In vivo fluxes of plasma cholesterol, phosphatidylcholine and protein into mini-pig aortic and pulmonary segments

Abstract

Levels of radioactive cholesterol found in the arterial wall after a few hours' in vivo exposure to plasma containing labelled cholesterol are so small that the contaminating plasma in the tissue may contribute significantly to the radioactivity measured in the wall. By the simultaneous use of [ 3H]- and [ 14C]cholesterol the contamination was found to be less than 15 nl of plasma/cm 2 of intimal surface of the thoracic aorta in mini-pigs, and this contamination accounted for less than 5% of the total radioactivity found in the intima—media tissue 7 h after the injection of serum with bioincorporated labelled cholesterol. The radioactivity in the arterial wall divided by the area below the plasma radioactivity versus the time curve was named the intimal clearance. For a particular mini-pig 2 nearly identical estimates were obtained for the intimal clearance of labelled plasma cholesterol, using an influx period of about 4 h for [ 3H]cholesterol and 7 h for [ l4C]cholesterol. This indicates a negligible loss of labelled cholesterol from the wall during a 7-h period. The intimal clearances as calculated for labelled plasma cholesterol were about 500, 200, 100 and 50 nl cm −2 h −1 for pulmonary trunk, aortic arch, proximal and distal thoracic aorta, respectively. The concomitantly measured intimal clearances of plasma [ 32P]phosphatidylcholine were about 1.1 times those for plasma cholesterol for the 4 segments. This suggests that the appearance of labelled plasma phosphatidylcholine and of cholesterol in the intima—media is essentially a consequence of a lipoprotein influx. Similarly, the intimal clearances of plasma [ 35S]albumin for the same 4 segments were about 1.5 times those of the labelled lipids, suggesting a non-specific mechanism for the transfer of plasma lipoproteins into the artery. The intimal clearances of [ 35S]albumin were within the same order of magnitude as capillary clearance values for albumin reported for mammalian tissues.