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Partially purified cell membranes were prepared from midterm and term placentas after sedimentation on a sucrose density gradient. Biochemical characterization showed that the sucrose density pellet was enriched 8-fold in alkaline phosphatase activity and also contained the majority of [125I]LHRH binding sites. This enrichment was also confirmed by electron microscopy. Specific binding of LHRH was then determined by incubating iodinated LHRH or two of its superanalogs with increasing doses of the corresponding radioinert ligand. Scatchard representation of the data showed curvilinear plots whose first component revealed, for both stages of pregnancy, saturable binding of [125I]LHRH and its agonists with similar association constants (Ka) that ranged between 5.5 X 10(5) M-1 and 1.1 X 10(7) M-1. When standardized per milligram of DNA content, the number of binding sites ranged between 225 and 310 X 10(-12) M. Specificity was evidenced by the inability of a biologically active LHRH antagonist, oxytocin, and TRH to inhibit [125I]LHRH binding. Short term placental cultures incubated with 1.5 X 10(-6)M LHRH had increased production rates of both immunoassayable and bioassayable hCG, and this effect was 4-fold higher in midterm placental cultures. Placental incubations with either buffer or equimolar concentrations of oxytocin or TRH had no effect on hCG production. These observations expand information on extrapituitary binding sites of LHRH and suggest a role for this peptide in the physiology of the human placenta.

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This study compares the functional consequences of levothyroxine (T4) treatment during pregnancy as well as the acute affects of triiodothyronine (T3) on spontaneous uterine contractile activities observed in vitro. Uterine biopsies were obtained from consenting women undergoing elective caesarean at term (n = 28). Spontaneous contractile activities from T4-treated pregnant women (n = 8) were compared with control patients (n = 20) by isometric tension measurements. Effects of acute T3 and T4 on control tissues were also monitored. Area under the curve, amplitude, time to peak, duration, and frequency were quantified. In uterine strips from women treated for hypothyroidism, phasic uterine contractions of larger amplitude (+77%) were observed, with a prolonged duration at 90% relaxation (+138%) and reduced frequency (-55%) compared with values of the control group. The addition of exogenous T3 in vitro on control strips induced a significant increase in the duration of the contractions and a significant decrease in frequency (P < 0.05), which partially mimics the results obtained in strips from T4-treated women. Significant modifications of contractile properties were observed in strips from pregnant women treated with levothyroxine, consistent with those observed with the addition of exogenous T3. Clinical practices of modern obstetrics should take into account the effect of thyroid hormones on uterine contractions' time course to ensure a tighter followup at the end of pregnancy to achieve safer delivery.

We have described a thyroid hormone receptor in synaptosomes of the chick embryo brain. To understand how the hormones exert their actions at this level, we performed a series of studies to demonstrate that this receptor could be linked to G proteins. Guanosine 5'-[gamma-thio]triphosphate (GTP gamma S)(100 muM) lowered the binding capacity of the receptor high affinity site from 8.9 +/- 1.3 to 3.4 +/- 1.3 ng T3/mg protein, a finding consistent with the coupling of receptor to G proteins. Furthermore, ADP ribosylation with pertussis toxin showed that thyroid hormones induced a dose-dependent increase in the inactive alpha 0-subunit of the G0 protein. This effect was detected at 10 pM, with a maximal increase (mean +/- SEM, 50 +/- 3.6%) at 100 nM, and T4 was as effective as T3. Both hormones also decreased the intrinsic guanine triphosphatase activity of G proteins by lowering the binding of GTP to the alpha-subunit and their rate of hydrolysis. This inhibition was greater with T4 (25 +/- 5%) than with T3 (14 +/- 2%), suggesting that the former could be the more active hormone at the synaptosomal level. The effect on guanine triphosphatase activity confirms that the synaptosomal thyroid hormone receptor is coupled to a G(zero) protein. These results demonstrate that thyroid hormones increase or favor the ADP ribosylation of G alpha(zero) by pertussis toxin. Thus, they enhance the alpha(zero)-GDP form of the G(zero) protein, namely its inactive conformation. By decreasing the activity of this protein, these hormones may modulate the formation of second messengers in synaptosomes and intervene in the regulation of neuronal proliferation and differentiation induced by several factors. Therefore, thyroid hormones may exert their action on brain maturation at least in part by modulating G alpha(zero) through their synaptosomal receptor.