The effect of palmitate and lactate on mechanical performance and metabolism of cat and rat myocardium

An Equivocal Final Link - Quantitative Determination of the Thermodynamic Efficiency of ATP Hydrolysis - Sullies the Chain of Electric, Ionic, Mechanical and Metabolic Steps Underlying Cardiac Contraction

Each beat of the heart completes the final step in a sequence of events commencing with electrical excitation-triggered release of Ca2+ from the sarcoplasmic reticulum which, in turn, triggers ATP-hydrolysis-dependent mechanical contraction. Given that Thermodynamics is inherently detail-independent, the heart can be thus be viewed as a mechanical pump - the generator of pressure that drives blood through the systemic and pulmonary circulations. The beat-to-beat pressure-volume work (W) of the heart is relatively straightforward to measure experimentally. Given an ability to measure, simultaneously, the accompanying heat production or oxygen consumption, it is trivial to calculate the mechanical efficiency: ε = W/ΔH where ΔH is the change of enthalpy: (W + Q), Q representing the accompanying production of heat. But it is much less straightforward to measure the thermodynamic efficiency: η = W/ΔG ATP , where ΔG ATP signifies the Gibbs Free Energy of ATP hydrolysis. The difficulty arises because of uncertain quantification of the substrate-dependent yield of ATP - conveniently expressed as the P/O2 ratio. P/O2 ratios, originally ("classically") inferred from thermal studies, have been considerably reduced over the past several decades by re-analysis of the stoichiometric coefficients separating sequential steps in the electron transport system - in particular, dropping the requirement that the coefficients have integer values. Since the early classical values are incompatible with the more recent estimates, we aim to probe this discrepancy with a view to its reconciliation. Our probe consists of a simple, thermodynamically constrained, algebraic model of cardiac mechano-energetics. Our analysis fails to reconcile recent and classical estimates of PO2 ratios; hence, we are left with a conundrum.

Differential depression of myocardial function and metabolism by lactate and H+

The effects of both high blood H+ concentration ([H+]) and high blood lactate concentration ([lactate]) under ischemia-reperfusion conditions are receiving attention, but little is known about their effects in nonischemic hearts. Isolated rat hearts were Langendorff perfused at constant flow with media at two pH values (7.4 and 7.0) and two [lactate] (0 and 20 mM) in various sequences (n = 6/group). Coronary flow and arterial O2 content were kept constant at levels that allowed hearts to function without O2 supply limitation. We measured contractility, O2 uptake, diastolic pressure, and at the end of the protocol, tissue [lactate] and pH. Perfusion with high [lactate] raised tissue [lactate] from 5.5 +/- 0.1 to 17.5 +/- 2.6 micromol/heart (P < 0.0001), whereas decreasing the pH of the medium decreased tissue pH from 6.94 +/- 0.02 to 6.81 +/- 0.06 (P = 0.002). Heart rate was not affected by high [lactate] but was reversibly depressed by high [H+] (P = 0.004). Developed pressure declined by 20% in response to high [lactate], high [H+], and high [lactate] + high [H+] (P = 0.002). After the high-[lactate] challenge was withdrawn, pressure continued to decline. In contrast, withdrawing the high [H+] challenge allowed partial recovery. The behavior of diastolic pressure mirrored that of developed pressure. Although unaffected by high [lactate], the O2 uptake was reversibly depressed by high [H+]. This suggests higher O2 cost per contraction in the presence of high [lactate]. We conclude that for similar acute contractility depression, high [lactate] induces irreversible damage, likely at some point in the pathway of O2 utilization. In contrast, the effect of high [H+] appears reversible. These differential behaviors may have implications for heart function during heavy exercise and ischemia-reperfusion events.

Substrate selection in the isolated working rat heart: effects of reperfusion, afterload, and concentration

A study of substrate selection in the isolated heart was made using 13C NMR isotopomer analysis, a method that unequivocally identifies relative substrate utilization. This technique has several advantages over conventional approaches used to study this problem. It detects the labeling of metabolic end-products present in tissue, as opposed to more indirect methods such as measurement of respiratory quotient, arteriovenous differences, or specific activity changes in the added substrate. It also has advantages over methods such as 14CO2 release, which may involve dilution of label with unlabeled pools before CO2 release. Furthermore, it can measure the relative oxidation of up to four substrates in a single experiment, which other labeling techniques cannot conveniently achieve. Substrate selection was considered in light of its effects on myocardial efficiency and recovery from ischemia. A mixture of four substrates (acetoacetate, glucose, lactate, and a mixture of long chain fatty acids), present at physiological concentration (0.17, 5.5, 1.2, and 0.35 mM, respectively), was examined. This is the first use of such a mixture in the study of substrate selection in an isolated organ preparation. At these concentrations, it was found that fatty acids supplied the majority of the acetyl-CoA (49%), and a substantial contribution was also provided by acetoacetate (23%). This suggests that the ketone bodies are a more important substrate than generally considered. Indeed, normalizing the relative utilizations on the basis of acetyl-CoA equivalents, ketone bodies were by far the preferred substrate. The relative lactate oxidation was only 15%, and glucose oxidation could not be detected. No change in utilization was detected after 15 min of ischemia followed by 40 min of reperfusion. The change in substrate selection with afterload was examined, to mimic the stress-related changes in workload found with ischemia. Only minor changes were found. Substrate selection from the same group of substrates, but employing concentrations observed during starvation, was also assessed. This represents the state during which most clinical treatments and evaluations are performed. In this case, acetoacetate was the most used substrate (78%), with small and equal contributions from fatty acids and endogenous substrates; the oxidation of lactate was suppressed.