• The Korean Journal of Physiology
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• v.29 no.2
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• pp.203-216
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• 1995

The effects of ${\alpha}_1-adrenergic$ receptor stimulation on membrane potential, intracellular $Na^+$ activity, and twitch force were investigated in ventricular muscles from guinea-pig hearts. Action potentials, intracellular $Na^+$ activity, and twitch force of ventricular papillary muscles were measured simultaneously under various experimental conditions. Stimulation of the ${\alpha}_1-adrenergic$ receptor by phenylephrine produced variable changes in action potential duration, a slight hyperpolarization of the diastolic membrane potential, a decrease in intracellular $Na^+$ activity, and a biphasic inotropic response in which a transient negative inotropic response was followed by a sustained positive inotropic response. These changes were blocked by prazosin, an antagonist of the ${\alpha}_1-adrenergic$ receptor, but not by atenolol, an antagonist of the ${\beta}-adrenergic$ receptor. This indicates that the changes in membrane potential, intracellular $Na^+$ activity, and twitch force are mediated by stimulation of the ${\alpha}_1-adrenergic$ receptor, but not by stimulation of ${\beta}-adrenergic$ receptor. The decrease in intracellular $Na^+$ activity was not observed in quiescent muscles, depending on the rate of the action pontentials in beating muscles. The intracellular $Na^+$ activity decrease was substantially inhibited by tetrodotoxin. However, the decrease in intracellular $Na^+$ activity was not affected by an inhibition of the $Na^+-K^+$ pump. Therefore, the decrease in intracellular $Na^+$ activity mediated by the ${\alpha}_1-adrenergic$ receptor appears to be due to a reduction of $Na^+$ influx during the action potential, perhaps through tetrodotoxin sensitive $Na^+$ channels. Our study also revealed that the decrease in intracellular $Na^+$ activity might be related to the transient negative inotropic response. The intracellular $Na^+$ activity decrease could lower intracellular $Ca^{2+}$ through the $Na^+-Ca^{2+}$ exchanger and thereby produce a decline in twitch force.

• The Korean Journal of Physiology and Pharmacology
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• v.2 no.6
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• pp.715-724
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• 1998

Frequency-force relationships (FFR) were studied in electrically field stimulated rat left atria (LA) by reducing the stimulation frequency from resting 3 Hz to test frequencies (0.1-1 Hz) for 5 minutes. The twitch amplitudes of LA elicited the typical negative staircases with 3-phased changes: the initial rapid increase, the second decrease and the following plateau at test frequencies. Verapamil $(3{\times}10^{-5}\;M)$ pretreatment elicited frequency-dependent suppression of the twitch amplitudes, exaggerating the negative staircase. Monensin pretreatment enhanced not the peak but the plateau amplitudes in a concentration-dependent manner. When the $Na^+-Ca^{2+}$ exchange was blocked by $Na^+\;and\;Ca^{2+}$ depletion in the Krebs Hensleit buffer (0 $Na^+-0\;Ca^{2+}$ KHB), the twitch amplitudes increased in a frequency-dependent manner, changing the negtive staircase into the positve one. Meanwhile, the 0 $Na^+-0\;Ca^{2+}$ KHB applicationinduced enhancement was strongly suppressed by caffeine (5 mM) pretreatment. Only dibucaine among the local anesthetics increased the basal tone during frequency reduciton. There were no differences in $^{45}Ca$ uptakes between 0.3 Hz and 3 Hz stimulation except at 1 min when it was significantly low at 0.3 Hz than 3 Hz, illustrating net $Ca^{2+}$ losses. Monensin pretreatment enhanced the rate of this $Ca^{2+}$ loss. Taken together, it is concluded that $Na^+-Ca^{2+}$ exchange extrudes more SR released $Ca^{2+}$ out of the cell in proportion to the frequency, resulting in the negative rate staircase in the rat LA.

• The Korean Journal of Pharmacology
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• v.31 no.1 s.57
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• pp.39-51
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• 1995

The roles of ${\beta}-adrenoceptor$ were well known in hyperthyroidal heart, but not with ${\alpha}-adrenoceptor$. So we studied the effects of phenylephrine on membrane potential, intracellular sodium activity ($a^{i}_{Na}$), twitch force, and intracellular pH ($pH_i$) by continuous intracellular recordings with ion-selective and conventional microelectrodes in the papillary muscles of hyperthyroid guinea pig heart. ${\alpha}_1-adrenoceptor$ stimulation by phenylephrine (10^{-5}\;or\;3{\times}10^{-5}M$) produced the following changes: variable changes in action potential duration, a hyperpolarization ($1.5{\pm}0.1mM$) of the diastolic membrane potential, an increase in $a^{i}_{Na}\;(0.4{\pm}0.15mM)$, a stronger positive inotropic effect ($220{\pm}15%$), an increase in $pH_i\;(0.06{\pm}0.002\;unit)$. These changes were flocked by prazosin and atenolol. This indicated that the changes in membrane potential, $a^{i}_{Na}$ twitch force, and $pH_i$ are mediated by a stimulation of the ${\alpha}_1-adrenoceptor$. Ethylisopropylamiloride ($10^{-5}$) also blocked the increase in $a^{i}_{Na}$ and twitch force. On the other hand, strophanthidin, tetrodotoxin, $Cs^+$ or verapamil did not block the increase in $a^{i}_{Na}$ and twitch force. Thus, it was suggested that ${\alpha}_1-adrenoceptor$ stimulation increased $a^{i}_{Na}\;and\;pH_i$ by stimulation of $Na^{+}-H^{+}$ exchange, thereby allowing intracellular alkalinization and $a^{i}_{Na}$ increase. These results were very different from euthyroidal heart which showed ${\alpha}_1-adrenoceptor$-induced decrease in $a^{i}_{Na}$ and initial negative inotropic effect. From the above results, it was concluded that ${\alpha}_1-adrenoceptor$ had a important role in hyperthy-roidal heart.

• Korean Journal of Veterinary Research
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• v.35 no.2
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• pp.245-254
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• 1995

The density of ATP-sensitive potassium($K_{APT}$) channels, that open as intracellular ATP concentration falls below a critical level, is very high in skeletal muscle surface membrane and those high density may imply that $K_{ATP}$ channels have very important physiological roles. To elucidate a role of $K_{ATP}$ in relation to fatigue, the modulating effects of potassium channel openers and blockers on the fatigue velocity(FV) of mouse extensor hallucis longus muscle(EHL) were investigated in vitro. Twitch contraction was induced by an electrical field stimulation (EFS: 24-48V, 20ms, 0.2-4Hz) and resulting contraction force was isometrically recorded. The twitch forces were gradually decreased to 25% of initial contraction force(ICF) in $37.52{\pm}1.55sec$($mean{\pm}s.e.m.$, n=135), indicating the fatigue phenomena. The mean velocity for development of the fatigue was measured during the period that twitch force decreased to half($FV_{0/0.5}$) and during the period from half to 25%($FV_{0.5/0.25}$) of ICF. The fatigue was induced once every one hour and the tissue response was stable for up to 4 hours. In control condition, ICF was $5.8{\pm}0.12g$ (n=144) and decreased to 50% of ICF with the mean fatigue velocity of $0.182{\pm}0.006g/sec$($FV_{0/0.5}$, n=135) and from 50% to 25% of ICF with $0.084{\pm}0.004g/sec$($FV_{0.5/0.25}$, n=135). Cromakalim($50{\mu}M$) significantly increased $FV_{0.5/0.25}$(n=4). Glibenclamide($IC_{50}>50{\mu}M$), $Ba^{2+}$($IC_{50}=10{\mu}M$), 4-aminopyridine($FV_{0/0.5}$, $IC_{50}=0.5mM$; $FV_{0.5/0.25}$, $IC_{50}=2mM$) decreased both $FV_{0/0.5}$ and $FV_{0.5/0.25}$ concentration-dependently up to 75%. $TEA^+$(30mM), E-4031($10{\mu}M$), tolbutamide(1mM) decreased $FV_{0.5/0.25}$, but apamin(300nM) and $TEA^+$(10mM) showed no significant effects. Our results suggest that activation of the $K_{ATP}$ channels may be major cause of $K^+$ outflux during development of the fatigue and the isolated EHL muscle could be an useful experimental preparation in studying the fatigue phenomena in skeletal muscle. In addition, the possibility of activation of delayed rectifier during the fatigue development remains to be studied further.

• Journal of Biomedical Engineering Research
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• v.11 no.2
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• pp.257-268
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• 1990

A system has been developed for monitoring the effect of neuromuscular blocking frugs and the neuromuscular function during anesthesia and surgery. This system is composed of software and hardware, the latter are nerve stimulator, force transducer, interface board(preamplifier, filter, peripheral input/output) and personal computer (apple ll) , the former are programmed in ASSEMBLY and BASIC language. The nerve stimulator which is controlled by personal computer is capable of delivering single shocks at o.)Hz, train of four at 2Hz and tetanic stimulation at 30, 100, 200Hz. The response, adduction of the thumb, is sensed by the force transducer. The output of the force transducer Is amplified, filtered, converted digital signal and then processed by the per- sonal computer. The personal computer quantia4es twitch and traln of four tesponse and calculates the 74 ratio (Ta/Tl )between the first and fourth response of train of four. This ratio is used to estimate the level of the neuromuscular block. This system has reaserch potential for determining the effect of newer neuromuscular blocking drugs for comparlsion with presently used drugs of alternatively, for delerminig the effects of blocking drugs in altered physilogical states.

• The Korean Journal of Physiology
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• v.26 no.2
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• pp.159-166
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• 1992

The present study was designed to evaluate effects of intermittent electrical stimulation of the sciatic nerve on the atrophic response of antigravity muscles, such as the soleus (slow m.) and medial gastrocnemius (fast m.) muscles. Rats (Sprague-Dawley, 245-255g) were subjected to a hindlimb suspension and divided into three groups : one was with hindlimb suspension (MS) and another with hindlimb suspension plus intermittent electrical stimulation of the sciatic nerve (HS ES). Control group (CONT) was kept free without strain of the hindlimb. After 7 days of hindlimb suspension, the soleus and medial gastrocnemius muscles were cut at their insertion sites, and were then connected to the force transducer to observe their mechanical properties. Optimal pulse width and frequency of electrical stimulation were 0.2ms, 20Hz for the soleus muscle and 0.3ms, 40Hz for the medial gastrocnemius muscle under supramaximal stimulation. Body weight and circumference of the hindlimb were significantly decreased in HS and HS-ES groups compared with the control group. In HS-ES group, however, the weight of the soleus muscle was not different from that in the control group while the weight of the medial gastrocnemius muscle was lower than that in the control group. In HS group, mechanical properties of muscle contraction including contraction time, half relaxation time, twitch tension, tetanic tension, and fatigue index of both muscles were significantly decreased compared with the control group except for twitch tension and tetanic tension of medial gastrocnemius muscle. The degree of atrophy of the soleus muscle in HS group was more prominent than that of the medial gastrocnemius muscle. Twitch tension and fatigue index of the soleus muscle and fatigue index of the medial gastrocnemius muscle in HS-ES group were not different from those of the control group. While mechanical properties of the soleus muscle examined were all significantly increased in HS-ES group compared with HS group, only contraction time and fatigue index of the medial gastrocnemius muscle were significantly increased in HS-ES group. These data indicate that intermittent electrical stimulation may be useful in prevention of muscle atrophy.

• Archives of Pharmacal Research
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• v.27 no.6
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• pp.646-652
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• 2004

The objective of this study was to determine the acute effect of trimetazidine (TMZ) on the pre-fatigue, fatigue and post-fatigue contractile characteristics and tension-frequency relationships of isolated rat diaphragm muscle. Muscle strips were taken from the ventral-costal aspects of the diaphragm muscle of rats killed by decapitation. The muscle strips were suspended in organ baths containing Krebs solution, with a gas mixture of 95% $O_2$ and 5% $CO_2$ at $37^{\circ}C$ and pH 7.35-7.45. After determining the thermoregulation and optimum muscle length the muscles were subjected to direct supramaximal stimulation with 0.05 Hz frequency square pulses for periods of 0.5 msec to obtain control values. After adding $5{\times}10^{-6}{\;}and{\;}5{\times}10^{-5}$ M trimetazidine solution to the respective bath media, the contractile parameters of the muscles were recorded. The contractile parameters were also recorded for both the trimetazidine and tri-metazidine-free media after application of the high frequency fatigue protocols. Later, the tension-frequency relationship was determined by applying stimulating pulses of 10, 20, 50 and 100 Hz to the muscle strips. Whilst the twitch tension obtained from the $5{\times}10^{-6}{\;}and{\;}5{\times}10^{-5}$ M trimetazidine media showed numerical increases compared to that of the controls, these were not statistically significant (p>0.05). The contraction time exhibited a dose dependent increase (p<0.001), whilst the contraction and relaxation rates did not differ significantly. The isometric contraction forces obtained with the different stimulating frequencies showed a significant increase in the tetanic contraction only at 100 Hz (p<0.05). A comparison of the pre- and post-fatigue twitch tensions in the trimetazidine media showed the post- fatigue twitch tensions to be significantly higher than those of the pre-fatigue contraction forces (p<0.05). In the $5{\times}10^{-6}{\;}and{\;}5{\times}10^{-5}$ M trimetazidine media the increases in the post-fatigue contraction force were 22 and 30%, respectively. These results demonstrated that in isolated rat diaphragm muscle, TMZ significantly limited the mechanical performance decrease during fatigue. It is our opinion that trimetazidine contributed to the observed fatigue tolerance by eliminating the factors of fatigue, due to preservation of intracellular calcium homeostasis, provision of the ATP energy levels needed by ATPase dependent pumps and especially by keeping the intracellular pH within cer-tain limits.

• The Korean Journal of Pharmacology
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• v.32 no.2
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• pp.189-199
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• 1996

Myocardial ${\alpha}_1-adrenoceptors$ have been shown to mediate a biphaslc inotropic response that was characterized by a transient decline followed by a sustained increasing phase in guinea pig ventricular muscle. Recently one group reported that an ${\alpha}_1-adrenoceptors-induced$ intracellular $Na^+$ decrease is linked to fast $Na^+$ channel inhibition and another group reported that it is linked to $Na^+$-$K^+$ pump activation by ${\alpha}_{1b}-adrenoceptors$. But until now, its mechanism is not clear. Therefore, to see whether the $Na^+$channel or $Na^+-K^+$ pump is related to a decrease in intracellular $Na^+$ activity and/or the negative inotropic response, and which ${\alpha}_1-adrenoceptor$ subtype was involved in the decrease in intracellular $Na^+$activity by phenylephrine, we used conventional and sodium selective microelectrodes, and tension transducer to determine the effects of ${\alpha}_1-adrenergic$ stimulation on membrane potential, intracellular $Na^+$ activity, and twitch force in guinea pig ventricular muscles. $10^{-5}$ M Phenylephrine produced a slight hyperpolarization of the diastolic membrane potential, a decrease or increase in $a_N^i_a$, and a biphasic inotropic response. The negative inotropic response accompanied by a decrease in intracellular $Na^+$activity, whereas in muscles showing a remarkable positive inotropic response without initial negative inotropic effect was accompanied by an increase in intracellular $Na^+$ activity. The decrease in intracellular $Na^+$ activity was apparently inhibited by WB4101, an antagonist of the ${\alpha}_{1a}-adrenoceptors$. The decrease in intracellular $Na^+$ activity caused by phenylephrine was not abolished or reduced by a block of the fast $Na^+$ channels. $V_{max}$ also was not affected by phenylephrine. Phenylephrine produced an increase in intracellular $Na^+$ activity in the presence of a high concentration of extracellular $Ca^{2+}$ (in quiescent muscle) or phorbol dibutyrate, a protein kinase C activator(in beating muscle). These suggest that the ${\alpha}_{1a}-adrenoceptors-mediated$ decrease in intracellular $Na^+$ activity may be related to the protein kinase C.

• The Korean Journal of Physiology and Pharmacology
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• v.4 no.3
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• pp.197-210
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• 2000

Suppressive role of $Na^+-Ca^{2+}$ exchange in myocardial tension generation was examined in the negative frequency-force relationship (FFR) of electric field stimulated left atria (LA) from postnatal developing rat heart and in the whole-cell clamped adult rat ventricular myocytes with high concentration of intracellular $Ca^{2+}$ buffer (14 mM EGTA). LA twitch amplitudes, which were suppressed by cyclopiazonic acid in a postnatal age-dependent manner, elicited frequency-dependent and postnatal age-dependent enhancements after $Na^+-reduced,\;Ca^{2+}-depleted$ (26 Na-0 Ca) buffer application. These enhancements were blocked by caffeine pretreatment with postnatal age-dependent intensities. In the isolated rat ventricular myocytes, stimulation with the voltage protocol roughly mimicked action potential generated a large inward current which was partially blocked by nifedipine or $Na^+$ current inhibition. 0 Ca application suppressed the inward current by $39{\pm}4%$ while the current was further suppressed after 0 Na-0 Ca application by $53{\pm}3%.$ Caffeine increased this inward current by $44{\pm}3%$ in spite of 14 mM EGTA. Finally, the $Na^+$ current-dependent fraction of the inward current was increased in a stimulation frequency-dependent manner. From these results, it is concluded that the $Ca^{2+}$ exit-mode (forward-mode) $Na^+-Ca^{2+}$ exchange suppresses the LA tension by extruding $Ca^{2+}$ out of the cell right after its release from sarcoplasmic reticulum (SR) in a frequency-dependent manner during contraction, resulting in the negative frequency-force relationship in the rat LA.

• The Korean Journal of Pharmacology
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• v.30 no.1
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• pp.67-73
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• 1994

Na-Ca exchange transports calcium ion either into (reverse mode Na-Ca exchange) or out of the cell (forward mode Na-Ca exchange) according to the direction of driving force produced by the changes in ratio of intra- and extra-cellular Na concentrations. Thus, Na-Ca exchange is regarded as the regulator of myocardial contraction. However, the existence of reverse mode Na-Ca exchange and its role in myocardial contraction is still questioned. Present study was performed to identify the presence of reverse mode Na-Ca exchange and its possible involvement in the regulation of myocardial contraction in rat heart. Using the left atria of rat, contraction was induced by electrical field stimulation (EFS, 0.5 msec duration and supramaximal voltage). Changing of the stimulation frequencies from resting 4 Hz to 0.4, 1 or 8 Hz caused typical negative staircase effect in twitch tension, but $^{45}Ca$ uptake showed bimodal increase. When the stimulation frequency was abruptly changed from 4 Hz to 0.4 Hz the atrial twitch tension showed three phased-enhancement, that is, the initial rapid increase (the first phase) followed by rapid decrease (the second phase) and stabilization (the third phase). $^{45}Ca$ uptake was equivalent to tension, i.e. initial significant increase in first 30 second and then decrease. Benzamil treatment abolished the first phase of increase in a dose dependent manner from $10^{-5}\;to\;3{\times}10^{-4}M.$ Bay k 8644 $(3{\times}10^{-5}M)$ treatment enhanced the inotropy induced by frequency reduction and abolished the second and third phase decreases. Benzamil treatment also suppressed the contraction stimulated by Bay K 8644. Although the contraction at 4 Hz stimulation was completely abolished by verapamil $3{\times}10^{-5}\;M$ pretreatment, the contraction reappeared as soon as the stimulation frequency was changed into 0.4 or 1 Hz and interstingly,$^{45}Ca$ uptake were significantly higher than no treatment. From these results, it is concluded that reduction of stimulation frequency causes calcium influx by the reverse mode Na-Ca exchange, resulting in initial rapid increase of twitch tension. then it turns into forward mode exchange to efflux the calcium, resulting in decrease of the twitch tension in left atria of rat.