Department of Physiological Sciences

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Browsing Department of Physiological Sciences by browse.metadata.advisor "Huisamen, Barbara"

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    The signaling pathways involved in the cardioprotection offered by insulin to the global low flow ischaemic/reperfused myocardium
    (Stellenbosch : Stellenbosch University, 2001-12) Louw, Rehette; Huisamen, Barbara; Van Rooyen, J.; Stellenbosch University. Faculty of Science. Dept. of Physiologi​cal Sci​ences.
    ENGLISH ABSTRACT: Introduction: It is well documented that insulin offers cardioprotection under ischaemic stress. In the past it was believed that the protective effects of insulin, such as the (a) recruitment of glucose transporters to enhance glucose entry into the cell, (b) stimulation of glycolysis, (c) enhancement of glycogen synthesis, (d) improved protein synthesis, and (e) positive inotropic and chronotropic properties, were metabolic of origin, but lately the emphasis has shifted towards the diverse signal transduction pathways elicited by insulin. Although these beneficial effects of insulin on ischaemia/reperfusion induced injury have been studied for many years, the exact protective mechanism is still not resolved. Aim: To investigate the influence of insulin on the signaling pathways as a possible protective mechanism against ischaemia/reperfusion and therefore to investigate the possible roles and cross signaling of cyclic adenosine monophosphate (cAMP), protein kinase B (PKB) and p38 mitogen activated protein kinase (p38 MAPK) in the cardioprotection offered by insulin to the reperfused, ischaemic myocardium. Materials and methods: Isolated rat hearts were perfused retrogradely in accordance with the Langendorff technique (95%02, 5% C02). After 30 min of stabilization, hearts were subjected to 30 min global low flow ischaemia (0,2 ml/min), followed by 30 min of reperfusion. Hearts perfused with standard Krebs Henseleit solution containing 5 mM glucose were compared to hearts perfused with a perfusion solution containing 5 mM glucose and 0,3 IlIU/ml insulin. Wortmannin was added during either ischaemia or reperfusion. Left ventricular developed pressure (LVDP), rate pressure product (RPP), tissue cAMP and PKB and p38 MAPK activation were measured. Results: Insulin treated hearts showed improved functional recovery (P<0.05) during reperfusion after ischaemia vs. non-insulin treated hearts (85.5±4.6% vs. 44.8±4.9%). However, the addition of wortmannin (a Pl3-kinase inhibitor) to the perfusion solution during either ischaemia or reperfusion abolished the improved recovery. At the end of ischaemia, cAMP levels of the insulin treated hearts were elevated significantly, while the cAMP content in the non-insulin treated hearts returned to control levels. Addition of wortmannin during ischaemia abolished this rise in cAMP. Wortmannin added during reperfusion only did not alter the levels of cAMP at the end of reperfusion. Activation of p38 MAPK was transient during ischaemia for both insulin and non-insulin treated hearts. Addition of wortmannin during ischaemia did not alter p38 MAPK levels at the end of ischaemia. P38 MAPK was activated significantly (P<0.001) in the non-insulin treated hearts vs. insulin treated hearts during reperfusion. Wortmannin, added at the onset of reperfusion, could partially abolish the effects of insulin to suppress p38 MAPK activation after 30 min of reperfusion. Activation of PKB in insulin treated hearts was significantly higher than non-insulin treated hearts during stabilization and early ischaemia. This activity was depressed by 30 min of ischaemia in both presence and absence of insulin. Wortmannin, when added before induction of ischaemia did not further lower this. The presence of insulin resulted in occurrence of strong PKB activation during reperfusion, peaking at 15 minutes and diminishing at 30 minutes. Wortmannin, added at the onset of reperfusion, abolished PKB activity measured at the end of reperfusion. Conclusion: Insulin exerted a positive inotropic effect and delayed the onset to ischaemic contracture. Inhibition of Pl3-kinase by wortmannin abolished the protective effects of insulin, arguing for an insulin stimulated PKB involvement in cardiac protection. Insulin also increased cAMP production and attenuated activation of p38 MAPK, both associated with improved recovery. This evidence suggested possible cross signaling between different signaling pathways.