RENATURALIZING THE SEXUALLY DIMORPHIC PROFILES OF CIRCULATING GROWTH-HORMONE IN HYPOPHYSECTOMIZED RATS

Hypophysectomy resulted in a total elimination of measurable circulating growth hormone with an associated loss of body weight gain. The typical sexually dimorphic plasma growth hormone patterns: pulsatile profiles in male rats and tonic-like secretion in female rats, were lost. The male- and female-dependent profiles of plasma growth hormone, monitored from serial blood collections, were restored by administering the hormone through a single electrically controlled external pump attached to an indwelling catheter, and by implanting osmotic pumps intraperitoneally, respectively. Restoring the natural patterns of plasma growth hormone in animals devoid of pituitaries, re-initiated body growth. However, the body weight gains in both sexes of hypophysectomized rats were much greater when rat growth hormone was introduced to the animals in a masculine (pulsatile) pattern that appeared to be independent of pulse frequency, rather than in a continuous feminine profile. Subcutaneous injections, the most commonly reported method of administration, produced low-amplitude, long-lasting plasma peaks that were not as effective as pulse infusion in restoring growth. The procedure allows manipulation of the hormone profile (i.e. number of pulses/day, pulse amplitude, and through duration in the pulsatile pattern, and plasma concentration in the tonic pattern) in order to identify, and thus study the presumed salient components of the pattern regulating growth hormone responses.