Lipoprotein lipase activity is stimulated by insulin and dexamethasone in cardiomyocytes from diabetic rats

Abstract

Type 1 diabetes mellitus reduces lipoprotein lipase (LPL) activity in the heart. The diabetic phenotype of decreased LPL activity in freshly isolated cardiomyocytes persisted after overnight culture (16 h). Total cellular LPL activity was 311 ± 56 nmol oleate released·h-1·mg-1cell protein in diabetic cultured cardiomyocytes compared with 661 ± 81 nmol oleate released·h-1·mg-1cell protein for control cultured cells. Diabetes also resulted in lower heparin-releasable (HR) LPL activity compared with control cells (111 ± 25 vs. 432 ± 63 nmol·h-1·mg-1cell protein). In kinetic experiments, the reduction in total cellular LPL and HR-LPL activities in cultured cells from diabetic hearts was due to a decrease in maximal velocity, with no change in apparent Kmfor substrate (triolein). LPL activity in primary cultures of cardiomyocytes from control rats is stimulated by the combination of insulin (Ins) and dexamethasone (Dex). Overnight treatment of cultured cardiomyocytes from diabetic rats with Ins+Dex elicited an 84% increase in cellular LPL activity (to 572 ± 65 nmol·h-1·mg-1cell protein) and a 194% increase in HR-LPL activity (to 326 ± 46 nmol·h-1·mg-1cell protein). This stimulation occurred at subnanomolar concentrations of the hormones, but neither hormone was effective alone. The amount of immunoreactive LPL protein mass in cultured cardiomyocytes from diabetic hearts was unchanged by Ins+Dex treatment. Addition of oleic acid (60 µM) to the overnight culture medium inhibited the already reduced HR-LPL activity in diabetic cultured cells by 73% (to 30 ± 4 nmol·h-1·mg-1cell protein). The presence of oleic acid also reduced hormone-stimulated HR-LPL activity. Increasing the glucose concentration in the culture medium to 26 mM had no effect on total cellular LPL or HR-LPL activities.Key words: lipoprotein lipase, cardiomyocytes, diabetes.