Clonidine induced endothelium-dependent tonic contraction in circular muscle of the rat hepatic portal vein.

Clonidine, an alpha2-agonist, has been shown to be useful in the treatment of hepatic portal hypertension in cirrhosis. The mechanism has been attributed to a clonidine-induced decrease in sympathetic activity. While clonidine has been shown to stimulate the alpha2-adrenoceptors of blood vessels, there is limited knowledge of the effects of clonidine on the circular muscle of the hepatic portal vein which regulates its blood flow. To investigate clonidine-induced contraction of the circular muscle of the hepatic portal vein and to clarify the possible role of the endothelium in the contraction, we examined the effects of clonidine on the isometric contraction of endothelium-intact and -removed ring preparations of the rat hepatic portal vein. In endothelium-intact preparations, clonidine caused a concentration-dependent increase in the amplitude of contractions. Inhibition of NO synthesis with Nomega-nitro-L-arginine (L-NNA) elevated the resting tone, and increased the amplitude of the clonidine-induced contractions. Inhibition of cyclooxygenase by diclofenac did not change the amplitude of the clonidine-induced contractions observed both in the presence and absence of L-NNA. Application of a single concentration of clonidine induced a clear increase in amplitude of both twitch and tonic contractions. Twitch and tonic contractions induced by clonidine were inhibited by yohimbine. When the endothelium was damaged by sodium deoxycholate, tonic contractions induced by clonidine were completely suppressed, whereas the increase in twitch contractions was not influenced by chemical damage of the endothelium. Neither SKF-96365, a nonselective cation channel blocker, nor superoxide dismutase, a free radical scavenger, in the presence of catalase, changed the tonic contraction induced by clonidine. These results indicate that stimulation of alpha2-adrenoceptors enhanced twitch contractions and induced tonic contractions in the circular muscle of the rat hepatic portal vein, especially in the absence of NO. The latter, but not the former, occurs through an endothelium-dependent pathway.     

Effects of Dai-kenchu-to on spontaneous activity in the mouse small intestine.

The effects of Dai-kenchu-to (DKT), a Chinese medicine, on spontaneous activity of mouse small intestine were investigated. Experiments were carried out with tension recording and intracellular recording. DKT contracted mouse longitudinal smooth muscles in a dose dependent manner (0.1-10 mg/ml). Low concentration of DKT (0.1 mg/ml) did not contract the longitudinal muscles of mouse small intestine. DKT (0.1 mg/ml) inhibited contraction elicited by transmural nerve stimulation (TNS). DKT (1 mg/ml) evoked relaxation before contraction. The initial relaxation was abolished by Nomega-nitro-L-arginine (L-NNA). DKT (10 mg/ml)-induced contraction had two components: a transient rapid contraction and a following slow contraction. Atropine inhibited DKT (1 mg/ml)-induced contraction to about 50% of control. In the presence of atropine, tetrodotoxin (TTX) inhibited the contraction elicited by DKT (1 mg/ml) to about 80%. DKT depolarized the membrane and decreased the amplitude of pacemaker potentials recorded from in situ myenteric interstitial cells of Cajal (ICC-MY) with no alteration to the frequency, duration and maximum rates of rise in the presence of nifedipine and TTX. The same results were obtained in slow waves recorded from circular smooth muscle cells. These results indicate that DKT evoked both contraction and relaxation by releasing acetylcholine, nitric oxide and other excitatory neurotransmitters in mouse small intestine. DKT had no effects on pacemaker mechanisms and electrical coupling between ICC-MY and smooth muscle cells in mouse small intestine. The results also suggest that DKT may contract smooth muscles by depolarizing the membrane directly.     

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.     

Effects of L-arginine on spontaneous contraction of the rat portal vein

The effects of L-arginine on spontaneous contraction of endothelium-denuded longitudinal preparations of the rat portal vein were studied. L-arginine increased the frequency of spontaneous contraction concentration-dependently between 10 microM and 1 mM. Changes in contraction amplitude and duration were not remarkable. D-arginine had a negligible effect on spontaneous contraction. N(omega)-nitro-L-arginine (1 mM) did not affect spontaneous contraction or the response to L-arginine. Addition of N(G)-monomethyl-L-arginine (1 mM), l-lysine (1 mM) or N-ethymaleimide (0.1 mM) increased the frequency of spontaneous contractions and inhibited the effect of L-arginine. Glibenclamide (10 microM) did not affect spontaneous contraction or the response to L-arginine. Spontaneous increase in concentration of intracellular Ca2+, estimated as the ratio of Fura-PE3 fluorescence occurred synchronously with spontaneous contraction. Spontaneous increase in concentration of intracellular Ca2+ occurred more frequently in the presence of L-arginine (1 mM). L-arginine (1 mM) also increased the number of action potential bursts/min in the longitudinal smooth muscle layer. L-arginine (1 mM) also depolarized cell membranes. This study indicates that L-arginine increases the frequency of spontaneous contraction of longitudinal muscle in the rat portal vein by membrane depolarization through mechanisms that do not involve nitric oxide or the inhibition of ATP-sensitive K+ channels.     

Evidence for the involvement of the cyclooxygenase-metabolic pathway in diclofenac-induced inhibition of spontaneous contraction of rat portal vein smooth muscle cells.

The effects of diclofenac, a cyclooxygenase (COX) inhibitor, were investigated on spontaneous phasic contractions of longitudinal preparations of the rat portal vein. Diclofenac produced a concentration-dependent decrease in the amplitude of these spontaneous phasic contractions. Diclofenac (30 microM) decreased the amplitude of the spontaneous phasic increase in the F340/F380 ratio of Fura PE3, an indicator of intracellular Ca2+ concentration. It also reduced the number of action potentials in each burst discharge without changing the resting membrane potential of longitudinal smooth muscle cells. The extent of the distribution of Lucifer Yellow injected into a smooth muscle cell was decreased in the presence of diclofenac (30 microM). Both AH6809, a prostanoid EP receptor antagonist, and SQ22536, an adenylate cyclase inhibitor, decreased the amplitude of the spontaneous contractions. On the other hand, neither ozagrel, a thromboxane synthase inhibitor, nor SQ29548, a prostanoid TP receptor antagonist, significantly affected spontaneous contractions. These results indicate that diclofenac inhibits the amplitude of spontaneous contractions of the rat portal vein through inhibition of electrical activity, which may be related to an inhibition of the cyclooxygenase pathway.     

Mechanisms of endothelin-induced augmentation of the electrical and mechanical activity in rat portal vein

Actions of porcine endothelin (ET) on the electrical and mechanical activity of the rat portal vein were investigated by means of the intracellular microelectrode and isometric tension recording techniques, ET (> 0.1 nM) enhanced the amplitude and frequency of the spontaneous contractions which ceased in the presence of 100 nM dihydropyridine derivatives (nifedipine or nicardipine). ET (0.15 nM) increased the frequency of the spontaneous action potentials, with no change in the basal membrane potential. Higher concentrations of ET (≧ 0.3 nM) further depolarized the membrane potential and increased the spike frequency. After blocking the spontaneous action potentials with nifedipine (100 nM), ET still depolarized the membrane. The depolarization was associated with a reduction in the electrotonic potential and was blocked in a Na-deficient solution (15.5 mM) but not in Ca-free, K-deficient or Cl-deficient solutions. In a Na-deficient solution, ET still evoked action potentials without depolarization. In Ca-free solution, ET depolarized the membrane potential with small oscillations, which were blocked by nifedipine (100 nM). The results indicate that in the rat portal vein, ET enhances electrical and mechanical responses through activation of the dihydropyridine-sensitive and voltage-dependent Ca channels. Acceleration of the Ca entry induced by ET can occur with or without depolarization of the membrane and can enhance the pacemaking mechanism.     

Membrane properties and innervation of smooth muscle cells in Hirschsprung's disease

Mechanical and membrane properties of smooth muscle cells and/or neuroeffector transmission in the aganglionic segment of the large intestine (Hirschsprung's disease) were compared with findings in the ganglionic segment. Tension-recording, microelectrode, and double sucrose gap methods were used. There was no difference in resting membrane potential of the longitudinal or circular muscle cell in these two segments, which were obtained at biopsy in Japanese children. In the ganglionic preparations, generations of regular prepotentials, with or without the spike, correlated well to the rhythmic contractions. However, in the aganglionic segment, irregular spike and contraction only were observed. In the circular or longitudinal muscle of the ganglionic segments, field stimulations evoked inhibitory junction potentials, excitatory junction potentials, or both and triggered initial relaxation and then a contraction of the tissue. In the aganglionic segment, however, field stimulation evoked only excitatory junction potentials followed by contraction. These results indicate that, in cases of Hirschsprung's disease, there may be a deficiency in the nonadrenergic inhibitory pathways. This is the first evidence for a lack of spasticity in muscle from the aganglionic segment of the large intestine obtained from children with Hirschsprung's disease.     

Selective suppression of some components of spontaneous activity in various types of smooth muscle by iproveratril (verapamil)

Summary Intra- and extracellular measurements of electrical activity were recorded simultaneously with tension development from isolated preparations of taenia coli, portal vein, stomach and ureter of the guinea-pig. Iproveratril (verapamil) inhibits electrical and mechanical activity in parallel in spontaneously active preparations of taenia coli and portal vein. Spike discharges and second-rhythm (SR) oscillations are both suppressed in the electrical activity. In portal vein, the relatively weak basic organ specific rhythm (BOR) of this tissues is also suppressed. The minute-rhythm (MR) appears less sensitive or resistant to iproveratril. In ureter and antrum preparations of the stomach there is an additional iproveratril resistant component: the basic electrical process of stomach and ureter peristalsis which, in the classification of smooth muscle rhythms, belongs to the type BOR. The superimposed spike potentials of the antrum and the spike-like oscillations of the ureter action potential are inhibited similarly as are the spike potentials in taenia coli and portal vein. The similarity between the iproveratril effects and the effect of Ca-deprivation indicates a Ca-antagonistic action of iproveratril. Conclusions concerning the hypothesis of Ca-spikes in smooth muscle are discussed.     

Relaxing action of sodium nitroprusside independent of membrane potential in the CCh-induced contracture of the guinea pig stomach muscle.

Carbachol (CCh, 10(-6) M) induced biphasic contraction of longitudinal muscle of the guinea pig stomach, consisting of rapid phasic contraction and contracture. The contracture was almost completely inhibited by sodium nitroprusside (SNP, 10(-6) M) and S-nitroso-N-acetyl penicillamine (SNAP, 10(-6) M). A membrane permeable analogue of cyclic GMP, 8Br-cGMP (10(-4) M), also inhibited the CCh induced contracture. Although a heme site inhibitor of nitric oxide-sensitive guanylyl cyclase, 1-H-[1, 2, 4] oxadiazolo-[4, 3 a]quinoxalin-1-one (ODQ; 10(-6) M), reduced the inhibitory action of SNP, it did not affect the inhibitory action of 8Br-cGMP, indicating that the effect of SNP was developed via cyclic GMP production in the presence of D600. Charybdotoxin (10(-7) M), an inhibitor of Ca2+ activated K+ channel, did not influence on the CCh induced contracture. On the other hand, CCh induced a depolarization of the longitudinal muscle cell membrane (from -60 mV to -45 mV) in the presence of 10(-6) M D600, but SNP did not affect the depolarization. These results suggest that in the presence of D600 SNP induces relaxation of CCh induced contracture of the longitudinal muscle of the guinea pig stomach via cyclic GMP but not membrane potential dependent mechanism.     

Dual concentration-dependent effects of phorbol 12, 13-dibutyrate on spontaneous and acetylcholine-induced electrical responses recorded from isolated circular smooth muscle of the guinea-pig stomach antrum.

Intracellular recordings of electrical activity were made from circular smooth muscle cells in small segments of tissue isolated from the guinea-pig stomach antrum. Every cell that was impaled exhibited a rhythmic generation of slow potentials. Experiments were carried out to test the effects of three different concentrations (1, 10 and 100 nM) of phorbol 12, 13-dibutyrate (PDBu) on these slow potentials and on the responses produced by acetylcholine (ACh), in the presence of nifedipine and N(omega)-nitro-L-arginine (nitroarginine), known inhibitors of L-type Ca-channels and nitric oxide synthase, respectively. The resting membrane potential was -62 +/- 7 mV, while the frequency and amplitude of the slow potentials were 1.6 +/- 0.1 cycle per min (cpm) and 33 +/- 1 mV, respectively. Application of 1 nM PDBu increased the frequency of slow potentials, with no significant change in the membrane potential and amplitude of slow potentials. At a concentration of 100 nM, PDBu depolarized the membrane by about 6 mV, and either decreased the amplitude and frequency of the slow potentials or abolished them. The amplitude and frequency of the slow potentials were not significantly changed in the presence of 10 nM PDBu. In the presence of chelerythrine (1-2 microM), a known inhibitor of protein kinase C (PKC), the increase in frequency of slow potentials by 1 nM PDBu and depolarization produced by 100 nM PDBu were not elicited. The increase in frequency of slow potentials by 100 nM ACh was inhibited by PDBu, in a concentration-dependent manner, and ACh-responses were abolished in the presence of 100 nM PDBu. These results indicate that PDBu has dual actions on the spontaneous activity of antral circular muscle, with low concentrations increasing and high concentrations inhibiting the frequency of the slow potentials. The former may be produced by activation of protein kinase C (PKC). As the ACh-induced excitation of slow potentials is inhibited by PDBu, a possible causal relationship between the inhibition and over-activation of PKC is considered.     

Regulation of voltage-dependent excitatory responses to alpha,beta-methylene ATP, ATP and non-adrenergic nerve stimulation by dihydropyridines in the guinea-pig vas deferens

Simultaneous recordings of mechanical and intracellular electrical activity were obtained from the guinea-pig vas deferens, where nerve stimulation, ATP and the stable nucleotide analogue alpha,beta-methylene ATP elicited excitatory responses. Excitatory junction potentials and action potentials were elicited by low-frequency (trains of pulses, generally less than or equal to 2 Hz) field stimulation. alpha,beta-Methylene ATP and ATP elicited only concentration-dependent depolarizations at low concentrations, while higher concentrations elicited a superimposed action potential discharge which was accompanied by mechanical contraction. The voltage threshold at which action potential discharge was initiated by these three stimuli was about -45 mV (resting membrane potential averaged -66 mV). Action potential discharges and contractile responses were antagonized by nifedipine and augmented by Bay K 8644 at concentrations (1 and 0.5 microM, respectively) which exhibited only small effects on either excitatory junction potential amplitudes or nucleotide-induced depolarizations. Bay K 8644 enhanced and nifedipine antagonized the repolarization (rectification) phase of action potential discharge elicited by nerve stimulation and drugs; after-hyperpolarizations were prominent in the presence of Bay K 8644 (0.1-5 microM). Excitatory junction potentials were antagonized after exposure to alpha,beta-methylene ATP. This antagonistic effect of alpha,beta-methylene ATP was also observed following depolarizations elicited in the absence and presence of nifedipine (1 microM). Noradrenaline was approximately 50-100 times less potent than alpha,beta-methylene ATP in eliciting action potential discharge and contraction. It was only when a high concentration of noradrenaline was used (about 60-100 microM) that the noradrenaline-induced depolarization attained the voltage threshold for action potential initiation. These results illustrate the similarity of the electrical components which underlie excitation by nerve stimulation and adenine nucleotides in the vas deferens, and demonstrate the ability of dihydropyridines to regulate voltage-dependent events associated with both the generation and inactivation of muscle action potentials. These are probably voltage-dependent calcium currents and calcium-activated potassium currents, respectively. Neither excitatory junction potentials nor the mechanism of desensitization of the ATP purinoceptor by alpha,beta-methylene ATP involve voltage-dependent calcium channels.     

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.     

Comparative effects of Mg, Ca, Sr, and verapamil on the uterine longitudinal muscle of spayed and estrogen-treated rats

A comparison was made of effects of Mg, Ca, Sr, or verapamil on the mechanical and electrical activities in the uterine longitudinal muscles of spayed and estrogen-treated rats. The muscle strips taken from spayed rat exhibited spontaneous rhythmic activity in the Locke solution which did not contain Mg, whereas spontaneous activity was less frequent in the preparation taken from estrogen-treated rat. The resting potentials were -54 and -61 mV in the spayed and the estrogen-treated preparations, respectively. An initial spike potential followed by plateau potential with abortive spikes on the top was generated in both spayed and estrogen-treated preparations. In the spayed preparation, the frequency of rhythmic contractions was reduced, and the base-line tension was lowered when 0.6 mM Mg was added to the solution. The base-line tension was elevated progressively when the external Ca concentration was raised, and reached a maximal value up to 10 mM. The amplitude of phasic contraction was progressively increased by increasing Ca concentrations in the range from 1.25 to 5 mM, and was reduced by Ca higher than 10 mM. In the estrogen-treated preparation, the amplitude of phasic contraction was increased by increasing Ca concentrations in the range from 1.25 up to 17.5 mM. When the amplitude of phasic contraction was increased, the duration of plateau potential became protracted. Substitution of the external Ca with Sr caused an increase in the spike activity generated on the top of plateau potential. However, the amplitude of phasic contraction was diminished in both the spayed and the estrogen-treated preparations. Verapamil (2 microM) caused a stronger depression of electrical and mechanical activity in the spayed preparation. Results were discussed in relation to the genomic effects of estradiol on the membrane properties so as to change the interaction with divalent cations.     

Comparative investigations of alpha- and beta-effects on the longitudinal and circular muscles of the pregnant rat myometrium.

Comparative effects of noradrenaline (10(-8)-10(-6) g/ml) and isoprenaline (10(-10)-10(-6) g/ml) on electrical and contractile activity were investigated in longitudinal and circular muscle strips of rat myometrium (13-19 days pregnant). When recorded with an intracellular microelectrode, spike potentials of longitudinal muscle discharged spontaneously in bursts. Treatment with either noradrenaline or isoprenaline hyperpolarized the membrane and increased membrane conductance, causing the cessation of spontaneous activity. Slow potential was dominant in the circular muscle, and it became prolonged after treatment with noradrenaline, leading to an increase in tension. Isoprenaline reduced the duration of the slow potential and depressed the contraction. The minimal concentration of isoprenaline needed to suppress the spontaneous activity in circular muscle was much higher than in longitudinal muscle. The excitatory effect of noradrenaline on circular muscle was antagonized by treatment with phentolamine, and the inhibitory effect of catecholamines on both longitudinal and circular muscles by propranolol. alpha-Adrenoceptor appears predominant in circular muscle, while beta-adrenoceptor in longitudinal muscle in rat myometrium during late-pregnancy.     

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 properties of the smooth muscle of mouse vas deferens.

1. Contractions of the mouse vas deferens in response to electrical stimulation differ form those recorded form the guinea-pig vas deferens in that they are abolished by tetrodotoxin. 2. Changes in membrane potentials were recorded form the smooth muscle of both preparations in response to stimulation with current pulses applied by an intracellular electrode and by alrge extracellular plate electrodes. 3. Both preparations behaved similarly in response to intracellular stimulation. Electrotonic potentials in response to extracellular current pulses spread in a longitudinal direction in the guinea-pig vas deferens in accordance with the cable-like properties of this preparation. In contrast, no longitudinal spread of eletrotonus was observed in the mouse vas deferens. 4. Responses to nerve stimulation differed in the two preparations. In the guinea-pig, single stimuli caused excitatory junction potentials (e.j.p.s) which gave rise to action potentials. Some cells from the mouse vas deferens showed similar e.j.p.s and action potentials, although the threshold for the initiation of action potentials was lower and more variable. 5. The majority of cells in the mouse vas deferens failed to show action potentials in response to a single stimuli even though the amplitude of e.j.p.s was from 35 to 40 mV. This was probably due to the large resting membrane potentials of these cells, as all-or-nothing action potentials could be evoked if successive e.j.p.s were allowed to sum with each other or if a depolarizing current pulse was applied at the peak of an e.j.p. 6. The nature of the response to nerve stimulation recorded from differnt cells in the mouse vas deferens could be correlated with the amplitude and time course of the response of the same cell to intracellular stimulation. 7. It is concluded that individual smooth muscle cells in both preparations are probably coupled electrically but that there are few, if any, low resistance pathways in the longitudinal direction in the mouse vas deferens.     

Caffeine-induced contraction of rat portal vein and effects of K-depolarization, Na-removal and low temperature.

Contribution of Ca influx from extracellular pool and Ca-release from store sites in caffeine-induced contraction of rat portal vein longitudinal muscle were examined. At 37 degrees C caffeine induced a phasic contraction and the contraction was inhibited by verapamil or in the absence of Ca. Under low temperature, it was not decreased remarkably by verapamil or by the removal of extracellular Ca. Na-removal potentiated caffeine-induced contraction in the absence of Ca. Caffeine-induced contraction was also potentiated by high-K-depolarization. These contractions were at both temperature inhibited greatly by ryanodine. Caffeine induced the burst of the action potential at 37 degrees C but it was not remarkable at 17 degrees C. These results indicate that both extracellular Ca influx and release of stored Ca are involved in the caffeine-induced contraction. However, dependence of the contraction on Ca sources are influenced by temperature, extracellular Na and membrane potential.     

The effects of sodium nitroprusside on mediator release and the functional properties of postsynaptic membranes in the neuromuscular synapse

Experiments on neuromuscular preparations of frog skin-thoracic muscle and sartorius muscle, using extracellular recording and two-electrode clamping of the muscle fiber membrane potential, were used to study the effects of the nitric oxide donor sodium nitroprusside on endplate currents. At a concentration of 100 μM, sodium nitroprusside sharply decreased the amplitude and quantum composition of the endplate currents, and also decreased the miniature endplate current frequency. The amplitude-time characteristics of miniature endplate currents, the voltage-dependent amplitude, and the decay time constant of miniature endplate currents did not change as compared with controls. However, unlike the situation with other secretion inhibitors, the decrease in endplate current amplitude was not accompanied by increased facilitation in response to rhythmic stimulation or changes in postsynaptic potentiation in conditions of application of pairs of stimuli to muscles. The suppression of acetylcholine secretion was not seen with inactivated sodium nitroprusside solution. These results provide evidence that nitric oxide can be a powerful inhibitor of both spontaneous and evoked transmitter secretion in the neuromuscular synapse, and that this is accompanied by decreases in the efficiency of presynaptic forms of short-term plasticity, while the functional characteristics of the postsynaptic membrane remain unchanged. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 85, No. 5, pp. 663–670, May, 1999.     

Physiological comparison of the longitudinal and circular muscles of the pregnant rat uterus.

A comparative investigation of the longitudinal and circular muscles in the pregnant rat uterus (10-15 days) was made by means of electrical and mechanical recordings. The response of the circular muscle strip was characterized in the following respects: application of stretch caused acceleration of spontaneous activity that was greater in extent than in the longitudinal muscle strip; tetanic contraction was not produced by repetitive stimuli in the range of 0.1-5 Hz; slow potential was dominant in the circular muscle cells. The longitudinal contraction of the uterine segment occurred in synchronization with the change in the intraluminal pressure. Either the longitudinal stretch or the increase in the intraluminal volume caused the acceleration of synchronized activity. Stimulation of the longitudinal muscle caused membrane response in the circular muscle cells and vice versa, suggesting electrical interference between longitudinal and circular muscle cells.     

Cellular mechanisms of myogenic activity in gastric smooth muscle

In many regions of the intestine, a thin layer of interstitial cells of Cajal (ICC) lie in the myenteric region, between the circular and longitudinal muscle layers. ICC are connected by gap junctions to surrounding ICC and also with circular and longitudinal smooth muscle cells, forming a large electrical syncytium. Damage of the ICC causes a disorder in the patterns of rhythmic activity. Isolated ICC produce a rhythmic oscillation of the membrane potential. All these observations have led to the suggestion that ICC may be the pacemaker cell responsible for intestinal activity. Gastric smooth muscles generate slow oscillatory membrane potential changes (slow waves) and spike potentials. The activity is considered to be linked to the metabolism in the cell. Three types of cells located in the gastric wall (circular and longitudinal smooth muscle cells and ICC) produce synchronized electrical responses with different shapes. The electrical responses appear to originate in ICC and then spread to the smooth muscle layers, indicating that ICC may also be the pacemaker cells responsible for gastric activity. However, isolated circular smooth muscle tissues spontaneously generate regenerative potentials, suggesting that there are at least two sites for the initiation of spontaneous activity in the stomach. Regenerative potentials persist in the presence of Ca-antagonists and are inhibited by agents which disrupt intracellular Ca(2+) homeostasis. Depolarization of the membrane elicits regenerative potentials after a long delay and the potentials have long refractory periods. This suggests that an unidentified 2nd messenger may be formed during the delay between membrane depolarization and the initiation of a regenerative potential. In gastric muscles of mutant mice which do not express inositol trisphosphate (InsP(3)) receptors, spike potentials but not slow waves are generated, suggesting the possible involvement of InsP(3) in the initiation of spontaneous activity.