Regulation of lipolysis in normoxic and hypoxic rat myocytes

Abstract

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 < 0.05, 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 < 0.05, n = 12 ) in hypoxic myocytes. On a molar basis glycerol output was significantly higher than the corresponding fall in TG ( P < 0.05, 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 < 0.05, 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 < 0.05). This indicates mass action of glycerol-3-phosphate on fatty acid-TG cycling under these conditions. There was no difference in the cellular content of cAMP between hypoxic and normoxic myocytes, and so the increased rate of lipolysis in response to hypoxia was not due to acivation of lipases through cAMP-dependent protein kinases. Furthermore, isoprenaline-induced glycerol output, both in normoxic and hypoxic myocytes, was mediated through an increased production of cellular glycerol-3-phosphate. Glycerol output was not reduced by adding 0.3 or 1.2 m 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.