Transfer of TRH through the Placenta and Metabolism in the Fetus of the Guinea‐Pig

Abstract

The permeability of the placenta to thyrotropin‐releasing hormone (TRH) and the fetal metabolism of this hormone were studied by giving iodine‐labelled synthetic TRH to pregnant guinea‐pigs during the first trimester. The radioactivities of various tissue fluids (serum, urine, amniotic fluid and maternal bile) and tissue samples (kidney, liver, small intestine, muscle and cortex) of both mother and fetus were counted. The biological activity of the transferred synthetic TRH was measured in fetal serum by bioassay. Direct measurements of placental permeability to TRH is presented. In the mother the metabolism seems to be the same as in mice and rats (Redding and Schally 1971, Virkkunen, Leppaluoto and Lybeck 1972). In the fetus the main elimination route of TRH is through the liver to bile, in contrast to the rapid urinary excretion in the mother. The half‐life of TRH in the fetal serum is about ten times as long as in the mother, reflecting a different elimination mechanism. Fetal tissue concentrations of TRH are comparable to those of the mother, and so may trigger the endocrine development of the fetal thyroid‐adenohypophyseal system.

Thyroid-pituitary function in eight anencephalic infants

“…It is not clear whether the development and secretion of these cells in anencephalic infants is autonomous or dependent on an extrahypothalamic source of TRH. The following hypotheses can be put for¬ ward: 1) foetal TSH secretion is partially autonom¬ ous or independent of the foetal hypothalamus, 2) foetal secretion is dependent on maternal TRH since it crosses the placenta readily (Lybeck 8c Virkkunen 1974). Recently, however, Tonooka & Greer (1977) have reported that the control of TSH secretion in the foetal rat is completely inde¬ pendent of maternal hypothalamic TRH, and 3) foetal TSH-secreting cells could be stimulated to grow and function by the TRH present in the amniotic fluid and in the placenta (Gibbons et al 1975).…”

Thyroid hormones: Metabolism and transportation in the fetoplacental unit

“…This hormone is located in the cytoplasm of the syncytiotrophoblast and is released into the fetal circulation (Bajoria & Babawale, 1998; Bajoria et al, 1997). Additionally, the placenta has aminopeptidases that metabolize TRH and regulate the passage of maternal TRH into the fetal circulation (Bajoria et al, 1997; Lybeck & Virkkunen, 1974). Presence of maternal TRH in the fetal circulation has been measured as early as 12 weeks of gestation (Jacobsen et al, 1977; Winters et al, 1974).…”

Hypothalamic-Like Peptides of the Placenta

“…23 The possibility that chorionic TRH is secreted into the maternal circulation and has some function during pregnancy also remains to be determined. Its possible transfer to the fetus might be considered, but whether the chorionic TRH, like synthetic TRH, [72][73][74] can pass the placenta, is not known.…”