Effect of hypoxia and fenigidin on the action potential duration and the strength of myocardial contraction in frogs

A 40-minute perfusion of the frog ventricular strip with a hypoxic Ringer solution brings about a decline in action potential duration by 37 plus or minus 10% whereas the contractility is completely suppressed by that time. The time constant of the contractility exponential decline constitutes 8 plus or minus 2 min. the effect of hypoxia is reversible. Complete blockade of the calcium channels by fenigidin (3.5 x 10(-5) M) irreversibly lowers action potential duration by 62 plus or minus 2%. Contraction is completely suppressed on attaining the given level of action potentials. Hypoxia does not virtually affect action potential after the preliminary blockade of the calcium channels by fenigidin. It is suggested that the hypoxic solution mainly blocks the inward current across the calcium channels without affecting the potassium permeability of the sarcolemma. Meanwhile complete suppression of the contractility under hypoxia cannot be accounted for only by the blockade of the calcium channels.     

Effects of simulated ischaemia on the electrical activity of ventricular myocardium in the presence of antiarrhythmic drugs.

The effects of a nutrient solution simulating the 'ischaemic milieu' (combined hyperkalaemia, hypoxia and acidosis) on the electrical activity of rabbit isolated ventricular myocardium were examined in the absence and presence of antiarrhythmic drugs. In such a simulated ischaemia the resting membrane potential, the rate of depolarization (Vmax) and the action potential duration (APD) were all diminished with a resultant decrease in conduction and a shortening of the effective refractory period (ERP). Sotalol, a Class 3 antiarrhythmic drug (5 x 10(-6)-10(-5) M/l) afforded a marked protection against the "ischaemic" abbreviation of the ERP and APD. To a much lesser extent, the same applied to propafenone, a predominantly Class 1C antidysrhythmic agent (1.65-3.3 x 10(-6) M/l). The 'ischaemia'-induced depression of Vmax was increased considerably by propafenone and diminished slightly by sotalol. The results are in keeping with the efficacy of propafenone and sotalol in ventricular arrhythmias of ischaemic origin and also with the proposal that the major mechanism by which propafenone inhibits postinfarction ventricular arrhythmias is a further depression of ischaemic myocardial cells resulting in conduction block of the reentrant wave front.     

Some physiological characteristics of myelinated and unmyelinated fibers with very low conduction velocity

The compound action potential arising in response to supramaximal stimulation of Aδ- or C-fibers of a cat cutaneous nerve (the saphenous nerve) was investigated by methods improving the signal/noise ratio in the record of the unit evoked response. By the use of optical and computer (BÉSM-3M) methods of coherent signal accumulation followed by averaging, potentials of nerve fibers ranging in amplitude from 20 to 0.05 µV and in duration from 10 to 0.4 msec were distinguished from the apparatus noise. A continuous distribution of nerve fibers by conduction velocity was found over the range from 80 to 0.15 m/sec. The conditions of appearance of low-amplitude action potentials of nerve fibers with a low conduction velocity are discussed.     

Tetrodotoxin Blockage of Sodium Conductance Increase in Lobster Giant Axons

Previous studies suggested that tetrodotoxin, a poison from the puffer fish, blocks conduction of nerve and muscle through its rather selective inhibition of the sodium-carrying mechanism. In order to verify this hypothesis, observations have been made of sodium and potassium currents in the lobster giant axons treated with tetrodotoxin by means of the sucrose-gap voltage-clamp technique. Tetrodotoxin at concentrations of 1 x 10^-7 to 5 x 10^-9 gm/ml blocked the action potential but had no effect on the resting potential. Partial or complete recovery might have occurred on washing with normal medium. The increase in sodium conductance normally occurring upon depolarization was very effectively suppressed when the action potential was blocked after tetrodotoxin, while the delayed increase in potassium conductance underwent no change. It is concluded that tetrodotoxin, at very low concentrations, blocks the action potential production through its selective inhibition of the sodium-carrying mechanism while keeping the potassium-carrying mechanism intact.     

Effect of primycin on some electric properties of the frog skeletal muscle

The effect of primycin, a guanidine-type antibiotic was studied on the electric properties and 42K+ uptake of the frog sartorius and semitendinosus muscle. Both in normal and choline chloride Ringer solution, primycin evoked a concentration and time dependent depolarization of the surface membrane of the muscle. This depolarization was significantly increased by Na ions. Primycin treatment was shown to evoke a dose-dependent decrease of the depolarization induced by 20 mM K+-Ringer. When the muscles were incubated in a Ringer solution containing choline chloride, during an incubation period of 30 min the uptake of 42K+ was decreased to 12% upon the exposure to 5 x 10(-6) mol primycin as compared to the control value. As the primycin-induced depolarization increased, the shape and amplitude of the action potentials elicited by square-wave electric impulses were altered and decreased, respectively. In sodium isaethionate Ringer 1--2 x 10(-6) M primycin induced a slow depolarization resulting in firing potentials. The results suggest that primycin depolarizes the surface membrane exclusively through the blockade of the resting K+ channels, the other phenomena being the results of this depolarizing effect.     

Effects of calcitonin gene-related peptide on neuromuscular transmission in the isolated rat diaphragm

The present study was undertaken to investigate a possible interaction between the cholinergic nerve neurotransmitter and CGRP on neuromuscular transmission in the isolated rat diaphragm. Electrical stimulation of the isolated phrenic nerve resulted in twitch contractions which were dose-dependently potentiated by CGRP in concentrations ranging from 1.2 x 10(-9) M to 3 x 10(-7) M. The potentiating action of CGRP (3 x 10(-7) M) disappeared in about 25 min. The same dose of CGRP 40 min later produced an augmentation of contraction amplitude similar to that observed prior to the administration of CGRP. The action of CGRP was dependent upon the stimulation pulse width ranging from 0.2 to 1.0 msec. Rat calcitonin (4.5 x 10(-7) M) caused a minimal change in the amplitude of twitch contractions. CGRP had no effect on the quiescent striated muscle. Twitch responses to direct electrical stimulation were also enhanced by CGRP (6 x 10(-8) M-6 x 10(-7) M) in the absence and presence of 10(-5) M d-tubocurarine. These results suggest that CGRP modulates the action of acetylcholine at the motor end plates of striated muscle.     

七叶莲对蟾蜍坐骨神经动作电位的影响

目的：观察七叶莲水煎液对蟾蜍离体坐骨神经复合动作电位的幅度及传导速度的影响,研究其对坐骨神经电生理特性的作用。方法：将制备的蟾蜍坐骨神经干分为4组,分别在任氏液和浓度为10％,20％,40％的七叶莲水煎液中浸泡,用MedLab生物信号采集处理系统引导神经干复合双相动作电位,并分别测定各组不同浸泡时间的坐骨神经干动作电位的幅度和传导速度两项电生理指标。记录不同浓度的七叶莲水煎液对蟾蜍离体坐骨神经复合动作电位的幅度和传导速度的影响。结果：10％,20％,40％3种浓度的七叶莲水煎液均使坐骨神经复合动作电位的幅度变小（P〈0．01）,传导速度变慢（P〈0．01）并最终使坐骨神经动作电位消失,且经过一段时间后动作电位的幅度和传导速度均能恢复。结论：七叶莲能可逆地阻滞神经动作电位的传导。     

Effect of chromium ions on the function of neuromuscular junctions

It has been shown on neuro-muscular preparations of frog sartorius muscle that chromium ions in the concentrations 1-4 x 10(-6) g/ml strengthen spontaneous and evoked transmitter release. Cr3+ ions in the concentrations above 4 x 10(-6) g/ml decrease the membrane potential of muscle fibres, decrease the quantum content of the end plate potentials. Experiments on a single Ranvier node have shown that Cr3+ ions decrease the amplitude, increase the rate and duration of the action potential of a nerve fibre. It is concluded that chromium ions produce a pronounced effect on synaptic transmission, which differs significantly from the action of manganese, cobalt and nickel ions.     

Effect of chlordimeform on nerve-muscle system of the larva of the waxmoth, Galleria mellonella

The action of chlordimeform on the nerve-muscle preparation of the larvae of the waxmoth Galleria mellonella has been studied by means of microelectrodes. Exitatory junction potential evoked by nerve stimulation is reversibly suppressed by 2 × 10^?3 M chlordimeform, and spike-like component is abolished. The resting membrane potential of the muscle fibre and the action potential from the nerve terminal are not affected at 5 × 10^?3 M chlordimeform. The depolarizing membrane response caused by outward current and the effective membrane resistance are not appreciably affected. It appears that chlordimeform exerts its blocking action on the neuromuscular junction rather than the conductance mechanism of muscle fibre membrane.     

Comparison of the capsaicin- and amino acid-sensitivity of dorsal root C fibres in the rat and the toad.

1. The C elevation of the compound action potential (CAP) was recorded with suction electrodes from dorsal roots of rats at 25 degrees C and toads (Bufo bufo) at 10 degrees C. The C fibre CAP had a conduction velocity of 0.5 +/- 0.07 SE M per sec (N = 10) and 0.25 +/- 0.04 M per sec (N = 8) in the rat and toad nerves respectively. 2. The depressant effect of applied drugs on the amplitude of the C fibres CAP was measured. Nerves from both species had similar sensitivities to GABA. EC50 5.0 microM +/- 0.5 SEM (N = 3) and 5.5 microM +/- 1.4 (N = 3) for the rat and toad respectively. Maximum depressant effects of GABA produced in rat and toad nerves were 35% +/- 5 SEM and 17% +/- 2.5 respectively. 3. In five out of ten of the rat nerves tested kainate had a clear depressant effect (maximum 36% +/- 4.3 SEM, EC50 6.8 microM +/- 0.9 SEM, N = 3) on the C fibre CAP. Kainate, at concentrations from 100 to 500 microM, had no effect on seven toad nerves. 4. Toad nerves were about 100 times less sensitive, than rat nerves, to capsaicin (ED50 values 430 microM +/- 190 SEM and 0.7 microM +/- 0.2 respectively, N = 4). 5. The similar sensitivity of nerves in both species to GABA and differing sensitivities to kainate and capsaicin suggests that amphibian C fibres specifically lack sensitivity to capsaicin and kainate.     

Role and differential expression of calpastatin mRNA and protein in cultured cardiomyocytes exposed to hypoxic stress

This study investigated the beneficial effects and mechanism of action of the juice of Momordica charantia in streptozotocin (STZ)-induced diabetes mellitus in rats. Diabetes mellitus was associated with significant (p < 0.01) time course reductions in body weight, plasma insulin and the number of insulin positive cells per islet and significant (p < 0.01) time course elevation in blood glucose and osmolarity and systolic blood pressure compared to age-matched healthy controls. Oral intake of M. charantia juice by STZ-induced diabetic rats partially reversed all the diabetes-induced effects measured. Daily oral administration of M. charantia juice to STZ-induced diabetic rates significantly (p < 0.01) reduced the Na+- and K+-dependent absorptions of glucose by the brush border membrane vesicles of the jejunum compared to the responses obtained in STZ-induced diabetic rat. Either insulin (100 MM) or the fruit juice lyophilised extract (5 microg x ml(-1)) can stimulate 14C-D-glucose uptake in L6 myotubes. These effects were completely blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. High concentrations (10-200 microg x ml(-1)) of M. charantia juice extract inhibited 14C-D-glucose uptake in L6 myotubes compared to the control response. The effect of M. charantia treatment was also investigated on myelinated fibre abnormalities in the tibial nerve of STZ-induced diabetic and control rats. The results show that diabetes was associated with significant (p < 0.05) reduction in the mean cross-sectional myelinated nerve fibres, axonal area, myelin area and maximal fibre area compared to end controls. Treatment of STZ-induced diabetic rats with M. charantia juice normalised the structural abnormalities of peripheral nerves. The results indicate that M. charantia can exert marked beneficial effects in diabetic rats, and moreover, it can regulate glucose uptake into jejunum membrane brush border vesicles and stimulate glucose uptake into skeletal muscle cells similar to the response obtained with insulin.     

Comparative research on synaptic transmission in the sympathetic ganglia of rabbits and frogs during acetylcholinesterase inhibition

Organophosphorus inhibitor of acetylcholinesterase (AChE) armin (1 x 10(-6) M) induced a variety of pre- and postsynaptic effects resulting from the AChE inhibition and subsequent accumulation of acetylcholine (ACh) in the synaptic cleft. The intensity of postsynaptic effects (level of neuron depolarization, degree of action potential depression) was shown to be different in the ganglia of frog and rabbit. This could be explained by differences in the total amount of ACh released in response to nerve stimulation as well as at rest. Both muscarinic and nicotinic cholinoreceptors were involved in the process of sustained depolarization of the neurons in the rabbit superior cervical ganglion after AChE inhibition. In frog ganglion neurons the nicotinic receptors did not participate in depolarization evidently due to their fast desensitization. The activation of presynaptic muscarinic receptors resulted in decrease of ACh released by nerve stimulation seems to weaken depolarization and blockade of synaptic transmission in sympathetic ganglia treated by AChE inhibitors.     

Adenosine selectively inhibits noncholinergic transmission in guinea pig bronchi

The neuromodulatory action of adenosine and ATP was investigated in isolated guinea pig bronchial strip chain preparations contracted with electrical field stimulation. The tissues were placed in organ baths containing physiological salt solution and stimulated at 8-Hz frequency, 0.5-ms pulse duration, and 30 V (approximately 100 mA) for 5 s. Electrical field stimulation evoked a biphasic contraction of bronchial muscle, consisting of an initial contraction followed by a sustained contraction, which was mediated by intramural cholinergic and noncholinergic nerve stimulations, respectively. Adenosine, at concentrations greater than M, caused a concentration-dependent inhibition in the height of the noncholinergically mediated contraction, accompanied by a very weak inhibition on the cholinergically mediated response. ATP (10(-5) to 3 x 10(-3) M) also produced a similar inhibitory effect on the noncholinergically mediated contraction, but the inhibitory potency was less than that of adenosine. The inhibitory response to adenosine was enhanced by the pretreatment with dipyridamole (2 x 10(-6) M) but antagonized with aminophylline (10(-5) M). Contractions of bronchial muscle evoked by exogenous acetylcholine (2 x 10(-6) M) or substance P (2 x 10(-7) M) were significantly inhibited by the adenosine (3 x 10(-4) M) pretreatment. These data suggest that in isolated guinea pig bronchi adenosine selectively inhibits noncholinergic neurotransmission through prejunctional P1-purinoceptors.     

3',5'-cyclic adenosine monophosphate as an intracellular second messenger of luteinizing hormone: application of the forskolin criteria

The role of adenosine 3',5'-cyclic monophosphate (cAMP) as an intracellular second messenger of luteinizing hormone (LH) was reinvestigated in vitro with diterpene forskolin, a highly specific activator of adenylate cyclase. Treatment of cultured testicular cells from adult hypophysectomized rats with increasing concentrations (10(7)-10(-4) M) of forskolin produced dose-dependent increments in cAMP and testosterone accumulation. Concomitant blockade of cAMP-phosphodiesterase activity with 3-isobutyl-1-methyl-xanthine (10(-4) M) resulted in significant (P less than 0.05) enhancement of the forskolin effect for all but the 10(-4) M forskolin dose. Potency evaluation as judged by half-maximal stimulation of testosterone accumulation revealed median effective doses (mean +/- SE) of 1.25 +/- 0.2 x 10(-5), 1.7 +/- 0.5 x 10(-5), and 2.5 +/- 0.4 x 10(-10) M for forskolin, N6, O2'-dibutyryl cAMP (Bt2cAMP), and human chorionic gonadotropin (hCG), respectively. Examination of the time requirements of forskolin disclosed time-dependent increments in the accumulation of extracellular cAMP and testosterone, the earliest significant (P less than 0.05) increases being noted by 6 hr of treatment. In comparison, a minimal time requirement of less than or equal to 12 hr was noted for hCG- and choleragen-stimulated androgen biosynthesis, whereas the apparent onset of action of Bt2cAMP was delayed to the 24-hr time point. Although 10(-7) M of forskolin by itself did not alter the accumulation of testosterone, its addition resulted in substantial amplification of the hCG effect, producing a 4.6-fold reduction in the median effective dose (ED50) of hCG. Moreover, concurrent treatment with this functionally inert dose of forskolin rendered steroidogenically inert doses of hCG (eg, 10(-11) or 3 x 10(-11) M) steroidogenically potent. However, combined treatment with maximally stimulatory doses of Bt2cAMP (10(-4) M) and one of several testicular cell agonists [forskolin (10(-4) M), choleragen (10(-9) M) or hCG (10(-9) M)] did not prove additive. Taken together, our findings indicate that forskolin, like LH, is capable of stimulating testicular cAMP generation as well as androgen biosynthesis and that a functionally inert low dose of forskolin can significantly amplify LH hormonal action. Inasmuch as forskolin-stimulated and forskolin-amplified hormonal action are acceptable as novel criteria of cAMP dependence, our observations provide new evidence in keeping with the notion that cAMP may be in intracellular second messenger of LH.     

Insect visceral muscle. Excitation and conduction in the proctodeal muscles

Bioelectrical potentials were studied from longitudinal muscle fibres of the cockroach proctodeum. The muscle bundle receives a polyaxonal innervation from both anterior and posterior branches of the anterior proctodeal nerve. Evoked post-synaptic potentials consisted of two independent, but similar components generated through the two branches. An action potential in the muscle fibre could be generated with single branch stimulation, and more readily by co-operation of excitation in the two nerve branches.Any part of the muscle was capable of acting as a pacemaker for myogenic rhythmic action potential, and the pacemaker region fluctuated with time. Excitation of the muscle could spread in two ways, directly myogenic and indirectly through nerve tracts. Myogenic conduction (2 cm/sec) was observed to be slower than neural conduction (35–38 cm/sec) in the muscle bundle.     

Active Uptake of Substance P Carboxy-Terminal Heptapeptide (5–11) into Rat Brain and Rabbit Spinal Cord Slices

We previously reported that nerve terminals and glial cells lack an active uptake system capable of terminating transmitter action of substance P (SP). In the present study, we demonstrated the existence of an active uptake system for SP carboxy-terminal heptapeptide, (5-11)SP. When the slices from either rat brain or rabbit spinal cord were incubated with [3H](5-11)SP, the uptake of (5-11)SP into slices was observed. The uptake system has the properties of an active transport mechanism: it is dependent on temperature and sensitive to hypoosmotic treatment and is inhibited by ouabain and dinitrophenol (DNP). In the brain, (5-11)SP was accumulated by means of a high-affinity and a low-affinity uptake system. The Km and the Vmax values for the high-affinity system were 4.20 x 10(-8) M and 7.59 fmol/10 mg wet weight/min, respectively, whereas these values for the low-affinity system were 1.00 x 10(-6) M and 100 fmol/10 mg wet weight/min, respectively. In the spinal cord, there was only one uptake system, with a Km value of 2.16 x 10(-7) M and Vmax value of 26.2 fmol/10 mg wet weight/min. These results suggest that when SP is released from nerve terminals, it is hydrolysed into (5-11)SP before or after acting as a neurotransmitter, which is in turn accumulated into nerve terminals. Therefore, the uptake system may represent a possible mechanism for the inactivation of SP.     

A comparison of the receptor constants of morphine and ethylketocyclazocine for analgesia and inhibition of gastrointestinal transit in the rat

The efficacies and dissociation constants of proposed mu and kappa receptor agonists (morphine and ethylketocyclazocine, respectively) were compared using the method of partial irreversible blockade (with buprenorphine) and Stephenson's theory of drug action. While there was good agreement between the dissociation constant (KA) of morphine in analgesia (3.3 x 10(-5) M) and in inhibition of gastrointestinal transit (1.1 x 10(-5) M), the KA of ethylketocyclazocine differed by an order of magnitude in these endpoints (3.2 x 10(-6) M and 6.7 x 10(-5) M, respectively). The efficacies of morphine were found to be similar for the two effects studied (4.23 and 5.26), while those for ethylketocyclazocine differed markedly (2.06 and 10.39). The fraction of receptors remaining unblocked after buprenorphine was consistent for the test but not for the agonist, indicating a different distribution of receptors for the two endpoints. Our results strongly suggest that morphine induces analgesia, and slows transit in the small intestine, through the same type of receptor. The same conclusion cannot be drawn for ethylketocyclazocine.     

Interaction of formamidines with octopamine-sensitive adenylate cyclase receptor in the nerve cord of Periplaneta americana L

Chlordimeform (CDM) and demethylchloridimeform (DCDM) mimic the action of octopamine in elevating adenylate cyclase activity in intact nerve cords of the American cockroach, Periplaneta americana. At a concentration of 1 x 10(-5)M, DCDM (13.5x increase within 20 minutes) is a more potent effector of the response than CDM (3x increase within 20 minutes), but both compounds show less efficacy than octopamine (23.5x increase within 15 minutes). DCDM also mimics the stimulatory effect of octopamine on adenylate cyclase activity in nerve cord homogenates whereas CDM has no demonstrable effect on this preparation. The octopamine- and DCDM-induced responses are competitively inhibited by phentolamine (1 x 10(-6)M) and cyproheptadine (1 x 10(-6)M) but not by propranolol (1 x 10(-6)M). DCDM and CDM inhibit the octopamine-induced activation of adenylate cyclase by 33% and 44% respectively. The results are discussed in light of the proposal that DCDM serves as a partial agonist and CDM as an antagonist of the octopamine receptor.     

Cardiac sympathetic nerve stimulation does not attenuate dynamic vagal control of heart rate via alpha-adrenergic mechanism

Complex sympathovagal interactions govern heart rate (HR). Activation of the postjunctional beta-adrenergic receptors on the sinus nodal cells augments the HR response to vagal stimulation, whereas exogenous activation of the presynaptic alpha-adrenergic receptors on the vagal nerve terminals attenuates vagal control of HR. Whether the alpha-adrenergic mechanism associated with cardiac postganglionic sympathetic nerve activation plays a significant role in modulation of the dynamic vagal control of HR remains unknown. The right vagal nerve was stimulated in seven anesthetized rabbits that had undergone sinoaortic denervation and vagotomy according to a binary white-noise signal (0-10 Hz) for 10 min; subsequently, the transfer function from vagal stimulation to HR was estimated. The effects of beta-adrenergic blockade with propranolol (1 mg/kg i.v.) and the combined effects of beta-adrenergic blockade and tonic cardiac sympathetic nerve stimulation at 5 Hz were examined. The transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with pure delay. beta-Adrenergic blockade decreased the dynamic gain from 6.0 +/- 0.4 to 3.7 +/- 0.6 beats x min(-1) x Hz(-1) (P < 0.01) with no alteration of the corner frequency or pure delay. Under beta-adrenergic blockade conditions, tonic sympathetic stimulation did not further change the dynamic gain (3.8 +/- 0.5 beats x min(-1) x Hz(-1)). In conclusion, cardiac postganglionic sympathetic nerve stimulation did not affect the dynamic HR response to vagal stimulation via the alpha-adrenergic mechanism.     

Local anesthetic action of phentolamine on insect mechanoreceptors

1. The effect of phentolamine on the response properties of insect mechanoreceptors and on the conduction in their axons was examined using electrophysiological techniques. 2. Phentolamine blocked conduction of action potentials along axons, an effect which exhibited 3 characteristics typical of local anesthetics: the effect was frequency-dependent, reversible and varied for nerves with different diameters. 3. The concentration of phentolamine required to block axonal conduction (1-2 x 10(-3) M) was significantly higher than that required to abolish the response of receptors to mechanical stimulation (3-5 x 10(-4) M). 4. All mechanoreceptors that were examined in Locusta migratoria and Periplaneta americana were inactivated by phentolamine (Table 1). The type I receptors (chordotonal, campaniform and hair sensilla) were inactivated within 5-15 min following phentolamine application. The only type II receptor examined (forewing stretch-receptor) underwent a phase of repetitive discharge before being inactivated. 5. Tolazoline and metoclopramide inactivated, like phentolamine, mechanoreceptors at lower concentrations than necessary to block axonal conduction. However, yohimbine and chlorpromazine inactivated mechanoreceptors and blocked axonal conduction at similar concentrations. 6. These findings suggest that phentolamine affects sense-organ specific ionic processes that are more sensitive to the drug than the ionic processes along the axons.