Prolactin in the Cat: I. Pseudopregnancy, Pregnancy and Lactation1

Abstract

Prolactin (Prl) secretion patterns were investigated during pregnancy, parturition, lactation and the weaning period of 8 cats by means of a heterologous radioimmunoassay (RIA) using canine Prl antisera and canine reference standards (NIAMDD). Prl concnetrations began to increase during the sixth week of pregnancy (gestation period, 9.5 weeks) and were significantly higher from the seventh week through the last week of pregnancy (X̄=31.2 ± 5.1 ng/ml) compared to the first 5 weeks (X̄=7.0 ± 0.3 ng/ml). Prl values averaged 43.5 ± 4.5 ng/ml for the last 3 days of gestation. In the 5 animals that lactated for 6 weeks postpartum, Prl levels remained high (40.6 ± 7.2 ng/ml) for the first 4 weeks postpartum. Prl concentrations averaged 27.8 ± 3.1 ng/ml during the last 2 weeks of lactation, a significant decline from the previous 4 weeks (P<0.05). Following weaning, Prl concentrations decreased to approximately 15 ng/ml during the first week with basal levels obtained by the end of the second week. In the 3 nonlactating animals, basal levels of Prl were achieved by 1, 3, and 10 days postpartum. During pseudopregnancy (the period of luteal production of progesterone, nonpregnant state) there were no significant changes in Prl concentrations observed with a blood sampling regimen of one daily morning sample (X̄=7.3 ± 0.6 ng/ml). From this study, the following conclusions were drawn: 1) It is possible to measure Prl levels in the cat with the canine Prl system. 2) Prl levels are elevated in the last one-third of pregnancy, presumably for the induction of mammary gland growth and lactogenesis. 3) With the suckling stimulus, Prl levels are elevated during the 6-week lactation period; however, during the last 2 weeks of lactation, Prl levels decline significantly to about one-half of the immediate postpartum levels. Following weaning, Prl concentrations return to basal levels within 1 to 2 weeks. 4) Coitusinduced pseudopregnancy (without lactation) has no significant effect on Prl levels.