Kinetics of removal of intravenous testosterone pulses in normal men.
BACKGROUND
Testosterone is secreted into the bloodstream episodically, putatively distributing into total, bioavailable (bio) nonsex hormone-binding globulin (nonSHBG-bound), and free testosterone moieties. The kinetics of total, bio, and free testosterone pulses are unknown. Design Adrenal and gonadal steroidogenesis was blocked pharmacologically, glucocorticoid was replaced, and testosterone was infused in pulses in four distinct doses in 14 healthy men under two different paradigms (a total of 220 testosterone pulses).


METHODS
Testosterone kinetics were assessed by deconvolution analysis of total, free, bioavailable, SHBG-bound, and albumin-bound testosterone concentration-time profiles.


RESULTS
Independently of testosterone dose or paradigm, rapid-phase half-lives (min) of total, free, bioavailable, SHBG-bound, and albumin-bound testosterone were comparable at 1.4+/-0.22 min (grand mean+/-S.E.M. of geometric means). Slow-phase testosterone half-lives were highest for SHBG-bound testosterone (32 min) and total testosterone (27 min) with the former exceeding that of free testosterone (18 min), bioavailable testosterone (14 min), and albumin-bound testosterone (18 min; P<0.001). Collective outcomes indicate that i) the rapid phase of testosterone disappearance from point sampling in the circulation is not explained by testosterone dose; ii) SHBG-bound testosterone and total testosterone kinetics are prolonged; and iii) the half-lives of bioavailable, albumin-bound, and free testosterone are short.


CONCLUSIONS
A frequent-sampling strategy comprising an experimental hormone clamp, estimation of hormone concentrations as bound and free moieties, mimicry of physiological pulses, and deconvolution analysis may have utility in estimating the in vivo kinetics of other hormones, substrates, and metabolites.
Veldhuis
Johannes D
JD
Endocrine Research Unit, Mayo School of Graduate Medical Education, Clinical Translational Science Center, Mayo Clinic, Rochester, Minnesota 55905, USA. veldhuis.johannes@mayo.edu
Keenan
Daniel M
DM
Liu
Peter Y
PY
Takahashi
Paul Y
PY
D000328:Adult; D002786:Cholesterol Side-Chain Cleavage Enzyme / Q000037:antagonists & inhibitors / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D004958:Estradiol / Q000097:blood; D006207:Half-Life; D006727:Hormone Antagonists / Q000494:pharmacology; D006801:Humans; D007262:Infusions, Intravenous; D007654:Ketoconazole / Q000494:pharmacology; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D013739:Testosterone / Q000008:administration & dosage* / Q000097:blood* / Q000378:metabolism; D055815:Young Adult
D006727:Hormone Antagonists; D013739:Testosterone; D004958:Estradiol; D003907:Dexamethasone; D002786:Cholesterol Side-Chain Cleavage Enzyme; D007654:Ketoconazole