Instability of endothelium-bound lipoprotein lipase activity in perfused rat hearts

The enzymatic activity of endothelium-bound lipoprotein lipase was measured in rat hearts perfused for 1 h at 37 °C. Viability parameters such as beating rate, flow rate, aortic pressure, and oxygen consumption were all kept strictly constant during the entire perfusion time. The lipase activity was determined by input–output difference of the triacylglycerol content in the nonrecirculating heart perfusate which contained either an artificial triolein emulsion or rat lymph chylomicrons. With either substrate, the lipase activity decreased with time: approximately 2% of initial lipase activity was lost per minute. The presence of rat serum (10%) in heart perfusate enhanced the rate at which the lipase activity disappeared. Only a small portion of the lipoprotein lipase activity, which was lost from the perfused heart, was recovered in the outflow perfusate. Our data demonstrate that, under our experimental conditions, the enzymatic activity of rat heart lipoprotein lipase does not remain constant during heart perfusion. Caution should therefore be taken by users of heart perfusion technique, especially for those who need constant lipoprotein lipase activity. The results suggest that the first step in the catabolic fate of endothelium-bound cardiac lipoprotein lipase is the loss of its catalytic activity. Whether the inactivated enzyme is released from the endothelium or remains in situ is not yet known.