EFFECTS OF EXTENDED LACTATION AND 9α-Fluoroprednisolone ACETATE ON SERUM THYROXINE AND LACTATIONAL PERFORMANCE

Abstract

The effects of extending lactation up to 48 days on serum thyroxine (T4), and its relationship to lactational performance, were studied with rats fed a ration containing 1\m=.\0 \g=m\gI\m=-\/g of feed. The effects of chronic administration of 9\g=a\-fluoroprednisolone acetate on serum T4 during prolonged lactation were also determined. Lactation was prolonged by substitution of foster litters at 8-day intervals. Serum T4 was measured by a competitive protei n\x=req-\ binding assay.Chronic administration of 9\g=a\-fluoroprednisolone acetate up to 32 days resulted in a significant increase in serum thyroxine in non-lactating rats. In saline-treated rats lactating up to 48 days, serum T4 was initially reduced by 30% at 16 days, but progressively increased as lactational performance (expressed as gain in weight of pups) decreased. This inverse correlation was attributed to a progressive decline in mammary gland competition for iodine and to a progressive depletion of corticosterone which may have exerted an antithyroid effect initially. Rats treated with 9\g=a\-fluoroprednisolone acetate and lactating up to 48 days, had serum T4 levels which were lower than values for corresponding saline-treated lactating rats, but were similar to those levels observed in saline-treated non-lactating rats. This may indicate a higher rate of thyroid hormone utilization in lactating rats treated with 9\g=a\-fluoroprednisolone acetate and may also explain the ability of these rats to maintain a better lactational performance than saline-treated lactating rats. Comparisons of each 8-day extended lactation period showed that in all cases lactational performance was greater for mothers treated with 9\g=a\-fluoroprednisoloneacetate; up to 113% more from 16 to 24 days lactation, up to 153% more from 24 to 32 days lactation, 146% more from 32 to 40 days lactation, and 317% more from 40 to 48 days lactation, despite the fact that by day 48 maternal body weight of lactating rats treated with 9\g=a\-fluoroprednisolone acetate was not different from that of saline-treated lactating rats. A hypothesis proposed for the hormonal maintenance of lactation suggests that the maintenance of euthyroid levels of serum T4 due to treatment with 9\g=a\-fluoroprednisolone

Lactation

“…Other investigations have shown that circulating T 4 was lower in lactating than in nonlactating cows (Heitzman and Mallinson, 1972;Vanjonack and Johnson, 1975), a result supported by our own work (Hart et aI., 1978; high yielders: early lactation, 30.7 ng/ml; dry period, 38.4 ng/ml; P <0.1; low yielders: early lactation, 39.8 ng/ml; dry period, 55.4 ng/ml; P < 0.001). Lorscheider's group (1971) observed significantly lower levels of T4 in rats suckling 12 as opposed to 3 pups and have since suggested that the lower circulating levels ofT 4 found during lactation may be the result of increased competition on the part of the mammary gland for dietary iodine (Lorscheider and Mears, 1973). Such findings are in agreement with those of Lorscheider et al (1969) and Vanjonack and Johnson (1975).…”

“…Mittra (1974) examined the mechanisms by which the mammary gland develops in hypothyroid rats and concluded that in the absence of thyroid hormones the mammotrophic effect of endogenous prolactin is markedly increased, i. e. that prolactin and T 4 are antagonistic at the level of the rat mammary gland. It should, however, be emphasized that similar symptoms may accompany hyperthyroidism (Arroyo and Aubert, 1972) and a study in rats has suggested that T 4 may even be a factor limiting milk production (Lorscheider and Mears, 1973). It should, however, be emphasized that similar symptoms may accompany hyperthyroidism (Arroyo and Aubert, 1972) and a study in rats has suggested that T 4 may even be a factor limiting milk production (Lorscheider and Mears, 1973).…”

Changes in Body Weight, Milk Production, Food and Water Consumptions and Vaginal Smears in Rats during Prolonged Lactation

“…Results of the present experiment are in good agreement with those of previous workers (Nicoll & Meites, 1959;Kurz, 1967;Thatcher & Tucker, 1968Lorscheider & Mears, 1973) in demonstrating a definite decline of lactational performance during prolonged lactation in the rat.…”

“…hormones required for the maintenance of lactation seems to be a direct cause of the decreased milk production in prolonged lactation as suggested by the previous workers (Kurz, 1967;Thatcher & Tucker, 1968and Lorscheider & Mears, 1973). However, the mechanism or the factors causing these hormonal changes have yet to be clarified.…”

Effects of tri-iodothyronine administration on the disposal of oral [1-14C]triolein, lipoprotein lipase activity and lipogenesis in the rat during lactation and on removal of the litter

“…Lactation induces a state of relative hypothyroidism; lactating rats show decreases in plasma thyroxine (T4) and T3 and an elevation of plasma thyrotropin [8][9][10]. The extra-thyroidal conversion of T4 into T3 in lactating rats is lower compared with non-lactating rats [10].…”