FREQUENCY OF LUTEINIZING-HORMONE PULSES AND CIRCULATING 17-BETA-ESTRADIOL CONCENTRATION IN COWS IS RELATED TO CONCENTRATION OF PROGESTERONEIN CIRCULATION WHEN THE PROGESTERONE COMES FROM EITHER AN ENDOGENOUS OR EXOGENOUS SOURCE

The present study was designed to test whether progesterone from exoge nous and endogenous sources modulates the frequency of luteinizing hor mone (LH) pulses in a differential fashion. Our working hypotheses wer e: (1) the frequency of secretion of LH pulses and concentration of 17 beta-oestradiol in circulation would be directly related to circulati ng concentration of progesterone; (2) progesterone from an exogenous s ource would have a different effect on frequency of LH pulses and circ ulating concentration of 17 beta-oestradiol than progesterone from an endogenous source. Cows were treated with graded doses of PGF(2 alpha) or progesterone to result in varying concentrations of progesterone i n circulation from endogenous and exogenous sources. Treatment with PG F(2 alpha) resulted in concentrations of circulating progesterone amon g animals in this group that ranged from 0.5 to 10.9 ng ml(-1) of plas ma. Treatment with progesterone resulted in circulating concentrations of progesterone among animals in this group that ranged from 0.8 to 1 1.8 ng ml(-1) of plasma. Concentrations of progesterone in circulation and frequency of LH pulses were negatively correlated (r = -0.59) in cows with functional corpora lutea and in cows treated with progestero ne without corpora lutea (r = -0.62). The frequency of LH pulses had t he same relationship to the varying concentrations of progesterone in both treatment groups. Cows with higher progesterone levels had fewer LH pulses and cows with lower progesterone had a greater frequency of LH pulses. Mean circulating concentrations of 17 beta-oestradiol also had the same relationship with varying concentrations of progesterone in both treatment groups. Cows with higher progesterone levels had low er 17 beta-oestradiol and cows with lower progesterone had higher 17 b eta-oestradiol levels. The results indicate that there is a direct rel ationship between circulating concentration of progesterone, frequency of LH pulses and circulating concentration of 17 beta-oestradiol. In addition, progesterone modulates the frequency of release of LH pulses from the pituitary and circulating concentration of 17 beta-oestradio l in a similar fashion when coming from either an endogenous or exogen ous source.