Effects of elevated extracellular potassium on the stimulation mechanism of diastolic cardiac tissue

During cardiac disturbances such as ischemia and hyperkalemia, the extracellular potassium ion concentration is elevated. This in turn changes the resting transmembrane potential and affects the excitability of cardiac tissue. To test the hypothesis that extracellular potassium elevation also alters the stimulation mechanism, we used optical fluorescence imaging to examine the mechanism of diastolic anodal unipolar stimulation of cardiac tissue under 4 mM (normal) and 8 mM (elevated) extracellular potassium. We present several visualization methods that are useful for distinguishing between anodal-make and anodal-break excitation. In the 4-mM situation, stimulation occurred by the make, or stimulus-onset, mechanism that involved propagation out of the virtual cathodes. For 8-mM extracellular potassium, the break or stimulus termination mechanism occurred with propagation out of the virtual anode. We conclude that elevated potassium, as might occur in myocardial ischemia, alters not only stimulation threshold but also the excitation mechanism for anodal stimulation.     

Influence of protective drugs on the elevation of extracellular potassium ion concentration in the brain during ischaemia

Influence of drugs on the changes of extracellular potassium ion concentration in the brain during total cerebral ischaemia was investigated. The aorta of the dogs was clamped twice with an intermittent reperfusion period of 60 min. In control experiments no significant difference was found in the elevation of extracellular potassium ion concentration of the brain during the first and second clampings. In the present study drugs were administered 10 min prior the second aorta occlusion. Verapamil in a dose of 0.125 mg/kg proved to be ineffective. Piridoxilate in a dose of 10 mg/kg and piracetam in a dose of 100 mg/kg delayed to a small extent the potassium outflow. The following drugs enhanced significantly the duration before the steep increase of potassium ion outflow: phenytoin in a dose of 10 mg/kg by 31.8 sec (p less than 0.01), ethyl-butyl-thiobarbital in a dose of 15 mg/kg by 30.2 sec (p less than 0.05), and lidocaine in a dose of 100 mg/kg by 115.8 sec (p less than 0.01). Comparing present results to our earlier data (obtained after 50 sec ischaemia) it can be concluded, that these protective influences become more effective during longer ischaemic period (2-5 min), when lidocaine, phenytoin and ethyl-butyl-thiobarbital were used. Moreover, in spite of the observation seen during shorter ischaemia, even piridoxilate and piracetam exerted some degree of protective effect. No such effect of verapamil could be detected in the present experimental model.     

Peculiarities of Ca<Superscript>2+</Superscript>-regulation of functional activity of myocardium of frog <Emphasis Type="Italic">Rana temporaria</Emphasis>

To elucidate role of intra- and extracellular Ca²⁺ in regulation of rhythm and strength of frog heart contractions, there were studied ECC and isometric contraction of myocardium preparations in response to verapamil, adrenaline, and blockers of α- and β-adrenoreceptors. It has been shown that after an intramuscular injection of verapamil (6 mg/kg), bradycardia develops, the heart rate (HR) decreasing by 50–70%. Further, the cardiac arrest occurred; however, administration to the animals of adrenaline (100 mg/kg) restored the cardiac rhythm for a short while. After an intramuscular injection of adrenaline at doses of 0.1–10 mg/kg, no essential changes were observed in the potential action amplitude and HR; an increase of the administered adrenalin concentration to 100 mg/kg was not accompanied by the cardiac rhythm stimulation, as this takes place in homoiothermal animals and human; on the contrary, an essential HR deceleration was revealed. Phentolamine (5 mg/kg) gradually decelerated HR rhythm by 32–45%. The potential amplitude changed insignificantly. A subsequent intracardiac injection of adrenaline (100 mg/kg) on the background of block of α-adrenoreceptors produced acceleration of the rhythm (by 15–21%) and fall of the electrogram amplitude. These results can indicate that in the frog heart phentolamine interacts predominantly with α 1-adrenoreceptors. An intracardial administration of propranolol (1 mg/kg) to frogs promoted inhibition of β-adrenergic receptors and produced a gradual cardiac rhythm deceleration. In experiments on assessment of verapamil effect on the character of contractions this preparation at a concentration of 150 μM was established to produce a significant dose-dependent decrease of the contraction strength. A rise of verapamil concentration in the sample to 200 μM led to a decrease of the amplitude, on average, by 68–70% and in individual preparations—by 80–85%; however, administration into the sample of adrenaline (10 μM) restored the cardiac contraction strength. Adrenaline (1 nM–100 μM) increased markedly the contraction amplitude. Phentolamine (10 μM) did not inhibit transmission of contractile signal to cardiomyocytes; this was manifested in that the contraction amplitude after addition of adrenaline (10 μM) into the sample was approximately the same as in the sample containing no phentolamine. Propranolol (10 μM) eliminated the stimulatory action of adrenaline (10 μM). The results of these experiments indicate that in the frog ventricular cardiomyocytes the main adrenaline acceptors are β-adrenoreceptors.     

Potassium current activated by depolarization of dissociated neurons from adult guinea pig hippocampus

Currents were generated by depolarizing pulses in voltage-clamped, dissociated neurons from the CA1 region of adult guinea pig hippocampus in solutions containing 1 microm tetrodotoxin. When the extracellular potassium concentration was 100 mM, the currents reversed at -8.1 +/- 1.6 mV (n = 5), close to the calculated potassium equilibrium potential of -7 mV. The currents were depressed by 30 mM tetraethylammonium in the extracellular solution but were unaffected by 4-aminopyridine at concentrations of 0.5 or 1 mM. It was concluded that the currents were depolarization-activated potassium currents. Instantaneous current-voltage curves were nonlinear but could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. Conductance-voltage curves could be described by a Boltzmann-type equation: the average maximum conductance was 65.2 +/- 15.7 nS (n = 9) and the potential at which gK was half-maximal was -4.8 +/- 3.9 mV (mean +/- 1 SEM, n = 10). The relationship between the null potential and the extracellular potassium concentration was nonlinear and could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. The rising phase of potassium currents and the decay of tail currents could be fitted with exponentials with single time constants that varied with membrane potential. Potassium currents inactivated to a steady level with a time constant of approximately 450 ms that did not vary with potential. The currents were depressed by substituting cobalt or cadmium for extracellular calcium but similar effects were not obtained by substituting magnesium for calcium.     

Disposition of methadone in the ovine maternal-fetal unit

The distribution of methadone between mother and fetus after a single dose and at steady state was determined using the chronic pregnant ewe preparation. Chronic indwelling catheters were placed in the maternal aorta and vena cava, umbilical vein and fetal aorta. Following a single i.v. dose (0.5 mg/kg) to the mother, methadone was rapidly distributed to the fetus, with peak concentration in the umbilical vein occurring within two min. An umbilical venous-arterial gradient existed for 10–15 min after drug administration, indicating uptake of methadone by fetal tissues. Methadone concentration in the fetus was 2–5 times lower than those in the mother even in the post-distribution phase. The terminal half-life of methadone in 4 animals was 57±7.6 (S.E.) min in the mother, and 58.5±10.0 (S.E.) min in the fetus. When methadone was infused at a constant rate to the mother (0.01 mg/kg/min), steady state was achieved in both mother and fetus by 4–5 hrs. In 5 animals, maternal steady state was found to be 203±18.8 (S.E.) ng/ml, and fetal steady state was found to be 29.7±2.9 (S.E.) ng/ml. These studies show that methadone is rapidly distributed to the fetus, but fetal concentration remain lower than maternal concentration at all times.     

Propranolol antagonism to the effect of furosemide on the compostion of endolymph in guinea pigs.

Furosemide, administered intravenously (50 mg/kg) to guinea pigs, caused an increase in the sodium concentration and a decrease in the potassium concentration of endolymph, and a fall in the endolymphatic potential. The furosemide-induced electrolyte changes were prevented by pretreatment of five guinea pigs with propranolol given intravenously (2 mg/kg). The fall in the endolymphatic potential was not prevented by propranolol. Local administration of furosemide to the perilymphatic or endolymphatic space caused a fall in the endolymphatic potential, but had no effect upon the concentrations os sodium and potassium of endolymph. These studies provide additional information suggesting the mutual independence of the endolymphatic potential and sodium and potassium concentration gradients.     

Extracellular calcium transients at single excitations in rabbit atrium measured with tetramethylmurexide

Extracellular calcium transients were resolved within the time course of single contraction cycles in rabbit left atrium using tetramethylmurexide (2 mM) as the calcium-sensitive dye (150-250 microM total calcium, 80-150 microM free calcium). Net extracellular calcium depletion began within 2-4 ms upon excitation; over the following 5-20 ms, depletion continued steeply and amounted to 0.2 mumol/kg wet weight X 10 ms (135 microM free extracellular calcium). In regularly excited muscles (0.5-2 Hz), net depletion slowed rapidly and stopped early during the rise of contractile motion monitored by transmitted light. Maximum depletions amounted to 0.2-0.5% of total extracellular calcium (0.2-0.5 mumol/kg wet weight with 135 microM free calcium). Replenishment of extracellular calcium began at the latest midway to the peak of the motion signal. Calcium replenishment could be complete for the most part by an early phase of relaxation or could take place continuously through relaxation. The maximal net depletion per beat decreased manyfold with a decrease of frequency from 1 to 0.05 Hz. During paired pulse stimulation (200-300-ms twin pulse separation at basal rates of 0.3-1 Hz), extracellular calcium accumulation was enhanced at the initial potentiated contraction; extracellular calcium depletion was prolonged at the low-level premature contraction. With quadruple stimulation (three premature excitations), the apparent rate of net extracellular calcium accumulation at potentiated contractions approached or exceeded the apparent rate of early net calcium depletion. Under the special circumstance of a strongly potentiated post-stimulatory contraction after greater than 5 s rest, repolarization beyond -40 mV occurred within 10 ms, net extracellular calcium accumulation began with the onset of muscle motion, and net extracellular calcium accumulation (1-3 microM/kg wet weight) coincided with a more positive late action potential in comparison with subsequent action potentials. Consistent changes of the apparent rate of early net calcium depletion were not found with any of the simulation patterns examined. In ryanodine-pretreated atria, the duration of depletion was clearly limited by action potential duration at post-rest stimulations; in the presence of 4-aminopyridine (2 mM), depletion continued essentially undiminished for up to 200 ms. The resulting net depletion magnitudes were greater than 10 times larger than the transient depletions found during steady stimulation.     

Biochemical characterization of dichlorvos-induced delayed neurotoxicity in rat

In vitro and in vivo studies were carried out to assess the delayed neurotoxicity potential of dichlorvos. In vitro, dichlorvos caused a concentration and time-dependent decrease in the activity of neuropathy target esterase (NTE). The Ki of dichlorvos for NTE was calculated to be 1.28 x 10(3) M-1 min-1. In vitro reactivation and ageing studies revealed that dichlorvos-inhibited NTE became refractory to activation by potassium fluoride after 5 min in the ageing medium, thus indicating the formation of an aged complex between dichlorvos and NTE. In vivo also, dichlorvos (200 mg/kg body wt) given as a single subcutaneous dose inhibited NTE in brain at various intervals after exposure (24 h, 10 days, and 21 days). The delayed neurotoxicity potential of dichlorvos was finally confirmed by the rota rod test, which revealed severe motor deficit in all the exposed animals.     

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.     

Stimulation induced recurrent epileptiform discharges block cortical and subcortical spreading depression in rats

Blockade of the spread depression [SD] in chemically or electrically stimulated areas of the cerebral cortex was analyzed in a series of experiments performed in 40 curarized, locally anaesthetised rats. Longlasting stimulation of the cerebral cortex (0.5 to 1.7 mA, 6 Hz, 0.1 ms) elicited recurrent episodes of enhanced evoked potentials propagating to remote cortical areas [Co], the caudate nucleus [Cd], the hippocampus [Hi] and the thalamus [Th] and was accompanied by marked slow potential shifts (4-6 mV amplitude, 2-3 min duration, at 2-5 min intervals]. The projected discharge interfered with SD initiation and propagation in all the examined structures. The SD blockade was most pronounced during the episodes and almost absent in the intervals between them. The block was manifested by reduced amplitude and duration of the slow potential of SD. Pentobarbital [20 mg/kg] suppressed the recurrent discharges and eliminated the corresponding SD blockade. Recurrent excitability changes induced by Cd and Th stimulation elicited similar effects but the threshold was higher in Cd and Th than in Co. SD was less effectively blocked by the projected discharge than by stimulation of the same structure, particularly in the vicinity of the recording electrodes, where the blockade could be observed even under pentobarbital. The SD blockade outlasted stimulation by a considerably longer period of time in Th (about 10 min) than in the Cd (about 3 min). The onset of stimulation and the projected episodes sometimes elicited SD waves but SD blockade prevailed with continued stimulation. The present findings support the hypothesis that excessive neural activity increases the potassium clearance and thus prevents the autoregenerative accumulation of potassium ions, mediating SD propagation.     

Positive and negative inotropic effects of elevated extracellular potassium level on mammalian ventricular muscle

The effect of moderate elevation in extracellular potassium concentration (up to 12 mM) on contraction of cat ventricular muscle was examined. Isometric force development was recorded from eight excised trabeculae and from six coronary-perfused in situ papillary muscle preparations. Contraction in the steady state was variably affected, sometimes decreasing monotonically, sometimes remaining unchanged, with increasing potassium level. In 11 of these 14 preparations, the steady state was preceded by a transient period in which the contraction was augmented. In addition, eight excised trabeculae were used in an experimental arrangement designed to distinguish between inotropic effects caused by potassium-induced alterations in the action potential and other, more direct, effects of this ion on contraction. The negative inotropic effect is attributable to a potassium-induced reduction in the amplitude and/or duration of the action potential plateau. The positive inotropic effect was found in experimental arrangements where effects of the potassium-rich medium on action potential time-course were effectively "buffered." The positive inotropic effect thus depends on the presence of the elevated potassium concentration and can occur independently of effects on the action potential time-course.     

Ethanol production with Saccharomyces cerevisiae under aerobic conditions at different potassium concentrations

The specific ethanol productivity with Saccharomyces cerevisiae grown aerobicly in a chemostat was found to be highly dependent on the ratio of intracellular to extracellular potassium concentration through variations in the energy consumption used for maintenance of the concentration gradient of potassium across the cell membrane. The specific ethanol productivity progressively rose from 0 to 20 mmol h(-1) g(-1) cell dry matter at a growth rate of 0.17 h(-1) when the ratio of intracellular to extracellular potassium concentration was increased from 10 to 80. The ethanol production under potassium limited growth conditions was caused neither by a reduction in the specific respiratory activity nor by variations in the potassium content in cell dry matter. Results which strongly that ethanol production under potassium limited growth conditions is brought about by changes in the ratio of pyruvate oxidase to pyruvate decarboxylase activity through changes in the intracellular pyruvate concentration are presented.     

The ionic basis of electrical activity in embryonic cardiac muscle

The intracellular sodium concentration reported for young, embryonic chick hearts is extremely high and decreases progressively throughout the embryonic period, reaching a value of 43 mM immediately before hatching. This observation suggested that the ionic basis for excitation in embryonic chick heart may differ from that responsible for electrical activity of the adult organ. This hypothesis was tested by recording transmembrane resting and action potentials on hearts isolated from 6-day and 19-day chick embryos and varying the extracellular sodium and potassium concentrations. The results show that for both young and old embryonic cardiac cells the resting potential depends primarily on the extracellular potassium concentration and the amplitude and rate of rise of the action potential depend primarily on the extracellular sodium concentration.     

Ketanserin pretreatment attenuates MDMA-induced dopamine release in the striatum as measured by in vivo microdialysis

Systemic administration of the amphetamine analogue, 3,4-methylenedioxymethamphetamine (MDMA) produced a dose-dependent increase in the extracellular concentration of dopamine (DA) in the striatum as measured by in vivo microdialysis in awake, freely-moving rats. The extracellular concentration of the DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), was significantly decreased in dialysate samples following the administration of MDMA (10 and 20 mg/kg, i.p.). The serotonin-2 (5-HT2) antagonist ketanserin (3 mg/kg, i.p.) had no effect on the extracellular concentration of DA or DOPAC in the striatum of vehicle- treated rats. The administration of ketanserin (3 mg/kg) 1 hr prior to MDMA (20 mg/kg) significantly attenuated the MDMA- induced increase in the extracellular concentration of DA without affecting the decrease in DOPAC concentrations. These data are suggestive that MDMA administration increases DA release in the striatum of awake, freely-moving rats. In addition, MDMA-induced increase in the extracellular concentration of DA in the striatum is mediated, in part, via 5-HT2 receptor mechanisms.     

Transmembrane electrical characteristics of cultured human skeletal muscle cells

Skeletal muscle explants from normal subjects were established from biopsy material on collagen. Cellular outgrowth appeared within 3-4 days, and fusion of myoblasts was observed in 5-10 days. Multinucleated myotubes were impaled under high optical magnification, at 37 degrees C, with conventional glass microelectrodes. The mean resting potential was -44.4 mV +/- 2.4 (n = 399); -33 +/- 2.3 mV at 9 days (n = 10) vs -48 +/- 2.5 mV (n = 15) at 27 days. The average input resistance (Rin) was 9.7 M omega (n = 83). Action potentials could be elicited by electrical stimulation and had a mean amplitude of 55.9 +/- 2.1 mV with a mean maximum rate of rise (Vmax) of 72.1 +/- 7.5 V/s. The mean overshoot was 13.9 +/- 2.3 mV, and the action potential duration determined at 50% of repolarization (APD50) was 8.0 msec (n = 7). The resting membrane potential showed a depolarization of 23 mV/decade for extracellular potassium ion concentration ([K]o) between 4.5-100 mM. Thus, we have established the normal resting potential and maximum rate of rise of the action potential for human myotubes in culture. We have shown that the values for these are less than those previously reported in cultured avian and rodent cells. In addition, we have shown that the response in our system of the resting potential to change in extracellular potassium concentration is blunted compared to studies using isolated muscle, suggesting an increase in ratio of sodium to potassium permeability. Cultured human muscle cells depolarized in the presence of ouabain.     

Some electrical properties of the membrane of the barnacle muscle fibers under internal perfusion

Summary Intracellular perfusion technique has been applied to the muscle fibers of the barnacle species,Balanus nubilus. In these fibers, generation and the form of the calcium spike was governed by the frequency of stimulation and intra- and extracellular calcium concentrations. Voltage-clamp experiments showed that the magnitude of the potassium outward current was controlled by the intracellular calcium concentration whose increase, nearly 10³-fold, raised the resting membrane conductance and the outward potassium current. On the other hand, application of 10mm zinc ions inside the muscle fiber had no effect on either the resting potential or the outward potassium current but suppressed the early inward calcium current. Similarly, the inward calcium current was decreased by low concentration of sodium ions in the extracellular fluid only when its ionic strength was made low by substituting sucrose for the sodium salt. Measurement of outward current with the muscle fiber in calcium-free ASW solution and intracellularly perfused with several cationic solutions established the selectivity sequence TEA相似文献   

Peculiarities of Ca2+ regulation of functional activity of myocardium of frog Rana temporaria

To elucidate role of intra- and extracellular Ca2+ in regulation of rhythm and strength of frog heart contractions, there were studied ECC and isometric contraction of myocardium preparations in response to verapamil, adrenaline, and blockers of alpha- and beta-adrenoreceptors. It has been shown that after an intramuscular injection of verapamil (6 mg/kg), bradycardia develops, the heart rate (HR) decreasing by 50-70 %. Further, the cardiac arrest occurred; however, administration to the animals of adrenaline (100 mg/kg) restored the cardiac rhythm for a short while. After an intramuscular injection of adrenaline at doses of 0.1-10 mg/kg, no essential changes were observed in the potential action amplitude and HR; an increase of the administered adrenalin concentration to 100 mg/kg was not accompanied by the cardiac rhythm stimulation, as this takes place in homoiothermal animals and human; on the contrary, an essential HR deceleration was revealed. Phentolamine (5 mg/kg) gradually decelerated HR rhythm by 32-45 %. The potential amplitude changed insignificantly. A subsequent intracardiac injection of adrenaline (100 mg/kg) on the background of block of alpha-adrenoreceptors produced acceleration of the rhythm (by 13-21%) and fall of the electrogram amplitude. These results can indicate that in the frog heart, phentolamine interacts predominanty with alpha-adrenoreceptors. An intracardiac administration of propranolol (1 mg/kg) to frogs promoted inhibition of beta-adrenergic receptors and produced a gradual cardiac rhythm deceleration. In experiments on assessment of verapamil effect on the character of contractions this preparation at a concentration of 150 microM was established to produce a significant dosedependent decrease of the contraction strength. A rise of verapamil concentration in the sample to 200 microM led to a decrease of the amplitude, on average, by 68-70 % and in individual preparations -- by 80-85 %; however, administration into the sample of adrenaline (10 microM) restored the cardiac contraction strength. Adrenaline (1 nM--100 microM) increased markedly the contraction amplitude. Phentolamine (10 microM) did not inhibit transmission of contractile signal to cardiomyocytes; this was manifested in that the contraction amplitude after addition of adrenaline (10 microM) into the sample was approximately the same as in the sample containing no phentolamine. Propranolol (10 microM) eliminated the stimulatory action of adrenaline (10 microM). The results of these experiments indicate that in the frog ventricular cardiomyocytes the main adrenaline acceptors are beta-adrenoreceptors.     

Deexcitation of cardiac cells.

Excitation and deexcitation are fundamental phenomena in the electrophysiology of excitable cells. Both of them can be induced by stimulating a cell with intracellularly injected currents. With extracellular stimulation, deexcitation was never observed; only cell excitation was found. Why? A generic model with two variables (FitzHugh) predicts that an extracellular stimulus can both excite the cell and terminate the action potential (AP). Our experiments with single mouse myocytes have shown that short (2-5 ms) extracellular pulses never terminated the AP. This result agrees with our numerical experiments with the Beeler-Reuter model. To analyze the problem, we exploit the separation of time scales to derive simplified models with fewer equations. Our analysis has shown that the very specific form of the current-voltage (I-V) characteristics of the time-independent potassium current (almost no dependence on voltage for positive membrane potentials) is responsible here. When the shape of the I-V characteristics of potassium currents was modified to resemble that in ischemic tissues, or when the external potassium concentration (K0) is increased, the AP was terminated by extracellular pulses. These results may be important for understanding the mechanisms of defibrillation.     

The influence of endogenous mineralocorticoids on the composition of fetal urine

Experiments were carried out in 10 chronically catheterised fetal sheep aged 121-128 days. In 3 animals infusion of aldosterone (5 micrograms/h) caused a fall in fetal urinary Na/K ratio; an effect that was reversed by spironolactone 2.5 mg/kg followed by an infusion of 100 micrograms/h per kg. In 9 fetal sheep which had no previous treatment the same doses of spironolactone had no effect on fractional sodium excretion although the fractional excretion of potassium decreased (P less than 0.05) and the urinary sodium potassium (Na/K) ratio rose (P less than 0.05). Amiloride had a variable effect on sodium excretion but the fractional excretion of potassium decreased markedly (P less than 0.05). Thus in chronically catheterised fetal sheep, endogenous mineralocorticoid activity altered urinary potassium excretion and the urinary Na/K ratio. However this activity was low, as distal blockade with amiloride further decreased the fractional excretion of potassium and increased the urinary Na/K ratio.     

The intracellular calcium and mechanisms of endocytosis of synaptic vesicles at the frog nerve ending

In the experiments on frog motor nerve endings of cutaneous pectoris muscle, made by extracellular recording of synaptic signals, it has been shown that the increase in intracellular calcium ion concentration in the nerve ending (by enhance of extracellular potassium ion concentration, or by addition of caffeine) leads to an increase in the miniature end-plate potential frequency, which is preserved over the whole period (about 10 min) of action of these substrates. The rhythmic stimulation of motor nerve (20 or 100 imp/s) quickly leads to a decrease in the end plate potentials amplitude. It has been shown by fluorescent microscopy with the use of endocytotic marker FM 1-43 that in the course of a short time exposition (5 min) in a high potassium solution (40 mM) or caffeine (5 mM), light spots appeared in the nerve ending. This shows that synaptic vesicles undergo intensive processes of endocytosis. During a longer exposition (30 min) no light spots were revealed, whereas the nerve ending width increased. This data allowed to propose that the process of endocytosis was blocked. In the presence of even lower concentrations of potassium ions and caffeine, and during a long rhythmic stimulation (20 or 100 imp/s) no blocking of endocytosis was revealed. It is concluded that high concentrations of intracellular calcium in the frog motor nerve ending leads to a reversible block of endocytosis, while exocytosis in synaptic vesicles is proceeding.