Turnover of free and conjugated serum catecholamines during hemodialysis

The response of the catecholaminergic system was evaluated in uremic patients maintained on a chronic outpatient hemodialysis (HD) program. Serum concns of free and conjugated catecholamines (CA) were measured prior to and following HD and in response to upright posture, and were compared to values obtained in a normal group of individuals submitted to the same stimuli. Before HD, the level of free plasma dopamine (DA) (155 +/- 58 pg ml-1) was comparable to that of normal individuals (38 +/- 12 pg ml-1) due to the large number of individuals of the 2 groups having undetectable serum levels. Free norepinephrine (NE) and epinephrine (E) serum levels were higher, before HD, in uremic patients (301 +/- 31 and 139 +/- 43 pg ml-1) than in normal individuals (206 +/- 14 and 34 +/- 4 pg ml-1). Serum concns of the 3 forms of conjugated CA, before HD, in uremic patients were markedly elevated (35,800, 9644 and 1374 pg ml-1 for DA, NE and EPI, respectively), compared to those in normal individuals (1885, 1374 and 65 pg ml-1). Only the serum level of free NE was elevated (from 301 to 600 pg ml-1) due to the rapid fluid loss that occurs during HD. On the other hand, a drastic decrease in the serum level of conjugated DA (16,020 vs 35,794 pg ml-1), NE (4282 vs 10,596 pg ml-1) and E (688 vs 1294 pg ml-1) occurred during HD. It is suggested that uremic patients lose their normal catecholaminergic response to upright posture at the end of HD treatment as a result of a progressive depletion of their free CA storage that cannot be supplemented from the large quantity of conjugated CA still available in the serum under such circumstances.