels and partly through activation of voltage-dependent Ca channels.