Endocrinology, Vol 122, 1751-1760, Copyright © 1988 by Endocrine Society
Relaxin and progesterone secretion as affected by luteinizing hormone and prolactin after hysterectomy in the pig
KJ Felder, J Klindt, DJ Bolt and LL Anderson
Department of Animal Science, Iowa State University, Ames 50011.
Plasma levels of relaxin and progesterone in hysterectomized and pregnant
gilts were determined from days 100-120 to evaluate the effects of purified
porcine (p) LH and pPRL on the secretory activity of the aging corpora
lutea. Gilts were bred on the second observed estrus or were
hysterectomized between 6-8 days after estrus (estrus = day 0) and were
assigned randomly to one of three treatment groups; saline-treated control,
im injections of pLH, and iv injections of pPRL from days 110-120. In
control, pLH-treated, and pPRL-treated animals, average gestation lengths
were 114 +/- 0.8, 116 +/- 1.9, and 115 +/- 0.5 days (+/- SE), respectively.
The relaxin level in mated gilts on day 100 was less than 2 ng/ml; it began
to increase after day 110 and peaked in control animals on day 113 (66
ng/ml), whereas in pLH- and pPRL-treated animals, prepartum peak values
were greater (P less than 0.01) and occurred on days 113 (104 ng/ml) and
114 (117 ng/ml), respectively. Relaxin dropped to basal levels (less than 1
ng/ml) by day 115 in controls and by day 116 in both pLH- and pPRL-treated
gilts. Although pLH and pPRL treatments markedly accentuated peak relaxin
secretion, they did not significantly accelerate or delay parturition or
delay the abrupt demise of the corpora lutea immediately postpartum. In
hysterectomized gilts, relaxin began to increase after day 110, peaked in
control animals on day 113 (27 ng/ml), and decreased abruptly thereafter to
less than 4 ng/ml. In contrast, pLH caused an immediate release of relaxin
on day 111 (23 ng/ml) and sustained elevated levels (P less than 0.01) of
relaxin until day 118, but the original corpora lutea regressed. Relaxin in
pPRL-treated animals increased steadily after day 110, reaching peak values
by day 115 (29 ng/ml), and remained consistently elevated (P less than
0.01) until day 120. Progesterone secretion was maintained in the
pPRL-treated hysterectomized gilts from days 110-120 by the original
corpora lutea and with no luteinization of follicles or formation of new
corpora lutea. It is evident from this study that administration of pPRL
starting on day 110 enhanced and prolonged the preprogramed release of
relaxin and maintained progesterone secretion by aging corpora lutea in
hysterectomized animals until day 120.
