Serum binding of triiodothyronine: theoretical and practical implications for in vitro triiodothyronine uptake

Negative regulation by dexamethasone of the potentiation of neuromedin C-induced growth hormone and prolactin release by estradiol in anterior pituitary cell aggregates

The bombesin-like peptide neuromedin C (NMC) stimulated the release of growth hormone (GH) and prolactin (PRL) from adult male rat anterior pituitary cell aggregates cultured in serum-free medium. This effect, particularly on GH release, was significantly increased when the cells were cultured in the presence of estradiol (E2). The GH response to NMC was not affected by 4 nM dexamethasone (Dex) but was enhanced in the presence of 80 nM Dex. However, the strong stimulatory action of E2 on the GH response to NMC was completely abolished by 4 nM Dex. In contrast, the PRL response to NMC was reduced by 4 nM Dex and this to a proportionally similar extent in the presence or absence of E2. The present data show that glucocorticoids can be both positive and negative regulators of the same response system.

Thyroid hormone resistance in hibernating ground squirrels, Spermophilus richardsoni. I. Increased binding of triiodo-L-thyronine and L-thyroxine by serum proteins

Thyroid function was examined during the annual cycle of Richardson's ground squirrel, Spermophilus richardsoni. A number of facets were measured to facilitate comparison of thyroid function in active, dormant, and aroused animals. This report discusses changes in the serum thyroid hormone and binding as assessed by radioimmunoassay, equilibrium dialysis, and competitive binding assays. During the hibernation phase (both dormant and aroused), total serum T3 (trioodo-L-thyronine) and T4 (L-thyroxine) are elevated over active levels, two- to fivefold and four- to sixfold, respectively. However, in dormant squirrels, both free T3 and free T4 are reduced compared with both active and aroused phases of the annual cycle, while in aroused squirrels there is an increase in free T3 but no change in free T4 compared with active squirrels. The difference between changes in total and free thyroid hormone levels in the three groups is due to changes in serum binding of thyroid hormone. There is a more than twofold increase in the capacity of a saturable T3-binding site in serum of both dormant and aroused squirrels, and there is an increase in serum binding affinity at the low core temperature of dormant squirrels (6 degrees). Therefore, even though serum total T3 and T4 are elevated during dormancy, free T3 and T4 levels are reduced to half of the levels in active squirrels as a consequence of increased serum binding capacity and affinity. In aroused animals, however, increased serum binding capacity only partially buffers the increase in total T3 and T4, so that free thyroid hormone levels exceed those of active squirrels.

Anomalous binding characteristics of human thyroxine binding globulin due to a dilution-dependent separation artefact

Contrary to the accepted view, a recent study using Sephadex column separation suggested that thyroxine binding globulin (TBG) binds T4 and T3 with similar affinity, but with a much larger capacity for T4 than T3. We have evaluated this finding by comparing this separation method with equilibrium dialysis, taking account of the effect of serum dilution with each method. Estimates of free T4 fraction by equilibrium dialysis (with magnesium chloride precipitation) were valid over a wide range of serum dilutions. In contrast, Sephadex column separation gave a major overestimate of free hormone (underestimate of binding) in less diluted serum, indicating that this method cannot be used to establish a value for T4 affinity independent of serum dilution. Such a systematic error will result in a greater underestimate of affinity for the ligand with higher affinity when two ligands are compared at a single serum dilution. By equilibrium dialysis at 37 degrees C, the affinity of T4 for TBG was approximately 13-fold higher than that of T3, while the capacity of TBG for both T4 and T3 was close to the concentration of immunoreactive TBG. The previous report of similar T4 and T3 affinities appears to be due to a dilution-dependent underestimate of T4 affinity inherent in Sephadex column separation. Direct comparison of binding kinetics of various ligands requires a separation method that is valid over a wide range of binding protein concentrations.

Variant thyroxine-binding globulin in serum of Australian aborigines: its physical, chemical and biological properties

Low serum total thyroxine (TT4) and triiodothyronine (TT3) is found in approximately 40% of Australian Aborigines. Studies were carried out to characterize the properties of thyroxine-binding globulin (TBG) in these Australian Aborigines to explain the observed reduction of thyroid hormone concentration in their serum. TBG from Aborigines with low serum TT4 concentrations was compared to TBG from Aborigines with normal TT4 concentration and Caucasians and American Blacks with normal or reduced serum TBG levels due to familial partial TBG deficiency. TBG from Aborigines with low serum TT4 concentrations had a reduced affinity for thyroid hormone (Ka). The Ka for T4 was 54% and for T3 30% of the Ka values for TBG from Aborigines with normal TT4 concentration or non-Aborigines. Maximal binding values were in agreement with TBG measurements by RIA for Aborigines with low or normal serum TT4 and for non-Aborigines. An increase in the rate of heat denaturation of TBG at temperatures from 54 to 60 C was also observed in sera from Aborigines with low TT4. The heat lability was lowered by 2 C. The low concentration of TT4 in serum of these Aborigines could not explain this higher heat lability of TBG since only addition of greater than 80-fold the physiologic T4 concentration obliterated the difference of heat inactivation by denaturation.(ABSTRACT TRUNCATED AT 250 WORDS)