Effects of hypoxic preconditioning on the hypoxic-reoxygenated atria from fed and fasted rats.

Hypoxic preconditioning (PC) was studied using rat atria set up isometrically in 10 mM dextrose medium and paced at 1 Hz, applying three different protocols wherein fed and 24-h fasted rats were used in protocols 1 and 2 and only the fed in protocol 3. In protocol 1, PC was achieved applying a 5 min hypoxia followed by 10 min of reoxygenation before the onset of a 60 min hypoxia and 60 min reoxygenation. In protocol 2 the 5 min and a posterior 45 min hypoxia were applied in the absence of dextrose whereas in the 10 min and 60 min reoxygenation periods dextrose was present. In protocol 3, two cycles of 5 min dextrose-free hypoxic periods were applied before the sustained hypoxia (dextrose-free) and reoxygenation periods (10 min and final 45 min, both in the presence of dextrose). In the control groups of all protocols, the equilibration periods were prolonged to compensate the duration of PC. In the control groups of protocols 1 and 2, the sustained hypoxia evoked greater disturbances of contractility and a smaller post-hypoxic recovery in the fasted than in the fed rat atria. In protocol 1, PC markedly reduced the rise in resting tension and improved the post-hypoxic recovery in the fasted rat atria whereas in the fed rat atria protective effects were small and brief. In protocol 2, PC evoked a small reduction of contracture only in the atria from fasted rats and in protocol 3, PC exacerbated the hypoxic disturbances. These data suggest that PC effects depend both on the severity of the PC stress and the sustained hypoxia; and that PC does not require coronary flow.

Lack of protective effects of adenosine on the hypoxic and reoxygenated atria and ventricular strips of the rat.

The aim of the investigation was to assess whether adenosine would ameliorate the hypoxic-induced disturbances of the isolated atria and ventricular strips from fed and 24 h fasted rats. Adenosine 100-50 microM exerted negative inotropic and chronotropic effects on the aerobic atria whereas 10 microM was ineffective. During hypoxia the atria underwent a decline of the peak developed tension and pacemaker frequency. Adenosine 50 microM was detrimental for the performance of hypoxic atria whereas a 10 microM neither affected the fall of peak tension nor the post-hypoxic recovery. Adenosine 100 microM did not affect the peak developed tension of the aerobic ventricular strips. Under hypoxia the ventricular strips from fed and fasted rats exhibited a pronounced depression of their peak developed tension together with the development of a strong contracture and a partial recovery after reoxygenation, which attained a similar extent in both nutritional states. Lactate output during hypoxia was lower in the group of fasted rats. Adenosine 100 microM did not exhibit any effect on the ventricular functions and glycolytic activity in both experimental groups. Results suggests that adenosine has no beneficial effects on rat isolated atria and ventricular strips in hypoxic conditions

Effects of pentanoate, 4-pentenoate, 3-hydroxybutyrate and insulin on the tissue-levels of long-chain acyl CoA and acylcarnitine in the oxygenated and hypoxic rat atria.

Under hypoxic conditions the atrial contents of long-chain acyl CoA (LCCoA) and long-chain acylcarnitine display a close correlation with the noxious effects of fasting on the atrial functions as well as with the amelioration effected by inhibitors of carnitine palmitoyltransferase I. These findings suggested that fatty acid oxidation was detrimental for the hypoxic atria. However, since changes of the LCCoA and LCCa levels which may occur together with the hypoxic disturbances attained under some other metabolic interventions had not been assessed yet, present investigation aimed to provide information about this issue. At the end of the prehypoxic equilibration period, all the treatments tested evoked a fall of the free-CoA levels whereas free-carnitine, LCCoA and LCCa remained unchanged. In the hypoxic atria, 4-pentenoate, an inhibitor of fatty acid beta-oxidation that also can be oxidized, did not change LCCoA and LCCa levels whereas the readily oxidizable pentanoate evoked a drop of LCCoA. These effects may be due to the trapping of CoA as the short-chain acyl esters of both substances. Since 4-pentenoate and pentanoate were noxious on the hypoxic atria even though they did not increase LCCoA and LCCa contents, it may be inferred that short-chain acyl esters might be deleterious during oxygen shortage. The exposure to 3-hydroxybutyrate, an oxidizable substrate whose availability increases during fasting, did not alter the LCCoA and LCCa contents, agreeing with the weak detrimental effects that it exerts on the hypoxic atria. On the other hand, insulin elicited a rise in the LCCoA and a fall in the LCCa contents. Inasmuch insulin had been shown to improve the performance of the hypoxic atria, these findings suggest that LCCoA might not be involved in the noxious effects of fatty acid oxidation whereas LCCa would be the major toxic catabolite.

Effects of fasting, hypoxia, methylpalmoxirate and oxfenicine on the tissue-levels of long-chain acyl CoA and acylcarnitine in the rat atria.

During hypoxia the atria from fasted rats exhibit a faster decline in the pacemaker and contractile activities than those from fed rats. Oxfenicine and methylpalmoxirate, inhibitors of carnitine palmitoyltransferase 1 (CPT 1), ameliorate these disturbances. Since the fasted rat atria have greater triacylglycerol stores and a faster lipolysis, and CPT 1 funnels fatty acid into beta-oxidation, the effects of fasting could be ascribed to the accumulation of amphipathic metabolites such as long-chain acyl CoA (LCCoA) and long-chain acylcarnitine (LCCa). Hence, this investigation aimed to assess whether the levels of these metabolites correlate with the effects of fasting and CPT 1 inhibitors. At the end of the prehypoxic equilibration period the fasted rat atria had a 6.5-fold greater content of LCCa than those of the fed rats and methylpalmoxirate impeded the increase. During hypoxia the LCCoA content increased 9-fold in the fasted rat atria, LCCa levels were 3.6-fold greater in the fasted than in the fed group, and free-CoA and free-carnitine showed a significant fall. The increases of LCCoA and LCCa as well as the fall in free-CoA were abolished by both inhibitors. The decrease of free-carnitine was impeded by methylpalmoxirate, but oxfenicine unexpectedly decreased its concentration in both nutritional groups. These data suggest that: (1) the atrial CPT 1 activity is enhanced during fasting, (2) in the hypoxic atria levels of LCCoA and LCCa were closely correlated with the noxious effects of fasting and the amelioration effected by CPT 1 inhibitors, and (3) the effects of amphipathic metabolites during oxygen deprivation can be attenuated by pharmacological interventions.

Effects of 3-hydroxybutyrate on the hypoxic and reoxygenated atria from fed and fasted rats.

When exposed to hypoxia, the isolated atria from fed rats released lactate into the medium and underwent a decline of the peak developed tension and pacemaker frequency. The atria from 24-h fasted rats showed a rise in the resting tension together with a greater decline of the pacemaker rate and a lower lactate output than those from fed rats. The exposure to 5 mM 3-hydroxybutyrate caused only a small and brief decline in the pacemaker rate in the fed rats atria indicating that ketone bodies are able to exert only a minor detrimental effect on the hypoxic atria. Since the lactate output remained unaffected, this effect cannot be ascribed to a lowering in the energy supply from anaerobic glycolysis. On the contrary, 3-hydroxybutyrate improved the post-hypoxic recovery of the peak tension in the atria from fasted rats. This finding may be reflecting an anaplerotic role of 3-hydroxybutyrate, thus suggesting that in addition to glucose a second substrate is needed to meet the energy demand in the reoxygenated atria from fasted rats.

Exogenous substrates as energy source for the contractile activity of the isolated rat tail artery.

The isolated rat tail artery underwent a pronounced depression of the contractile responses to adrenaline during the incubation in a glucose-free medium containing 2-deoxyglucose and/or oxfenicine in order to inhibit the utilization of glycogen and/or endogenous triacylglycerol. When glucose was returned after 90 min of exposure to oxfenicine, the contraction strength recovered completely. In the medium with 2-deoxyglucose the addition of palmitate or hexanoate produced a recovery level 28% and 16% below the control values respectively. The effect of palmitate was nearly abolished and that of hexanoate partially decreased in the medium containing both inhibitors. Under this condition pyruvate reestablished the extent of the contraction to about 80% of the control value whereas beta-hydroxybutyrate produced a weak and transient recovery. These data suggest that in the tail artery the major portion of the energy needed to sustain the contractile activity is supplied by the oxidation of the more important plasmatic substrates with the exception of ketone bodies. However the Embden-Meyerhof pathway seems necessary to maintain at least a fraction of the contraction strength.

Assessment of the energetic role of endogenous substrates in the tail artery of rat by means of enzyme inhibitors.

The rat tail artery during a 180 min incubation period in a medium containing glucose plus oxfenicine (an inhibitor of fatty acid oxidation) did not show changes in the contractile responses to adrenaline. In a substrate-free medium the extent of the contractions underwent a slight decrease during the last 60 min of incubation. When the substrate-free medium contained 2-deoxyglucose (an inhibitor of glycolysis and glycogenolysis) or oxfenicine, the decline of the contractile activity developed faster and attained a similar extent with each inhibitor. When the substrate-free medium contained 2-deoxyglucose together with oxfenicine or methylpalmoxirate (an inhibitor of fatty acid oxidation) the arteries displayed a pronounced and early fall in the contraction strength. These data suggest that in the presence of glucose the reserve substrates are not necessary as fuel source for the arterial contractions. However, in substrate-free conditions they constitute an important energy source. Furthermore, glycogen and triacylglycerol share the supply of energy and there does not seem to be any other reserve material in the smooth muscle of the rat tail artery.

Effects of L-carnitine on the hypoxic atria from fed and fasted rats.

The aim of the investigation was to assess whether L-carnitine, an essential cofactor in the mitochondrial transfer of fatty acids, would ameliorate the hypoxic-induced disturbances in the isolated rat atria. During hypoxia, the atria released lactate into the bathing medium and underwent a rise in resting tension and a decline of the peak developed tension and pacemaker frequency. The atria from 24-h fasted rats, which oxidize faster their endogenous triacylglycerol stores>> exhibited greater functional disturbances during hypoxia and a smaller recovery after reoxygenation, with respect to the fed rats' atria. Furthermore, at the end of the hypoxic incubation the fasted rats atria displayed a reduction of the free CoA content together with a 3-fold increase in the content of long-chain fatty-acyl CoA, in comparison with those of fed rats. The addition of 5 mM L-carnitine 60 min before the onset of hypoxia did not exert any effect on the hypoxic atria. In contrast, 20 mM L-carnitine accelerated the decline of the pacemaker activity in the fasted rat atria and worsened the contracture development in both nutritional states. The fall of the peak tension and the posthypoxic recovery as well as the levels of free CoA and long-chain fatty-acyl CoA, and lactate output, were not affected by 20 mM L-carnitine treatment. These data suggest that L-carnitine is not beneficial for the hypoxic rat atria, even in the fasted state, wherein the atrial fatty acid catabolism is increased.

Beneficial effects of oxfenicine on the hypoxic rat atria.

During hypoxia the isolated rat atria released lactate into the bathing medium and underwent a rise of resting tension and a decline of the peak developed tension and pacemaker frequency. The atria from 24 h fasted rats, which oxidize faster their endogenous triacylglycerol pool, showed greater functional disturbances during hypoxia and a smaller recovery after reoxygenation than those from fed rats. Oxfenicine, which is a selective inhibitor of carnitine palmitoyltransferase I, attenuated the rise of resting tension and improved the post-hypoxic recovery of peak tension in the atria from fasted rats. The decline of the pacemaker frequency as well as the lactate output were not altered by the inhibitor. Present data show that oxfenicine ameliorated some of the hypoxic functional disturbances. Inasmuch lactate output did not change and these effects manifested only in the atria predisposed to the utilization of endogenous lipids, it may be inferred that oxfenicine preserved the atrial functions through the inhibition of fatty acid oxidation.

Effects of oxfenicine on the atria from fed and fasted rats.

The aim of the investigation was to assess whether endogenous triacylglycerol contributes to the maintenance of the atrial functions. To attain this information, the atria from fed and fasted rats were treated with oxfenicine which is a cardioselective inhibitor of carnitine palmitoyltransferase I. In the presence of glucose, oxfenicine suppressed lipolysis without affecting the pacemaker and contractile activities. When exposed to 2-deoxyglucose in a substrate-free medium, the atria displayed a progressive fall of the contractile strength and pacemaker rate. The dysfunctions appeared faster in the atria from fed rats coinciding with a smaller triacylglycerol mobilization. Under this condition, oxfenicine abolished the triacylglycerol breakdown, increased the fall in the peak tension, elicited a rise in the resting tension and accelerated the decline of the pacemaker rate, leading in a significant number of atria to a complete cessation of the spontaneous contractions. These effects proceeded faster in the fed rats atria. Present data suggest that glucose oxidation is sufficient to meet the atrial energy demand when the fatty acid catabolism is impeded. The noxious effects of oxfenicine, attained after the glucose metabolism was eliminated, lend direct evidence to the notion that endogenous triacylglycerol supports, at least partly, the atrial functions.

Effects of methyl palmoxirate on hypoxic rat atria.

Isolated rat atria in hypoxia released lactate into the bathing medium and underwent a decline of the contraction frequency which, in some cases led to a complete cessation of the pacemaker activity. A pronounced fall in the peak developed tension and a rise in the resting tension also appeared. The atria from 24 h fasted rats, which oxidize faster their reserve lipids than those from fed rats, exhibited greater functional disturbances during hypoxia, a lower lactate output and a smaller recovery of peak tension upon reoxygenation. Methyl palmoxirate, which is a selective inhibitor of carnitine palmitoyltransferase I, attenuated the decline of the beating rate and the rise of the resting tension in both groups of rats and the incidence of atrial arrest in the fasted rat group. The fall in the peak tension, lactate output and recovery upon reoxygenation were not altered by the inhibitor. These data indicate that methyl palmoxirate alleviates some of the hypoxic functional derangements. Hence, it may be inferred that inhibiting the oxidation of the fatty acid derived from the endogenous triacylglycerol is beneficial during oxygen-limited conditions and that these effects could not be ascribed to changes in the glycolytic flux.

Effects of methylpalmoxirate on isolated rat atria.

The aim of the investigation was to assess whether endogenous triacylglycerol contributes to the maintenance of the contractile and pacemaker activities of the isolated atria from fed and fasted rats. To attain this information, the atria were treated with methylpalmoxirate which is a potent inhibitor of carnitine palmitoyltransferase I. In the presence of glucose, methylpalmoxirate abolished the lipolysis without affecting peak developed tension or the atrial rate. When exposed to a substrate-free medium containing 2-deoxyglucose, the atria displayed a progressive fall of the pacemaker frequency, a pronounced decay of contractile strength and the appearance of contracture. These derangements appeared faster in the atria from fed rats coinciding with a smaller triacylglycerol mobilization. Methylpalmoxirate suppressed triacylglycerol breakdown, increased the contracture strength, accelerated the fall of the atrial rate and in a significant number of fasted atria it led to a complete cessation of the spontaneous contractions. The decline of the peak tension was not altered by the inhibitor, probably because the contractile strength was too weak in the glucose-free medium, so that additional negative inotropic effects were not detectable. These data suggest that exogenous glucose in addition to that derived from glycogen meet the atrial energy requirements when the fatty acid oxidation is hindered. The deleterious effects exerted by methylpalmoxirate after the glucose metabolism was eliminated indicate that endogenous triacylglycerol supports, at least partly, the atrial functions.