Metabolic clearance and plasma disappearance rates of human pancreatic tumor growth hormone releasing factor in man.

The metabolic clearance rate (MCR) and plasma disappearance rate (t1/2) of human pancreatic tumor growth hormone releasing factor [hpGRF(1-40)] was determined in normal adult male subjects by single injection and constant infusion techniques. Single injections of 1, 3.3, and 10 micrograms/kg hpGRF(1-40) were administered intravenously, plasma immunoreactive (IR) GRF levels were measured during the subsequent 180 min, and biexponential curve analysis was performed. Graded, dose-constant infusions of hpGRF(1-40) at rates of 1, 3.3, 10, and 33 ng/kg per min were administered and the MCR was calculated from measurement of steady state plasma IR-GRF levels at each infusion rate. The postinfusion disappearance rate was determined by linear regression analysis of plasma IR-GRF levels during the 120-min period after cessation of the infusion. The calculated MCR during the single injection study was 194 +/- 17.5 liters/m2 per d and was not significantly different from the calculated value during the constant infusion study (202 +/- 16 liters/m2 per d). The disappearance rate during the single injection study was subdivided into two linear phases: an initial equilibration phase (7.6 +/- 1.2 min) and a subsequent elimination phase (51.8 +/- 5.4 min). The latter was similar to the linear disappearance rate observed (41.3 +/- 3.0 min) after cessation of the constant infusion. The chromatographic and biologic characteristics of plasma IR-GRF, 30 min after injection, were similar to those of synthetic hpGRF(1-40). The results have been discussed in relation to the MCR of other hypothalamic hormones and have been used to extrapolate secretion rates of GRF in patients with ectopic GRF production.