A protective effect of insulin on reperfusing the ischaemic rat heart shown using 31P-NMR

(1) The effect on the recovery of mechanical function, ATP, phosphocreatine, P_i and pH of various lengths of total global ischaemia in the insulin-treated, perfused rat heart has been studied using ³¹P-NMR. (2) Insulin-treated hearts recovered stable mechanical function after 18 min ischaemia when their intracellular pH was 6.0 and 70% of the pre-ischaemic ATP remained. Hearts perfused without insulin fail to recover after 18 min ischaemia, having an intracellular pH of 6.3 and 40% of ATP remaining (Bailey, I.A., Seymour, A.-M.L. and Radda, G.K. (1981) Biochim. Biophys. Acta 637, 1–7). Thus, ATP maintenance in ischaemia is more important to recovery on reperfusion than is maintaining intracellular pH. (3) The importance of this observation in devising biochemical strategies for the clinical protection of the myocardium is discussed.     

Adenine nucleotide synthesis from inosine during normoxia and after ischaemia in the isolated perfused rat heart

[14C]inosine in a range of concentrations of 20 microM to 1 mM was administered to the isolated perfused rat heart for 30 min. The incorporation of the nucleoside into myocardial adenine nucleotides increased for extracellular concentrations of the precursor up to 50 microM, reaching a plateau at 60 nmol . g-1 X 30 min-1 with concentrations ranging between 50 and 200 microM. The supply of 500 microM and 1 mM of inosine induced a further increase in cardiac adenine nucleotide synthesis to about 200 nmol . g-1 X 30 min-1. When supplied during low flow ischaemia (0.5 mL . min-1, 30 min.), 1 mM of inosine protected the heart against ATP degradation, while 100 microM of inosine was inefficacious. In the presence of 1 mM of inosine on reperfusion the adenine nucleotide content of the heart was similar to that observed in the absence of the nucleoside. The incorporation of [14C]inosine into adenine nucleotides was, in this last condition, below the value measured before ischaemia. Inosine administration was effective in protecting the heart against ischaemic breakdown of glycogen and favoured postischaemic restoration of glycogen stores.     

Proteins released from liver after ischaemia induced an elevation of heart resistance against ischaemia-reperfusion injury: 1. Beneficial effect of protein fraction isolated from perfusate after ischaemia and reperfusion of liver

OBJECTIVES: Numerous mechanisms have been proposed to participate in adaptation of heart to ischaemia by ischaemic preconditioning. We have described previously a release of cardio-protective protein fraction during ischaemic preconditioning of dog heart. In the current study the effect of high soluble protein fraction (HS fraction) released from isolated perfused rat liver after ischaemia and reperfusion was examined on isolated perfused rat heart during ischaemia-reperfusion injury. METHODS: Livers were subjected to 30 or 60 min ischaemia followed with 120 min reperfusion. HS fraction was isolated using ammonium sulphate precipitation and dissolved in perfusion solution before Langendorf perfusion of isolated rat hearts. The protein pattern of HS fraction was detected with SDS-PAGE and western blot with ConA and anti ConA antibody. Hearts were then subjected to 20 min ischaemia followed by 20 min reperfusion. During reperfusion, the haemodynamic parameters of hearts were measured. Heart levels of adenine nucleotide were measured in HClO4 extracts using HPLC on C18 column. RESULTS: Liver ischaemia induced changes in protein pattern of HS fraction released from the liver during reperfusion period. Particularly, we registered an increase in amount of several low-molecular weight proteins and decreased amount of high-molecular weight proteins. Proteins in this fraction isolated from perfusate after liver ischaemia interact with ConA with lower intensity as proteins isolated from perfusate after control non-ischaemic condition. HS fraction isolated from perfusate after ischaemia and reperfusion of liver had beneficial effect on heart function during 20 min ischaemia and subsequent 20 min reperfusion, documented by: i) decrease of arrhythmia score from 2 to 1 in 5 min of reperfusion and from 2 to 0 in 10 min of reperfusion; ii) improved heart contractility monitored as stabilised [dP/dt]max and increased Q parameter; iii) increased coronary flow. Proteins isolated from liver perfused under control non-ischaemic condition did not induce similar effects. The stabilisation of heart haemodynamics, observed after administration of HS proteins isolated from perfusate after ischaemia and reperfusion was associated with slight increase in ATP and ADP levels as well as decrease in AMP level.     

Studies of acidosis in the ischaemic heart by phosphorus nuclear magnetic resonance.

1. Phosphorus-nuclear-magnetic-resonance measurements were made on perfused rat hearts at 37 degrees C. 2. With the improved sensitivity obtained by using a wide-bore 4.3 T superconducting magnet, spectra could be recorded in 1 min. 3. The concentrations of ATP, phosphocreatine and Pi and, from the position of the Pi resonance, the intracellular pH (pHi) were measured under a variety of conditions. 4. In a normal perfused heart pHi = 7.05 +/- 0.02 (mean +/- S.E.M. for seven hearts). 5. During global ischaemia pHi drops to 6.2 +/- 0.06 (mean +/- S.E.M.) in 13 min in a pseudoexponential decay with a rate constant of 0.25 min-1. 6. The relation between glycogen content and acidosis in ischaemia is studied in glycogen-depleted hearts. 7. Perfusion of hearts with a buffer containing 100 mM-Hepes before ischaemia gives a significant protective effect on the ischaemic myocardium. Intracellular pH and ATP and phosphocreatine concentrations decline more slowly under these conditions and metabolic recovery is observed on reperfusion after 30min of ischaemia at 37 degrees C. 8. The relation between acidosis and the export of protons is discussed and the significance of glycogenolysis in ischaemic acid production is evaluated.     

Effects of increased heart work on glycolysis and adenine nucleotides in the perfused heart of normal and diabetic rats

1. In the isolated perfused rat heart, the contractile activity and the oxygen uptake were varied by altering the aortic perfusion pressure, or by the atrial perfusion technique (;working heart'). 2. The maximum increase in the contractile activity brought about an eightfold increase in the oxygen uptake. The rate of glycolytic flux rose, while tissue contents of hexose monophosphates, citrate, ATP and creatine phosphate decreased, and contents of ADP and AMP rose. 3. The changes in tissue contents of adenine nucleotides during increased heart work were time-dependent. The ATP content fell temporarily (30s and 2min) after the start of left-atrial perfusion; at 5 and 10min values were normal; and at 30 and 60min values were decreased. ADP and AMP values were increased in the first 15min, but were at control values 30 or 60min after the onset of increased heart work. 4. During increased heart work changes in the tissue contents of adenine nucleotide and of citrate appeared to play a role in altered regulation of glycolysis at the level of phosphofructokinase activity. 5. In recirculation experiments increased heart work for 30min was associated with increased entry of [(14)C]glucose (11.1mm) and glycogen into glycolysis and a comparable increase in formation of products of glycolysis (lactate, pyruvate and (14)CO(2)). There was no major accumulation of intermediates. Glycogen was not a major fuel for respiration. 6. Increased glycolytic flux in Langendorff perfused and working hearts was obtained by the addition of insulin to the perfusion medium. The concomitant increases in the tissue values of hexose phosphates and of citrate contrasted with the decreased values of hexose monophosphates and of citrate during increased glycolytic flux obtained by increased heart work. 7. Decreased glycolytic flux in Langendorff perfused hearts was obtained by using acute alloxan-diabetic and chronic streptozotocin-diabetic rats; in the latter condition there were decreased tissue contents of hexose phosphates and of citrate. There were similar findings when working hearts from streptozotocin-diabetic rats with insulin added to the medium were compared with normal hearts. 8. The effects of insulin addition or of the chronic diabetic state could be explained in terms of an action of insulin on glucose transport. Increased heart work also acted at this site, but in addition there was evidence for altered regulation of glycolysis mediated by changes in tissue contents of adenine nucleotides or of citrate.     

A protective effect of insulin on reperfusing the ischaemic rat heart shown using 31P-NMR

The effect on the recovery of mechanical function, ATP, phosphocreatine, Pi and pH of various lengths of total global ischaemia in the insulin-treated, perfused rat heart has been studied using 31P-NMR. Insulin-treated hearts recovered stable mechanical function after 18 min ischaemia when their intracellular pH was 6.0 and 70% of the pre-ischaemic ATP remained. Hearts perfused without insulin fail to recover after 18 min ischaemia, having an intracellular pH of 6.3 and 40% of ATP remaining (Bailey, I.A., Seymour, A.-M.L. and Radda, G.K. (1981) Biochim, Biophys. Acta 637, 1-7). Thus, ATP maintenance in ischaemia is more important to recovery on reperfusion than is maintaining intracellular pH. The importance of this observation in devising biochemical strategies for the clinical protection of the myocardium is discussed.     

Effect of prior exercise and insulin on potential thermogenic systems in rat skeletal muscle.

We previously reported that insulin stimulates oxygen consumption by the perfused rat hindquarter after high-intensity exercise. The purpose of the present study was to examine whether fructose 6-phosphate-fructose 1,6-bisphosphate cycling or an uncoupling of mitochondrial respiration contributes to this phenomenon. Hindquarter skeletal muscle was analyzed after perfusion in the absence or presence of insulin (150-200 microU/ml) for high-energy phosphate content, fructose 6-phosphate-fructose 1,6-bisphosphate cycling of glucose before incorporation into glycogen, and mitochondrial respiratory control. Muscle from exercised rats perfused with insulin did not display greater rates of glucose cycling or mitochondrial uncoupling; in fact, insulin decreased the rate of fructose 6-phosphate cycling and tended to increase respiratory control in skeletal muscle mitochondria. In addition, the concentrations of ATP and creatine phosphate and the calculated free ADP level in muscle of previously exercised rats perfused with insulin were similar to those of control rats. The results do not exclude the possibility that localized subcellular changes in ADP occurred, however. In conclusion, the results suggest that insulin-induced increases in other substrate cycles, ion transport systems, and/or as yet unidentified energy-requiring processes account for the 25-30% increase in hindquarter oxygen consumption after intense exercise.     

Regulation of glycolytic flux in the heart of the fetal guinea pig

Functional glycolytic capacity and its regulation have been studied in the fetal guinea-pig heart during O2 deprivation in situ and in the Langendorff perfused heart. Anaerobic glycolytic flux, at 2 mumol/min per g wet wt. was similar in the 48-50 and 60-65 days fetal and adult guinea-pig heart, despite lower fetal phosphofructokinase activity. During O2 deprivation in situ and in the perfused heart glucose was the major substrate, with glycogen making a smaller contribution. Glycolytic capacity became more tightly regulated during fetal heart development. Thus at 48-50 days glycolysis was increased during O2 deprivation by substrate supply, but at 60-65 days activation of phosphofructokinase was required also. Low malate/aspartate cycle activity in the fetal heart was suggested by the absence of an increase in malate and alanine at the expense of aspartate. The large proportion of aerobic glycolytic flux converted to lactate concurred with this. Because of the low O2 consumption and relatively high aerobic glycolytic flux, the proportion of glycolytically-derived ATP was 3-4 fold higher in the fetal than adult heart, and may explain its functional resistance to O2 deprivation.     

Cytophotometric analysis of glycogen,protein and DNA of a glycogen-storing rat hepatoma (N13) cell line

This study examines the behavior of glycogenstoring rat hepatoma (N13) in vitro using cytophotometric techniques. A significant increase in glycogen is observed in these cells after 30 min incubation in a buffered solution containing 0.1 mM glucose, that is 80 times lower than the physiological glucose concentration in rat blood. N13 hepatoma cells grow exponentially in culture using RPMI 1640 tissue culture medium supplemented with 10% fetal bovine serum. During the first day in culture these cells store a large amount of glycogen and this increase is also observed in serum-free cultures. In more prolonged cultures the amount of glycogen per cell gradually becomes lower, although the culturing conditions are maintained. Similar variations of protein are also observed during the initial period of culture. DNA distribution does not show significant changes, although in serum-free cultures an increase in the proportion of cells in S and G₂/M phases is observed. The addition of glucagon, epinephrine and cyclic AMP derivatives to serum-free cultures does not impede the storage of glycogen. Nevertheless, addition of either 2 mM N⁶,O²-dibutyryl cyclic AMP or 0.1 mM 8-(4-chlorophenylthio)-cyclic AMP blocks the cell cycle at G₀/G₁ and glycogen content does not decrease after the first day in culture. We believe that this cell line offers an appropriated model to study glycogen metabolism and its involvement in the neoplastic process.     

Control mechanisms in the acceleration of hepatic glycogen degradation during hypoxia

Hepatic glycogen metabolism in aerobic and hypoxic conditions has been assessed with respect to glycogenolysis, phosphorylase a activity and nucleotide content. Insulin did not inhibit glycogen breakdown nor stimulate lipogenesis in the aerobic perfused liver.Partial ischaemia induced glycogen breakdown, release of glucose and changes in nucleotide content in the perfused liver. Phosphorylase a content increased within 2 min in response to total ischaemia, in vivo and in the perfused liver. This change was paralleled by an increase in hepatic AMP. Glycogen synthase a activity decreased, as did the hepatic content of both cyclic AMP and cyclic GMP.     

Influence of vanadate on glycolysis,intracellular sodium,and pH in perfused rat hearts

Vanadium compounds have been shown to cause a variety of biological and metabolic effects including inhibition of certain enzymes, alteration of contractile function, and as an insulin like regulator of glucose metabolism. However, the influence of vanadium on metabolic and ionic changes in hearts remains to be understood. In this study we have examined the influence of vanadate on glucose metabolism and sodium transport in isolated perfused rat hearts. Hearts were perfused with 10 mM glucose and varying vanadate concentrations (0.7100 M) while changes in high energy phosphates (ATP and phosphocreatine (PCr)), intracellular pH, and intracellular sodium were monitored using 31P and 23Na NMR spectroscopy. Tissue lactate, glycogen, and (Na+, K+)-ATPase activity were also measured using biochemical assays. Under baseline conditions, vanadate increased tissue glycogen levels two fold and reduced (Na+, K+)-ATPase activity. Significant decreases in ATP and PCr were observed in the presence of vanadate, with little change in intracellular pH. These changes under baseline conditions were less severe when the hearts were perfused with glucose, palmitate and b-hydroxybutyrate. During ischemia vanadate did not limit the rise in intracellular sodium, but slowed sodium recovery on reperfusion. The presence of vanadate during ischemia resulted in attenuation of acidosis, and reduced lactate accumulation. Reperfusion in the presence of vanadate resulted in a slower ATP recovery, while intracellular pH and PCr recovery was not affected. These results indicate that vanadate alters glucose utilization and (Na+, K+)-ATPase activity and thereby influences the response of the myocardium to an ischemic insult.     

Effects of L-carnitine and its acetyl and propionyl esters on ATP and PCr levels of isolated rat hearts perfused without fatty acids and investigated by means of 31P-NMR spectroscopy

³¹P-NMR in vivo spectroscopy is a non-invasive and non-hazardous technique which investigates chemical composition and metabolism of living objects, for example by determining phosphocreatine (PCr) and ATP concentrations. In the present study we investigated the influence of L-carnitine, acetyl-L-carnitine and propionyl-L-carnitine on the energetic state of the Langendorff rat heart subjected to an ischemic period of 20 min followed by a reperfusion period of 60 min. To avoid an overlapping of the effects of fatty acids and glucose, the hearts were perfused with a Tyrode solution containing no fatty acids. Ischemia causes a rapid decrease in the PCr signal, followed by a decrease in the ATP signal after a prolonged period of ischemia. At the same time, a drastic increase in the P_i signal was observed. A partial recovery of the ATP and PCr signals was observed in the reperfusion period. With L-carnitine a markedly improved recovery of the high energy phosphates (e.g. increased PCr/P_i ratios) was found. With acetyl-L-carnitine this effect was enhanced in the first postischemic phase. It was followed, however, by a more rapid decrease in the PCr/P_i ratio in the late reperfusion period. The effect of propionyl-L-carnitine was not significantly improved in the first minutes of the reperfusion period, but during the whole reperfusion phase a stabilization of the PCr/P_i ratio was observed. Intracellular pH can be calculated from determination of the P_i-chemical shift. This shows that L-carnitine and its derivatives have a protective effect against intracellular pH decrease during ischemia. L-carnitine improves the energetic state of the heart, which leads to increased ischemia tolerance. Hearts under L-carnitine were able to tolerate up to four ischemia-reperfusion periods in succession, whereas the controls were not able to do so. These NMR results confirm the hypothesis that L-carnitine and its esters have a protective effect in the reperfusion period of the ischemic rat heart. This could be of importance for the treatment of ischemic cardiac diseases.     

The effects of cyanide and iodoacetate intoxication and ischaemia on enzyme release from the perfused rat heart

Isolated rat hearts perfused in the presence of iodoacetate show inhibition of glycolysis and release enzymes into the perfusate. Hearts perfused with cyanide, a mitochondrial inhibitor, show acceleration of glycolysis and no enzyme release. The adenine nucleotide content of the iodoacetate, but not the cyanide-perfused hearts was reduced. These results indicate that the membranes were permeable in the former treatment group. The adenylate energy charge and the ATP content of both the cyanide and iodoacetate treatment groups were similar but, as the extent of enzyme release was quite different, it appears that the energy state of the cell was not the prime factor controlling membrane integrity. Isolated perfused hearts were rendered ischaemic by placing a one-way ball valve in the aortic outflow tract. ATP concentration declined, as did ADP after an initial rise of short duration. AMP concentrations rose as the time of ischaemia increased. At the time at which enzyme release was first determined, the intracellular total adenine nucleotide content began to decline, suggesting that the membrane had become permeable to both small and large molecules. Glycolysis was stimulated by the hypoxia induced in the preparation and then this increase became inhibited. The point at which this inhibition was observed was also the point at which membrane permeability was evident. Taken together, the data from these experiments suggest that the energy derived from the activity of the glycolytic pathway may be important to the heart for maintenance of membrane function, particularly in ischaemia.     

Effect of growth hormone on glycogenesis in cerebral cortical slices

The uptake of glucose by cerebral cortical slices of rats was found to be enhanced by insulin by Rafaelsen (1961) and Genes and Charnaya (1966). This was confirmed by Prasannan and Subrah-manyam (1965) and more recently by Nelson , Schultz , Pasoneau and Wry (1968). Eisenberg and Seltzer (1962) and Gotistein , Held , Sebenng and Walpurger (1965) obtained evidence for a direct effect of insulin on the entry of glucose into brain and on its metabolism in this tissue. A marked resynthesis of glycogen was demonstrated with glucose as substrate by Lebaron (1955) and Mcilwain and Tresize (1956) in cerebral cortical slices of the guinea pig. Prasannan and Subrahmanyam (1965) obtained evidence for a similar resynthesis of glycogen in cerebral cortical slices of the rat. Addition of 0.2 unit of insulin per 3.5 ml of incubating medium gave rise to an increase of 60 per cent in the resynthesis of glycogen in these slices. The incorporation of ¹⁴C from labelled glucose into glycogen and CO₂ by cerebral cortical slices of normal and alloxan diabetic rats and the stimulation of the incorporation into glycogen by insulin in vitro was reported by Visweswaran , Prasannan and Subrahmanyam (1969). An insulin-like action of growth hormone on the carbohydrate metabolism was reported by Ketterer , Randle and Young (1967) and Manchester and Young (1961). It was believed to be due to the formation of a polypeptide breakdown product of growth hormone which has biological insulin-like properties. Park , Brown , Cornbluth , Daughaday and Krahl . (1952) reported an increased uptake of glucose by isolated rat diaphragm due to the action of growth hormone which is similar to that of insulin. Hence, it was considered appropriate to study the incorporation of ¹⁴C from labelled glucose into glycogen and CO₂ by cerebral slices of growth hormone treated rats and the effect of growth hormone treatment on the activities of the enzymes concerned with glycogenesis in rat cerebral cortex.     

Synthesis of adenine nucleotides from exogenous adenosine in the perfused rat heart under normoxic conditions and after ischemia

The rate of synthesis of myocardial adenine nucleotides from exogenous adenosine was studied in the isolated rat heart perfused under normoxic conditions and following ischaemia. The rate of incorporation of adenosine depended on the extracellular concentration of the precursor, following Michaelis-Menten kinetics with a apparent Km of 51.3 microM and a maximal rate of incorporation of about 1 100 nmol g-1 (wet wt.) 30 min-1. The adenosine uptake induced an increase in ATP concentration (+ 20%) when the exogenous concentration of precursor exceeded 10 microM. Following low-flow ischaemia (0.5 ml/min, 30 min), the rate of incorporation of 5 microM adenosine was diminished (-23%), but adenine nucleotide level restoration was favoured by the nucleoside administration. After total ischaemia (24 min), the extent of the decrease in adenosine incorporation was the same as in the case of moderate ischaemia but adenine nucleotide content was not restored.     

Glycogen as a fuel: metabolic interaction between glycogen and ATP catabolism in oxygen-independent muscle contraction

The main role of muscular oxygen-independent glycolysis, starting from glycogen as the initial substrate, is the production of three ATP molecules from ADP and P_i per glucosyl moiety transformed into two lactate molecules. During this catabolic process not only there is no proton release, but one proton is consumed. Metabolic acidosis occurs because the three ATP molecules are immediately hydrolysed by myosin ATPase back to 3P_i and 3ADP, to sustain contraction. As a consequence of this ATP turnover, the ATP pool (~5?mmol?kg^?1 wet weight) should remain constant. However, a bulk of experimental evidence has clearly shown that depletion of the muscular ATP pool, and accumulation of ATP catabolites occur even during short sprint bouts. In the present article the interrelationship between glycogen and ATP catabolism in anaerobic contracting muscle is discussed. It is shown how myosin ATPase plays a role not only in the mechanisms of ATP recycling through glycogen anaerobic catabolism, but also in the process of ATP depletion.     

The effect of ischaemia and reperfusion on sarcolemmal inositol phospholipid and cytosolic inositol phosphate metabolism in the isolated perfused rat heart

In this study the mass of polyphosphoinositides as well as the turnover of [³H]inositol phospholipids and [³H]inositol phosphates during ischaemia and short periods of reperfusion were studied in the isolated perfused rat heart. Since the phosphoinositides located within the sarcolemma are precursors for release of inositoltrisphosphate (InsP₃) and diacylglycerol, sarcolemmal membranes (rather than whole tissue) isolated at the end of the experimental procedure, were used. Hearts were prelabelled with [³H]inositol and subsequently perfused with 10 mM LiCI to block the phosphatidylinositol (PI) pathway. The results showed that 20 min of global ischaemia depressed the amount of [³H]inositol present in both sarcolemmal phosphatidylinositol-4-phosphate (PI-4-P) and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P₂), as well as in the cytosolic [³H]inositol phosphates, [³H]InsP₂ and [³H]InsP₃. The mass of the sarcolemmal inositol phospholipids remained unchanged during ischaemia. Reperfusion caused an immediate (within 30 sec) increase in the amount of [³H]inositol in sarcolemmal PI, PI-4-P and PI-4,5-P₂. PI-4-P levels showed a transient increase after 30 seconds postischaemic reperfusion, while the mass of the other sarcolemmal inositol phospholipids, PI and PI-4,5-P₂, remained unchanged. [³H]Insp, [³H]InsP₂ and [³H]InsP₃ also increased significantly in comparison to ischaemic hearts after only 30 sec postischaemic reperfusion.In summary, the results obtained indicate inhibition of the PI pathway during ischaemia with an immediate significant stimulation upon reperfusion. In view of the capacity of InsP3 to mobilize Ca²⁺ the possibility exists that stimulation of this pathway during reperfusion may play a role in the intracellular Ca²⁺ overload, characteristic of postischaemic reperfusion.     

The effect of ischaemia-reperfusion on [3H]inositol phosphates and Ins(1,4,5)P3 levels in cardiac atria and ventricles — a comparative study

In this study incorporation of [³H]inositol into inositol phosphates and phosphoinositides as well as tissue Ins(1,4,5)P₃ levels of the atria and ventricles of isolated, perfused rat hearts were compared. Although the incorporation of [³H]inositol into the phosphoinositides of atria and ventricles was similar, significantly higher (2–3 fold) incorporation rates into inositol phosphates were observed in atrial tissue. Using a D-myo-[³H]Ins(1,4,5)P₃ assay system, the Ins(1,4,5)P₃ levels observed in atria from perfused rat hearts were also significantly higher than those obtained under the same experimental circumstances in the ventricles.Since previous studies on whole hearts showed inhibition of the phosphatidylinositol (PI) pathway during ischaemia with an immediate significant stimulation upon reperfusion [12, 20], the effects of ischaemia and 1 min postischaemic reperfusion were also examined separately in atria and ventricles. The results showed that 20 min of global ischaemia significantly depressed Ins(1,4,5)P₃ levels as well as incorporation of [3H]inositol into ventricular InSP₂ and InSP₃. Reperfusion caused an immediate (within 1 min) increase in Ins(1,4,5)P₃ levels and also [³H]inositol incorporation into all three cytosolic inositol phosphates in the ventricles. However, the effect of ischaemia and reperfusion on Ins(1,4,5)P₃ levels as well as the incorporation of [³H]inositol into the inositol phosphates were less prominent in the atria. It therefore appears that the differential responses of the atria and the ventricles to an oxygen deficiency [41] are also reflected in the differences in PI metabolism during ischaemia-reperfusion.     

Heart glycogen content and isoprenaline-induced myocardial lesions

The role of glycogen content in the heart for the development of isoprenaline-induced myocardial lesions (IML) was studied in Wistar rats and in two inbred rat strains: In IR rats (resistant to the development of IML) and in IS rats (sensitive to IML development).Glycogen content in the heart can be dramatically lowered or increased by various interventions. IML develop during the period of very low heart glycogen content (about 0.6 mg.g^–1) induced by isoprenaline administration. In animals with increased resistance to IML, either due to genetic factors or induced by isoprenaline pretreatment a high glycogen content in the heart is found (up to 7.5 mg.g^–1).The increase of resistance to IML development and increased glycogen content induced by isoprenaline pretreatment were accompanied by lower basal or ISO-, guanylylimidodiphosphate- (Gpp/NH/p) and forskolin-stimulated activities of adenylyl cyclase. On the other hand, these parameters did not differ between IR and IS rats in spite of the presence of significant differences in the resistance to the development of IML and in heart glycogen content in these two rats strains.These results suggest that genetically determined differences between two inbred rat strains in the resistance of the heart to the development of IML and in the heart glycogen content are caused by factors which are independent of the receptor-adenylyl cyclase complex and are therefore different from those involved in the increase of resistance and glycogen content due to isoprenaline pretreatment.