Electrical and mechanical activity of isoproterenol-damaged frog heart

The electrical and mechanical activities of myocardial strips from Rana pipiens were studied in the steady state at various stimulation frequencies and after a period of rest. The temperature of the bath was varied between + 12 and 35 degrees C. Normal myocardium was compared with that damaged by isoproterenol (ISO). The percentage change of action potential durations (APD50) and isometric force (P) was similar in ISO-damaged and control hearts at various bath temperatures with steady state stimulation rates between 20 and 100/min. At low stimulation rates (3-6/min) the prolongation of the action potential (AP) was more pronounced and the P-decrease was less in ISO-damaged myocardium compared to controls. These differences became more apparent at low bath temperatures. After 10 min of rest, APD50-90 was significantly prolonged in ISO-damaged heart and P was increased by a factor of 2 compared to controls (stimulation rate 20/min). Steady state values, on the other hand, were nearly the same in both groups. These findings are interpreted as indicating a temperature or ISO-dependent increase of electrogenic trans-sarcolemmal Ca2+-uptake during low frequency, or post-rest stimulation, either directly by an increase of the slow inward current (Isi), or indirectly by decreased K+-permeabilities.     

Effects of taurine on the electrical and mechanical activities of embryonic chick heart

The effect of taurine (2-aminoethanesulphonic acid) on myocardial slow action potentials (APs) and accompanying contractions was examined in isolated perfused chick hearts and reaggregated cultured cells. Isoproterenol (ISO), histamine (HIS), or tetraethylammonium (TEA) induced slow APs and contractions in hearts whose fast Na+ channels had been inactivated by elevated K+. Taurine (10 mM) not only failed to induce slow APs, but actually decreased ISO (10(-8) M), HIS (10(-4) M), or TEA (10 mM) induced slow APs and contractions transiently (about 30s-2 min after the addition of taurine). The properties of the slow APs recovered to control levels by 7-13 min after the addition of the taurine; at this time, there was an increase in developed tension of the contraction accompanying the slow APs. These results suggest that the positive inotropic action of taurine is not mediated through an increase in the slow inward Ca2+ current. However, the transient depression of Ca2+-dependent slow APs by taurine probably explains the transient negative inotropic effect of taurine.     

Influence of prostaglandins on electrical and mechanical activities of gastric muscles of Bufo marinus

1. Study was performed to compare the role of prostaglandins in regulating gastric contractile activity in an amphibian model, Bufo marinus, with mammalian models. 2. The prostaglandin synthesis inhibitor, indomethacin, had little effect on spontaneous mechanical activity, but increased the force and frequency of contractions stimulated by acetylcholine. 3. PGE2 reversed the effects of indomethacin and reduced the force and frequency of contractions. These effects were concentration-dependent. 4. Intracellular measurement of membrane potential demonstrated that the effects of PGE2 could be explained by basic effects on membrane potential and slow wave activity. 5. The data shown that many similarities exist between amphibian and mammalian gastric muscles in terms of the regulatory role played by endogenous PGs. It also appears that the mechanisms of PGE2 action are similar.     

Effect of ryanodine on the electrical and mechanical activity of frog atrial fibers

The effect of ryanodine on the action potential, slow inward current and mechanical activities of frog atrial fibres was studied by means of the double sucrose gap technique. Ryanodine was shown to reduce the amplitude of the slow inward current, to cause an intracellular Ca accumulation and to decrease the tonic component of the tension.     

Effects of selenium on altered mechanical and electrical cardiac activities of diabetic rat

Since selenium compounds can restore some metabolic parameters and structural alterations of diabetic rat heart, we were tempted to investigate whether these beneficial effects extend to the diabetic rat cardiac dysfunctions. Diabetes was induced by streptozotocin (50mg/kg body weight) and rats were then treated with sodium selenite (5 micromol/kg body weight/day) for four weeks. Electrically stimulated isometric contraction and intracellular action potential in isolated papillary muscle strips and transient (I(to)) and steady state (I(ss)) outward K(+) currents in isolated cardiomyocytes were recorded. Sodium selenite treatment could reverse the prolongation in both action potential duration and twitch duration of the diabetic rats, and also cause significant increases in the diminished amplitudes of the two K(+) currents. Treatment of rats with sodium selenite also markedly increased the depressed acid-soluble sulfhydryl levels of the hearts. Our data suggest that the beneficial effects of sodium selenite treatment on the mechanical and electrical activities of the diabetic rat heart appear to be due to the restoration of the diminished K(+) currents, partially, related to the restoration of the cell glutathione redox cycle.     

Effects of octylguanidine on the electrical and mechanical events of xenopus heart

The effects of octylguanide on the electrical and mechanical events of Xenopus heart perfused at the rate of 5 ml/m with oxigenated Ringer of ph 7,4 at 24 degrees C were studied. The early effects on the action potential resemble those of the tetrodotoxina, the later effects looks like instead as those observed with metabolic poisons. The cardiac preparations poisoned by octylguanidine still have a metabolic reserve available to furnish energy for the contractile process.     

Influence of serosal Cl on transport properties and cation activities in frog skin

Summary Treatment of resident peritoneal macrophages of rats with small unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC SUV) potentiated their activation for tumor cell lysis by endotoxins. The fluorescence polarization of diphenylhexatriene (DPH) embedded in rough endoplasmic reticulum membranes isolated from DPPC SUV-treated macrophages was enhanced. The average fluorescence lifetime of DPH and the rotational correlation time deduced from anisotropy decay were unchanged, whereas the residual anisotropy and hence the order parameter were increased. The measurement of the fluorescence anisotropy of DPH as a function of the temperature showed a phase transition. No phase transition was observed in the rough endoplasmic reticulum membranes of macrophages either treated or not treated with cholesterol/DPPC SUV (1/1; mol/mol). The synergistic effect of DPPC SUV on the tumoricidal activity of macrophages induced by endotoxins appears to be correlated with the changes in the properties of the rough endoplasmic reticulum membranes. Both effects were transient; they had the same kinetics of induction and reversion, and they were both inhibited by cholesterol.     

Actions of some cations on the electrical properties and mechanical threshold of frog sartorius muscle fibers

With the use of a point voltage-clamp technique, the effects of Zn²⁺, UO₂ ²⁺, tetraethylammonium, and several other homologous quaternary ammonium ions on the electrical properties of the frog sartorius muscle and its mechanical threshold were studied. None of the agents separated the voltage thresholds for mechanical activation and delayed rectification. However, Zn²⁺, UO₂ ²⁺, and TEA, which are known to potentiate the twitch, caused some inhibition of the normal increase in potassium conductance during delayed rectification. Zn²⁺ and UO₂ ²⁺ also slowed the rate of development of the outward current. A strength-duration relation was studied for depolarization pulses capable of initiating contraction. With a depolarizing pulse of 2.5 msec the mechanical threshold is about -13 mv at about 20°C. UO₂ ²⁺, 0.5 µM, which markedly reduced the outward current produced by such a short pulse, did not raise the mechanical threshold. All findings indicate that there is no direct causal relation between delayed rectification and mechanical activation.     

Effect of temperature on the electrical and mechanical activity of lizard ventricular myocardium

Intracellular action potentials and isometric twitches were recorded from lizard ventricles electrically driven at 20 and 4 beats/min and submitted to temperatures changes between 10.5 and 21 degrees C. It was found that cooling induced a depolarization of the diastolic membrane potential ER, which below 15 degrees C exceeded that predictable for a diffusion potential; on the contrary, during the recovery from hypothermia ER underwent a transitory hyperpolarization. Other effects of the low temperature were a decrease of the maximum rate of depolarization, a lengthening of both the action potential duration and the time to peak contraction, an increase of the strength of contraction, in the hearts driven at 20/min it became apparent also an increase of the action potential overshoot. The hypothesis is discussed that the positive inotropic effect of low temperatures may be due not only to a slowing down of the repolarization of the action potential, but also to an increase of the slow inward current intensity.     

The action of ADP ribose on the mechanical and bioelectrical activity of the frog heart]

In the frog isolated heart, cyclic perfusion of ADP-ribose induced a dose-dependent decrease in the heart rate and the contraction force, a decrease in the AP duration as well as in the rate of rise in the sinus node. It also shortened the atrial AP and exerted no significant effect upon multicellular ventricular preparations. In conditions of systemic administration in unanesthetised frogs, the ADP-ribose induced a reversible increase in the heart rate due, probably, to a sympathetic effect.     

Effects of adrenergic and cholinergic drugs on electrical and mechanical activities of the rat cauda epididymidis in vitro

Electrical and mechanical activities of the rat epididymis (at 29 +/- 1.1 cm from the junction of the vas deferens) were recorded in vitro. The frequency of the spontaneous activity was 2.7 +/- 0.15/min. Adrenaline, phenylephrine, isoprenaline and carbachol increased the basal tension, frequency and amplitude of the contractions. Phentolamine, an alpha-adrenergic blocking agent, abolished the stimulatory effects of adrenaline and isoprenaline, but not those of carbachol. Propranolol and metoprolol, beta-adrenergic blocking agents, did not inhibit the stimulatory effects of isoprenaline. Atropine abolished the response to carbachol. The results suggest that alpha-adrenergic receptors but not beta-receptors are present in the rat epididymis.     

Combined effects of fatigue and temperature manipulation on skeletal muscle electrical and mechanical characteristics during isometric contraction

Peripheral fatigue and muscle cooling induce similar effects on sarcolemmal propagation properties. The aim of the study was to assess the combined effects of muscle temperature (Tm) manipulation and fatigue on skeletal muscle electrical and mechanical characteristics during isometric contraction. After maximum voluntary contraction (MVC) assessment, 16 participants performed brief and sustained isometric tasks of different intensities in low (Tm(L)), high (Tm(H)) and neutral (Tm(N)) temperature conditions, before and after a fatiguing exercise (6s on/4s off at 50% MVC, to the point of fatigue). During contraction, the surface electromyogram (EMG) and force were recorded from the biceps brachii muscle. The root mean square (RMS) and conduction velocity (CV) were calculated off-line. After the fatiguing exercise: (i) MVC decreased similarly in all Tm conditions (P<0.05), while EMG RMS did not change; and (ii) CV decreased to a further extent in Tm(L) compared to Tm(N) and Tm(H) in all brief and sustained contractions (P<0.05). The larger CV drop in Tm(L) after fatigue suggests that Tm(L) and fatigue have a combined and additional effect on sarcolemmal propagation properties. Despite these changes, force generating capacity was not affected by Tm manipulation. A compensatory mechanism has been proposed to explain this phenomenon.