Diabetes mellitus decreases hippocampal release of neurotransmitters: an in vivo microdialysis study of awake, freely moving rats

Possible diabetes mellitus-induced changes in hippocampal monoaminergic activities were studied to understand the relationships between neurotransmitter levels and various abnormalities in freely moving diabetic rats. We used both experimentally (STZ rats) and spontaneously diabetic rats (WBN/Kob rats) as the diabetic animal model, and compared the findings with those obtained from non-diabetic rats (C rats). Measurement of neurotransmitters (serotonin and dopamine) was carried out using an in vivo microdialysis method. We found that: 1) the basal level of serotonin in the hippocampus was lowest in WBN rats, followed by STZ rats, then by C rats. The level of serotonin in WBN rats was about a half of that in C rats; 2) the basal level of dopamine was also significantly lower in the diabetic WBN and STZ rats than in C rats. The data show that diabetes mellitus decreases in the monoamine release from the hippocampus in both experimentally and spontaneously diabetic rats.

Postrest contraction in the ventricular papillary muscle of spontaneously diabetic WBN/Kob rat

In the present study, we investigated the characteristics of the postrest contraction (PRC) in chronic diabetic ventricular muscle. We used WBN/Kob rats of 7-8 weeks as the spontaneously diabetic animal and Wistar rats of 7-8 weeks as the control. We found: (1) No significant differences were seen in the amplitude, the contracting speed, and the relaxing speed of electrically stimulated twitch tension between control and WBN/Kob rats. In addition, the relationship between amplitude of twitch tension and stimulus cycle lengths (0.2-5 sec) was very similar in both animals. (2) The ratios of the first twitch tension (T1) of PRC with various rest intervals (5-600 sec) to the steady-state tension (Tss) were significantly smaller in the diabetic rats than in the controls. (3) When the preparation was stimulated at shorter cycle lengths, the recovery process of PRC was separated into at least two components (fast and slow components). In the diabetic rats, the time constant (tau) of both components was significantly longer than in controls. (4) After caffeine (10(-3) M) treatment, tau of the fast component in the control rats became longer, whereas it remained unchanged in diabetic rats. These findings suggest a dysfunction of the intracellular calcium handling system in spontaneously diabetic heart that is likely to include impaired calcium sequestration and/or extrusion.

Potassium chloride depolarization enhances MPP+-induced hydroxyl radical generation in the rat striatum

We determined that extracellular potassium ion concentration, [K+]o-induced depolarization, enhances 1-methyl-4-phenylpyridinium ion (MPP+)-induced hydroxyl radical (*OH) generation in the rat striatum. Rats were anesthetized, and sodium salicylate in Ringer's solution (0.5 nmol/microl/min) was infused through a microdialysis probe to detect the generation of *OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Induction of high concentration KCl (70 mM) drastically increased formation of *OH trapped as DHBA by the action of MPP+. When dopamine (DA) was administered to the high KCl-treated animals, a marked elevation of DHBA was observed, compared with MPP+-only-treated animals, that showed a positive linear correlation between DA and *OH formation trapped as DHBA (R2 = 0.979) in the dialysate. When corresponding experiments were performed with iron (II), the same results were obtained: a positive linear correlation between the release of iron (II) and DHBA (R2 = 0.988) in the dialysate. These results suggest that [K+]o-induced depolarization enhances the formation of *OH products of efflux/oxidation due to MPP+.

Electrophysiological properties of ventricular muscle obtained from spontaneously diabetic mice

The electrophysiological properties of cardiac muscle in KK/Ta mouse (hereafter referred to as KK mouse), an animal model of human non-insulin-dependent diabetes mellitus, were investigated, and the findings compared with those obtained from a non-diabetic control mouse (C57BL/6J mouse; referred to as B6 mouse). The ages of the B6 mice were 23.9 +/- 5.4 weeks (n = 24) and those of the KK mice used were 25.7 +/- 10.8 weeks (n = 34). The KK mice had mild obesity, hyperglycemia and hyperinsulinemia. Ventricular muscles from both mice were examined by light microscopy. Partial myocardial fibrosis and filament disorder in the ventricular muscles were found only in the KK mice. The resting membrane potential of the ventricular muscle was less negative in the KK mice than in the control mice. The maximum rate of rise in the upstroke of the action potential was significantly decreased in the KK mice compared with that of the control mice. These suggest a decrease in a time-independent K+ current (IK1) in the KK mice. The duration of the action potential (APD) at all levels of repolarization was significantly longer in the KK mice than in the B6 mice. A blocker of transient outward current (I(to)), 4-aminopyridine, significantly prolonged the APD of the B6 mice, but failed to prolong it in the KK mice, suggesting that Ito in the diabetic mice is very small. A Ca2+ channel blocker, CoCl2, dramatically lengthened all levels of APD in both groups, suggesting that there is no difference between B6 mice and KK mice in L-type Ca2+ current via Ca2+ channels. These suggest the malfunction or deficiency of ionic channels which carry, at least Ito and IK1 in diabetic mice.

Protective effect of histidine on hydroxyl radical generation induced by potassium-depolarization in rat myocardium

We investigated the efficacy of histidine on potassium-depolarization induced hydroxyl radical (*OH) generation in the extracellular fluid of rat myocardium by a flexibly mounted microdialysis technique (O system). After the rat was anesthetized, a microdialysis probe was implanted in the left ventricular myocardium, and then sodium salicylate in Ringer's solution (0.5 nmol/microl per minute) was infused to detect the generation of *OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Infusion of KCl (70 mM) clearly produced an increase in *OH formation. However, when KCl in the presence of a high concentration of histidine (25 mM) was infused through the microdialysis probe, KCl failed to increase the 2,3-DHBA formation. To examine the effect of histidine on ischemia-reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the levels of 2,3-DHBA was observed in the heart dialysate. However, when corresponding experiments were performed with histidine (25 mM)-pretreated animals, histidine prevented the ischemia-reperfusion induced *OH formation trapped as 2,3-DHBA. These results indicate that histidine may protect against K+-depolarization-evoked *OH generation in rat myocardium.

Protective effect of histidine on iron (II)-induced hydroxyl radical generation in rat hearts

We investigated the efficacy of histidine on iron (II)-induced hydroxyl radical (.OH) generation in extracellular fluid of the rat myocardium using a flexibly mounted microdialysis technique (O system). Rats were anesthetized and a microdialysis probe was implanted in the left ventricular, followed by infusion of sodium salicylate in Ringer's solution (0.5 nmol/microL/min) to detect the generation .OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Iron (II) clearly produced a concentration-dependent increase in .OH formation. A positive linear correlation between iron (II) and the formation of 2,3-DHBA (R2 = 0.987) was observed. However, histidine (25 mM) was infused through a microdialysis probe; iron (II) failed to increase the 2,3-DHBA formation obtained. To examine the effect of histidine on ischemia-reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the levels of 2,3-DHBA was observed in the heart dialysate. When corresponding experiments were performed with histidine (25 mM)-pretreated animals, histidine prevented the ischemia-reperfusion induced .OH generation trapped as 2,3-DHBA. These results indicate that histidine protects the myocardium against ischemia-reperfusion damage by .OH generation.

Antiarrhythmic effects of magnesium on rat papillary muscle and guinea pig ventricular myocytes

1. Despite widespread use of magnesium ion (Mg2+) for antiarrhythmic purposes, little direct information is available regarding its antiarrhythmic mechanisms. To elucidate the possible cellular mechanism, the effects of Mg2+ on early afterdepolarization (EAD), delayed afterdepolarization (DAD), triggered activity (TA), transient inward current (TI) and aftercontraction (AC) were examined in various cardiac preparations. The effects of Mg2+ on myoplasmic Ca2+ concentration were also studied. 2. The effects of Mg2+ on AC, induced by overdrive stimulation, were studied in isolated rat ventricular papillary muscle superfused with low K+ solution. In enzymatically isolated guinea pig myocytes, EAD, DAD and/or TA were induced after overdrive stimulation under conditions of superfusion with low K+ solution, using the whole-cell current-clamp method, and TI was also induced by the whole-cell voltage-clamp method. 3. Immediately after changing the solutions, containing varying concentrations of Mg2+, the effects of Mg2+ were examined. In addition, effects of Mg2+ on Ca transient were studied, using fura-2. 4. We found that: (1) in the rat papillary muscle, 10 mM Mg2+ effectively inhibited AC, which was produced after stimulation at both 3.3 Hz and 5 Hz, although 5 mM Mg2+ was without effect in the case of AC induced after 5-Hz stimulation; (2) in the myocytes, 5 mM Mg2+ did not inhibit DADs, EADs and TA, but 10 mM Mg2+ inhibited them completely; (3) the amplitude and frequency of TI decreased significantly in the presence of 10 mM Mg2+; and finally (4) 10 mM Mg2+ inhibited the Ca transient underlying DAD and/or TA. 5. The findings suggest, but do not prove unequivocally, that Mg's actions are probably due to a combination of a shift of the threshold of various ion channels to less negative potentials, a decrease in Ca2+ influx via Ca channels, a block of several K channels, and/or a block of Na-Ca exchanger. In conclusion, the present study indicates that extracellular Mg2+, via whatever mechanism, exerts antiarrhythmic activities.

Capsaicin does not inhibit the intracellular calcium handling process in rat ventricular papillary muscle

1. We studied the effects of capsaicin, a pungent agent extracted from red pepper, on rested-state contraction (RSC) of isolated rat ventricular papillary muscles. 2. The RSC was induced by stimulation, after a rest interval of 5 sec to 10 min, after the twitch tension of the muscle preparation stimulated at the regular stimulus frequency of cycle lengths of 5, 1 or 0.2 sec attained the steady state. 3. Drug effects were evaluated on the RSC in the presence of capsaicin 10(-5) M, caffeine 10(-2) M or ryanodine 10(-7) M, respectively. 4. All drugs inhibited the RSC but to different degrees. Ryanodine was the most effective in inhibiting RSC, followed by caffeine and capsaicin, in that order. However, the inhibitory mode varied, depending on the drugs. 5. These findings suggest that capsaicin may not inhibit the function of intracellular Ca2+ store in rat cardiac muscle.

Inhibition of contractile tension by capsaicin in isolated rat papillary muscle

1. We examined effects of capsaicin (10(-9) - 10(-5) M), a pungent agent extracted from red pepper, on the contractile tension of rat ventricular papillary muscles stimulated at various cycle lengths (0.2, 0.5, 1, 2 and 5 sec). 2. Capsaicin produced a marked concentration-dependent decrease in the amplitude, the rate of rise (dp/dt) and the rate of relaxation (dr/dt) of the tension. 3. However, the half relaxation time and the time to peak tension of the tension were slightly affected by the agent. 4. The negative inotropic effect of capsaicin was stimulus cycle length (CL) dependent. In particular, IC50 (50% inhibitive concentration) of the agent in the amplitude of the tension was stimulus CL dependent. That is, the values of IC50 were around 10(-7) M at longer CLs (1, 2 and 5 sec), and the value of IC50 at CL 0.2 sec was 4 x 10(-6) M. 5. These capsaicin-induced negative inotropic effects were reversible. Other studies from our laboratory show that the negative inotropic effects may be largely due to a decrease in Ca2+ current.

Bepridil prolongs the action potential duration of guinea pig ventricular muscle only at rapid rates of stimulation

1. We examined the electromechanical effects of the calcium antagonist, bepridil (1-20 microM), on isolated guinea pig ventricular muscles, driven at various stimulus frequencies (0.1, 0.5, 1, 2 and 5 Hz) in Tyrode's solution containing various K+ concentrations (1.4-43.2 mM). 2. Conventional microelectrode and tension-recording techniques were used. 3. We found that bepridil decreased the maximum upstroke velocity (Vmax) of the action potential with no change in the resting membrane potential (RMP). 4. The former effect depended on both stimulus frequency and the drug concentration used. 5. Bepridil lengthened the duration of the action potential at the level of 25% repolarization (APD25) at the highest frequency (5 Hz), but shortened it at lower frequencies (< or = 2 Hz). 6. The drug also lengthened the APD90 at the highest frequency (5 Hz) but without significant effect at lower frequencies (< or = 2 Hz). 7. Bepridil depolarized the RMP at relatively low extracellular K+ concentrations (< or = 2.7 mM), accompanied by a prolongation of APD90. 8. There were no such effects at much higher K+ concentrations (> or = 5.4 mM), and the drug markedly depressed the Vmax and the action potential amplitude. 9. The drug eliminated the positive staircase phenomenon of twitch contraction, in a concentration-dependent manner. 10. All these findings taken together suggest that bepridil prolongs the action potential duration by inhibiting outward potassium currents (IK and IK1), at rapid rates of stimulation (approximately 300/min), which is comparable to the physiological heart rate of a guinea pig. 11. The prolongation of APD seemed to be secondary to the bepridil-induced reduction of intracellular Ca2+ concentration, [Ca2+]i.

The negative inotropic effects of amiodarone on isolated guinea pig heart: a possible role of Na(+)-Ca2+ exchange

1. The negative inotropic effects of amiodarone (AM) were studied in isolated, isometrically contracting ventricular papillary muscles from guinea pigs. 2. AM, 4.4 x 10(-5) M, significantly decreased ouabain (10(-6) M)-induced increase in the developed tension. 3. Manganese (10(-2) M), a partial blocker of Na(+)-Ca2+ exchange, attenuated the AM's negative inotropy. 4. Theophylline (1.5 x 10(-2) M), an inhibitor of phosphodiesterase, produced a marked increase in the tension (about twice compared to the ouabain effect). 5. However, the magnitude of decrease by AM in the tension in the presence of theophylline was similar to that in the case of ouabain. 6. Tetrodotoxin (TTX) decreased the contraction by about a half, and then subsequent addition of AM in the presence of TTX led to a further decrease in the tension. 7. Eventually co-existence of TTX and AM led to a decrease in tension of same degree, compared to the decrease in tension by AM alone. 8. The results suggest that a large portion of negative inotropic action of AM may, at least, reflect interference with the Na(+)-Ca2+ exchange mechanism. 9. This interference with the Na(+)-Ca2+ exchange mechanism may exert a strong negative inotropic effect of the drug, in combination with a decrease in Ca2+ influx via Ca2+ channels and/or an impairment of Ca(2+)-sequestration.

Effects of amiodarone on triggered activity induced by overdrive stimulation in Ca2+ overloaded ventricular muscle from guinea pig

1. The acute effects of amiodarone, a powerful antiarrhythmic drug, on transient depolarizations (TDs) and/or triggered activity (TA) induced by an overdrive stimulation in the condition of low potassium (2.7 mM) and high calcium (5.4 mM) solution were evaluated on isolated ventricular papillary muscles from guinea pig, by means of conventional microelectrode techniques. 2. The amplitude of the induced TDs was enhanced by increase in stimulus number and frequency during overdrive stimulation, and the coupling interval of TDs was shortened. 3. Amiodarone (4.4 x 10(-5) M) significantly decreased the amplitude of TDs, and prolonged the coupling interval. 4. On the other hand, superfusion with a higher concentration (4.4 x 10(-4) M) of amiodarone tended to induce automatic activity. 5. Possible implications with respect to the antiarrhythmic activity of amiodarone are discussed.

[Antiarrhythmic effects of magnesium on single ventricular myocytes]

Despite magnesium ions (Mg)'s widespread use for antiarrhythmic purposes, little is known concerning its antiarrhythmic mechanisms. We, therefore, examined Mg effects on delayed afterdepolarization (DAD), early afterdepolarization (EAD), triggered activity (TA) and aftercontraction (AC), using isolated ventricular cells and/or ventricular papillary muscle. In the experiments using the multicellular preparation, ACs were measured, with the use of a strain gauge. ACs were induced in isolated rat papillary muscle superfused with low K+ (0.5 mM) medium, after a train stimulation (2-5 Hz, 10-30 beats). In the experiments concerning cardiac myocytes, the effects of Mg on transient inward current (TI), which is responsible for DAD, were studied using a whole-cell voltage-clamp method applied on isolated guinea pig ventricular cells. And action potentials were also induced by current clamp (10 ms) and the effects of Mg on DAD, EAD and TA were examined. TI, DAD, EAD and TA were induced by use of pipette solution with high Ca2+ (0.7 mM) and low EGTA (0.1 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine reverses isoproterenol-induced depression of Vmax in residual Na channel-dependent action potentials of guinea-pig cardiac muscles

We examined effects of acetylcholine (ACh) on isoproterenol (ISP)-induced changes of the upstroke velocity of the action potential (Vmax) in isolated 13.5 mM K(+)-depolarized atrial muscles from guinea-pigs, using conventional glass microelectrode techniques. In some experiments, ventricular muscles were also used, for purposes of comparison. ISP (0.1 microM) decreased the fast component of Vmax (Vmax,fast) and increased the slow component of Vmax (Vmax, slow) of the atrial muscle, as has been noted in ventricular muscle. ACh (0.1 microM) reversed or antagonized these effects of ISP. However, in the presence of atropine (0.1 microM), the antagonism disappeared. In the presence of the Ca2+ channel blocker, D600 (1 microM), the depressant effect of ISP on the Vmax,fast was augmented while ACh exerted a much less restorative effect on the ISP-induced, depressed Vmax,fast. Similar findings were obtained in low (0.36 and 0.072 mM) Ca2+ media. To investigate the possible involvement of GTP-binding protein (Gi) on these ACh effects, we performed similar experiments using atrial muscles obtained from guinea pigs pre-treated with pertussis toxin (150 micrograms/kg) for 48 h. In these preparations, the depressant effect of ISP on the Vmax,fast remained unaffected, while the reversing effect of ACh on the ISP-induced depression of Vmax,fast either specifically diminished or disappeared. These results show that ACh antagonizes the ISP-induced Vmax changes via stimulation of muscarinic ACh receptors and that this effect is presumably mediated by Gi and modified by intracellular Ca2+. Clinical implications are discussed.

Increased susceptibility to hypoxia of prolonged action potential duration in ventricular papillary muscles from diabetic rats

The action potential duration (APD) of ventricular muscles obtained from diabetic animals is reported to be prolonged. We studied the effect of varied periods of diabetes on APD prolongation using isolated ventricular papillary muscles from streptozocin-injected rats. We found that a diabetic period greater than 30 wk was necessary for the evolution of significant prolongation of APD. We then studied the effect of repeated hypoxia (PO2 40 mmHg) and normoxia (PO2 300 mmHg) on prolonged APD of diabetic muscles and compared the findings with those from control rats. Transmembrane potentials were recorded with conventional glass microelectrodes. Under normoxic conditions, the APDs of diabetic muscles were significantly prolonged, the maximum upstroke velocity of action potentials tended to be decreased, and the resting membrane potential was not changed significantly compared with controls. The first hypoxia (20 min) shortened the APD in both diabetic and control rats but more so in diabetic rats, thereby making the APD of diabetic rats virtually identical to the control rat APD during the hypoxia. On subsequent reoxygenation (30 min), these hypoxia-induced changes were almost recovered. The second hypoxia (20 min) produced more severe shortening of the APD in both muscle types, and the effect was again far greater in diabetic rats than controls, ending with a reversed sequence of APDs and the APD of diabetic muscles much shorter than the controls. Excessive shortening of APD in diabetic muscles during hypoxic conditions was speculated to be due to greater increases in the outward K+ current through ATP-regulated K+ channels that may be secondary to the more severe reduction of intracellular ATP concentrations in diabetic versus control hearts.

Chronic diabetes mellitus prolongs action potential duration of rat ventricular muscles: circumstantial evidence for impaired Ca2+ channel

STUDY OBJECTIVE

The aim of the study was to investigate the effects of chronic diabetes mellitus on electromechanical properties of ventricular papillary muscles.

DESIGN

Conventional glass microelectrodes and tension recording techniques were used in isolated hearts of rats made diabetic for 30-40 weeks by single intravenous injections of streptozotocin.

SUBJECTS

Experimental animals were male Wistar rats of 200-250 g. Diabetic rats (n = 14) were given streptozotocin 65 mg.kg-1; controls (n = 15) were given vehicle only.

MEASUREMENTS AND MAIN RESULTS

(1) The maximum upstroke velocity of the action potential duration of diabetic muscles was decreased compared to control, with no difference in the resting potential. (2) At all stimulation frequencies (0.2, 1 and 5 Hz), and particularly the lower ones, the action potential duration of diabetic muscles was longer than control. (3) In diabetic muscles, frequency dependent shortening of the late phase of action potential duration (APD75, APD90) was more pronounced, and frequency dependent lengthening of the early phase (APD25, APD50) was less pronounced. (4) A blocker of transient outward current, 4-aminopyridine, lengthened the early phase of action potential durations by the same amount in diabetic and control muscles. (5) A Ca2+ channel blocker, CoCl2, dramatically shortened all levels of action potential duration, with much greater effect on diabetic muscles. (6) Ryanodine lengthened the early phase of action potential duration and shortened the late phase in both diabetic and control muscles. It enhanced the difference between the groups in the early phase. (7) Developed tension in the presence of ryanodine (ryanodine resistant tension component) was greater in diabetic muscles than in control.

CONCLUSIONS

The findings suggest that altered Ca2+ current, but not altered Na(+)-Ca2+ exchange current or altered transient outward current, significantly prolongs action potential duration in diabetic rat ventricular muscles.

Negative inotropic effects of amiodarone on isolated guinea pig papillary muscle

STUDY OBJECTIVE

The aim of the study was to investigate the inotropic effects on isolated papillary muscle of acute and chronic exposure to amiodarone.

DESIGN

Papillary muscles were obtained from untreated and amiodarone pretreated guinea pigs, and tension recording and conventional glass microelectrode techniques were used to assess the effects of amiodarone.

SUBJECTS

Guinea pigs of either sex weighing 300-500 g were used: 65 untreated and 18 amiodarone pretreated (20 mg.kg-1.d-1 for 3 1/2 weeks).

MEASUREMENTS AND RESULTS

Acute amiodarone exposure (4.4 X 10(-6)-1.1 X 10(-3) mol.litre-1) caused a concentration dependent negative inotropic effect, which was also frequency dependent (0.1-5 Hz), with greater effect at higher stimulation frequencies. Time to peak tension and resting tension increased, while rate of development and relaxation of twitch contraction were slower. Amiodarone caused a dose dependent depression or abolition of slow response action potentials and of contraction in both untreated and amiodarone pretreated specimens. In amiodarone pretreated muscle, contractile forces did not differ significantly from control preparations. Chronic exposure to amiodarone attenuated the negative inotropic effects of acute amiodarone superfusion.

CONCLUSIONS

The negative inotropic action of amiodarone may reflect a decrease in Na+ influx via Na+ channels and/or Ca2+ influx via Ca2+ channels, and/or an impairment of Ca2+ sequestration, alone or in combination. The differences in contractile response in relation to acute and chronic amiodarone exposure may be relevant to clinical actions of amiodarone.

The inhibitory actions of amiodarone on rested-state contraction in isolated guinea-pig ventricular muscle

1. To assess the mechanism(s) of the negative inotropic effects of amiodarone (AM), an effective anti-arrhythmic agent, the effects of AM on rested-state contraction (RSC) were studied in isolated ventricular papillary muscle from control (untreated) and AM-pretreated guinea-pigs. 2. The RSC was induced by stimulation, following a 30 sec- or 10 min-rest period. 3. The drug's effects were evaluated on an initial response (Ti) and steady-state response (Tss) of the RSC at the end of 1 min-stimulus train at 3.3 Hz. 4. In normal physiological solution, the magnitude of Ti was 2.1 times that of Tss in papillary muscles from untreated animals, but, 1.3 times, in AM-pretreated ones. 5. The effects of superfused AM (4.4 x 10(-5)M) were also evaluated in both muscle types. 6. The drug markedly decreased Ti and Tss in both control and AM-pretreated specimens. 7. However, the depression by superfused AM of RSC in control specimens was more likely to be marked than in AM-pretreated ones. 8. Further, verapamil and caffeine, which affect SR function, also depressed the RSC. 9. These results suggest that the negative inotropic actions of AM, at least in part, reflect a decrease in Ca2+ via Ca2+ channels and an impairment of Ca2(+)-sequestrating system.

The making of diabetic guinea pigs by streptozotocin and high incidence of triggered activity in the ventricular muscle

Myoplasmic Ca2+ metabolism is reported to be impaired in diabetic rat heart. We studied the possibility that the ventricular muscles of diabetic guinea pig are prone to develop delayed afterdepolarizations (DADs) and triggered activity (TA), because DADs and TA are believed to be a possible index of increased level of intracellular Ca2+ concentration. To establish an experimental diabetic model from the guinea pig, male animals were divided into four groups: 1) control group: intracardiac injection of citrate buffer; 2) IP group: intraperitoneal injection of streptozotocin (STZ, 200 mg/kg); 3) IC group: intracardiac injection of STZ; and 4) Ins-IC group: intracardiac injection of STZ after pretreatment with insulin (20 IU/kg). We found that: 1) only in the Ins-IC group was the fasting plasma glucose concentration (determined 40 days after STZ injection) significantly higher than in the control group; 2) oral glucose tolerance test performed 40 days after treatment also showed glucose intolerance in the Ins-IC group. These findings evinced the successful making of diabetic guinea pigs by an intracardiac one-shot injection of STZ during development of insulin-induced hypoglycemia. In vitro electrophysiological experiments were performed on ventricular papillary muscle from diabetic animals (Ins-IC group) by conventional glass microelectrode techniques. Transmembrane action potentials were elicited by pulse trains with various rates (2-5 Hz) and durations (10-30 stimuli) in the presence of ouabain (1 microM) and various Ca2+ concentrations (1.8-7.2 mM). The incidence of TA in the muscles from diabetic animals was significantly higher (chi 2-test, p less than 0.05) than that from controls. The findings gave evidence that Ca2+ homeostasis in the myocardium of diabetic guinea pigs is impaired, and this may be a cause of arrhythmia.

Electrophysiologic effects of a short-chain acyl carnitine, L-propionylcarnitine, on isolated canine Purkinje fibers

The effects of L-propionylcarnitine (PC), a derivative of L-carnitine, on action potentials of canine Purkinje fibers in vitro, were studied under acidic conditions (pH 6.9), using conventional microelectrode techniques. The concentrations of 10(-5) M to 3 X 10(-3) M had no significant effect on action potential amplitude, maximal upstroke velocity of phase 0, and resting potential. However, higher concentrations of PC (10(-2) M and 3 X 10(-2) M) decreased some of these action potential parameters and such high concentrations of PC consistently prolonged the action potential duration (APD), most of which was attributed to the lengthening of phase 3. Under hypoxic conditions (PO2 less than or equal to 40 mm Hg) the drug (10(-2) M) also lengthened the APD with eventual cancellation of the hypoxia-induced shortening in the APD. The drug also prolonged the APD of slow response with a slight decrease in Vmax. In addition, PC significantly (p less than 0.05) depolarized the maximal diastolic potential of the fibers only at low [K+]o (less than or equal to 2.7 mM) and not at high [K+]o (greater than or equal to 5.4 mM). These observations suggest that the PC-induced prolongation of APD is, at least in part, due to a decrease in membrane K+ conductance, an effect partly shared with other amphiphilic intermediates of lipid metabolism, such as palmitylcarnitine or lysophosphatidylcholine.

Suggestive evidence for inhibitory action of amiodarone on Na+, K+-pump activity in guinea pig heart

1. Effects of amiodarone, a powerful antiarrhythmic agent, on Na+, K+-pump activity were examined on ventricular papillary muscle of guinea pig, by means of conventional microelectrode technique. 2. The activity of Na+, K+-pump was measured by two methods. One of them was to measure the amplitudes of depolarization observed during overdrive stimulation (3.3 Hz) and of hyperpolarization observed after the overdrive stimulation (post-overdrive hyperpolarization), and the other, to measure the hyperpolarization observed following introduction of 10 mM K+, after exposure to K+ free solution for a certain duration. 3. Amiodarone significantly decreased the amplitude of depolarization during overdrive stimulation and the amplitude of post-overdrive hyperpolarization. 4. In the latter method, the deactivation process of hyperpolarization recorded by the introduction of 10 mM K+ following K+ depletion slowed down by amiodarone. 5. These findings suggest that amiodarone may inhibit, at least in part, the Na+, K+-pump activity in the ventricular muscle of guinea pig.

Effects of amiodarone on barium-induced automatic activity in guinea pig ventricular muscle

1. The electrophysiological effects of a potent antiarrhythmic agent, amiodarone, on BaCl2-induced automaticity of guinea pig ventricular papillary muscle were studied by means of conventional microelectrode techniques. 2. BaCl2 (5 x 10(-4) M) depolarized the maximum diastolic potential of muscle preparation to about -62 mV and induced the spontaneous activity at rate of about 62 beats/min. 3. The application of amiodarone (4.4 x 10(-5) M to 1.1 x 10(-3) M) to the BaCl2-induced automatic muscle fibers decreased the spontaneous rate and slope of slow diastolic depolarization in a dose-dependent manner, associated with insignificant changes in action potential duration. 4. Amiodarone may exert the inhibitory effects of BaCl2-induced automaticity by decreasing steady-state conductance for the Na+ and Ca2+, probably thereby permitting the manifestation of antiarrhythmic properties in combination of its other actions.

Tetrodotoxin-sensitive component in action potential plateau of guinea pig Purkinje fibers: comparison with the papillary muscle

1. The effects of tetrodotoxin on action potentials of isolated guinea pig purkinje fibers were examined and compared the findings with those obtained in the ventricular papillary muscle, by use of conventional microelectrode techniques. 2. Tetrodotoxin (5 x 10(-7)-10(-5) M) decreased the amplitude, overshoot, and maximum upstroke velocity of action potentials of the Purkinje fibers, and shortened the duration of action potential at all levels of repolarization concentration- and stimulus cycle length-dependently. 3. The longer the stimulus cycle length, the greater the shortening by the drug of the action potential duration. 4. In particular, the plateau potential of the Purkinje fibers exposed to tetrodotoxin was remarkably depressed, and which occurred even in case of blockade of K+ conductance, using tetraethylammonium. 5. On the other hand, a high concentration (10(-5) M) of tetrodotoxin did not significantly affect the papillary muscle action potentials. 6. These findings suggest that there is a tetrodotoxin-sensitive component of Na+ current in plateau voltage range of the Purkinje fibers, but little in the papillary muscle, and that the component plays an important role to maintain the plateau of Purkinje fibers action potential.

Differential electrophysiological effects of amiodarone on ventricular muscle and Purkinje fibers in canine one-day-old myocardial infarction

1. We examined the electrophysiological effects of acute exposure to amiodarone (AM) on ischemic myocardium. 2. Regional myocardial ischemia was performed by occlusion on left anterior descending coronary artery in dog heart. 3. Conventional glass microelectrode techniques were used for electrophysiological investigation of regional ischemia. 4. The effects of AM on action potentials of subendocardial Purkinje fibers and ventricular muscle excised from ischemic area were studied and compared the findings with those obtained from non-ischemic area. 5. Acute exposure to AM, 4.4 x 10(-5) M, prolonged the total duration of action potential in the ischemic ventricular muscle and decreased the maximum upstroke velocity of action potentials significantly. 6. On the other hand, in the ischemic Purkinje fibers, AM produced no significant actions. 7. These findings suggest that AM's antiarrhythmic activity is, at least in part, due to its differential effects on repolarization of ischemic Purkinje fibers and ventricular muscle.

Inhibition of tetrodotoxin-sensitive plateau sodium current by amiodarone in guinea pig cardiac muscles

1. The acute effects of amiodarone (AM), a potent antiarrhythmic drug, on tetrodotoxin (TTX)-sensitive component of action potentials of Purkinje fibers from guinea pig were studied by use of conventional microelectrode techniques, and compared the findings with the results obtained in the papillary muscle. 2. The present study showed that the action potentials of Purkinje fibers (PF) were more sensitive to AM, compared to those of papillary muscle. 3. Acute exposure (30 min) to 4.4 x 10(-5) M AM led to a depression of plateau potential of PF action potential, but the drug did not affect the total action potential duration. 4. TTX (2-4 x 10(-6) M) shortened the PF action potential duration at all levels of repolarization. 5. The depression of PF plateau potential in the presence of AM was, at least in part, involved in a decrease in TTX-sensitive plateau sodium current, because no further depression of the plateau potential was observed by addition of TTX in the presence of AM.

Electrophysiological effects of lidocaine on isolated guinea pig Purkinje fibers: comparison with its effects on papillary muscle

1. The effects of lidocaine on action potentials of isolated guinea pig Purkinje fibers were examined and compared with those on the papillary muscle, by use of conventional microelectrode techniques. 2. Lidocaine 2.1 x 10(-4) M significantly depressed the resting membrane potential, amplitude, overshoot, maximum upstroke velocity, and duration of action potential of the Purkinje fibers stimulated at cycle length (CL) 1 sec, while the lower concentration (2.1 x 10(-5) M) produced no significant effect. 3. Various parameters of action potentials from the papillary muscle at CL = 1 sec were also depressed by lidocaine 2.1 x 10(-4) M, but the extent of the depression was less marked than in Purkinje fibers. 4. When the Purkinje fibers were stimulated at various CLs (0.3-10 sec), the shortening of action potential duration induced by lidocaine was more evident at slower stimulus rate.

Comparative effects of tetrodotoxin, lidocaine, and amiodarone on Vmax in guinea pig cardiac Purkinje and papillary muscle fibers

1. Electrophysiological effects of three drugs (tetrodotoxin, lidocaine, and amiodarone), which are known to depress Na channels in cardiac tissues, were studied on isolated guinea pig Purkinje fibers, by means of microelectrode techniques, and compared the findings with their effects on the ventricular papillary muscle. 2. Special attention was paid with regard to their effects on the maximum upstroke velocity (Vmax) of action potentials. 3. In Purkinje fibers, tetrodotoxin was the most inhibitory for Vmax, then in descending order, amiodarone, and lidocaine. 4. That is, half maximal inhibition (IC50) of Vmax by tetrodotoxin, lidocaine and amiodarone was approximately 1.7 x 10(-5) M, 1.5 x 10(-4) M and 2.8 x 10(-4) M, respectively. 5. As for papillary muscle, higher concentrations of the three drugs were needed to get similar potency. 6. The relationships between the depression in the Vmax and action potential duration are discussed in conjunction with their antiarrhythmic activities.

Effects of L-propionylcarnitine on electrical and mechanical alterations induced by amphiphilic lipids in isolated guinea pig ventricular muscle

We examined the effects of L-propionylcarnitine (Prop. C), a short-chain acylcarnitine, on amphiphile (L-lysophosphatidylcholine or L-palmitoylcarnitine)-induced electrophysiological and ultrastructural changes in isolated guinea pig ventricular papillary muscles, under acidic conditions (pH 6.9). Conventional microelectrode, tension-recording, and electron microscope techniques were used. Both amphiphiles, at a concentration of 10(-4) M, significantly decreased the resting membrane potential, action potential amplitude, and action potential duration, but increased the developed and resting tension. Such amphiphile-induced electrical changes were not observed in muscles pretreated with the beta-blocker, atenolol, although the mechanical changes remained unaffected. The application of Prop. C (10(-2) M), in the continued presence of the amphiphiles caused a return of the action potential duration and the developed tension to the control level. However, the resting potential and action potential amplitude remained unaffected; in fact, the maximum upstroke velocity (Vmax) of the action potential tended to decrease further. Pretreatment with Prop. C prevented all the amphiphile-induced electrophysiological and mechanical changes, except for Vmax. Electron microscopic studies revealed that amphiphile-induced ultrastructural changes were prevented, at least in part, in the presence of Prop. C. Thus, Prop. C antagonizes some of deleterious effects of amphiphiles, such as lysophosphatidylcholine and palmitoylcarnitine, upon the electrical and mechanical activities of the ventricular muscle, under acidic conditions.

Multiple electrophysiological actions of amiodarone on guinea pig heart

The cellular electrophysiologic effects of acute exposure to amiodarone (AM) on guinea pig papillary muscle (PM) and Purkinje fibres (PF) were investigated by means of conventional microelectrode techniques. Superfusion with AM less than 1.1 x 10(-4) mol/l reduced the maximum rate of rise (Vmax) of the action potential (AP) upstroke phase 0 of both PM and PF stimulated at 1 Hz, without changing resting membrane potential (RMP) or action potential duration (APD). AM greater than 1.1 x 10(-4) mol/l decreased APD at all levels, accompanied by decreases in Vmax and RMP. PF AP's were much more sensitive to AM than PM. In contrast, chronic exposure (20 mg/kg/day, 3 1/2 weeks) prolonged PM APD at all levels and decreased Vmax. In addition, acute exposure shifted steady state inactivation of Vmax by 4-7 mV to more negative potentials. The decrease of Vmax was frequency- and concentration-dependent. Half-maximal inhibition (IC50) of Vmax by AM was affected by K+-induced membrane depolarization (in 4 mmol/l K+, IC50 congruent to 2.3 x 10(-4) mol/l; in 8 mmol/l K+, IC50 congruent to 9 x 10(-5) mol/l). Frequency-dependent inhibition of closed Na+ channels by AM was demonstrated and AM increased the time constant for recovery from Na+ channel blockade. Depression of PF plateau by AM was similar to the effects of tetrodotoxin (TTX). Finally, AM depolarized RMP of PM exposed to low K+. The multiplicity of changes suggests that AM exerts inhibitory effects on a number of ionic current components, including at least fast Na+ current, slow inward current, TTX-sensitive plateau and outward K+ currents. Possible implications with respect to the broad spectrum of antiarrhythmic activity exhibited by AM are considered.

Acute effects of amiodarone on action potentials of isolated canine Purkinje fibers: comparison with tetrodotoxin effects

1. We compared the acute electrophysiological effects of amiodarone (AM) and tetrodotoxin (TTX) on action potentials of isolated canine Purkinje fibers. All two drugs suppressed action potential amplitude, overshoot, and maximum rate of upstroke of action potential, and shortened action potential duration (APD). 2. However, higher concentrations (4.4 x 10(-4) M) of AM showed differential effects on APD, compared with TTX. 3. These differences between effects of AM and TTX suggest that in case of high concentration of AM the APD-shortening by AM might be masked by AM's other action(s), while in low concentration the APD-shortening effect of AM, probably due to decrease in Na+ and slow Ca2+ current, was predominant.

Does acute exposure to amiodarone prolong cardiac action potential duration?

1. The acute effects of a potent antiarrhythmic agent, amiodarone (AM), were studied in isolated guinea pig ventricular muscle. Transmembrane action potentials were recorded by conventional microelectrode techniques, and isometric contractile tension, by a strain gauge. 2. Short-term (30 min) exposure to 4.4 x 10(-5) M AM did not significantly effect action potential characteristics, whereas the peak developed tension was significantly depressed. 3. On the other hand, long-term (3-5 hr) exposure to the same concentration of AM led to a significant decrease in the resting membrane potential, amplitude of action potential, overshoot of action potential, maximum upstroke velocity of action potential, and the peak developed tension. However, the duration of action potential at all levels of repolarization was not significantly effected by 4.4 x 10(-5) M AM during the entire duration of the experiment. 4. These results suggest that the acute AM's antiarrhythmic actions reported may be not due to APD-lengthening action, but probably due to other actions (mainly, inhibition of Na+ channels, Ca2+ channels).

Inhibition by amiodarone of slow response action potentials and contraction in guinea pig ventricular muscle

1. Effects of an antiarrhythmic agent, amiodarone (AM), on slow response action potentials and contraction of guinea pig ventricular muscle were examined. 2. AM (4.4 x 10(-5) M, 1.1 x 10(-4) M) significantly inhibited the maximum upstroke velocity of slow response action potentials and contraction. 3. As for action potential duration (APD), higher concentration (1.1 x 10(-4) M) of AM significantly shortened the APD, while low concentration (4.4 x 10(-5) M) had little effect on it. 4. These findings strongly suggest that AM blocks slow inward current inflow via Ca2+ channels in the ventricular muscle.

Acute effects of amiodarone on action potentials of isolated guinea-pig ventricular muscle exposed to simulated ischemic solution and metabolic inhibitors

1. Acute effects of an antiarrhythmic agent, amiodarone (AM), on action potentials from isolated guinea-pig ventricular papillary muscles exposed to simulated ischemic solution and Tyrode's solution containing NaCN or dinitrophenol (DNP) were examined. 2. In the papillary muscles exposed to simulated solution (K+ = 6 mM, glucose = 2.3 mM, pH = 7.1, PO2 less than 40 mmHg) or metabolic inhibitors (10(-3) M NaCN or 10(-4) M DNP), resting membrane potential (RMP) and maximum upstroke velocity (Vmax) of action potential were decreased with increasing time to the exposure, and action potential duration (APD) was shortened. In these conditions, addition of 4.4 x 10(-5) M AM produced transient and small increase in the APD and Vmax, followed by progressive APD-shortening and decrease in Vmax and RMP. 3. These results suggest that acute AM's antiarrhythmic effects reported are not due to APD prolonging action (Vaughan Williams Class III) of the drug, but mainly due to its local anesthetic action (Class I).

Differential effects of L-propionylcarnitine on the electrical and mechanical properties of guinea pig ventricular muscle in normal and acidic conditions

We studied the effects of L-propionylcarnitine (PC) on transmembrane action potentials and isometric contractile tension in isolated guinea pig ventricular papillary muscles. The effects of 5 concentrations of PC (10(-5), 10(-4), 10(-3), 10(-2) and 3 X 10(-2)M) were examined in both normal (pH 7.4) and acidic (pH 6.9) conditions. The concentrations of 10(-5) to 10(-2)M had no significant effect on action potential amplitude, maximum upstroke velocity of phase 0 and resting potential, in either condition. At pH 7.4, action potential duration (ADP) was significantly (P less than 0.05) or insignificantly shortened by the drug depending upon the concentration used. At pH 6.9, however, the APD was prolonged by moderate PC concentrations (10(-3) and 10(-2)M), in which the effective refractory period (ERP) was also lengthened, associated with an increased ERP/APD ratio. In both pH conditions, the highest concentration (3 X 10(-2)M) significantly (P less than 0.05) decreased all these action potential parameters. PC had a biphasic effect on the developed tension. In both pH conditions, low PC concentrations (10(-5) to 10(-3)M) produced an initial augmentation of the contraction, followed by subsequent reduction. The initial augmentation disappeared by pretreatment with reserpine or propranolol, suggesting the involvement of beta-adrenoceptors. In the steady state, all PC concentrations produced a negative inotropic effect at pH 7.4, while at pH 6.9 only high concentrations (10(-2)M and 3 X 10(-2)M) had this effect. These results suggest that the effects of PC in an acidic condition differ considerably from those in a normal pH condition and that limited concentrations of PC (10(-3) to 10(-2)M) may prevent re-entrant arrhythmias from developing under acidic conditions via lengthening of the ERP, without deleterious effects on the contractile force.

Amiodarone-induced block of sodium current in isolated cardiac cells

Sodium current (INa) block by amiodarone (AMI) was investigated in isolated single Purkinje and ventricular myocardial cells using the single suction-pipette voltage-clamp technique. AMI produced marked resting block that was enhanced at low holding potentials, findings consistent with a shift in the steady-state INa availability curve to more negative potentials (-16 +/- 3 mV). Resting block was not associated with any change in the time course of INa decay during a depolarizing clamp step. AMI also produced use-dependent block in conjunction with increases in rate (0.5-5.0 Hz) and pulse duration (2-200 msec). These changes are consistent with a slowing of the recovery from inactivation of the sodium channel. Brief depolarizing pulses produced little use-dependent block, suggesting that the onset of drug-induced block is slow. Thus, AMI blocks INa and shifts the availability curve in isolated myocytes, both of which contribute to the net tonic block. The results suggest that both rested state and inactivated state sodium channel block are factors in AMI's antiarrhythmic efficacy.

Na,K-ATPase activity and repolarization of ventricular action potentials in simian hearts

We measured Na,K-ATPase activity and ATP content of 4 different areas of the left ventricular muscle of Japanese monkeys (Mucaca fuscata): The apex, base, and epicardium (Epi) and endocardium (Endo) of the free wall. We compared those values with electrical parameters such as action potential duration (APD) and the level of resing potentials. APDs of base and Endo were significantly longer than those of apex and Epi, respectively. There was no significant difference in ATP contents among the four tissues while Na,K-ATPase activity in Epi was significantly higher, thereby indicating that the maximum capacity of the Na,K-pump activity is greater in Epi than in Endo. This observation serves to explain the following differences in electrophysiology characteristics found between Epi and Endo: greater shortening of APD in Endo when the cycle length of stimulation was shortened (from 2,000 to 200 ms) or when ouabain (0.5-1 microM) was applied; larger amplitude of post-overdrive hyperpolarization in Epi; and marked ouabain-induced depolarization of the resting potential in Endo, as compared to Epi. We conclude that the difference in Na,K-ATPase activity accounts for the different electrical behavior observed in Epi and Endo of the simian ventricle.

Pretreatment with coenzyme Q10 protects guinea pig ventricular muscle from hypoxia-induced deterioration of action potentials and contraction

Effects of coenzyme Q10 (CoQ10) on hypoxia-induced changes in transmembrane action potentials and isometric tension were studied in isolated guinea pig ventricular papillary muscles. Guinea pigs were pretreated with CoQ10 (60 mg/kg/day i.p., n = 4) or the solvent (n = 4) for 3 consecutive days before the study. The hypoxia for 30 min (Po2 = 40 mmHg) was induced to the preparation twice with a 20 min normoxic perfusion (Po2 = 300 mmHg) intervention. The hypoxia markedly shortened action potential duration (APD) and decreased the developed tension, the effects being more pronounced during the second than the first-induced hypoxia. Pretreatment with CoQ10 or the solvent did not affect the membrane potentials and contractile tension under normoxic conditions. The decreases in APD and the developed tension produced by hypoxia were partially but significantly suppressed in the preparation obtained from CoQ10-pretreated animals. The results suggest that the pretreatment with CoQ10 partially protects the isolated ventricular muscle subjected to hypoxia from the deterioration of action potentials and contraction.

Antiarrhythmic effects of nicorandil on canine cardiac Purkinje fibers

Antiarrhythmic efficacy of nicorandil (SG-75) (1-100 microM), a coronary vasodilator, was investigated electrophysiologically with regard to the action potentials of canine Purkinje fibers. The main results obtained are as follows: (a) nicorandil suppressed three kinds of automaticities, i.e., spontaneous or low-K+-induced automaticity and electrical depolarization-induced automaticity; (b) the drug increased the effective refractory period relative to action potential duration and decreased differences between action potential duration and effective refractory period; and (c) in the presence of nicorandil (50 microM), membrane potentials at which the earliest premature response could be elicited were significantly more negative than control, thus leading to a much faster upstroke velocity of the premature response. These changes in electrophysiological properties suggest that the drug may be effective for treating selected cardiac arrhythmias due to both enhanced automaticity and reentry, particularly in the presence of a decreased membrane K conductance. These effects of nicorandil could be mostly attributed to an increase in membrane K conductance.

Isotachophoretic evidence for energy-preservating effect of coenzyme Q10 on isolated guinea-pig cardiac muscle

Effects of coenzyme Q10 (CoQ10) on hypoxia-induced changes in ATP, NAD and NADH levels were studied in the isolated atrial and ventricular muscles of guinea-pigs. Guinea-pigs were pretreated with CoQ10 (60 mg/kg/day, i.p.) or the solvent for 3 consecutive days before initiation of study. The concentrations of ATP, NAD and NADH were determined by isotachophoresis. The concentrations of ATP and NAD contained in the atrial or ventricular muscle decreased with increasing incubation time with hypoxic Tyrode's solution (pO2 not equal to 160 mmHG), but that of NADH increased. However, the ATP and NAD concentrations of atrial and ventricular muscles from CoQ10-pretreated animals tended to be higher than those from solvent-pretreated ones. Moreover, the increase in NADH concentration during hypoxia tended to be less in the CoQ10-pretreated preparation than in the solvent-pretreated one. These results suggest that the pretreatment with CoQ10 leads to the increase in CoQ10 content in mitochondria of heart muscle, thereby permitting the improvement of oxidative phosphorylation in the mitochondria during hypoxia.

Lysophosphatidylcholine decreases single channel conductance of inward rectifier K channel in mammalian ventricular myocytes

Effect of lysophosphatidylcholine (LPC) on the inward rectifier K channel of the isolated guinea pig ventricular cell was studied using patch clamp technique. In that LPC (100 microM) decreased the magnitude of the single channel conductance from 48 +/- 5 pS (mean and S.D., n = 5) to 12 +/- 9 pS (n = 8), this event may be the prime factor related to the alleged LPC-induced depolarization of cardiac tissues.

Electrocardiogram and His bundle electrogram of Japanese monkeys (Macaca fuscata)

Electrocardiogram (ECG) was taken from Macaca fuscata in the supine position under secobarbital anesthesia. The ECGs were recorded in the leads of standard (I, II, III), augmented unipolar limb (aVR, aVL, aVF) and unipolar chest (V4R, V1, V2, V3, V4, V5, V6,). The ECG was similar to that of humans as well as the monkeys of other species in their general patterns and the voltage of each wave (P, Q, R, S, T), though the heart rate was considerably different. The direction of T wave was consistent with that of human ECG. The PR, QRS and QT intervals and the duration of P wave were in good agreement with those of monkeys of other species, and amounted to about 50% of those of human ECG. His bundle electrogram (HBE) was recorded with a tripolar electrode catheter inserted into the right ventricle via the femoral vein, and PA, AH, HV and PV intervals were measured. All of the values were also approximately 50% and PV intervals were measured. All of the values were also approximately 50% of those reported in human HBE. Such short time intervals observed in Macaca fuscata may be due to a smaller heart size of the monkey as compared to the human's. The measurements of ECG and HBE reported in the present study may be used as a normal reference in ECG and HBE of Macaca fuscata under anesthesia. The study suggests that the Macaca fuscata is useful as an animal model to investigate the repolarization process of human ECG.

Nature of "residual fast channel" dependent action potentials and slow conduction in guinea pig ventricular muscle and its modification by isoproterenol

The nature of "residual fast channel" dependent action potentials and conduction was studied in guinea pig ventricular papillary muscle in which the resting potential was reduced to 58 +/- 1 mV in high K+ (16.7 mM) Tyrode's solution, with or without isoproterenol (0.1 microM). In the absence of isoproterenol, the action potential had a slur on the upstroke and the maximum rate of rise (Vmax) was composed of 2 separable peaks: the early large (32 +/- 7 V/s, Vmax,fast) and the late small (10 +/- 2 V/s, Vmax,slow) ones. The conduction velocity ranged from 30 to 35 cm/s. The Vmax,fast was selectively depressed by l-verapamil (1 microgram/ml), indicating that the impulses were transmitted through incompletely inactivated ("residual") fast channels. Lidocaine (2 micrograms/ml) depressed Vmax,fast with reduction of conduction velocity to about 20 cm/s just before the conduction block. Isoproterenol quadrupled the Vmax,slow but decreased Vmax,fast. As a result, the Vmax,slow overrode the Vmax,fast without change in the conduction velocity. In the absence of isoproterenol, there was no rate-dependent change in the action potential duration and the effective refractory period among the rates of 0.1, 0.5, and 0.9 Hz. Isoproterenol produced rate-dependent shortening in action potential duration with rate-independent shortening of postexcitation refractoriness, thereby resulting in a significant shortening in the effective refractory period at faster rates (0.5 and 0.9 Hz). Results suggest that the "residual fast channel" could produce "slow conduction" and that the ionic channels (fast or slow) responsible for the slow conduction may alternate according to local concentrations of tissue catecholamines.

Effects of SG-75 (nicorandil) on electrical activity of canine cardiac Purkinje fibers: possible increase in potassium conductance

The effects of various concentrations (1-1000 microM) of SG-75 nitrate or nicorandil) were investigated in action potentials of canine Purkinje fibers with microelectrodes. In normal fibers stimulated at the standard cycle length of 1500 msec, the drug (1-100 microM) produced dose-dependent shortening in action potential duration without changes in the resting potential, the maximum rate of rise of phase 0 and the slope of phase 4 depolarization. The drug-induced percentage of shortening in the action potential duration was more pronounced at lower rates of stimulation and lower external K+ concentrations, but the shortening was unchanged in lower external Na+ concentrations. Depolarizations produced by Ba++ (0.04 mM) and low K+ (less than or equal to 1.35 mM) solution were partially reversed by the drug, whereas there were no effects on the high K+ (5.4-24.7 mM)-induced depolarizations. Stretch-induced depolarization was also partially recovered in the presence of the drug. SG-75 shortened the duration of the slow response produced by isoproterenol (1 microM) in the presence of 24.7 mM K+, with little change in the maximum rate of rise. We concluded that SG-75 increases membrane K+ conductance in cardiac Purkinje fibers probably by modifying ix1 and iK1 channels without changing the slow inward currents and that the effects are not mediated by muscarinic receptors. The computed action potentials indicated that this view was reasonable.

Na, K-ATPase activity, ATP and Pi concentrations in various regions of monkey heart and their relation to post-overdrive hyperpolarization

The relationship among Na, K-ATPase activity, ATP and inorganic phosphate (Pi) concentrations in functionally different regions of cardiac and skeletal muscles were studied in the Japanese monkey (Macaca fuscata). The activity of Na, K-ATPase was determined by the method of FIske and SubbaRow, and the concentrations of ATP and Pi, by isotachophoresis. The ATP concentrations in several regions of cardiac and skeletal muscles correlated positively with the Na, K-ATPase activities and Pi concentrations, whereas there was no significant correlation between the Na, K-ATPase activities and the Pi concentrations. Therefore, the ATP concentrations in functionally different regions of the tissues seemed to be a feasible measure of Pi concentrations and the Na, K-ATPase activities. The Na, K-ATPase activity was significantly higher in the epicardium than in the endocardium of the ventricle. The post-overdrive hyperpolarization was compared in these two tissues, using microelectrode methods. The hyperpolarization which followed overdrive stimulation (3.3-3.8 Hz) in the epicardium exceeded that in the endocardium, and was attributed to the difference in Na, K-ATPase activity. Our findings suggest that tissues with a higher Na, K-ATPase activity may have a more potent Na, K-pump activity, in association with higher tissue concentrations of ATP and Pi.

Electrophysiological and inotropic effects of Coenzyme Q10 on guinea pig ventricular muscle depolarized by potassium under hypoxia

The electrophysiological and inotropic effects of Coenzyme Q10 (CoQ10) on isoproterenol or barium-induced slow responses in ventricular papillary muscle, depolarized by high K+ concentration (21.6 mM) under hypoxia (PO2 = 40 mmHg), were studied with microelectrode techniques. For the isoproterenol-induced slow response, application of CoQ10 (50 micrograms/ml), which was emulsified with the aid of a special solvent, increased the maximum rate of rise of action potentials (Vmax), an indicator of the slow inward current, by about 40%, with no consistent effect on the action potential duration and developed tension. Application of the solvent alone produced a significant decrease in both Vmax and developed tension. However, in these solvent-pretreated preparations, CoQ10 produced a significant (by about 50%) recovery in both Vmax and developed tension. The action potential duration was not changed by either the solvent alone or the application of CoQ10. The developed tension of the slow response consisted of early and late components. CoQ10 produced significantly more recovery in the late component than in the early one, suggesting that the recovery effect was due to increased slow inward Ca2+ current. CoQ10 did not reverse any parameter in the slow response induced by BaCl2 (0.2 mM). The results suggest that CoQ10 has significant but limited reversing effects on the hypoxia-induced deterioration of the slow response, and that the recovery is presumably due to increased availability of slow channels via increased production of ATP.

Surface layer ATP-related contraction in isolated, superfused canine ventricular papillary muscle: an isotachophoretic analysis

The relationship among contractile tension, ATP content and concentration was examined in isolated, superfused ventricular papillary muscle under normoxic (Po2 not equal to 300 mmHg) and hypoxic (Po2 not equal to 100 mmHg) conditions, using capillary isotachophoresis. The muscle preparations were exposed to each condition for 30 min, and the contractile tension was recorded with a strain gauge. Immediately after the recordings, the preparations were homogenized and the metabolites (ATP, ADP, AMP, creatine phosphate, inosine monophosphate, NAD, NADH, glucose-6-phosphate, pyruvate, and inorganic phosphate) were extracted in 50% methanol-1.25 mM EDTA solution at -20 degrees C for 4 days. The supernatant of the extract was used for the isotachophoretic analysis. Hypoxia markedly depressed ATP content and concentration in the tissue. Under conditions of normoxia, but not hypoxia, the developed tension positively correlated with the ATP content and concentration. Under normoxic conditions, the tension tended to be proportional to the estimated surface area of the preparation, while in hypoxia it tended to be inversely proportional thereto. The ATP concentration appeared to be inversely proportional to the muscle weight, thereby suggesting that the outer layer of the preparation contains more ATP than the inner. In fact, an isotachophoretic analysis of the tissue revealed significantly higher ATP concentrations in the outer layer. Our findings indicate that there is a central anoxic core in the isolated canine papillary muscle superfused with oxygenated Tyrode solution and that surface layer ATP probably plays a pivotal role in the initiation of contraction.

Isotachophoretic analyses of metabolites of cardiac and skeletal muscles in four species

We studied ATP and concentrations of other metabolites in cardiac and skeletal muscles from different species (frog, hamster, guinea-pig, and dog), using analytical capillary isotachophoresis. The method had several advantages for quantitative analysis of tissue metabolites: short separation time, high sensitivity, high resolution, and good reproducibility. It was also possible to detect a number of compounds simultaneously, including ATP, ADP, AMP, cyclic AMP, creatine phosphate (CP), inosine monophosphate, NAD, NADH, glucose-6-phosphate, pyruvate, and inorganic phosphate. Generally, the concentrations of high-energy phosphates (ATP and CP) in the cardiac muscle were significantly lower than those in the skeletal muscle, in all species tested, except for hamster where the concentration of CP in the skeletal muscle was comparable to that in the cardiac muscle. In mammals, ATP and CP concentrations were comparable in the atrium and ventricle while the concentration in the frog ventricle was significantly higher than that in the atrium. The data obtained by this isotachophoretic analysis were comparable to the data reported by the use of other conventional analytical methods. The significance and reliability of isotachophoresis in the determination of the various metabolites in the cardiac muscle were discussed.

Reactivation process of calcium current-independent tonic tension in bullfrog atrium

The time constant of reactivation process of ICa-independent tonic tension in bullfrog atrial muscle was temperature-dependent and cyanide-sensitive. The cyanide effect and the Q10 value (2.8) of the time constant of the reactivation process suggest that the tonic tension is at least partially mediated by some metabolic process.

The time-dependent and dose-dependent effects of the sulfhydryl blocker N-ethylmaleimide on the tonic tension in bullfrog atrium

The kinetics of the inhibitory action by N-ethylmaleimide (NEM) on ICa-independent tonic tension in bullfrog atrial muscle was studied under voltage-clamped conditions, using the double sucrose gap method. The higher the concentration of NEM became, the larger was the rate of decrease of the tonic tension. In addition, the tension decreased with time of exposure to NEM. When the rate of the decrease of the tonic tension was plotted as a function of NEM concentration, the curve showed saturation kinetics of the Michaelis-Menten type with respect to NEM concentration, following the characteristics of a carrier-mediated process. Moreover, in the presence of excess Ca or half quantity of Na in Ringer solution, the curve showed sigmoidal kinetics. This probably indicates that the condition of either an excess of Ca or a reduction of Na in Ringer solution is apparently a cooperative effector of the inhibitory action of NEM, suggesting that the carrier protein responsible for the generation of tonic tension may be an allosteric protein. Finally, the relationship between ICa-independent tonic tension and the Na-Ca exchange mechanism which is supposed to be a powerful candidate for generating tension was discussed.