Possible role of phospholipase C in the induction of Ca(2+)-paradox in rat heart.
In order to investigate the involvement of phosphoinositide-specific phospholipase C (PLC), an enzyme associated with phosphoinositide signal transduction pathway, for the occurrence of Ca(2+)-paradox (loss of contractile activity associated with contracture), rat hearts perfused with Ca(2+)-free medium (1 to 5 min) were reperfused (5 to 10 min) with medium containing 1.25 mM Ca2+. Crude membranes isolated from hearts perfused with Ca(2+)-free medium exhibited a significantly increased activity of PLC, whereas normal activity was detected in hearts reperfused with Ca(2+)-containing medium. A significant rise in PLC activity was observed at 1 min of Ca(2+)-free perfusion; maximal increase was seen at 4 min of Ca(2+)-free perfusion. Minimal concentration of Ca2+ in the perfusion medium required for showing an increase in PLC activity was 10 microM, whereas that required for the occurrence of Ca(2+)-paradoxic changes in heart function upon reperfusion was 50 microM. Perfusion of the hearts with Ca(2+)-free medium in the presence of low Na+ or at low temperature, which prevents the occurrence of Ca(2+)-paradox upon reperfusion, did not prevent the increase in PLC activity. An increase during Ca(2+)-free perfusion similar to that seen for PLC was also observed for two other enzymes, namely the phosphatidylinositol (PI) 4-kinase and the PI-4-monophosphate (PIP) 5-kinase, which synthesize the PLC substrate, phosphatidylinositol 4,5-bisphosphate (PIP2). No alteration of the alpha-adrenoreceptors was observed after 5 min of Ca(2+)-free perfusion. On the other hand, the observed changes in PLC activity during Ca(2+)-free perfusion appear to be due to some redistribution of the enzyme in the myocardium. These results suggest a possible role of the phosphoinositide/PLC pathway in the induction of Ca(2+)-paradox via mechanisms which do not appear to be associated with changes in the characteristics of alpha-adrenergic receptors.
Division of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, Manitoba, Canada.
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S
Persad
A
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Vrbanova
J T
JT
Meij
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V
Panagia
N S
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Dhalla
D000818:Animals; D002118:Calcium / Q000502:physiology*; D008297:Male; D009200:Myocardial Contraction / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D010738:Type C Phospholipases / Q000502:physiology*
D011942:Receptors, Adrenergic, alpha; D010738:Type C Phospholipases; D002118:Calcium