The hypothesis that 17β-estradiol suppresses dopamine secretion into hypophysial portal blood was tested. Portal plasma concentrations of dopamine were significantly lower in proestrous rats (1.0 ± 0.1 ng/ml; mean ± SE) than in estrous rats (1.9 ± 0.38 ng/ml). To deplete the animal of endogenous steroid hormones, proestrous rats were adrenalectomized (Adx) and ovariectomized (Ovx). Twenty-four hours later, hypophysial portal blood was collected for 60 min, and the plasma from this blood was analyzed for dopamine. Arterial plasma from these rats was assayed for 17β-estradiol and progesterone. The concentrations of dopamine in the portal plasma of sham-operated rats and bilaterally Adx-Ovx rats were similar to those in estrous animals. The concentration of dopamine in portal plasma of Adx-Ovx rats injected 24 h earlier with 50 μg 17β-estradiol was 1.0 ± 0.31 ng/ml, which was comparabExposure of adipocytes from young rats (2–3 months old) to dexamethasone in vitro results in 40–50% inhibition of glucose transport and metabolism. ComparabDose-response curves were obtained for the production of androgen-binding protein (ABP) by Sertoli cells prepared from testes of 20-day-old rats and treated in culture with either FSH or testosterone (T). FSH stimulated ABP production by up to 3.5 times control levels. For NIH-FSH-Sll, the ED50 was 3 ng/ml, and for highly purified ovine FSH, the ED50 was 0.066 ng/ml. Addition of T produced a stimulation of up to 3 times control levels; half-maximal response was obtained at a dose of 4 nM. The presence of small numbers of contaminating Leydig cells in some preparations resulted in production of endogenous T, especially when high doses of NIH-FSH, which contains some LH, were employed. A modified preparatDissociation of [125I]iodoinsulin from adipocyte insulin receptors was studied in the presence or absence of the insulin derivatives, desoctapeptide insulin and desalanine desasparagine insulin. When cells were allowed to associate with a tracer concentration (10-10 M) of [125I]iodoinsulin and dissociation was studied in either insulin-free buffer or buffer containing 100 ng/ml unlabeled insulin, dissociation was accelerated in the presence of unlabeled insulin. This is consistent with negatively cooperative site-site interactions. On the other hand, when dissociation studies were performed in the presence of high concentrations of desoctapeptide insulin or desalanine desasparagine insulin, dissociation rates were slower than those observed in insulin-free buffer. In marked contrast, when cells were allowed to achieve a high fractional receptor occupancy by associating with high concentrations of either desoctapeptide insulin or desalanine desasparagWe have postulated the existence of a thyroid hormone (T3 or T4)-mediated short loop feedback mechanism in thyroid based on studies wherein T3 or T4 inhibited TSH-induced ornithine decarboxylase (ODC) activity in both intact and hypophysectomized rats (J Clin Invest 57: 745, 1976). Since it has been suggested that some or all of this inhibition may be secondary to T3- or T4−-induced suppression of endogenous TSH, it was felt to be of interest to perform further studies. In contrast to our initial findings, thyroid ODC in the hypophysectomized rat was generally unresponsive to exogenous TSH (2 U) administration. A normal response could be restored by prior injection of a low dose (0.2–1 U) of TSH, which did not influence thyroid ODC activity per se, 16 h before administration of the 2-U TSH test dose. Similarly, pretreatment with dibutyryl cAMP (4–15 mg) plus aminophylline (5 mg) at -16 h was also effective in restoring the response to exogenous TSH (2 U) to near normal levels. T3 pretreatment effectively inhibits TSH-induced thyroid ODC activity in intact rats. However, when intact rats were pretreated with 1 U TSH together with T316 h before administration of the 2-U TSH test dose, no inhibition of ODC activity was observed. Additionally, when hypophysectomized rats were pretreated at -16 h with TSH (0.5–1 U) together with T3 or T4, no inhibition of TSH-induced ODC was seen. Goitrogen treatment increases rat thyroid cAMP-dependent protein kinase activity (CNDPK) by 60% over control, whereas addition of T4 to the drinking water (5 mg/liter) decreases the activity by 35%. Thyroid CNDPK was 50% lower in hypophysectomized rats than in intact rats; pretreatment of hypophysectomized rats with TSH restored the CNDPK levels to near normal. Goitrogen treatment increases rat thyroid adenylate cyclase activity by 130% over control, whereas hypophysectomy lowers basal activity by 35%. It was concluded that TSH exerts a tonic regulatory effect on thyroid function and the previously reported inhibition of TSHinduced ODC by T3 or T4 in intact rats may, totally or in part, have been due to endogenous TSH suppression. © 1979 by The Endocrine Society.
