On the roles of calcium ion during potassium induced contracture in the smooth muscle cells of the rabbit main pulmonary artery

The half decay time of the K-induced contracture of rabbit main pulmonary artery following pretreatment with Ca-free EGTA containing solution was 110 sec. A Ca-free K-solution did not generate contraction while noradrenaline, acetylcholine and prostaglandin F2alpha-containing solution did evoke contracture. The decays of the chemically induced mechanical response in Ca-free solution against the exposure times could be classified into three components (2 min, 28 min and over 100 min, respectively). When the membrane depolarization produced by excess K+ was simulated in Krebs solution by application of current, the generated mechanical response was smaller than that produced by 118 mM K+. When the membrane potential was clamped at the resting level before, during and after application of the excess K+, and excess K+ still evoked contracture. The amplitudes of contracture depended on [K]o. The effects of various [K]o on the length constant of the tissue were also observed in relation to the clamping condition. It is postulated that the mechanical response of the pulmonary artery induced by excess K is mainly due to influx of Ca++ and the depolarization plays only a minor role. This means that release of stored Ca by depolarization is not an essential factor in generation of K-induced contracture in this tissue.     

Electrical and mechanical properties of longitudinal and circular muscles of the guinea-pig ileum.

The membrane potential of the electrically quiescent circular muscle fibre is higher than that of the spontaneously active longitudinal muscle fibre. The length constant of circular muscle fibres is longer than that of the longitudinal muscle. The electrical activity of the longitudinal muscle membrane is enhanced by caffeine and PGE1 is suppressed for the circular muscle. Isotonic K Krebssolution generates a large tonic response of the contracture of longitudinal muscle tissue, but in circular muscle it is small. Following an inward current pulse, a large rebound contraction of the circular muscle was generated during K-INDUCED CONTRACTURE BUT IT WAS SMALL IN THE LONGITUDINAL MUSCLE TISSUE. Caffeine suppressed the K-induced contracture of both tissues but enhanced the rebound contraction of the circular muscel. Verapamil and lanthanum suppressed the mechanical responses of both muscle tissues but rebound contraction evoked in isotonic K Krebs solution was of longer duration in circular muscle tissue than in longitudinal muscle tissues. It is postulated that both muscle tissues possess different electrical and mechanical properties, and that these differences are not caused by nervous activity.     

Some differences in contractile responses of isolated longitudinal and circular muscle from the guinea-pig stomach.

The mechanical and electrical properties of the longitudinal (fundus and corpus) and circular (antrum) muscle fibres of the guinea-pig stomach were investigated. 1. In the longitudinal but not in the circular muscle isotonic K Krebs and Na-free (sucrose) Krebs solutions produced a contracture with a tonic component. The different mechanical responses were not accompanied by different membrane responses. Verapamil abolished both phasic and tonic components of K-induced contracture. 2. During the tonic response of the K-induced contracture, repolarization of the membrane by current pulses relaxed the tissue; after cessation of the current pulse, rebound contracture occurred. In the circular muscle, the Q10 value for the rate of relaxation induced by inward current pulse was 3-1 and for the development of rebound contracture was 2-4. 3. After the tissue had been immersed in Ca-free isotonic K Krebs solution, application of Ca produced a large contracture in the longitudinal muscle, but contracture in the circular muscle was small or absent. However, the amplitude of subsequent carbachol-induced contracture in the above solution was enlarged in proportion to the durations of Ca treatment in both tissues. 4. Direct tetanic electrical stimulation could produce tension in both tissues. With low frequency of stimulation (0-1 Hz) a positive staircase was observed in the circular but not in the longitudinal muscle. 5. It is concluded from these results that topical differences of the motility in the stomach may be due not only to the activity of nervous elements, but also to differences in the properties of the muscle fibres themselves.     

Effects of sodium depletion on contractions evoked in intact and skinned muscles of the guinea-pig mesenteric artery

In the guinea-pig mesenteric artery, reduction in [Na]o by 30 mM (substituted by choline or sucrose; 137 mM [Na]o in Krebs solution) generated contraction with no change in membrane potential. In NaCl-free solution (15 mM [Na]o), the amplitude of phasic contraction reached 0.8 times the contraction evoked by 118 mM [K]o with only a slight depolarization. In NaCl-free solution, the amplitude of phasic contraction evoked by noradrenaline (NA) 5 X 10(-5) M or caffeine 5 mM increased to roughly twice the amplitude of the contraction evoked in the control solution. In Ca-free solution, the K-, NaCl-free- or Na-free-induced contractions rapidly ceased, but NA-induced contraction ceased within 5 min and the caffeine-induced contraction persisted for more than 15 min. In a skinned fiber, increase of [Na]o from 10 to 60 mM suppressed the pCa-tension relationship in the ranges of 10(-7) and 10(-5) M free Ca but not with a dose of 30 mM [Na]o. NA (10(-5) M) had no effect on skinned fibers. Increase in Na concentration (60 mM) had no effect on Ca accumulation in the store site or on Ca release by caffeine. Possible Na-related mechanisms on the development of mechanical response are discussed in relation to Ca on the surface and in the internal membrane structure. The NaCl-free-induced contraction in smooth muscles of the guinea-pig mesenteric artery is postulated to be due to influx of Ca through the Na channel, rather than the Ca channel.     

Comparison of Mg, Mn, and Co ions affecting the beta-adrenoceptor-mediated membrane response in the guinea-pig taenia caeci

Electrical activity was recorded intracellularly from the muscle cell of guinea-pig taenia caeci in Locke solution. Membrane potential was -46.4 mV, and spike potentials were discharged spontaneously. Isoprenaline (3 microM) hyperpolarized the membrane and suppressed the spike discharge. The hyperpolarization by isoprenaline was increased at low K (2 mM), while decreased at high K (11.8, 29.5, 59 mM). The hyperpolarization by isoprenaline was potentiated in the presence of external Mg ions, depending on the concentration of Mg (0-9.6 mM). Forskolin (3 microM) and papaverine (30 microM) hyperpolarized the membrane; the effects were augmented by 1.2 mM Mg. The hyperpolarization in response to 3 microM isoprenaline or 100 microM papaverine was inhibited by Mn, Co, and low Ca (1 mM) whereas it was not affected by high Ca (7.5 mM). Verapamil (0.5, 2 microM) had no influence of the hyperpolarization caused by isoprenaline. It was discussed that extracellular and/or intracellular Mg and Ca ions played important roles in the beta-adrenoceptor-mediated action on the smooth muscle membrane of taenia caeci.     

Changes in contractility and calcium binding of guinea pig taenia coli by treatment with enzymes which hydrolyze sialic acid.

The effects of neuraminidase and phospholipase C on the contractility and the Ca++ -binding of guinea pig taenia coli were investigated. Potassium contracture or histamine-induced contracture of taenia coli was inhibited by treatment with neuraminidase, though acetylcholine-induced contracture was not. Treatment with phospholipase C markedly inhibited the contracture induced by isotonic potassium, histamine or acetylcholine. By treatment with neuraminidase for 4 hr, about 40 mumol/100 mg wer wt of sialic acid was released from taenia coli. This corresponded to two-fifths of total content of sialic acid. By treatment with phospholipase C for 2 hr, a similar amount of sialic acid to that produced by neuraminidase treatment was released. The Scarchard plot of Ca++-binding was a biphasic pattern indicating the presence of two types ofthe Ca++ -binding site with different affinity constants. Neuraminidase produced a 57% decrease in the amount of bound Ca++. The Scatchard plot of Ca++ -binding changed to a monophasic pattern indicating the disapperance of thel ow affinity Ca++ -binding site. Phospholipase C caused a 59% decrease of bound Ca++. The Scatchard plot also indicated the disappearance of the low affinity Ca++ -binding site. From these results, we speculated that sialicacid residue of surface membrane of the muscle cell was first site in the Ca++ -influx mechanism.     

The membrane properties of the smooth muscle of the guinea-pig portal vein in isotonic and hypertonic solutions

The membrane properties of the longitudinal smooth muscle of the guinea-pig portal vein were investigated under various experimental conditions.1. In isotonic Krebs solution, the membrane potential (-48.7 mV), the maximum rates of rise and fall of the spike (4.6 and 2.3 V/sec respectively), the space constant (0.61 mm), the conduction velocity of excitation (0.97 cm/sec) and the time constant of the foot of the propagated spike (18.4 msec) were measured.2. The various parameters of the muscle membrane in the isotonic solution were compared with those in the hypertonic solution prepared by the addition of solid sucrose (twice the normal tonicity).3. When the muscles were perfused with hypertonic solution, marked depolarization of the membrane and increased membrane resistance occurred. These were probably due to reduction of the K permeability, increased internal resistance of the muscle and shrinkage of the muscle fibre.4. The membrane potential in isotonic and hypertonic solutions was analysed into two components, i.e. the metabolic (electrogenic Na-pump) and the ionic (electrical diffusion potential) component in the various environmental conditions.(a) In isotonic and hypertonic solutions, the membrane was depolarized by lowering the temperature or by removal of K ion from the solutions. When the tissues were rewarmed or on readdition of K ion, the membrane was markedly hyperpolarized. These hyperpolarizations of the membrane were suppressed by treatment with ouabain (10(-5) g/ml.), by warming to only 20 degrees C and by K-free solution.(b) The relationships between the membrane potential and the [K](o) in isotonic Krebs, in the hypertonic (sucrose) Krebs, in the Na-free (Tris) Krebs and in the Cl-deficient (C(6)H(5)SO(3)) Krebs were observed. The maximum slopes of the membrane depolarization against tenfold changes of [K](o) were much lower than that expected if it behaved like a K electrode.(c) In Na-free (Tris) solution, the membrane was not depolarized in isotonic condition but it was depolarized in hypertonic condition.5. The low membrane potential in hypertonic solution (-37 mV) compared with isotonic solution (-49 mV) was thought to be mainly due to suppression of K permeability of the membrane and not due to suppression of the metabolic component.The electrogenic Na-pump and the membrane potential of the portal vein was discussed in relation to other excitable cell membranes.     

The effect of sodium and calcium on the action potential of the smooth muscle of the guinea-pig taenia coli

1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea-pig taenia coli.2. Replacement of external NaCl with sucrose (leaving 10 mM-Na in the buffer) caused hyperpolarization and stopped spontaneous activity within 10 min. Spikes could, however, be evoked for 2-3 hr. The amplitude, the overshoot and rate of rise of the spike were increased.3. In 10 mM-[Na](o) the intracellular Na concentration was reduced from 35 to 24 mM, shifting the Na-equilibrium potential from +34 to -22 mV.4. Excess Ca (12.5 mM) caused hyperpolarization and increased membrane conductance. The amplitude and the rate of rise of the spike were increased, the threshold was raised and the latency of the spike evoked by threshold stimulation became shorter.5. The effect of reducing the external Ca concentration depended on the Na concentration present, being greater with higher external [Na](o). When the membrane was depolarized and spikes deteriorated in low Ca (0.2-0.5 mM) reduction of Na to 10 mM caused repolarization and recovery of the action potential.6. Mn (0.5-1.0 mM) blocked spontaneous spike discharge after 20 min. Higher concentrations (more than 2.0 mM) were required to block the evoked action potential.7. The results indicate that the smooth muscle spike in taenia is due to Ca-entry and that Na influences spike activity indirectly by competing with Ca in controlling the membrane potential.     

Effects of cytochalasins on the mechanical and membrane responses in the isolated longitudinal myometrium of pregnant rat with reference to the EGTA-resistant contraction

Effects of cytochalasins B and D (CB, CD) were examined for the contractions of the longitudinal myometrium of pregnant rat exposed to Krebs solution, 40 mM K Krebs, and Ca-free (40 mM K) solution containing 5 mM Mg. The Ca-free contraction was evoked by applying 3 mM ATP. Application of CB caused a prompt inhibition of the contractions: 3 microM CB depressed the twitch contraction generated in Krebs solution by 12%, the K-contracture by 6%, and the ATP-induced contraction in the Ca-free solution by 59%. CD (3 microM) depressed the K-contracture by 31%, and the ATP-induced contraction in the Ca-free solution by 81%. CB and CD in 3 microM hyperpolarized the membrane and depressed the generation of action potential. From the above results, it was discussed that depressant effects of cytochalasins on twitch contractions in Krebs solution are at least in part due to the depression of membrane excitability, whereas contractions evoked in high K solutions are depressed by cytochalasins due to their effects on cytoskeleton.     

Noradrenaline-induced afterdepolarization in cat sympathetic preganglionic neurons in vitro

Sympathetic preganglionic neurons of the intermediolateral nucleus were identified by antidromic stimulation in the slice of the T2 or T3 segment of the cat spinal cord. In normal Krebs solution, the action potential of these neurons had a shoulder on the repolarization phase and was followed by a long-lasting afterhyperpolarization (AHP). The AHP had a fast and a slow component. Superfusion of the slice with noradrenaline (NA), 10-50 microM, resulted in depression of the shoulder on the repolarization phase of the action potential, in the appearance of an afterdepolarization (ADP), which was absent in control conditions, and in depression of the slow component of the AHP. These effects were present whether the membrane potential of the sympathetic preganglionic neurons was decreased, increased, or not changed by NA. A typical ADP had time to peak of 50 ms and decay time of 200-500 ms; the amplitude was variable and large ADPs could be suprathreshold, causing repetitive firing. The amplitude and duration of the ADP increased with NA concentration. The appearance of the ADP seemed to be independent of the depressant effect of NA on the slow AHP. The ADP was associated with a decrease in neuron input resistance and was voltage dependent, being depressed in nonlinear fashion by membrane hyperpolarization. The ADP decreased in amplitude or disappeared within a range of membrane potentials from -70 to -90 mV. The ADP was reversibly suppressed by the Ca-channel blocker cobalt (2 mM), by low Ca Krebs (0.25 mM), and by iontophoretic injection of ethyleneglycol-bis(B-aminoethyl-ether)-N,N'-tetraacetic acid into the cell. Increasing Ca concentration from 2.5 to 10.0 mM had no effect. The ADP was unaffected by tetrodotoxin, at a concentration blocking the Na spike, but was suppressed in Na-free medium, even when the Ca spike was prolonged by tetraethylammonium 20 mM. Changes in external K concentration from 3.6 to 2.5 or 10.0 mM did not change the ADP. Increasing intracellular Cl concentration or decreasing extracellular Cl concentration had no effect on the ADP. It is concluded that the ADP, evoked by NA, is due to an increase in membrane conductance involving Na and Ca ions, possibly a Ca-activated Na conductance. The ADP provides a mechanism with which NA may modulate sympathetic preganglionic neuron responsiveness to excitatory synaptic inputs.     

The effect of Mn on the electrical and mechanical activities in the guinea-pig stomach

The effects of Mn on electrical and mechanical activities in the circular muscle of the guinea-pig stomach were investigated. A low concentration of Mn (0.1 mM) in Krebs solution caused a prolongation of the first component, resulting in an increase in frequency of the second component. However, a high concentration of Mn (more than 1 mM) had a similar effect on the electrical activities in the early period, but subsequently inhibited them. The membrane was depolarized by Mn from 69 (S.D., +/- 6) to 63 (S.D., +/- 6) mV at 0.1 mM and to 42 (S.D., +/- 5) mV at 2 mM in the Krebs solution. The membrane resistance slightly decreased with 0.1 mM Mn, but slightly increased with 2 mM Mn. A low concentration of Mn (0.1 mM) also increased resting tone and phasic contraction. These contractile responses were completely inhibited when Ca was removed from the bathing solution. The enhanced, later phasic contraction was reduced by verapamil. All these electrical and mechanical responses for Mn were not related to the nervous system. Excess Ca (15 mM) inhibited the actions of Mn on the spontaneous slow waves, the resting potential, and the duration of the second component. Thus, it is concluded that Mn displaces or releases Ca from its stored sites, which are loosely-bound pools, and then induces membrane depolarization, resulting in subsequent stimulation of the Ca-channel, which is blocked by a high concentration of Mn.     

Effects of Ca removal on the smooth muscle of the guinea-pig taenia coli

1. The effects of removing the external Ca ions on the spontaneous and evoked activity of the smooth muscle of the guinea-pig taenia coli were investigated with the double sucrose-gap method.2. In Ca-free Locke solution the membrane was depolarized, the membrane resistance became low, the spike amplitude became small and the mechanical response decreased. In most preparations the electrical and mechanical activity was abolished within 10 min, but in some preparations the electrical activity continued for more than 30 min.3. In Ca-free solution containing 0.1 mM-EGTA, the membrane was depolarized and the electrical and mechanical activity was abolished within 5 min in every preparation. When NaCl was replaced with sucrose, the effects of Ca removal on the spike activity and contraction appeared very slowly and the membrane potential and membrane resistance remained unchanged.4. When Ca was replaced with Mg (2 mM) the spike was blocked within 1 min without depolarization or reduction of the membrane resistance. In Na-deficient (sucrose) solution, the presence of Mg accelerated the disappearance of the spike caused by Ca removal.5. In Ca-free solution containing 0.5 mM-Mg, a spike-like activity was observed without accompanying mechanical response. This activity was blocked by increasing the Mg concentration above 2 mM. It was Na-dependent, since it was abolished by removing Na from the external solution, but it was not influenced by tetrodotoxin (2 x 10(-6) g/ml.).6. It was concluded that calcium has at least two functions, one as current carrier for the action potential and another as controller of the Na permeability of the membrane. It was also suggested that the Ca which is bound at the membrane may be utilized as a source of Ca ions to carry the current for the action potential.     

Activation of the contractile mechanism in the anterior byssal retractor muscle of Mytilus edulis.

1. The electrical and mechanical responses of the anterior byssal retractor muscle (ABRM) of Mytilus edulis to acetylcholine (ACh), high [K]O or the removal of external Ca were examined under a variety of conditions. 2. ACh (10(-6)--10(-3)M) produced contracture tensions larger than those produced by high [K]O (30-300 mM) for a given amount of depolarization. In Ca-free solution, the rate of decline of ACh-contractures was much smaller than that of K contractures, though both ACh- and K-contractures eventually disappeared. 3. 5-HT (10(-4)M) of procaine (1 mM) markedly reduced the height of ACh-contractures, but had little or no effect on K-contractures. The height of K contractures was markedly decreased by Mn ions (20 mM) or low pH (4-5), while ACh-contractures remained unaffected. 4. Partial replacement of [Na]o by choline (30-100 mM) reduced both ACh-induced depolarization and contracture tension, whereas K-contractures remained unchanged even after total replacement of [Na]o by choline. 5. ACh could produce little or no tension when applied during the relaxation phase of K-contractures, while high [K]o produced the maximal contracture tension when applied during the relaxation phase of ACh-contractures. 6. Following the removal of external Ca from solutions containing less than 10 mM-Mg, the ABRM showed a marked tension development associated with repetitive electrical activity superimposed on a gradual decline of membrane potential. 7. These results suggest that ACh-contractures are mainly due to the release of intracellularly stored Ca, while K-contractures are mainly associated with the inward movement of external Ca.     

Interaction of 28Mg with Ca and K in the smooth muscle of guinea-pig taenia coli

1. The uptake of (28)Mg, and the total tissue content of Mg, Ca, Na and K have been determined in the smooth muscle of the guinea-pig taenia coli. The Mg content was 6.56 m-mole/kg fresh wt. immediately after dissection, falling slowly to 5.11 after 6 hr immersion in Krebs solution at 37 degrees C.2. The Mg content rose to 15.4 m-mole/kg fresh wt. during immersion in isotonic sucrose containing only MgCl(2). It was independent of the Mg(2+) concentration in this solution, but was depressed when K(+) or Ca(2+) ions were added.3. (28)Mg uptake showed three separate phases, extracellular, intermediate and slow. The size of the extracellular phase was proportional to the Mg(2+) concentration in the solution, but the size of the slow phase was constant. The size of the intermediate phase, exchanging with a half-time of a few minutes, was depressed when K(+) or Ca(2+) ions were added.4. The results are compatible with a competition between Mg in the intermediate phase of tracer exchange, and K(+) or Ca(2+) ions for fixed anionic sites in the tissue.     

The effect of external calcium and magnesium ions on the response of denervated muscle to acetylcholine.

1. The effect of external Ca and Mg on the membrane depolarization and contracture of rat denervated muscle in response to acetylcholine, ACh, was studied. 2. Raising external Ca concentration reduced the rate of rise and the amplitude of the ACh contracture, and prolonged its time course. 3. Increasing external Ca reduced the membrane depolarization in response to ACh. The dose increment required to elicit depolarization in high Ca concentration increased with external Ca, and was greater for depolarization than for contracture. 4. External Mg was less effect than Ca in reducing ACh depolarization but was more effective in reducing contracture. In contrast to Ca, Mg did not alter the time course of relaxation. 5. It is concluded that external Ca has two opposing effects on the ACh contracture: one of stabilizing the membrane and the other of increasing intracellular Ca concentration. External Mg may interfere with Ca influx and hence reduce contractility.     

Contribution of intracellular stored calcium to contractile activation in contractures of stomach circular muscle of Bufo vulgaris formosus

Contractile responses of stomach circular muscle of Bufo to high-K, to acetylcholine (ACh) in normal Ringer or in high-K solution, and to calcium in Ca-free high-K solution showed a phasic contraction which relaxed completely in 30-45 sec. K-induced contracture was abolished in Ca-free solution containing 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) within 20 sec, while ACh-induced contracture was not abolished and 10-25% of control tension was kept up to 40 sec. This response increased to 40-50% when all extracellular Na was replaced with tris(hydroxymethyl)-aminomethane (Tris). K-induced contracture was inhibited completely by 1 mM La. ACh-induced contracture in the muscle depolarized by high-K solution was dependent on the depolarization time, 0-10, 60-70, and nearly 100% of control after 1, 3, and 10 min depolarization, respectively. These ACh-induced contractures were not inhibited by 1 mM EGTA or La. All contractures mentioned above were markedly inhibited by 5 mM procaine. These results suggest that activation of both contractures induced by high-K and ACh were, at least partly, dependent on the Ca at the intracellular Ca storage sites. Ca-induced contracture was dependent on depolarization time as was ACh-induced contracture, when the muscle was depolarized by Ca-free high-K solution without pre-treatment with Ca-free Ringer solution. These results suggest that activation of Ca-contracture is also dependent on intracellular stored Ca.     

Effects of ionic calcium on the responses of canine testicular polymodal receptors to algesic substances

1. To explore possible mechanisms of the responses to algesic substances (bradykinin, hypertonic saline, and high K+ solution) of polymodal receptors in the canine testis, the Ca2+ concentration was varied in vitro. 2. After 1 min in Ca2+-free media, the responses to both high K+ solution (60 mM K+) and hypertonic saline (0.6 M Na+) were significantly augmented and tended to increase further with time; return to normal Ca2+ concentration quickly reversed these changes. These augmenting effects were blocked by the substitution of Mg2+ for Ca2+. The excitation produced by 60 mM K+ was decreased by increasing Ca2+ in a concentration-dependent fashion. 3. Reducing the bath concentration of K+ decreased responses evoked by 9 X 10(-8) M bradykinin (BK), whereas increased K+ concentration had the opposite effect. 4. The excitatory effects of BK were significantly suppressed in extracellular Ca2+-free condition. The suppression was not affected by the addition of Mg2+. Prostaglandin E2, which has been known to be released by BK and to augment the BK response, failed to restore the suppressed response by either preapplication (2.8 X 10(-7) M) or simultaneous application in high concentration (1.4 X 10(-5) M). 5. On the basis of these observations, it was postulated that Ca2+ concentration-dependent changes of the responses to 60 mM K+ and 0.6 M Na+ results from Ca2+-dependent "membrane surface potential" changes. The suppressed response to BK by Ca2+ depletion may be explained by the intervention of Ca2+-dependent processes other than PG production.     

Effects of manganese on the electrical and mechanical properties of frog skeletal muscle fibres

1. The effects of Mn on the electrical and mechanical properties of frog muscle fibres have been studied.2. In normal saline 10 or 20 mM-Mn hyperpolarized the fibres and had no effect on the membrane resistance. In isotonic K(2)SO(4) saline, Mn increased the membrane resistance indicating that this agent reduced the conductance to K.3. The action potential is prolonged by Mn while the overshoot amplitude is unaffected. The threshold of the action potential is shifted to more positive values of membrane potential.4. The isometric twitch is reduced by 45% in 10 mM-Mn; this effect is observed within 8 sec of the application.5. Mn (10 mM) reduced K contractures induced by 40 or 75 mM-K (constant [K].[Cl] product) and shifted to the right in a parallel manner the curve tension vs. log K concentration. The calculated mechanical threshold for K contractures was shifted from -48 to -33 mV.6. Caffeine contractures (3-4 mM) and supramaximal K contractures (190 mM-K) were unaffected by 10 mM indicating that contractile proteins and the ability of the sarcoplasmic reticulum to release Ca are not impaired.7. It is concluded that Mn is mainly affecting the excitation-contraction coupling by altering the mechanical threshold. Since Mn reduces the permeability to Ca in several excitable membranes, it is suggested that the mechanical threshold depends on the entry of Ca to the muscle.     

Effects of Na removal and readmission on the mechanical response in the guinea-pig tracheal smooth muscle

In the guinea-pig tracheal smooth muscle, tonic contraction, maintained in normal medium, was abolished by Ca removal or by indomethacin (10(-6) M). Removal of Na produced a transient contraction followed by a gradual decrease in tension to a level lower than the control. When Na was readmitted, a rapid relaxation was observed. The degree of the relaxation was enhanced with the exposure time to Na-free solution, and with the concentration of Na readmitted. At 5.9 mM K, the contraction produced by Ca readmission with simultaneous Na removal became smaller and slower as the external Na was reduced before Ca readmission, while at 40 mM K, the Ca-induced contraction was less dependent on the Na concentration. The rate of relaxation on Ca removal was slower when the Na concentration was reduced both at 5.9 and 40 mM K. Verapamil (10(-5) M) had a weak suppressing effect in normal solution, but suppressed markedly in 40 mM K medium. In the presence of verapamil, the difference between the effects of Na removal and of Na readmission on Ca-induced contraction was markedly potentiated, particularly at 40 mM K. It was concluded that the Na-Ca exchange mechanism may contribute to the transient response on Na removal and to the relaxation phase following Na readmission. At 5.9 mM K, removal of Na may increase the Ca conductance and also reduce the contribution of prostaglandins. On the other hand, when depolarized by excess K, the effect of Na is less significant; probably due to a high Ca conductance.     

Ultrastructure and contractures of the pigeon iris striated muscle

1. The ultrastructure of adult pigeon iris muscle fibres has been described with emphasis on the distribution of the sarcoplasmic reticulum (SR). Contractures due to superfusion with solutions of different [K(+)] (3-150 mM) and acetylcholine (ACh) and their modification by alteration of external [Ca(2+)] and [Mg(2+)] were studied in isolated pigeon iris.2. The arrangement of the contractile myofilaments was like that of vertebrate skeletal fibres. The SR is well developed in the I-band and sparse at the A-band level.Tubular elements (T-system) which form triads with the SR were seen at all levels of the sarcomere though usually adjacent to the A-I junction.3. K(+) contractures developed monotonically to a steady level which was maintained for the duration of the high [K(+)] superfusion. The response to a standard [K(+)] stepwise change was not altered by conditioning the preparation with various [K(+)].4. Decreasing external [Ca(2+)] from 20 mM to Ca(2+)-free (i.e. no Ca(2+) added), enhanced iris contractures at all [K(+)] and in ACh enriched solutions. The K(+) response was abolished when the iris was superfused with Ca(2+) free solution plus EDTA (2 mM) for 45 min. Increasing [Mg(2+)] had little or no effect on iris contracture.5. Reducing external [Ca(2+)] from 3 to 0.3 mM caused a reduction of 3-7 mV in resting membrane potential and an increase from 3 to 10 mM-Ca(2+) caused 3 to 7 mV membrane hyperpolarization. Muscle fibre input resistance was not affected.6. It is concluded that in the pigeon iris, Ca(2+) required for contractile activation is obtained from internal stores, that membrane potential determines the degree of contractile activation and that the maintenance of the contracture is dependent on the failure of the Ca(2+) releasing mechanism to inactive. In addition, it is speculated that because the iris muscle has only sparse SR at the A-band level of the sarcomere, there may be slow Ca(2+) reaccumulation.