Actions of prostaglandins and indomethacin on the electrical and mechanical properties of smooth muscle cells of the guinea-pig ileocecal junction

The effects of prostaglandins (PGs) and indomethacin on the mechanical and membrane properties of the smooth muscle cells of the guinea-pig ileocecal junction were studied using microelectrode and tension recording techniques and radioimmunoassay to determine levels of PGs. In the guinea-pig ileocecal junction, we found two distinct cell populations-cells with, and without spontaneous electrical and mechanical activities. PGs (PGE₁, PGE₂, PGF₂) in low concentrations suppressed spontaneous mechanical activity. Correspondingly, PGs (>10^–7 M) suppressed both spontaneously generated spikes and evoked spikes, presumably due to an increase in the threshold for generation of action potential. On the contrary, indomethacin evoked rhythmic spontaneous contractions in the mechanically quiescent muscle preparations and reduced the PGE and PGF content of the muscle. Spontaneous spike discharges occurred during the indomethacin-induced contractions. The spontaneous electrical and mechanical activities induced by indomethacin were suppressed by PGs, in low concentrations. These results indicate that under physiological conditions, the endogenous PGs in the muscle may play an important role in regulating muscle tone as well as the membrane properties, thereby contributing to the regulation of motility of the intestine and possibly the sphincter.     

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.     

General features of electrical and mechanical properties of smooth muscle cells in the guinea-pig abdominal aorta

The membrane potential in smooth muscle cells of the guinea-pig's abdominal aorta has a mean value of –57.2 mV. These cells are electrically connected and the space and time constant are 0.66 mm and 180 ms respectively. An increased K-concentration elicited a contraction at 20 mM and the maximum was reached at 77mM. The maximum depolarization produced by a tenfold increase of [K]₀ was 45 mV. Tetraethylammonium at concentrations exceeding 2 mM, depolarized the membrane, increased the membrane resistance and reduced the rectifying properties of the membrane. Only at 20 mM a small active response could be induced by outward current pulses. Low concentrations of noradrenaline (<10^–8 M) hyperpolarized the membrane, while higher concentrations (10^–7 M) depolarized. Isoprenaline at concentrations below 10^–7 M also hyperpolarized, but it depolarized from 10^–5 M onward. Acetylcholine at concentrations over 10^–8 M hyperpolarized the cells without exerting an effect on the resting tension, but it reduced a noradrenaline induced contraction. Low concentrations of caffeine (2 mM) hyperpolarize the membrane, while higher concentration (5 mM) depolarize. Caffeine is found to be a more efficient releaser of cellular Ca than noradrenaline. This might be due to the weak -agonist action of noradrenaline appearing at high noradrenaline concentrations. The hypothesis is supported by the finding that a -stimulation increases the Ca-uptake in the intracellular store. The study of the electrophysiological effects of different stimuli do not suggest an important role for electromechanical coupling in this tissue.     

吗啡对离体兔小肠平滑肌电变化及环肌和纵肌收缩活动的影响

本实验同时观察吗啡对离体兔十二指肠平滑肌电活动,肠腔内压变化以及纵肌收缩活动三方面的影响。结果显示:吗啡可使离体兔十二指肠节律性环肌收缩加强,纵肌收缩减弱,以及平滑肌峰电位幅度,数量和峰电位发生率增加。吗啡的小肠兴奋作用可被纳洛酮或阿托品阻断。吗啡对纵肌的抑制作用不被心得安阻断,但可被酚妥拉明减弱。实验结果表明吗啡对兔小肠环肌和纵肌作用相反,它兴奋环肌,抑制纵肌。吗啡是通过肠道阿片受体起作用的。乙酰胆碱参与了吗啡对小肠的作用。     

Quantitative aspects of electrical and mechanical responses to anisosmolar solutions in the smooth muscle of the rat portal vein

The electrical and mechanical activity of the rat portal vein were studied quantitatively under prolonged exposure to solutions in which osmolality was varied by changes in sucrose content. Reducing osmolality by 15 or 30 mosm/kg below the control value of 290 mosm/kg caused enhanced electrical and mechanical activity whereas hyperosmolality up to 390 mosm/kg led to inhibition as demonstrated earlier. These responses faded under prolonged exposure. In hypoosmolality integrated mechanical activity returned to control after about 10–15 min while spike activity remained somewhat increased. Prolonged hyperosmolality was associated with return of spike discharge towards control frequency whereas the integrated contractile force reverted from initial inhibition to levels above control after some 10–15 min. Therefore, the integrated force per spike was increased by prolonged hyperosmolality and decreased by hypoosmolality. Variations in osmolality had similar effects on the amplitude of K⁺ contractures. The time course of the osmotic responses are discussed in relation to the dynamic effects of passive stretch and shortening in the portal vein. The relation that may exist between “inotropic” effects of osmolality and the contractures obtained in strongly hypertonic media is considered.     

Neural regulation of electrical and mechanical activities in the rat tail artery

Stimulation of the perivascular nerves elicited two types of electrical responses in the rat tail artery—excitatory junction potentials (e.j.p.s) and slow depolarization—and two types of mechanical responses—fast and slow contractions. Fast phasic contractions were triggered whenever action potentials were generated from either the e.j.p. or the slow depolarization reaching threshold. Slow tonic contractions and slow depolarizations were sensitive to -adrenergic blockade. However the slow contraction always preceded the slow depolarization. Bolus doses of exogenous noradrenaline also induced slow contraction and slow depolarization and the development of tension also preceded the membrane potential change. Increasing the external KCl also induced membrane depolarization however, contractions were not observed until the membrane was depolarized positive of –49 mV. In contrast, tension developed readily with membrane potential more negative than –49 mV with exogenous noradrenaline and neural stimulation, suggesting that the action of noradrenaline was not mediated by electromechanical coupling. It was concluded that vascular activity in the rat tail artery could be regulated by the e.j.p., the slow depolarization and also by pharmacomechanical coupling.     

Effects of chloride on the electrical and mechanical properties of guinea pig ventricle

The purpose of this study was to investigate the influence of chloride on the electrical and mechanical properties of the guinea pig ventricular myocardium. Bathing media were made chloride free by substituting the relatively impermeant anion gluconate, isethionate, or sulfate. Removal of chloride increased contractility and decreased the duration of the action potential. Additional experiments explored the influence of chloride free media on electrogenic calcium influx estimated from the magnitude of the action potential in cells partially depolarized by potassium (the slow response). In the absence of chloride, transient increases occurred in the magnitude of the slow response while the positive inotropic effect was maintained. These experiments suggest that the effects of chloride free media are mediated secondarily by an enhanced calcium influx.     

Electrical and mechanical properties and neuro-effector transmission in the smooth muscle layer of the guinea-pig ileocecal junction

Electrical and mechanical properties and neuro-effector transmission were studied in circular strips of smooth muscle taken from the ileocecal junction of guinea-pigs in relation to sphincter action, using the microelectrode, and tension recording methods. The membrane potential of the smooth muscle was low (–43 mV) compared with the membrane potential of circular muscle cells of the ileum or caecum (–58 mV or –62mV). Only small populations of the muscle cells (about 5%) generated spontaneous action potentials.Field stimulation of the tissue produced an initial slight relaxation followed by a contraction, and the mechanical responses were accompanied by membrane hyperpolarization (i. j. p.) followed by repolarization with rebound spikes. Treatment with atropine increased the amplitude of i.j. ps and decreased the amplitue of the rebound repolarization. Propranolol or phentolamine did not affect the amplitude of i. j. p., however, phentolamine slightly reduced the amplitude of the rebound repolarization.These results indicate that the ileocecal junction is predominantly controlled by non-adrenergic, non-cholinergic inhibitory nerve fibres and that the distribution of adrenergic and cholinergic excitatory nerve fibres is sparse.     

Electrical and mechanical properties of spontaneous contraction in hypertensive rat portal vein

Contractile and electrical activities of longitudinal smooth muscle of portal vein from normotensive Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were compared. Amplitude and duration of spontaneous contraction of SHRSP portal vein were greater than those of WKY portal vein. No significant differences were observed in the resting membrane potentials between these preparations. Spontaneous spike activity appeared as a form of bursts. Duration of the burst and the number of spikes in each burst was greater in the portal vein of SHRSP than that of WKY. The amplitude of phasic and tonic components of K-contracture was also greater in SHRSP portal vein. Adrenergic and cholinergic nerves were not involved in the differences in contractions of the portal vein of these animal strains. Cross-sectional area of the longitudinal muscle layer was greater in SHRSP portal vein. These results suggest that the differences in spontaneous electrical activity are the cause of the differences in force and duration of the spontaneous contraction of portal vein from WKY and SHRSP, although the difference in excitation-contraction coupling of smooth muscle may be involved in much less extent.     

Effect of guinea-pig purified immunoglobulin G1 on the responsiveness of tracheal, aortic, vas deferens and ileum smooth muscles

Background Smooth muscles hyperresponsiveness is a common feature in anaphylaxis and allergic diseases. Objective The aim of the present work was to investigate the effect of in vitro passive sensitization with highly purified immunoglobulin GI (IgGl) on the responsiveness of tracheal, aortic, vas deferens and ileum smooth muscles. Method Firstly, IgGl, obtained from actively sensitized BFA guinea-pigs, was purified by Protein A-Sepharose column and characterized by enzyme-linked immunosorbent assay (ELISA) and immunoelectrophoresis analysis. Concentration-response curves to spasniogens (acelylcholine for trachea and vas deferens, noradrenaline for aorta and histamine for ileum) were established before and after in vitro passive sensitizalion with IgGl. Results Contractile responses and maximal contractions were significantly enhanced after passive sensitization for all the organs. Maximal contractions were significantly increased in the trachea (+46.7%), aorta (+51%), vas deferens (+114.2%) and ileum (+ 117.2%). At the end of the experiments, the application of the sensitizing antigen induced a significant Schultz-Dale reaction of the smooth muscles. Conclusion The present results show that the in vitro application of purified IgGl can produce non-specific smooth muscle hyperreactivity and hypersensitivity. So, IgGl can be considered as the main factor involved in the genesis of sensitization-induced hyperresponsiveness, and probably play a great role in hyperreactivity observed during allergic diseases and anaphylaxis.     

Effects of thymol on the electrical and mechanical properties of the guinea-pig taenia coli

Effects of thymol (0.02-2 mM) on the electrical and mechanical activities of the smooth muscle cells of the guinea-pig taenia coli were investigated with either micro-electrode or double sucrose gap methods.1. Thymol, in a concentration of more than 0.03 mM, reduced the amplitude and maximum rate of rise of the spikes without any change of the membrane potential. When the concentration was increased to 0.3 mM, thymol completely blocked the spontaneous and evoked spike activities. In a concentration of more than 0.1 mM, thymol reduced the membrane resistance in proportion to the concentration without any change of the membrane potential.2. Ionic mechanisms involved in the effects of thymol on the membrane resistance were investigated in various ionic environments. The results showed that in concentrations below 0.5 mM thymol might selectively increase the Cl-conductance of the membrane. Participations of Na and K ion in the effects of thymol on the membrane resistance could be eliminated. However, at more than 1 mM, thymol increased the membrane conductance non-selectively. Excess Ca in the external solution partly suppressed the action of thymol on the taenia coli.3. Potentiation of the twitch tension was not observed on treatment with any concentration of thymol.4. After pre-treatment with thymol (0.5 mM), isotonic K Krebs solution depolarized the membrane and reduced the membrane resistance as observed in the absence of thymol. However, thymol completely suppressed the K-induced contracture.5. Application of excess Ca and acetylcholine during the maintained contracture evoked by isotonic K Krebs solution induced further development of the contracture. However, on pre-treatment with thymol, neither excess Ca nor acetylcholine could evoke a mechanical response in isotonic K Krebs solution.6. The results obtained from the present experiments are discussed in relation to the roles of Ca on the smooth muscle cells.     

Role of protein kinase C in the excitatory action of cholinergic nerve stimulation on spontaneous activity of circular smooth muscle isolated from the guinea-pig stomach antrum

Following inhibition of NO production with nitroarginine, circular muscle isolated from the guinea-pig gastric antrum generated periodic slow potentials and unitary potentials. Transmural nerve stimulation (TNS) during the interval between slow potentials evoked an apamin-sensitive inhibitory junction potential (IJP) followed by an atropine-sensitive depolarization; the latter was either a transient depolarization with enhanced generation of unitary potentials or a slow potential. After inhibition of unitary potentials and slow potentials with 1 mM caffeine, TNS evoked an IJP and subsequent cholinergic depolarization, the latter developing slowly and lasting for about 10 s. TNS was unable to elicit a slow potential until a certain period of time had elapsed following the cessation of a slow potential. The period during which TNS could not evoke slow potentials (termed the high-threshold period) was about 10 s, and this period was increased by chelerythrine and decreased by phorbol esters. It is concluded that cholinergic nerve-mediated excitation of gastric muscle involves the activation of protein kinase C (PKC), and that the high-threshold period, during which the generation of slow potentials by TNS is inhibited, may be a consequence of reduced activity of PKC.     

A comparison of the electrical properties and morphological characteristics of the smooth muscle of the rat and guinea-pig vas deferens

Summary Microelectrodes were used to compare a variety of electrophysiological parameters of the rat and guinea-pig vas deferens. In comparison to the guinea pig, spontaneous junction potentials in the rat tissue were of shorter duration and occurred with greater frequency and amplitude. Action potentials induced by nerve stimulation could be observed in the smooth muscle of both species. However, in the rat tissue the majority of action potentials were generated in the impaled cell while 60% of the action potentials in the guinea-pig vas deferens were propagated. When current was intracellularly applied, spike potentials could be induced in approximately 90% of the cells of the rat vas deferens but in less than 10% of the cells of the guinea-pig vas deferens, The space constant was 1.48 mm for the guinea-pig vas deferens, but less than 0.5 mm for the rat vas deferens. Electronmicroscopic examination of the homologous tissues indicates that the differences in electrical properties can be accounted for in part by differences in morphology. The incidence and intimacy of neuromuscular contacts was greater in the rat vas deferens while the incidence of nexuses between smooth muscle cells was greater in the guinea-pig tissue.Supported by grants from the National Institute of Neurological Diseases and Stroke (NS 08300) and the West Virginia University Medical Corporation     

Effects of ruthenium red on membrane ionic currents in urinary bladder smooth muscle cells of the guinea-pig

 Three major ionic currents, Ca²⁺-dependent K⁺ current (I _K-Ca), delayed rectifier type K⁺ current (I _kd) and Ca²⁺ current (I _Ca), were activated by depolarization under whole-cell clamp in single smooth muscle cells isolated from guinea-pig urinary bladder. Externally applied ruthenium red (RuR) reduced the amplitude of I _K-Ca and I _Ca at 0 mV (IC₅₀ values were 4.2 and 5.6 μM, respectively), but did not affect I _Kd. Spontaneous transient outward currents (STOCs) and caffeine-induced outward currents (I _caf) at –30 mV were reduced by external 10 μM RuR. When 10 μM RuR was added to the pipette solution, I _K-Ca during depolarization, STOCs and I _caf significantly decreased with time. RuR did not change the unitary current amplitude of the large-conductance Ca²⁺-dependent K⁺ (BK) channels, but reduced the open probability of the channel under excised patch-clamp recording mode. RuR reduced the channel activity more effectively from the cytosolic face than from the other. This inhibition decreased when the cytosolic Ca²⁺ concentration was increased. These results indicate that RuR blocks BK and Ca²⁺ channels in urinary bladder smooth muscle cells. The decrease in I _K-Ca, STOCs and I _caf by RuR is attributable to the direct inhibition of BK channel activity, probably in addition to the inhibition of Ca²⁺ release from storage sites. The direct inhibition of BK channel activity by RuR may be related to the interaction of RuR with the Ca²⁺-binding sites of the channel protein. Received: 15 October 1997 / Received after revision and accepted: 25 November 1997     

Inhibitory actions of indomethacin on electrical and mechanical responses produced by nerve stimulation in circular smooth muscle of the guinea-pig gastric fundus.

The effects of indomethacin on electrical and mechanical responses produced by transmural nerve stimulation (TNS) were investigated in isolated circular smooth muscle of the guinea-pig gastric fundus. TNS evoked a cholinergic excitatory junction potential (e.j.p.). The e.j.p.s were inhibited by 1-10 microM indomethacin, in a concentration-dependent manner, with no marked alteration of the resting membrane potential. Exogenously applied acetylcholine caused a depolarization of the membrane that was not altered by indomethacin. TNS evoked a cholinergic twitch contraction at low frequencies (0.1 Hz). A train of TNS's at high frequency (1 Hz) produced a transient contraction with a subsequent sustained relaxation. Indomethacin reduced the resting tension and inhibited these TNS-induced contractions. Application of Nomega-nitro-L-arginine (NOLA), an inhibitor of nitric oxide (NO) synthesis, increased the amplitude of twitch contractions, and altered transient contractions to tetanic contractions during TNS at a frequency of 1 Hz, also with an increased amplitude. In the presence of NOLA, indomethacin (5 microM) again reduced the resting tension and inhibited TNS-induced contractions. This inhibition was greater for twitch contractions than for tetanic contractions. Nifedipine reduced the TNS-induced contractions, while addition of indomethacin further reduced the amplitude of contractions. Contractions produced by low concentrations of acetylcholine (0.1 microM) were inhibited by indomethacin, while those produced by 1 microM were not. These results indicate that the inhibitory actions of indomethacin on TNS-induced contractions do not involve enhanced production of NO or selective inhibition of voltage-gated Ca-channels. Prejunctional autoregulatory mechanisms may also not be altered by indomethacin. As indomethacin inhibits the enzyme cyclooxygenase, it is speculated that endogenously produced prostaglandins exert excitatory actions on gastric smooth muscle, and act mainly postjunctionally to facilitate spontaneous and neurogenic electrical and mechanical activity.     

Influence of thrombin concentration on the mechanical and morphological properties of cell-seeded fibrin hydrogels

Fibrin is a biopolymer that has been used in a variety of biomaterial, cell delivery and tissue engineering applications. The enzyme thrombin catalyzes the formation of fibrin microfibrils, which form a three-dimensional mesh in which cells can be directly embedded at the time of gel formation. In this study, fibrin hydrogels containing vascular smooth muscle cells were created using varying concentrations of thrombin. Over 7 days in culture, all gels decreased in volume as the fibrin matrix compacted, and the degree of gel compaction increased as thrombin concentration decreased. The material modulus and ultimate tensile stress of the gels also increased with decreasing thrombin concentration. Addition of thrombin to similar constructs made using collagen Type I did not show an effect on gel compaction or mechanical properties, suggesting that these effects were a result of thrombin's action on fibrin polymerization, and not cellular functions. Cell proliferation in fibrin hydrogels was not significantly affected by thrombin addition. Matrix examination using scanning electron microscopy showed increasing fibrin fiber diameters as thrombin concentration decreased. Confocal microscopic imaging of the actin cytoskeleton showed that cell morphology on two-dimensional substrates of fibrin showed marked changes, with higher thrombin concentrations producing cells with longer cellular projections. However, these morphological changes were not as apparent in cells embedded in three-dimensional (3-D) matrices, in which cells exhibited a similar morphology independent of thrombin concentration. These results relate features of the matrix and cellular components of 3-D fibrin constructs to mechanical properties, and contribute to the understanding of structure-function relationships in cell-seeded, 3-D protein hydrogels.     

Effects of K-free solution on tension development and Na content in vascular smooth muscles isolated from guinea-pig,rat and rabbit

The effects of K-free solution on tension development and cellular Na content were investigated in aortic strips isolated from guinea-pig, rat and rabbit and in rabbit ear artery. Experiments were conducted in the presence of phentolamine (5×10^–7 to 10^–6 M) in order to eliminate the possible influence of endogenous catecholamine. K-free solution produced contractions and increased cellular Na content in these vascular smooth muscle preparations. The rate of increase in K-free contraction of rabbit aorta and ear artery was much slower than that of guinea-pig and rat aortae. The rate of accumulation of Na in rabbit arterial smooth muscle preparations was also much slower than that in guinea-pig and rat aortae. When the relationship between tension development and cellular Na content was compared in these arteries, it was found that an increase in cellular Na produced a greater increase in tension in guinea-pig and rat aortae than in rabbit aorta and ear artery. It is suggested that the slow and small responses of rabbit arterial smooth muscles to K-free solution might be due to a slow accumulation of Na and to a smaller role of transmembrane Na gradient in the regulation of cellular Ca.     

Effect of sodium concentration and of atropine on the contractile response of the guinea-pig ileum to potassium ions

The possibility that the guinea-pig ileum's contractile response to K⁺-rich solutions is partly mediated by acetylcholine release from the intramural nervous tissue was examined by studying the inhibition of that response by atropine at different values of [Na⁺]₀. In a medium in which the NaCl was replaced by iso-osmotic glucose, the response to high [K⁺]₀ was not greatly affected, while the responses to acetylcholine and to other agonists were significantly reduced. In control medium (149 mM Na⁺), atropine (10^–7 M) partly inhibited the responses to K⁺-rich solutions and to agonists such as histamine, 5-hydroxytryptamine and bradykinin. When [Na⁺]₀ was reduced to 12 mM, by iso-osmotic substitution of glucose for NaCl, the response to high K⁺ was no longer inhibited by atropine, which still partly inhibited the contractions elicited by the three agonists and totally blocked the response to acetylcholine. It is proposed that atropine's inhibition of the response to high K⁺ and to agonists is not due to its specific anti-muscarinic effect, but to an unspecific action, which in the case of the agonists is independent of [Na⁺]₀. In addition, the inhibition of the response to high K⁺ would result from a different Na⁺-dependent mechanism, possibly involving the stimulation of the Na-K pump by atropine. This is supported by the observation that this drug partly relaxed ileum preparations that were contracted by ouabain.     

The differential effect of cooling on the responses of splenic capsular and vascular smooth muscle to nerve stimulation and noradrenaline

The responses of the capsular and vascular smooth muscle of the dog's spleen to splenic nerve stimulation and to infused noradrenaline have been studied in the isolated, blood-perfused preparation at 37°C, at 27°C and again after rewarming to 37°C. It was found that cooling per se had no effect on perfusion pressure but reduced splenic arterial blood flow, and caused no appreciable alteration in spleen volume. The increase in splenic flow resistance in response to nerve stimulation and noradrenaline was significantly greater at 27°C than at 37°C, but the concomitant reduction in spleen volume was significantly reduced.The enhanced effect of splenic nerve stimulation and noradrenaline on splenic flow resistance is discussed in terms of the relative contributions of an increased smooth muscle response and increased blood viscosity. The different effects of cooling on the responses of splenic vascular and capsular smooth muscle to nerve stimulation and noradrenaline are discussed in the context of the effect of cooling on other vascular and non-vascular smooth muscle.This work was supported by a grant from the Medical Research Council     

Effects of low-frequency electrical stimulation on fast and slow muscles of the rat

Leg muscles of adult rats were stimulated chronically at a low-frequency, and the histochemical reactions of various enzymes (succinic dehydrogenase, mitochondrial -glycerophosphate dehydrogenase, phosphorylase, alkali-ATPase and acid-ATPase), capillary density, resistance to fatigue, and contractile properties were studied. Following stimulation, the histochemical properties of muscle fibres in the fast extensor digitorum longus (EDL) and tibialis anterior (TA) muscles became similar to those of the majority of fibres in the slow soleus muscle. In the soleus muscle, the histochemical properties of the few fast type fibres became similar to the majority of slow fibres so that its fibre composition was homogeneously slow. The stimulated fast muscles also had higher capillary density and were more resistant to fatigue than normal. Despite the prolonged stimulation, the twitch duration of the fast muscles was little changed. This result differs from the findings obtained previously for the rabbit and cat, which show that slowing of contraction can be achieved by low-frequency activity of similar duration. Thus it may be that there is a species difference regarding the readiness with which the transformation of fast to slow muscles can be brought about.