benzofurans and Acidosis

The role of the Na+/H+ exchanger in rat hearts during ischemia and reperfusion was investigated by measurements of intracellular Na+ concentration ([Na+]i) and intracellular and extracellular pH. Under our standard conditions (2-Hz stimulation), 10 min of ischemia caused no significant rise in [Na+]i but an acidosis of 1.0 pH unit, suggesting that the Na+/H+ exchanger was inactive during ischemia. This was confirmed by showing that the Na+/H+ exchange inhibitor methylisobutyl amiloride (MIA) had no effect on [Na+]i or on intracellular pH during ischemia. However, there was a short-lived increase in [Na+]i of 8.2 +/- 0.6 mM on reperfusion, which was reduced by MIA, showing that the Na+/H+ exchanger became active on reperfusion. To investigate the role of metabolic changes, we measured [Na+]i during anoxia. The [Na+]i did not change during 10 min of anoxia, but there was a small, transient rise of [Na+]i on reoxygenation, which was inhibited by MIA. In addition, we show that the Na+/H+ exchanger, tested by sodium lactate exposure, was inhibited during anoxia. These results show that the Na+/H+ exchanger is inhibited during ischemia and anoxia, probably by an intracellular metabolic mechanism. The exchanger activates rapidly on reperfusion and can cause a rapid rise in [Na+]i.

Topics: Acidosis; Amiloride; Animals; Benzofurans; Benzopyrans; Ethers, Cyclic; Female; Fluorescent Dyes; Heart Ventricles; Hydrogen-Ion Concentration; Hypoxia; Muscle Fibers, Skeletal; Myocardial Ischemia; Myocardium; Naphthols; Organ Culture Techniques; Rats; Rats, Sprague-Dawley; Rhodamines; Sodium; Sodium Lactate; Sodium-Hydrogen Exchangers; Ventricular Function, Left

1. In skeletal muscle there is generally a slowing of relaxation with increasing tetanus duration and it has been suggested that this is due to Ca2+ loading of parvalbumin (PA). To study this we have produced prolonged tetani in intact, single fibres from a mouse foot muscle which contain a high concentration of PA. We measured the rate of tension relaxation and also various aspects of Ca2+ handling. 2. During 'interrupted' tetani (15 repeated cycles of 100 ms with stimulation and 50 ms without) we observed a marked slowing of the relaxation both under control conditions and in acidosis (obtained by increasing the bath CO2 content). This slowing was not accompanied by any reduction of the initial rate of decline of the free myoplasmic Ca2+ concentration ([Ca2+]i), which was measured with indo-1. 3. The functioning of the sarcoplasmic reticulum (SR) pump after tetani of various durations was analysed by plotting d[Ca2+]i/dt vs. [Ca2+]i during the final slow decline of [Ca2+]i after tetani. This analysis showed that the rate of SR Ca2+ pumping after a 1 s tetanus is less than half of that after a 100 ms tetanus. 4. The amplitude of the tail of [Ca2+]i 250 ms into relaxation was measured after tetani of various durations. This amplitude increased with tetanus duration and could be fitted to the sum of one exponential and one linear function. The exponential component increased with a time constant of 0.17 s and probably reflects Ca2+ loading of PA. 5. Ca2+ binding to PA will displace Mg2+ and hence the free myoplasmic concentration of Mg2+ ([Mg2+]i) will increase. To study this we used the fluorescent Mg2+ indicator furaptra. The results showed an increase of [Mg2+]i during prolonged tetani which, after removing the Ca2+ component of the fluorescent signal, amounted to about 0.5 mM. 6. A model of Ca2+ movements and tension production in skeletal muscle was used. The model showed that the increase of the amplitude of [Ca2+]i tails after tetani of various durations can be explained by the combined effect of Ca2+ loading of PA and slowed SR Ca2+ pumping. In contrast to the experimental data, the model predicted a slight reduction of the initial rate of [Ca2+]i decline with increased tetanus duration. 7. In conclusion, we observed a marked slowing of relaxation during prolonged tetanic stimulation. Altered Ca2+ handling, including Ca2+ loading of PA, seems not to be important for this slowing. Thus, the slowing appears to be due to altered cross-bridge kinetics.

Topics: Acidosis; Animals; Benzofurans; Calcium; Electric Stimulation; Fluorescent Dyes; Fura-2; In Vitro Techniques; Indoles; Magnesium; Male; Mice; Muscle Fibers, Skeletal; Muscle Relaxation; Muscle, Skeletal; Oxazoles; Parvalbumins; Sarcoplasmic Reticulum; Troponin

Topics: Acidosis; Animals; Benzofurans; Blood Glucose; Chickens; Chromatography, Thin Layer; Citrate (si)-Synthase; Citric Acid Cycle; Fatty Acids, Nonesterified; Hydroxybutyrates; Liver; Liver Glycogen; Mitochondria, Liver; Plant Extracts; Plants; Succinate Dehydrogenase; Time Factors

Topics: Acidosis; Benzofurans; Humans; Milk Sickness; Plant Extracts