Development Of Intracellular Acidosis Research Articles

Na+/Ca++ exchanger and myocardial ischemia/reperfusion.

Myocardial hypoxia and ischemia are characterized by the depletion of ATP and the development of intracellular acidosis, which alter cellular ionic homeostasis. Specifically, elevated cytosolic free Ca++ concentrations cause cellular injury during hypoxia/ischemia and lead to irreversible myocardial damage during reoxygenation/reperfusion. An increase in the intracellular Na+ concentration has been shown to correlate with Ca++ overload. Although inhibition of Na+/K+ exchange because of decreased ATP production may be involved, it is more likely that intracellular acidosis drives Na+ into the cells via Na+/H+ exchange. Experimental evidence supports the notion that Na+/H+ exchange is primarily responsible for Na+ influx during hypoxia/ischemia. The accumulation of intracellular Na+ may then activate the Na+/Ca++ exchanger causing Ca++ overload. Therefore, the Na+/Ca++ exchanger plays a crucial role in cellular injury during hypoxia/ischemia and in cell death during reoxygenation/reperfusion. In the past few years, the Na+/Ca++ exchanger has been cloned and the structure/function relationship studied intensively. Agents which inhibit the Na+/Ca++ exchanger may have therapeutic potential for the treatment of ischemic heart disease. These advances will greatly accelerate the understanding of the cellular and molecular mechanisms underlying the role of the Na+/Ca++ exchanger in the development of myocardial damage during hypoxia/ischemia and reoxygenation/reperfusion.

Effects of Stilbene Derivatives SITS and DIDS on Development of Intracellular Acidosis during Ischemia in Isolated Guinea Pig Ventricular Papillary Muscle In Vitro

Using ion-selective microelectrode techniques, we investigated the effects of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which are known as Cl−-HCO3− exchange blockers, on action potentials and intracellular pH (pHi) in guinea pig ventricular papillary muscles subjected to simulated ischemia. Simulated ischemia was produced by stopping the flow of superfusing solution and then covering the preparations with mineral oil. Simulated ischemia induced a progressive decrease in the maximum upstroke rate and resting membrane potentials, shortened action potential duration, and resulted in cessation of action potentials within 10–12 min after the onset of simulated ischemia. The pHi-measurements revealed progressive intracellular acidosis during the period of simulated ischemia. SITS (0.5 mM) or DIDS (0.1 mM) delayed the onset of ischemia-induced deterioration of action potentials and prolonged the time to cessation of action potentials. SITS or DIDS (0.1–0.5 mM) induced an increase in pH in HCO3−-buffered solution and suppressed the development of intracellular acidosis during ischemia. Under the external CP-free condition, the time to cessation of action potentials caused by ischemia was significantly delayed, and the development of intracellular acidosis during ischemia was attenuated. The present results indicate that activation of the Cl−-HCO3− exchange system would be involved, in part, in the development of intracellular acidosis during cardiac ischemia.

Effect of lidocaine on contracture, intracellular sodium, and pH in ischemic rat hearts.

The relationships among intracellular Na concentration ([Na+]i), intracellular pH, [ATP], and contracture during global ischemia were studied in isolated, perfused rat hearts. Intracellular Na was monitored by 23Na nuclear magnetic resonance (NMR) spectroscopy using thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonate) (TmDOTP5-) as the paramagnetic shift reagent. High-energy phosphates and pH were monitored under the same conditions using 31P-NMR spectroscopy. Lidocaine (130 microM), a class IB fast Na channel blocker known to protect ischemic myocardium, prolonged the time to contracture in both unpaced and paced hearts (240 beats/min). After 10 min of global ischemia in paced hearts, [Na+]i was lower in the lidocaine-treated group compared with untreated hearts. The addition of lidocaine also significantly attenuated the depletion of ATP as well as development of intracellular acidosis. At the time of contracture, however, there was no difference in [Na+]i or pH between the two groups. Interestingly, the effect of lidocaine on Na+i accumulation during ischemia was manifested during the first 5-10 min of ischemia, while its effect on pH occurred after 9 min. This finding suggests that a mechanism other than the Na-H exchanger may play a role in the accumulation of Na+i early in the course of ischemia.

Contributions of nuclear magnetic resonance to study of acute renal failure.

Nuclear magnetic resonance (NMR) has contributed considerably to our understanding of experimental acute renal failure. Changes in energy metabolism which are caused by ischemia, urinary obstruction, and nephrotoxic drugs have been characterized with NMR spectroscopy. Data from our laboratory and others utilizing 31P NMR have demonstrated that levels of adenosine triphosphate fall rapidly with ischemia, and that the ability of the kidney to regenerate ATP correlates with ultimate functional recovery. Additionally, development of intracellular acidosis appears to occur early with ischemia and may, if severe enough, predict poor functional recovery. Urinary obstruction is associated with the rapid development of a large peak resonating in the phosphodiester region of the P-31 NMR kidney spectrum which is attributable to increases in urinary inorganic phosphate. Nephrotoxic acute renal failure with a variety of nephrotoxins is associated with little to no changes in high energy phosphates. Renal transplant allograft rejection is associated with energy metabolic changes similar to those seen with ischemia; however, the intracellular pH remains normal. These findings allow causes of experimental acute renal failure to be differentiated among each other in both native and transplanted kidneys. With recent advances in NMR software and hardware, the application of this methodology to human acute renal failure is now possible.

A polymeric prostaglandin (PGB x) attenuates adenine nucleotide loss during global ischemia and improves myocardial function during reperfusion

Effects of a synthetic, prostaglandin (PGBx) on energy metabolism in isolated, guinea pig hearts were studied using P-31 nuclear magnetic resonance spectroscopy (NMR) or by direct chemical analysis. Polymeric prostaglandin (500 and 750 ng/ml) attenuated the reduction of ATP and adenine nucleotides during 35 min of total transient ischemia. This occurred despite the absence of any significant preischemic changes in heart rate, contractility or coronary vascular resistance. Preischemic perfusion with PGBx extended the time taken to reach 50% reduction in dP/dt following the first few seconds of ischemia. PGBx had no effect on the development of intracellular acidosis during ischemia. Reperfusion resulted in normalization of phosphocreatine but not ATP concentrations in control and experimental groups. Prostaglandin (750 ng/ml) caused faster and more complete recovery of left ventricular dP/dt following reperfusion. In contrast to untreated hearts, dP/dt in PGBx-treated hearts was significantly higher than preischemic values despite incomplete restoration (70% of control) of ATP levels. These results suggest that the beneficial effects of PGBx observed during myocardial ischemia are unrelated to functionally-induced alterations and that PGBx probably has some direct cellular effect on energy metabolism.