Osteocalcin metabolism in the pulmonary circulation

The aim of this study was to evaluate the role of the pulmonary vessel endothelium in the removal of circulating osteocalcin, by measuring the osteocalcin levels in serum from pulmonary and radial artery blood from 39 patients undergoing aorto-coronary bypass. Because of the discrepancies between methods of measurement, two methods were used. Significant differences were observed in group A (n = 18), tested with heterologous radioimmunoassay (2.85 +/- 0.67 mu g l(-1) in the pulmonary versus 2.69 +/- 0.67 mu g l(-1) in the radial artery serum, P < 0.001) and in group B (n = 21), tested with a two-site immunoradiometric assay (5.22 +/- 1.46 versus 4.93 +/- 1.36 mu g l(-1), P < 0.01). The percentage differences were -5.54 +/- 4.76% (P < 0.001) in group A and -4.99 +/- 8.13% (P < 0.01) in group B; the comparison between the percentage differences was not significant. These different osteocalcin concentrations between the two vascular compartments were considered a marker of osteocalcin degradation. Therefore, the study seems to demonstrate that, as well as kidney, liver and bone, the lung is a relevant site of osteocalcin catabolism. The proteolytic activity of pulmonary vessel endothelium seems to involve about 5% of the circulating peptide.