Myocardial flow distribution. II : Empty beating heart, ventricular fibrillation and cardiac arrest

Myocardial oxygen consumption (MVO2) and coronary blood flow (CBF) distribution were studied in 21 isolated, metabolically supported dog hearts. Measurements of MVO2 and CBF distribution were carried out in three different experimental conditions : empty beating heart (EBH), ventricular fibrillation (VF) and high potassium-induced cardiac arrest (CA). MVO2 was approximately the same in EBH and VF (4.09 +/- 0.77 and 4.28 +/- 0.68 ml O2 min-1 100 g-1 respectively), and significantly lower in the group with CA (2.40 +/- 0.18 ml O2 min-1 100 g-1, P less than 0.05). Total CBF showed no significant differences among the three groups (84 +/- 7 ml/min in EBH; 78 +/- 7 ml/min in VF and 83 +/- 7 ml/min in CA). Subendocardial CBF per unit of tissue mass was significantly lower in hearts with VF (0.43 +/- 0.01 ml/min-1 g-1, P less than 0.05) when tested against the other two groups of experiments (0.69 +/- 0.03 ml min-1 g-1 in EBH and 0.65 +/- +/- 0.04 ml min-1 g-1 in CA). This was also reflected in the endo/epi ratio, that was significantly lower in VF (1.41 +/- 0.07, P less than 0.05) with respect to the other two groups (2 +/- 0.09 in EBH and 2.21 +/- 0.07 in CA). From data presented here we can conclude that cardioplegia, even in absence of hypothermia, is a method that will assure myocardial protection providing : (1) a lower subendocardial MVO2; (2) a higher subendocardial CBF, which helps for a prompt recovery during reperfusion.     

Endothelial NO formation does not control myocardial O2 consumption in mouse heart

To test whether endothelium-derived nitric oxide (NO) regulates mitochondrial respiration, NO was pharmacologically modulated in isolated mouse hearts, which were perfused at constant flow to sensitively detect small changes in myocardial O2 consumption (MVO2). Stimulation of NO formation by 10 microM bradykinin (BK) increased coronary venous nitrite release fivefold to 58 +/- 33 nM (n = 17). Vasodilatation by BK, adenosine (1 microM), or papaverine (10 microM) decreased perfusion pressure, left ventricular developed pressure (LVDP), and MVO2. In the presence of adenosine-induced vasodilatation, stimulation of endothelial NO synthesis by BK had no effect on LVDP and MVO2. Also, inhibition of NO formation by NG-monomethyl-l-arginine (l-NMMA, 100 microM) did not significantly alter LVDP and MVO2. Similarly, intracoronary infusion of authentic NO 2 microM were contractile dysfunction and MVO2 reduction observed. Because BK-induced stimulation of endothelial NO formation and basal NO are not sufficient to impair MVO2 in the saline-perfused mouse heart, a tonic control of the respiratory chain by endothelial NO is difficult to conceive.     

Reduced oxygen supply explains the negative force-frequency relation and the positive inotropic effect of adenosine in buffer-perfused hearts

In isolated rat hearts perfused with HEPES and red blood cell-enriched buffers, we examined changes in left ventricular pressure induced by increases in heart rate or infusion of adenosine to investigate whether the negative force-frequency relation and the positive inotropic effect of adenosine are related to an inadequate oxygen supply provided by crystalloid perfusates. Hearts perfused with HEPES buffer at a constant flow demonstrated a negative force-frequency relation, whereas hearts perfused with red blood cell-enriched buffer exhibited a positive force-frequency relation. In contrast, HEPES buffer-perfused hearts showed a concentration-dependent increase in left ventricular systolic pressure [EC50 = 7.0 +/- 1.2 nM, maximal effect (Emax) = 104 +/- 2 and 84 +/- 2 mmHg at 0.1 microM and baseline, respectively] in response to adenosine, whereas hearts perfused with red blood cell-enriched buffer showed no change in left ventricular pressure. The positive inotropic effect of adenosine correlated with the simultaneous reduction in heart rate (r = 0.67, P < 0.01; EC50 = 3.8 +/- 1.4 nM, baseline 228 +/- 21 beats/min to a minimum of 183 +/- 22 beats/min at 0.1 microM) and was abolished in isolated hearts paced to suppress the adenosine-induced bradycardia. In conclusion, these results indicate that the negative force-frequency relation and the positive inotropic effect of adenosine in the isolated rat heart are related to myocardial hypoxia, rather than functional peculiarities of the rat heart.     

Myocardial oxygen consumption regulation in isolated mouse heart: assessment by intracoronary administration of exogenous nitric oxide

In our previous work we have shown that in mouse heart basal level of endothelial produced nitrite, as a marker of nitric oxide (NO) formation, was 9.7 nmol l(-1). Bradykinin (10 microl l(-1)) induced a 5-fold rise in nitrite release, the coronary venous effluent concentration being 58 nmol l(-1), but there was no effect on myocardial oxygen consumption (MVO2). The aim of this study was to assess the levels of authentic nitric oxide solution, exogenously applied, on myocardial oxygen consumption. Isolated mouse hearts (n=36) were paced (500 imp./min) and perfused at constant flow (16.0 +/- 0.3 ml g(-1) min(-1)). When coronary vasculature resistance was carefully controlled by adenosine (1 micromol l(-1)), authentic nitric oxide solution, in a concentration less than 5 micromol l(-1) did not alter myocardial oxygen consumption. Only concentrations of nitric oxide higher than 5 micromol l(-1) induced reduction in myocardial oxygen consumption. Thus in the saline perfused mouse heart, with carefully controlled vasodilatation, modulating myocardial nitric oxide levels using an arterial application of authentic nitric oxide, concentrations higher than 5 micromol l(-1) of nitric oxide were required to induce a decrease in myocardial oxygen consumption.     

Measurement of the activation time of oxidative phosphorylation in isolated mouse hearts

The purpose of this study was to develop a technique for determination of the dynamic regulation of oxidative myocardial metabolism in the mouse. The response time of myocardial oxygen consumption (MVO(2)) to a step in heart rate was determined in Langendorff-perfused mouse hearts. We examined the effect of glucose-only perfusate and glucose combined with 1, 3, or 6 mM pyruvate. Left ventricular systolic pressure (LVSP) decreased, yet the rate-pressure product (RPP) and MVO(2) increased with upward steps in heart rate. Pyruvate increased LVSP, RPP, and MVO(2) at the lower concentrations; however, when 6 mM pyruvate was added, LVSP and RPP became depressed while MVO(2) remained elevated. The mean response time of oxygen consumption to a step in heart rate from 270 to 350 beats/min was 9.8 s (n = 7) in the glucose-only perfused hearts. Perfusion with glucose plus 6 mM pyruvate decreased the response time to 5.3 s. These results are similar to those found in the rabbit heart and lay the groundwork for further examination of the dynamic regulation of oxidative myocardial metabolism in genetically altered mice. We concluded that the activation time of oxidative phosphorylation in the mouse is similar to that in larger species, despite the high mitochondrial content and natural heart rate of the mouse.     

Temporal dynamics of inotropic, chronotropic, and metabolic responses during beta1- and beta2-AR stimulation in the isolated, perfused rat heart

During the beta-adrenergic receptor (beta-AR)-mediated stress response in the heart, the relations between functional responses and metabolism are ill defined, with the distinction between beta1- and beta2-AR subtypes creating further complexity. Specific outstanding questions include the temporal relation between inotropic and chronotropic responses and their metabolic correlates. We sought to elucidate the relative magnitudes and temporal dynamics of the response to beta1- and beta2-AR stimulation and the energy expenditure and bioenergetic state related to these responses in the isolated perfused rat heart. Inotropic [left ventricular developed pressure (LVDP) and dP/dt], chronotropic [heart rate (HR)], and metabolic responses were measured during beta1- (n = 9; agonist: norepinephrine) and beta2- (n = 9; agonist: zinterol) AR stimulation. Myocardial oxygen consumption (MVO2) was measured using fiber-optic oximetry, and high-energy phosphate levels and intracellular pH were measured using 31P NMR spectroscopy. A multiple-dose protocol was used, with near-maximal beta-AR stimulation at the highest doses. In both beta1 and beta2 groups, there were dose-dependent increases in LVDP, dP/dt, HR, and MVO2. The inotropic response showed more rapid onset, washout, and variation during dose than did the chronotropic response and was closely correlated with MVO2. This suggests that the myocardial bioenergetic state is more closely related to the inotropic response than to the chronotropic response. In addition, beta1-AR stimulation resulted in a greater magnitude and rate of onset of inotropic and MVO2 responses than did beta2-AR stimulation during maximal stimulation. However, a similar decrease in intracellular energy charge was seen in the two groups, consistent with a greater rate of oxidative phosphorylation during beta1- than during beta2-AR stimulation.     

Hepatic dysoxia commences during O2 supply dependence.

Hepatic O2 consumption (VO2) remains relatively constant (O2 supply independent) as O2 delivery (DO2) progressively decreases, until a critical DO2 (DO2c) is reached below which hepatic VO2 also decreases (O2 supply dependence). Whether this decrease in VO2 represents an adaptive reduction in O2 demand or a manifestation of tissue dysoxia, i.e., O2 supply that is inadequate to support O2 demand, is unknown. We tested the hypothesis that the decrease in hepatic VO2 during O2 supply dependence represents dysoxia by evaluating hepatic mitochondrial NAD redox state during O2 supply independence and dependence induced by progressive hemorrhage in six pentobarbital-anesthetized dogs. Hepatic mitochondrial NAD redox state was estimated by measuring hepatic venous beta-hydroxybutyrate-to-acetoacetate ratio (beta OHB/AcAc). The value of DO2c was 5.02 +/- 1.64 (SD) ml.100 g-1.min-1. The beta-hydroxybutyrate-to-acetoacetate ratio was constant until a DO2 value (3.03 +/- 1.08 ml.100 g-1.min-1) was reached (P = 0.05 vs. DO2c) and then increased linearly. Peak liver lactate extraction ratio was 15.2 +/- 14.1%, occurring at a DO2 of 5.48 +/- 2.54 ml.100 g-1.min-1 (P = NS vs. DO2c). Our data support the hypothesis that the decrease in VO2 during O2 supply dependence represents tissue dysoxia.     

Functional hepatocyte heterogeneity. Vascular 2-oxoglutarate is almost exclusively taken up by perivenous, glutamine-synthetase-containing hepatocytes

1. In isolated perfused rat liver maximal rates of 2-[1-14C]oxoglutarate uptake were about 0.4 mumol.g-1 .min-1; half-maximal rates of 2-[14C]oxoglutarate uptake were observed with influent concentrations of about 100 microM. 2-[14C]Oxoglutarate uptake by the liver was not affected by the direction of perfusion, but was decreased by about 80-90% when Na+ in the perfusion fluid was substituted by choline+, suggesting a Na+-dependence of hepatic 2-oxoglutarate uptake. In the absence of added ammonia, [14C]oxoglutarate uptake by the liver was about twice the net oxoglutarate uptake, indicating a simultaneous release of unlabeled oxoglutarate from perfused rat liver. 2. 14C-Labeled metabolites derived from [1-14C]oxoglutarate and recovered in the effluent perfusate were 14CO2 and 14C-labeled glutamate and glutamine; they accounted for 85-100% of the radiolabel taken up by the liver. 14CO2 was the major product (more than 70%) from [1-14C]oxoglutarate taken up the liver, provided glutamine synthesis was either inhibited by methionine sulfoximine or the endogenous rate of glutamine production was below 40 nmol.g-1.min-1. 3. Stimulation of glutamine synthesis by ammonia did not affect [14C]oxoglutarate uptake by the liver, but considerably increased net hepatic oxoglutarate uptake, indicating a decreased release of unlabeled oxoglutarate from the liver. Stepwise stimulation of hepatic glutamine synthesis led to a gradual decrease of 14CO2 production and radiolabel was recovered increasingly as [14C]glutamine in the effluent. At high rates of glutamine formation (i.e. about 0.6 mumol.g-1.min-1), about 60% of the [1-14C]oxoglutarate taken up by the liver was recovered in the effluent as [14C]glutamine. 14CO2 and [14C]glutamine production from added [1-14C]oxoglutarate were dependent on the rate of hepatic glutamine synthesis but not on the direction of perfusion. Extrapolation of 14C incorporation into glutamine to maximal rates of hepatic glutamine synthesis yielded an about 100% utilization of the [14C]oxoglutarate taken up by the liver for glutamine synthesis. This was again true for both the antegrade and the retrograde perfusion directions. On the other hand, addition of ammonia did not affect 14CO2 production from labeled oxoglutarate, when glutamine synthetase was inhibited by methionine sulfoximine. 4. The data suggest that vascular oxoglutarate is almost exclusively taken up by the small perivenous hepatocyte population containing glutamine synthetase, i.e. a cell population comprising only 6-7% of all hepatocytes. Thus, the findings demonstrate the existence of a, to date, uniquely zonally distributed oxoglutarate transport system which is probably Na+-dependent in the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dobutamine alters carnitine metabolism in the neonatal piglet heart

The use of inotropic agents to support the neonatal heart after sepsis or hypoxia increases cardiac energy demand. Carnitine plays a vital role in energy, fuel metabolism. To test the hypothesis that inotropic agents affect carnitine metabolism, hearts from sow-fed piglets were isolated and perfused with an oxygenated buffer containing glucose and palmitate. Increasing dosages of dobutamine (DOB 2.5-15 microg/Kg body wt per min, 0.007-0.044 micromol/kg per min) or saline vehicle (SAL) were administered. Heart rate (HR), left ventricular systolic (LVSP) and end diastolic pressures (LVEDP) were measured. Left ventricular developed pressure (LVDP = LVSP-LVEDP) and pressure-rate product (LVDP x HR) were calculated. Coronary effluent was collected to measure flow and metabolites. Heart tissue samples were collected for metabolite analysis. RESULTS: DOB increased HR, LVEDP and the pressure-rate product [LVDP x HR]. Mean lactate production increased in DOB, but not in SAL control hearts, and was correlated with heart acylcarnitine, but not with coronary flow. Tissue acylcarnitine levels were higher in the DOB than in the SAL group. Plasma total carnitine was correlated with [LVDP x HR] and LVDP, but not with HR. The findings demonstrate that DOB alters myocardial carnitine metabolism and suggest that carnitine status may affect cardiac response to inotropic agents.     

Cytosolic adenylates and adenosine release in perfused working heart. Comparison of whole tissue with cytosolic non-aqueous fractionation analyses

Free cytosolic adenylates were examined in relation to adenosine plus inosine released from perfused working guinea-pig hearts. Whole-tissue adenylate data from freeze-clamped hearts were quantitatively compared with corresponding values obtained by subcellular fractionation of homogenized myocardium in non-aqueous media. Adenosine and inosine in venous cardiac effluents were measured by high-performance liquid chromatography. Hearts, perfused at their natural flows, were subjected to various workloads, substrates and catecholamines to alter myocardial energy metabolism and respiration over a wide physiological range. Non-aqueous cytosolic ATP and creatine phosphate (CrP) accounted for more than 80% of the respective total myocardium content. The cytosolic CrP/Pi ratio was in near-quantitative agreement with the overall tissue CrP/Pi ratio when the latter parameter was corrected for extracellular Pi. This was conclusive evidence that ATP, CrP and Pi were predominantly located in the cytosol of the well-oxygenated cardiomyocyte. Measured myocardial oxygen uptake (MVO2) was reciprocally related to the phosphorylation state of CrP [( CrP]/[Cr] X [Pi]) and hence that of ATP [( ATP]/[ADP] X [Pi]) assuming the creatine kinase at near-equilibrium at a near-constant pH of 7.2. On the other hand, calculated mean free cytosolic ADP concentrations increased essentially linearly up to threefold with increasing MVO2 in the presence of virtually unchanged or only slightly decreased ATP levels; this was found both according to the whole tissue and the special subcellular fractionation data. Employing the myokinase mass-action ratio and substituting total cardiac ADP by the mean free cytosolic ADP concentrations, the mean free cytosolic AMP concentrations proved to be in the nanomolar range, i.e. up to three orders of magnitude lower than the overall tissue AMP content. We propose, therefore, that in the normoxic heart, AMP is located predominantly in the mitochondrial compartment. Nevertheless, both free cytosolic AMP concentration and release of adenosine plus inosine were apparently square or even higher-power functions of the rate of cardiac respiration. On the other hand, the mean purine nucleoside release seemed linearly correlated (r = 0.920) with the calculated free cytosolic AMP concentration. Our observations seem to suggest that the concentrations of free ADP and AMP in the cytosol are major determinants of the production of inosine and coronary vasodilator adenosine.(ABSTRACT TRUNCATED AT 400 WORDS)     

Rosiglitazone treatment improves cardiac efficiency in hearts from diabetic mice

Isolated perfused hearts from type 2 diabetic (db/db) mice show impaired ventricular function, as well as altered cardiac metabolism. Assessment of the relationship between myocardial oxygen consumption (MVO(2)) and ventricular pressure-volume area (PVA) has also demonstrated reduced cardiac efficiency in db/db hearts. We hypothesized that lowering the plasma fatty acid supply and subsequent normalization of altered cardiac metabolism by chronic treatment with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist will improve cardiac efficiency in db/db hearts. Rosiglitazone (23 mg/kg body weight/day) was administered as a food admixture to db/db mice for five weeks. Ventricular function and PVA were assessed using a miniaturized (1.4 Fr) pressure-volume catheter; MVO(2) was measured using a fibre-optic oxygen sensor. Chronic rosiglitazone treatment of db/db mice normalized plasma glucose and lipid concentrations, restored rates of cardiac glucose and fatty acid oxidation, and improved cardiac efficiency. The improved cardiac efficiency was due to a significant decrease in unloaded MVO(2), while contractile efficiency was unchanged. Rosiglitazone treatment also improved functional recovery after low-flow ischemia. In conclusion, the present study demonstrates that in vivo PPARgamma-treatment restores cardiac efficiency and improves ventricular function in perfused hearts from type 2 diabetic mice.     

Exercise training increases coronary transport reserve in miniature swine

Female yucatan miniature swine were trained on a treadmill (ET) or were cage confined (C) for 16-22 wk. The ET pigs had increased exercise tolerance, heart weight-to-body weight ratio, and skeletal muscle oxidative capacity. After anesthesia the left anterior descending coronary artery was cannulated and pump perfused with blood while aortic, central venous, and coronary perfusion pressures, electrocardiogram, heart rate, and coronary blood flow were monitored. Capillary permeability-surface area product (PS) for EDTA was determined with the single-injection indicator-diffusion method by use of an organ model based on the Sangren-Sheppard equations for capillary transport. Coronary blood flow (CBF) and PS were compared before and during maximal adenosine vasodilation with coronary perfusion pressures at 120 mmHg. Results indicate that there were no differences in base-line CBF or PS between C and ET groups. alpha-Receptor blockade with phentolamine and/or prazosin, before adenosine vasodilation, produced increases in PS in C pigs but had little effect in ET pigs. During maximal vasodilation with adenosine, ET pigs had greater CBF (447 +/- 24 vs. 366 +/- 27 ml.min-1.100 g-1) and greater PS (83 +/- 9 vs. 55 +/- 7 ml.min-1.100 g-1) than the C group. It is concluded that ET induces an increased coronary transport capacity in miniature swine that includes a 22% increase in blood flow capacity and a 51% increase in capillary exchange capacity.     

Dobutamine responsiveness,PET mismatch,and lack of necrosis in low-flow ischemia: is this hibernation in the isolated rat heart?

The clinical hallmarks of hibernating myocardium include hypocontractility while retaining an inotropic reserve (using dobutamine echocardiography), having normal or increased [18F]fluoro-2-deoxyglucose-6-phosphate (18FDG6P) accumulation associated with decreased coronary flow [flow-metabolism mismatch by positron emission tomography (PET)], and recovering completely postrevascularization. In this study, we investigated an isolated rat heart model of hibernation using experimental equivalents of these clinical techniques. Rat hearts (n = 5 hearts/group) were perfused with Krebs-Henseleit buffer for 40 min at 100% flow and 3 h at 10% flow and reperfused at 100% flow for 30 min (paced at 300 beats/min throughout). Left ventricular developed pressure fell to 30 +/- 8% during 10% flow and recovered to 90 +/- 7% after reperfusion. In an additional group, this recovery of function was found to be preserved over 2 h of reperfusion. Electron microscopic examination of hearts fixed at the end of the hibernation period demonstrated a lack of ischemic injury and an accumulation of glycogen granules, a phenomenon observed clinically. In a further group, hearts were challenged with dobutamine during the low-flow period. Hearts demonstrated an inotropic reserve at the expense of increased lactate leakage, with no appreciable creatine kinase release. PET studies used the same basic protocol in both dual- and globally perfused hearts (with 250MBq 18FDG in Krebs buffer +/- 0.4 mmol/l oleate). PET data showed flow-metabolism "mismatch;" whether regional or global, 18FDG6P accumulation in ischemic tissue was the same as (glucose only) or significantly higher than (glucose + oleate) control tissue (0.023 +/- 0.002 vs. 0.011 +/- 0.002 normalized counts. s-1x g-1x min-1, P < 0.05) despite receiving 10% of the flow. This isolated rat heart model of acute hibernation exhibits many of the same characteristics demonstrated clinically in hibernating myocardium.     

Adenosine induced direct negative inotropic effect is abolished during global ischemia: role of protein kinase C and prostacyclin

Adenosine acts as a cardioprotective agent by producing coronary vasodilation, decreasing heart rate and by antagonizing the cardiostimulatory effect of catecholamines; adenosine also exerts a direct negative inotropic effect. Myocardial ischemia is known to be associated with enhanced levels of adenosine, increased protein kinase C (PKC) activity and prostacyclin (PGI2) release. The present study was conducted to determine if myocardial ischemia alters the cardioprotective effect of adenosine by increasing PKC activity and PGI2 release in the isolated rat heart perfused at 10 ml/min with Krebs-Henseleit buffer (KHB; 95% O2+5% CO2). Adenosine (10 mmol/min) reduced myocardial contractility as indicated by a decrease in contractility (dp/dtmax), heart rate (HR) and coronary perfusion pressure (PP). In hearts subjected to 30 min of ischemia (without perfusion) and then reperfused with KHB, adenosine failed to decrease dp/dtmax, HR or PP. However, during infusion of PKC inhibitor H-7 (1-(5-Isoquinolinesulfonyl)-2-methylpiperazine hydrochloride) (H-7; 6 mmol/min), which commenced 10 min before ischemia and continued throughout reperfusion, adenosine produced a decrease in dp/dtmax, HR and PP, similar to that before ischemia. Infusion of the PKC activator phorbol 12,13-dibutyrate (PDBu; 2 nmol/min) but not an inactive analogue in non-ischemic hearts prevented the adenosine induced decrease in dp/dtmax. During infusion of H-7, PDBu failed to block the direct negative inotropic effect of adenosine in non-ischemic hearts. In addition, pretreatment with H-7 or indomethacin (cyclooxygenase inhibitor) significantly reduced the PGI2 release following ischemia. This data suggest that PKC and PGI2 regulate the direct negative inotropic effect of adenosine, which is abolished during ischemia.     

Relation between myocardial substrate utilization, oxygen consumption and regional oxygen balance in the dog heart in vivo

The interaction between myocardial function, oxygen consumption and energy production was examined in the left ventricular myocardium during various physiological conditions. Myocardial function was measured by both LV dP/dTmax and by local contractile tension. Coronary blood flow was measured from the coronary sinus; regional coronary blood supply was recorded using a thermistor placed on the epicardial surface. Intracellular oxygen balance was estimated using NADH fluorescence. Myocardial oxygen consumption and utilization of glucose, pyruvate, lactate and free fatty acids were calculated from their concentrations in the arterial and coronary sinus blood. The effects of tachycardia at 180 and 240 bpm, noradrenaline infusion (25 micrograms kg-1 min-1), and increased coronary blood flow caused by hypopneic respiration were examined. During pacing, contractile force, coronary flow and NADH fluorescence increased. At 240 bpm, the lactate/pyruvate ratio increased from 5.98 +/- 0.92 to 8.76 +/- 1.41 and NADH fluorescence increased from 50 to 71.7 +/- 3.73 (as compared to control), indicating impairment of myocardial oxygenation. Hypopneic respiration produced a marked elevation of coronary blood flow. Both noradrenaline infusion and hypopnea produced a decrease in both NADH fluorescence and the lactate/pyruvate ratio. No significant difference was found between the FORCE/ATP, FORCE/MVO2 and ATP/MVO2 ratios during pacing and noradrenaline. However, during hypopnea, the amount of ATP apparently formed (as calculated by substrate utilization assuming the formation of 3 ATP molecules per oxygen) was disproportionately greater than contractile force and oxygen consumption. It is suggested that this discrepancy may be due to the uncoupling of oxidative phosphorylation.     

Effect of increased Hb-O2 affinity on VO2max at constant O2 delivery in dog muscle in situ

We investigated the effect of increasing hemoglobin- (Hb) O2 affinity on muscle maximal O2 uptake (VO2max) while muscle blood flow, [Hb], HbO2 saturation, and thus O2 delivery (muscle blood flow X arterial O2 content) to the working muscle were kept unchanged from control. VO2max was measured in isolated in situ canine gastrocnemius working maximally (isometric tetanic contractions). The muscles were pump perfused, in alternating order, with either normal blood [O2 half-saturation pressure of hemoglobin (P50) = 32.1 +/- 0.5 (SE) Torr] or blood from dogs that had been fed sodium cyanate (150 mg.kg-1.day-1) for 3-4 wk (P50 = 23.2 +/- 0.9). In both conditions (n = 8) arterial PO2 was set at approximately 200 Torr to fully saturate arterial blood, which thereby produced the same arterial O2 contents, and muscle blood flow was set at 106 ml.100 g-1.min-1, so that O2 delivery in both conditions was the same. VO2max was 11.8 +/- 1.0 ml.min-1.100 g-1 when perfused with the normal blood (control) and was reduced by 17% to 9.8 +/- 0.7 ml.min-1.100 g-1 when perfused with the low-P50 blood (P less than 0.01). Mean muscle effluent venous PO2 was also significantly less (26 +/- 3 vs. 30 +/- 2 Torr; P less than 0.01) in the low-P50 condition, as was an estimate of the capillary driving pressure for O2 diffusion, the mean capillary PO2 (45 +/- 3 vs. 51 +/- 2 Torr). However, the estimated muscle O2 diffusing capacity was not different between conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Duration of ischaemia determines matrix metalloproteinase-2 activation in the reperfused rabbit heart

It has been hypothesised that activation of matrix metalloproteinase-2 (MMP-2) contributes to reversible myocardial dysfunction (stunning) following short-term ischaemia and reperfusion. Gelatin zymography was used to measure release of both pro-MMP-2 (72 kDa) and MMP-2 (62 kDa), into the coronary effluent from isolated, perfused rabbit hearts during 90 min aerobic perfusion (control), or low-flow ischaemia (15 or 60 min at 1 mL/min), followed by 60 min reperfusion. In controls, pro-MMP-2 was detected in the coronary effluent throughout the first 30 min of aerobic perfusion, but MMP-2 was not detected. In contrast, MMP-2 was detected in the coronary effluent during reperfusion after both 15 and 60 min ischaemia. However, while left ventricular systolic function was impaired after both 15 min and 60 min ischaemia, a significant increase in the release of MMP-2 was only detected in hearts following 60 min ischaemia. The dissociation between mechanical function and MMP-2 levels suggest that MMP-2 does not contribute to myocardial stunning in this model, but may contribute to myocardial dysfunction following prolonged ischaemia.     

Training-induced muscle adaptations: increased performance and oxygen consumption

An isolated perfused rat hindlimb preparation was used to study the impact of local muscle adaptations induced by endurance exercise training on muscle performance and peak muscle oxygen consumption. Rats were trained for 12-15 wk by a running program (30 m/min up a 15% grade for 1 h/day 5 days/wk) shown previously to increase muscle mitochondrial enzyme activity. Sedentary (n = 11) and trained (n = 11) hindlimbs of similar size were perfused with a similar inflow (12.1 ml/min) at a similar oxygen content (18.1 ml O2/100 ml blood). Tetanic contractions (100 ms at 100 Hz) at 4, 8, 15, 30, 45, and 60/min were elicited in consecutive order. Initial tension was better maintained by muscles of trained animals at all frequencies above 4 tetani/min (P less than 0.05). Oxygen consumption (mumol.min-1.g-1) increased similarly in both groups at the lower contraction frequencies but was greater (P less than 0.05) in the trained [3.52 +/- 0.32 (SE)] than in the sedentary (2.44 +/- 0.31) group at 60 tetani/min. The peak oxygen consumption of the trained group (3.93 +/- 0.27) was 20% greater (P less than 0.05) than that of the sedentary group (3.28 +/- 0.28) when peak values for each animal, irrespective of the contraction condition, are compared. Blood flows to the contracting muscle (approximately 100 ml.min-1.g-1) and, therefore, oxygen deliveries (mumol.min-1.g-1) were not different between sedentary (7.99 +/- 0.56) and trained groups (8.35 +/- 0.61). Thus the 20% higher peak oxygen consumption was achieved by a greater oxygen extraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heat production, blood flow, O2 uptake and CO2 output in the secretary process of the dog submandibular gland (author's transl)

1. Under normal circulation of the dog submandibular gland, the electrical stimulation induced a massive salivary secretion (about 0.35 ml . min-1.g-1 gland weight) and an increase in the glandular temperature (about 0.2 degrees C). The heat production was calculated of about 60 mW.g-1. 2. Clamping of the glandular artery made both of secretion and heat production to be transient. The early peak of secretion was about 0.12 ml.min-1.g and that of heat production was 7 approximately 10mW,g-1. Then each 1 ml secretion followed about 4.6 J heat production. 3. Under constant blood flow in the glandular circulation, the secretory process was divided clearly into 2 phases of peak and plateau. The glandular temperature increased about 0.12 degrees C with an early temperature drop. In the secretory plateau phase, the secretary rate was about 0.043 ml.min-1.g-1, the heat production was about 5 approximately 7 mW.g-1 and each 1 ml secretion caused about 8.2 J heat production. 4. The rate of oxygen uptake was about 20.9 microl.min-1g-1 at the resting state. The maximum during secretion was about 192 microliter.min-1.g-1. THe half time of the recovery process of O2 uptake tended slightly longer than that of heat production. 5. THe rate of CO2 output was about 21.8 microliter.min-1.g-1 at resting. The maximum during secretion was about 142 microliter.min-1.g-1 R. Q. were about 1 at resting and about 0.74 under secretion.     

Relationships between cardiac hyperactivity, oxygen tension, and release of vasodilator material in coronary autoregulation of guinea-pig hearts

Increases in cardiac activity induce autoregulatory coronary vasodilation. The intermediate steps which trigger this process are thought to be myocardial hypoxia which induces the release of vasodilator mediator(s). The present study examines the relationships between mechanical activity, oxygen tension, and release of vasodilator material in isolated perfused hearts. Guinea-pig isolated hearts were perfused in series, the effluent from donor hearts being regassed prior to entry to recipient hearts. Histamine (1 microgram) and isoproterenol (10 ng) increased the rate and tension of donor hearts and produced predominant coronary vasodilator responses which were followed by the appearance of vasodilator material in the recipient (falls in perfusion pressure, 9.8 +/- 1.1 and 9.1 +/- 2.5 mmHg) (1 mmHg = 133.322 Pa). Exposure of donor hearts to hypoxia also caused vasodilatation and release of vasodilator material (fall in pressure, 11.4 +/- 1.6 mmHg). Pacing-induced tachycardia (6 Hz) of donor hearts promoted the release of vasodilator material, the fall in recipient heart pressure being 11.5 +/- 1.8 mmHg. This was abolished by beta-adrenoceptor blockade and when donor hearts were from reserpine-pretreated guinea pigs. In was concluded that pacing released endogenous catecholamines which in turn released the vasodilator material. Pacing per se did not cause vasodilatation or release of the vasodilator. The Po2 of perfusates from donor hearts was reduced by pacing at 5 Hz (25.7 +/- 5.2 mmHg) and by isoproterenol (10 ng, 32.0 +/- 3.7 mmHg), indicative of an elevated oxygen extraction. The isoproterenol-induced falls in Po2 were abolished by beta-adrenoceptor blockade. However, the pacing-induced falls in Po2 persisted, the values occurring before (25.7 +/- 5.2 mmHg) and after propranolol (45.7 +/- 4.5 mmHg) and before (32.1 +/- 1.1 mmHg) and after practolol (27.3 +/- 4.1 mmHg) not differing significantly (p greater than 0.05). These falls in perfusate Po2 were not accompanied by coronary vasodilatation or release of vasoactive material. Perfusate Po2 changes could therefore be dissociated from the coronary vasodilatation and vasoactive material release, suggesting that hypoxia may not be a prerequisite for the metabolic autoregulatory vasodilatation in response to myocardial hyperactivity induced by cardiac stimulants.