Electrical restitution in rat ventricular muscle

The mechanism of electrical restitution was studied in isolated rat ventricular muscle using drugs that inhibit specific ion currents. The effect of transient changes in cytosolic Ca concentration and Na/Ca exchange in relation to the restitution process was also studied in single ventricular cardiomyocytes. Conventional microelectrode techniques were applied to record action potentials having gradually increasing coupling intervals, each evoked following a train of stimuli with a frequency of 1 Hz. Ion currents were recorded from enzymatically isolated cells using the whole cell patch clamp technique. Ca transients were monitored in myocytes loaded with the fluorescent dye, indo-1. The electrical restitution process in multicellular rat ventricular preparations at 37 °C was described as a sum of three exponential components: an early positive component, a subsequent fast negative component and a late negative component, having time constants of 21.9±1.9, 73.1±6.0 and 1053±61 ms, respectively (n=9). Inhibition of the transient outward K current, the delayed rectifier K current, or the chloride current did not substantially alter these time constants. The early positive and fast negative components were fully abolished by nifedipine or MnCl₂. In the presence of caffeine, the fast negative component was absent, while the time constant of the early positive component increased to 39.5±5.8 ms (n=5). In single myocytes loaded with indo-1, the Ca transients decayed with a time constant of 151±12 ms at room temperature (n=5). These Ca transients were accompanied by inward current tails, identified as a Na/Ca exchange current, having a decay time constant of 140±4.5 ms. It is concluded that electrical restitution in rat ventricular muscle is relatively little affected by recovery from voltage-dependent inactivation of ion channels, it is rather governed by transient changes in cytosolic Ca concentration possible via Ca-dependent inactivation of the L-type Ca current and activation of the Na/Ca exchange current.     

Calcium conductance and tension in mammalian ventricular muscle

Voltage, membrane current and contraction were simultaneously measured in voltage clamp experiments (single sucrose gap) on cat ventricular trabeculae. The pulse programs allowed the determination of the potential dependence of the steady state activation and inactivation as well as the restoration of the calcium-carrying system (slow inward current). 1. The steady state activation variable (d infinity) rose in a sigmoid manner from -50 mV (d infinity nearly 0) to 0 mV (d infinity nearly 1). The experimental values can be described by the function 1/1 + exp [(Vh-V)/S] where half activation (Vh) = -22.5mV and S = 7.6 mV. 2. The steady state inactivation variable (f infinity) declined from 1 at -60mV to 0 at 10mV. The best fit curve is nearly a mirror image of the activation curve with Vh = -28 mV and s = -8.3 mV. 3. The voltage dependence of the (normalized) peak tension was well described by the steady state conductance variables except at potentials positive to +20mV. A "steady state" tension (superimposed on "tonic tension") was found in the potential range where a steady state conductance is predicted by the curves describing steady state activation and inactivation. 5. Following inactivation, the time courses of restoration of the calcium-carrying system and tension were identical. Time courses were exponential with tau = 118 msec at -80 mV, 144 msec at -60 mV, and 198 msec at -40 mV. 6. Two possible models of excitation-contraction coupling in line with the present results are discussed.     

DNP-induced dissipation of ATP in anoxic ventricular muscle

1. During aerobic incubation in 5 mM glucose medium, 10(-5) M-DNP reduced the action potential duration and amplitude and the developed tension of guinea-pig ventricular muscle more rapidly and to a greater extent than anoxia.2. The DNP effect on electrical and mechanical activity was even more pronounced following prolonged anoxic incubation. Since the action potential duration and developed tension of anoxic ventricular muscle have previously been shown to be dependent on glycolytic ATP, and since the effects of DNP could not be duplicated with NaCN, it was concluded that DNP was exerting an effect in addition to its uncoupling of oxidative phosphorylation.3. Anoxic muscle was incubated with 10(-4) M-IAA or with 10(-4) M-IAA + 10(-4) M-DNP. The ATP content of IAA-treated muscle was significantly lower than control but in the presence of both IAA and DNP there was a further reduction in ATP and an increased lactate production.4. Sodium azide (10(-2) M), a potent inhibitor of mitochondrial ATPase, did not prevent the reduction of ATP in DNP-treated anoxic muscle.5. Ouabain (10(-7) M) partially prevented the rapid decline of action potential duration and developed tension of DNP-treated anoxic muscle. In addition, the glycoside partially blocked the DNP-induced break-down of ATP and stimulation of lactate production.6. Oligomycin (10 mug/ml.) partially prevented the reduction in action potential duration and developed tension of DNP-treated anoxic muscle.7. It was concluded that DNP induces an ;energy leak' by actively promoting the hydrolysis of an high energy glycolytic intermediate at least one step beyond the sites of ATPase inhibition by ouabain and oligomycin.     

Left ventricular muscle mass regression after aortic valve replacement

Implanting a valve that will reduce left ventricular mass is critical in aortic stenosis. Regression of left ventricular hypertrophy in 46 aortic valve replacement (AVR) patients receiving a St. Jude Medical (SJM) valve was assessed by serial electrocardiographic and echocardiographic studies during the preoperative, immediate, and late postoperative periods. The patients were divided into three groups according to valve size; 19 mm group (n=9), 21 mm group (n=20), and 23+mm group (n=17). There was no surgical mortality. The NYHA functional class improved from an average of 2.2+/-0.8 preoperatively to 1.3+/-0.5 post-operatively. Left ventricular muscle mass index (LVMI) regression failed to reach statistical significance in the 19 mm group, whereas in the other two groups a steady decrease in the LVMI occurred with follow up. ECG findings were less remarkable showing insignificant differences in voltage among the three groups (p=0.000). In conclusion, the current data suggest that the 19 mm SJM valve may not result in satisfactory left ventricular muscle mass regression despite adequate function, even in small patients. Therefore, additional procedures to accommodate a larger valve may be warranted in the aortic annulus smaller than 21 mm.     

Current-voltage relations in ventricular muscle preparations from different species

Current-voltage relations were determined in ventricular muscle preparations from dog, sheep, pig, guinea pig, bull and cat hearts. The single sucrose gap voltage clamp method was used to apply 2 s steps or slow speed voltage ramps (3-4 mV/s). 1. The current-voltage relations obtained with 2 s steps were similar to those obtained with ramps. 2. Negative slope regions were readily apparent in the current-voltage relations of preparations from all species except cat although the latter did display marked anomalous rectification. 3. Increasing the external potassium concentration from 3 to 10 mM increased the slope of the current-voltage relation around the resting potential and resulted in the "crossing-over" and "crossing-back" of the relations if all tissues. 4. The two extreme cases of bull (marked negative slope region) and cat (no negative slope region) were compared with regard to transgap intracellular and extracellular resistances; they were quite similar. This suggests that the current-voltage relations reflect differences in membrane properties rather than, for example, different proportions of non-membrane leakage current. 5. In bull ventricular preparations, the application of D-600 (2 X 10(-6) M), 30 min) had only a small effect on the current-voltage relation. In contrast, cesium (10-20 mM) nearly abolished the negative slope region suggesting that this characteristic of the current-voltage relation is due to potassium channel rectification. 6. Possible explanations for action potential durations in bull ventricular fibres being 50 to 100% longer than in cat are considered.     

Metabolism and the electrical activity of anoxic ventricular muscle

1. The action potential duration of anoxic guinea-pig ventricular muscle was related to ATP generated by glycolysis. In 50 mM glucose medium the action potential duration was maintained; in 5 mM glucose medium the action potential duration shortened, the glycolytic rate declined and the ATP content was reduced.2. The action potential amplitude was related to the metabolic state of the muscle but not to the intracellular sodium concentration.3. It is suggested that changes in the action potential duration and overshoot in anoxic muscle may be due to an influence of metabolism on the slow inward current.4. Anoxic muscle incubated for 8 hr in 5 mM glucose medium had an E(m) of -77.1 mV compared to -81.1 mV in fresh muscle. The calculated E(k) of anoxic muscle was -47.4 mV.5. The resting potential of anoxic muscle was separated into two components, one dependent on potassium distribution and the other on the activity of an electrogenic sodium pump.6. The electrogenic pump component was stimulated upon raising the glucose concentration of the medium or upon raising the external potassium concentration.7. The electrogenic pump component was inhibited by ouabain or by reduction of the temperature from 35 to 8 degrees C.     

Effects of procaine on the ventricular muscle of bullfrog.

The effects of procaine on the contractility of the bullfrog's ventricular muscle were investigated. The addition of 10(-5) g/ml of procaine potentiated the twitch tension which was accompanied by an elevation as well as a prolongation of the action potential plateau. This positive inotropism of procaine was not induced by endogenous catecholamine because a beta-blocking agent did not influence this twitch potentiation. The twitch potentiation was increased in proportion to the external Ca concentration, suggesting the possibility of augmentation of Ca influx during the action potential. In normal Ringer solution, procaine suppressed potassium contracture which was composed of two components: an initial phasic component and a late tonic one. Potassium contracture after perfusion with Ca-free solution was also suppressed by procaine. However, potassium contracture which had been treated previously with La was composed of only a tonic component and was potentiated by procaine in spite of perfusion with Ca-free solution. The tonic component of potassium contracture may be considered to occur with intracellular Ca. Procaine may increase the Ca inward current, acting on the intracellular Ca storage site and consequently accelerate the excitation-contraction coupling in frog ventricular muscle.     

Cow dung powder poisoning

Cow dung, which has germicidal property, was used in ancient days to clean living premises in South India. Nowadays, people are using commercially available synthetic cow dung powder. It is locally known as “saani powder” in Tamil Nadu. It is freely available in homes and is sometimes accidentally consumed by children. It is available in two colors - yellow and green. Cow dung powder poisoning is common in districts of Tamil Nadu such as Coimbatore, Tirupur, and Erode. We report two cases of yellow cow dung powder poisoning from our hospital.     

Effects of hypoxia on passive electrical properties of canine ventricular muscle

The effect of hypoxia, either in the presence or in the absence of glucose, on the passive electrical properties of canine ventricular muscle fibers was examined, employing a single sucrose gap method. The significant changes after 30 min of hypoxia (PO ₂=35–45mm Hg) were an increase in the specific internal longitudinal resistance (R _i) and a decrease in the space constant (). The values during the control (PO ₂>450mm Hg) were 198±77 cm forR _i and 0.81±0.15 mm for , and they changed to 245±90 cm and 0.70±0.10mm, respectively, after 30min of hypoxia. Hypoxia decreased the specific membrane resistance (R _m), but the changes were not statistically significant. The membrane time constant ( _m ) and capacity (C{imm}) were not affected significantly. The absence of glucose during hypoxia was found to cause more profound changes than hypoxia alone in the passive electrical properties, especiallyR _i and , suggesting that glucose might counteract the effects of hypoxia on these parameters of ventricular muscles.     

Acetylcholine inhibits positive inotropic effect of cholera toxin in ventricular muscle

Acetylcholine (ACh, 10(-6) M) had no effect on basal adenylate cyclase activity (3.4 +/- 0.56 pmol cyclic AMP . min-1 . mg wet wt-1), adenosine 3',5'-cyclic monophosphate (cyclic AMP) content (0.88 +/- 0.09 pmol/mg wet wt), or the force of contraction in paced (2.5 Hz) chick embryo right ventricles superfused with Tyrode solution. After 60-180 min of superfusion in the presence of cholera toxin (5 x 10(-6) g/ml), adenylate cyclase activity (1.7 times), cyclic AMP content (2.4 times), and contractility (2.4 times) had increased significantly above basal levels. ACh reversed the positive inotropic effect of cholera toxin but did not change the increased activity of adenylate cyclase and content of cyclic AMP obtained in cholera toxin. Stimulation of adenylate cyclase by isoproterenol (ISO) was inhibited by ACh in the absence and presence of cholera toxin. ACh did not change guanosine 3',5'-cyclic monophosphate (cyclic GMP) content in the absence or presence of cholera toxin. Cholera toxin has actions on chick embryo ventricle similar to those of the beta-adrenergic agonist, ISO, and the phosphodiesterase inhibitor, isobutylmethylxanthine. The ability of ACh to reverse the positive inotropic effect of cholera toxin without preventing the accumulation of cyclic AMP may involve the prevention or reversal of cyclic AMP-dependent phosphorylation. In this regard, reduction of Ca2+ influx through voltage-sensitive membrane channels may be an essential component of muscarinic inhibition.     

Use of vectorcardiography in determination of the left ventricular muscle mass

Many studies have investigated different ECG and vectorcardiographic (VCG): criteria for diagnosis of left ventricular hypertrophy (LVH). In some investigations VCG was more sensitive than ECG in this respect. This study was performed to elucidate whether it is possible also to determine the degree of LVH using VCG. Eighty cardiovascularly healthy subjects aged 15-39 were investigated with ECG, VCG (Frank system) and echocardiography. The echocardiographic left ventricular (LV) mass has been shown by others to correlate closely to the anatomical and the angiographically determined LV mass and was used as reference standard. Thirty-eight of the subjects were endurance sportsmen and had a LV mass above standard reference limits. The measured ECG variables were R-amplitude in a VL, I, V5, V6, S-amplitude in V1 and SV1 + RV5/V6 and the VCG variables were QRS spatial area and circumference and left maximal spatial vector. The sensitivity and specificity of single criteria tested were similar for ECG and VCG in the quantitative determination of LVH. The correlations between ECG-amplitudes and the magnitude of the LV mass were weak. The correlations were higher with the VCG-variables, QRS spatial circumference being superior to the others, but not good enough to permit an estimation of the LV mass in individual subjects. In conclusion, normal VCG variables were highly specific for a normal LV mass but in individuals with LVH, VCG was not useful for the estimation of the LV mass.