ATTENUATION OF MYOCARDIAL ISCHEMIC-INJURY 24 H AFTER DIACYLGLYCEROL TREATMENT IN-VIVO

Previous work suggests that delayed protection against infarction foll owing ischaemic preconditioning or rabbit myocardium may involve the a ctivation of protein kinase C (PKC). Preconditioning in the presence o f chelerythrine, an inhibitor of PKC, abolished the late anti-infarct effect of preconditioning. In the studies described here, we tested th e hypothesis that direct PI(C activation with an analogue of diacylgly cerol, the physiological activator of PI(C, would invoke an adaptive m echanism leading to enhanced myocardial tolerance to ischaemia 24 h la ter. Rabbits were treated with i.v. injections of 1,2-dioctanoyl-sn-gl ycerol (DiC8) 5 mg/kg or 15 mg/kg or an equivalent volume of vehicle s olution. Twenty-four h after pretreatment, the animals were anaestheti sed and underwent 30 min coronary artery occlusion with 120 min reperf usion. Infarct size was determined by triphenyltetrazolium staining. I n vehicle pretreated rabbits, infarct-risk zone ratio was 32.8 +/- 2.6 %. Pretreatment with DiC8 5 mg/kg did not significantly affect. infarc t size (26.3 +/- 4.0%), but pretreatment with DiC8 15 mg/kg resulted i n a marked reduction in infarct size (18.0 +/- 3.4%, P<0.05, I-way ANO VA). Reduction in infarct size with the higher dose of DiC8 was indepe ndent of myocardial risk zone size and systemic haemodynamic parameter s during coronary occlusion. The haemodynamic effects of acute adminis tration of DiC8 15 mg/kg were examined in a separate cohort of pentoba rbitone-anaesthetised rabbits. The compound was found not to affect sy stolic blood pressure or heart rate under these conditions, We examine d the possibility that increased ischaemic tolerance might be due to i nduction of the 27 and 72 kDa heat shock proteins (hsp27 and hsp70i) w hich are known to be cytoprotective and are upregulated by ischaemia a nd other stressful stimuli. Western blot analysis of left ventricular tissue revealed that neither protein was induced 24 h after treatment with DiC8 15 mg/kg. Thus, infarct limitation 24 h after DiC8 treatment did not appear to be due to increased tissue content of these protein s. The mechanisms of DiC8-induced delayed myocardial protection remain unclear. However, these data are compatible with the hypothesis that activation of PKC isoenzymes is an important intermediate signal of su b-acute cellular adaptation, and results in enhanced tolerance to isch aemia-reperfusion injury in myocardium many hours later. (C) 1997 Acad emic Press Limited.