Abstract
Colchicine injection was used as a tool to potentiate the increase in intracellular lipoprotein lipase (type L hormone-sensitive lipase) activity normally seen with fasting to determine if elevation of enzyme activity by this method produced a reduction in endogenous triacylglycerol (TG) in rat heart. Both fasting and fasting+colchicine treatment increased total lipoprotein lipase (LPL) activity from a control value of 80 units/g to approx. 144 units/g. The initial control value was obtained at 08:00 h after overnight feeding and the final values were obtained at 17:00 h, after 9 h of fasting. Fasting alone increased activity in both the capillary-bound LPL and type L hormone-sensitive lipase (HSL) fractions of cardiac muscle. In contrast, colchicine treatment, by blocking the export of enzyme from the cell as a result of microtubular disruption, restricted the increase in enzyme activity to the intracellular fraction of the heart. There was a highly significant (P less than 0.001) negative relationship (r = -0.73) between type L HSL activity and TG content in hearts of fasting and fasting+colchicine-treated rats. At a time when type L HSL activity was increased and TG content decreased, the cyclic AMP concentration of heart remained unchanged, ruling out the possibility that cyclic AMP might be activating any one of the identified cardiac TG lipases. These data provided indirect evidence that type L HSL is 'seeing the intracellular TG droplet' and that this enzyme may play a role in the regulation of myocardial lipolysis.