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The mechanism involved in ischemia-induced myocardial lipolysis is still a matter of controversy. To elucidate the regulation of lipolysis at the cellular level, we incubated isolated rat myocytes in normoxic or hypoxic medium containing 11.1 m m glucose. Rates of lipolysis (glycerol output) were significantly ( P n = 12) higher in hypoxic than in normoxic myocytes ( 34.9 ± 3.9 vs. 17.7 ± 3.4 nmol 10 6 cells·30 min ). However, there was no change in the content of cellular triacylglycerol (TG) in normoxic myocytes whereas it fell slightly ( 8 ± 2 nmol 10 6 cells·30 min , P ) in hypoxic myocytes. On a molar basis glycerol output was significantly higher than the corresponding fall in TG ( P n = 12, both normoxic and hypoxic myocytes). This difference (glycerol output — TG reduction) amounted to 17.1 ± 3.4 nmol 10 6 cells·30 min in normoxic myocytes and 27.6 ± 5.1 nmol 10 6 cells·30 min in hypoxic myocytes ( P n = 12, normoxic vs. hypoxic). The hypoxia-induced rise in glycerol output was paralleled by an increased intracellular level of glycerol-3-phosphate. Both these responses were, however, dose-dependently inhibited by addition of pyruvate to the incubation medium, giving rise to a close correlation between cellular glycerol-3-phosphate and glycerol output ( r = 0.75, P m oleic acid to the incubation medium, and therefore product inhibition of lipases was not evident. These results show that the rate of glycolysis is a primary regulatory factor of lipolysis (glycerol output) in oxygen-deprived myocytes. They also imply that the lipolytic effect of isoprenaline is secondary to an increased supply of glycerol-3-phosphate.

The induction of heat shock proteins in the myocardium has been suggested as a possible intervention to allow for enhanced cardioprotection. We have postulated that a brief period of retrograde hyperthermic perfusion would be sufficient to induce Hsp 70 mRNA and protein accumulation. To investigate this hypothesis, rat hearts (n = 45) were perfused at 37 degrees C for 30 min, then perfused for 15 min at 42 degrees C, and allowed to recover at 37 degrees C for 120 min. Control hearts (n = 23) were perfused at 37 degrees C continuously. Northern analysis indicated that in hearts treated with a brief period of retrograde hyperthermic perfusion Hsp 70 mRNA levels were increased 2.85 +/- 0.02-fold (P0.01) by 10 min, 4.88 +/- 0.94-fold (P0.01) by 15 min, 9.19 +/- 0.62-fold (P0.001) by 30 min, 9.4 +/- 0.52-fold (P0.001) by 60 min, 9.45 +/- 0.57-fold (P0.001) by 90 min and 9.66 +/- 0.99-fold (P0.001) by 120 min of normothermic recovery as compared to control hearts. Western analysis revealed that the heat inducible Hsp 72 kDa protein was increased 2.37 +/- 0.45-fold (P0.01) at 60 min, 2.53 +/- 0.25-fold (P0.01) at 90 min, and 2.7 +/- 0.6-fold (P0.01) at 120 min when compared to control hearts. Our results indicate that the induction of the heat shock protein Hsp 70 can be rapidly achieved through retrograde hyperthermic perfusion of the myocardium.

We have recently shown that a triacylglycerol (TG)-fatty acid cycle is operating in rat myocardial cells incubated in a hypoxic, glucose-containing incubation medium (Myrmel et al., 1991a). In the present study we investigated whether this cycle occurred in hypoxic, glucose-deprived myocytes, and whether high TG levels would increase TG-fatty acid cycling and thereby energy consumption. Myocytes with elevated contents of TG were obtained from the hearts of streptozotocin-induced diabetic rats (diabetic myocytes) and from normal rat myocytes prepared in the presence of oleic acid (TG-loaded myocytes). The TG content of diabetic and TG-loaded myocytes prior to hypoxic incubations was more than two times higher (P0.05) than that of their respective controls (123.8 +/- 20.6 and 125.3 +/- 12.7 vs 56.8 +/- 6.0 and 58.6 +/- 9.4 nmol/10(6) cells, mean +/- S.E., n = 7). Only diabetic and TG-loaded myocytes expressed marked reductions in TG content during glucose free incubations. There were no differences in TG-fatty acid cycling between the various myocyte groups, calculated as the difference between glycerol output and the concomitant decrease in TG (range: 36.7 +/- 8.1- 48.9 +/- 9.7 nmol TG/10(6) cells.2h). Apparently, the cycle was continuous throughout the whole incubation period despite falling ATP levels, contracture (rounding up) of myocytes, as well as cessation of glycogenolysis after about 40 min incubation. The cellular content of glycerol-3-phosphate, known to control TG-fatty acid cycling, increased continuously and to the same extent throughout the 2 h incubation period. Futile energy consumption associated with TG-fatty acid cycling, amounted to approximately 30% of total cellular energy consumption for the whole incubation period. In conclusion, hypoxic glucose deprived rat myocytes show TG-fatty-acid cycling, even after cessation of glycogenolysis. The extent of cycling, and thus the energy cost associated with it, was not influenced by the initial level of TG in the myocytes. We propose that glycerol-3-phosphate needed to fuel the TG-fatty acid cycle after exhaustion of the glycolytic supply is derived from phospholipid degradation.