title: Negative feedback control of LH by testosterone: a quantitative study in male rats.
abstract: Adult male rats were castrated and implanted sc with testosterone (T)-filled Silastic capsules of different sizes which produce stable plasma T levels proportional to capsule length.  T and LH levels were determined in blood samples obtained at 3-day intervals for 21 days.  Plasma T values greater than 1.8 ng/ml suppressed plasma LH to levels below the intact range, while a plasma T of less than 0.5 ng/ml yeilded no significant inhibition of the post-castration LH rise.  Animals with a plasma T of about 0.7 ng/ml, however, showed intact, castrate, or immediate LH levels.  The feedback-effective range for all animals was between 0.5 ngT/mg (castrate-like LH response) and 1.8 ng/ml (LH at or below intact levels), but individual rats seemed to have a more restricted feedback range than that indicated by these group data.  To test this hypothesis further, rats were implanted at the time of castration with five 5-mm T-filled capsules.  One capsule was removed each week, producing step function drops in plasma T, and allowing examination of the LH responses of individual animals.  Plasma t levels greater than 1.6 ng/ml maintained LH values at or below intact levels.  However, when plasma T dropped to 1.1 ng/ml, some rats showed castration-like LH increase and others did not respond until after a further drop to 0.6 ng/ml.  It was concluded that the feedback-effective range of plasma T is very restricted and differs in individual rats.
mesh_terms: D000818:Animals; D002369:Castration; D005246:Feedback; D007986:Luteinizing Hormone / Q000097:blood*; D008297:Male; D051381:Rats; D013739:Testosterone / Q000097:blood / Q000494:pharmacology / Q000502:physiology*; D013997:Time Factors
chemical_list: D013739:Testosterone; D007986:Luteinizing Hormone