GROWTH-HORMONE (GH) BINDING-PROTEIN AND GH INTERACTIONS INVIVO IN THE GUINEA-PIG

GH binding proteins (GHBPs) are present in the blood of several species and by complexing with circulating GH may alter its clearance and distribution. The actions of human GHBP (hGHBP) have hitherto been studied indirectly via their effects on the clearance of hGH. In the present experiments, recombinant preparations of hGHBP and a shorter variant lacking exon 3 (DELTA3hGHBP) were tested in vivo, either alone or after preincubation with recombinant hGH, recombinant 20K methionyl-hGH, or rat GH. Multiple serial blood sampling was performed in conscious chronically cannulated guinea pigs and both GHBP and GH variants monitored in the same samples by specific RIAs, unaffected by endogenous activities in this species. hGHBP, DELTA3hGHBP, hGH, and 20K met-hGH (10-40 mug) were all cleared rapidly when injected alone (t1/2 = 11-20 min). However, when the same amounts of hGHBP and hGH were incubated together for 60 min and then injected, the clearance of both proteins was greatly prolonged (t1/2 hGHBP = 75-94 min, hGH = 80-137 min). Similar results were obtained for hGH complexed with DELTA3hGHBP, and over a range of GH:GHBP ratios from 0.3:1 to 4:1. The effect was specific for 22K hGH; neither rat GH nor 20K met-hGH altered hGHBP clearance. nor were their clearances slowed by preincubation with hGHBP. Rapid complex formation also could be demonstrated in vivo. Injections of hGH 30 min after DELTA3hGHBP, or vice versa, altered the established plasma disappearance curve of hGHBP, and hGH clearance was slowed in parallel. During a continuous infusion of hGH to steady state, injections of DELTA3hGHBP increased plateau hGH concentrations by forming a DELTA3hGHBP/hGH complex in the circulation. These experiments show that the conscious chronically cannulated guinea pig provides a useful model in which the in vivo interaction between hGHBP and hGH can be studied dynamically. The results imply that the location, extent, and rate of complex formation between GHBP and GH may be an important determinant of the passage of GH between the intra- and extravascular compartments, which could affect the pattern of tissue exposure to GH.