Lack of hepatic metabolism of C-telopeptides of type I collagen

BACKGROUND

Bone biochemical markers have been used in dynamic studies of bone metabolism, and, for accurate interpretation of measured marker levels, it is essential to have information of extra-skeletal metabolism. Therefore, the objective of the present study was to investigate if the circulating C-telopeptides of type I collagen (CTX) was subject to hepatic extraction.

METHODS

Splanchnic plasma flow, total plasma volume and the plasma concentration of CTX was determined in an artery and the liver vein of eight healthy female volunteers. For comparison, the concentration of N-terminal propeptide of type I collagen, PINP, was measured.

RESULTS

No change in plasma level of CTX could be detected over the liver. In contrast, PINP decreased from an average of 52.9 ng/ml in the artery to 42.4 ng/ml in the vein, corresponding to a 19.8% reduction.

CONCLUSIONS

The C-telopeptides of type I collagen (CTX) are not subject to hepatic metabolism.

Splanchnic extraction and clearance of tissue plasminogen activator (t-PA) after injection of recombinant t-PA (Actilyse) in resting healthy subjects is proportional to the arterial concentration

The purpose of the study was to evaluate the splanchnic extraction of tissue plasminogen activator (t-PA) in normal healthy fasting subjects after the injection of recombinant t-PA (rt-PA; Actilyse). In nine healthy volunteers (five male, four female), 21-29 years of age, the concentration of t-PA was determined in plasma samples taken simultaneously from a femoral artery and a large liver vein after a bolus injection (5, 10 or 20 mg) of rt-PA. The splanchnic plasma flow rate, the plasma volume, and the splanchnic extraction fraction of t-PA were determined. After the rt-PA injection, the measured arterial concentration of t-PA decreased from 36 750 to 45 pmol/l for t-PA antigen and from 50 700 to 17 pmol/l for active t-PA. The splanchnic extraction fraction decreased from 0.95 to 0.02 for t-PA antigen and from 0.78 to 0.08 (n = 3) for active t-PA. The extraction fraction was proportional to the arterial concentration of t-PA when the arterial concentration of t-PA was above about 300 pmol/l (both t-PA antigen and active t-PA). The median splanchnic plasma flow rate was 911 ml/min (range, 651-1149 ml/min). In the individual subject, the splanchnic plasma flow rate remained constant during the experimental period. The main conclusion of the study is that the splanchnic clearance and extraction fraction of t-PA, following an injection of rt-PA in the resting fasting steady state, depends on the arterial concentration of t-PA. The higher the arterial concentration of t-PA, the higher the extraction fraction of t-PA.