Adenohypophyseal receptors for LHRH, pituitary content of gonadotropins and plasma concentrations of LH in cyclic, pregnant and postpartum beef cows.

Adenohypophyseal concentrations of LHRH receptors, pituitary content of LH and FSH, and plasma concentrations of LH were determined in thirty Hereford, Angus or Hereford-Angus heifers that were randomly assigned by breed and weight to five periods including day 3 of the estrous cycle (CY), pregnant day 120 (P120), 200 (P200), 275 (P275), or day 2 postpartum (PP). Jugular blood samples were collected at 10-min intervals for 8 hr from all cows. Within 2 hr after completion of blood sampling, animals were slaughtered and the pituitary gland frozen at -196 C. LH pulse frequency/8 hr was reduced (P less than .05) during gestation (.5, .2, and 1.5 +/- .5/8 hr, for P120, P200, and P275, respectively) and PP (.5 +/- .5/8 hr) compared to CY (7.8 +/- .5/8 hr). Frequency of LH pulses/8 hr was not different (P greater than .1) among P120, P200 or PP periods but was different (P less than .05) between P200 and P275. There were no differences in LH pulse height (P greater than .1) among periods; however, pulse amplitude was greatest (P less than .05) at P120 (1.3 +/- .2 ng/ml) and lowest between P200 and PP (.6 to .8 +/- .2 ng/ml). Baseline concentrations of plasma LH did not differ (P greater than .1) among P and PP periods (.3 +/- .1 ng/ml), but were lower (P less than .05) than in CY animals (.7 +/- .1 ng/ml). Concentration of adenohypophyseal LHRH receptors was approximately two-fold greater (P less than .05) at P120 (25.85 +/- 2.2 fmol/mg) than at all other periods (9.5 to 14.9 +/- 2.2 fmol/mg).(ABSTRACT TRUNCATED AT 250 WORDS)