PubMed Health⌕ Search

Biomedical subjects

J G Eales

Publications and source records attributed to J G Eales.

At least 19 recordsLinked to original sources
PubMed1992

Effects of short-term 17 beta-estradiol treatment on the properties of T4-binding proteins in the plasma of immature rainbow trout, Oncorhynchus mykiss.

To determine the effects of 17 beta-estradiol (E2) on the properties of the plasma proteins that bind L-thyroxine (T4) immature rainbow trout, Oncorhynchus mykiss, were injected intraperitoneally on days 0 and 3 with 0.5 mg E2-3-benzoate/100 g body weight, and plasma was sampled on days 4, 7, or 12. Control trout received peanut oil alone. E2 caused a small but significant decrease in the free T4 index. Saturation analysis on miniature G-25 Sephadex columns revealed at least two major T4-binding sites. Filtration on agarose Bio-gel A 1.5 also indicated two major T4-binding protein fractions with molecular weights of 150 and 55 kDa with a small proportion of T4 binding to a 1,500-kDa site presumed to be lipoprotein. Addition of unlabeled T4 displaced [125I]T4 from the 55-kDa site and unmasked an adjacent site of higher molecular weight. E2 increased the proportion of T4 bound to the low-affinity (150 kDa) site relative to that bound to the high-affinity (55 kDa) site, increased the level of protein associated with the 1,500-kDa site and its T4 binding, and also initiated the production of presumed vitellogenin (VTG), which bound a small amount of T4. It is concluded that the E2-induced depression in FT4 is caused by a shift in T4 binding between high-affinity and low-affinity sites, and also by binding of small amounts of T4 to presumed lipoprotein and VTG.

Animals↗
PubMed1992

Intra- and extra-cellular sources of T3 binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei of immature rainbow trout, Oncorhynchus mykiss.

The sources of extracellular and intracellular 3,5,3'-triiodo-L-thyronine (T3) binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei were determined in vivo for immature rainbow trout at 12 degrees C. Both [131I]T3 and [125I]T4 were injected intraperitoneally, the plasma and tissues were examined at isotopic equilibrium at 20 h, and the proportions of intracellular [125I]T3 and extracellular [131I]T3 saturably bound in the nucleus were determined. Comparable total amounts of T3 were saturably bound in the nuclei of liver (7.2), kidney (8.0), and gill (9.7 moles x 10(-13) .mg DNA-1), but the percentage of nuclear T3 generated within the target cell was greater for gill (76%) than for liver (50%) and kidney (28%). Both gill and liver possess a low Km T4 5'monodeiodinase which could be responsible for the high proportion of the nuclear T3 generated within those tissues.

Animals↗
PubMed1992

HPLC analysis of in vitro hepatic deiodination products of thyroid hormones in the rainbow trout, Oncorhynchus mykiss.

Reverse-phase HPLC employing five different solvent systems was used to determine the 125I-labeled products formed in rainbow trout by in vitro incubation at 12 degrees of the hepatic microsome fraction with L-thyroxine (T4) labeled with 125I in the outer phenyl ring. Trout were starved for 2 weeks or fed a 2% ration. The only labeled products identified during incubation for 7.5-70 min over a T4 substrate range of 0.03-0.5 nM were 125I- and 3,5,[125I]3'-triiodo-L-thyronine (T3). These products in combination with the parent [125I]T4 accounted for over 96% of the total chromatographic radioactivity. Neither 3,[125I]3'-diiodo-L-thyronine nor 3,[125I]3',5'-T3 (reverse T3) was detected, suggesting negligible inner-ring deiodination of T4 or T3. Essentially equal production of 125I- and [125I]T3 validated the use of 125I- production as a measure of [125I]T3 generation in assays for hepatic 5'-monodeiodinase activity. However, in some experiments the 125I- level slightly exceeded the [125I]T3 level, indicating that outer-ring deiodination of T3 may occur to a limited degree. In conclusion, the present data for liver support earlier observations from in vivo studies in showing that for trout at 12 degrees deiodination pathways are geared primarily toward T4 outer-ring monodeiodination to produce T3 with undetectable inner-ring deiodination of T3 or T4 and limited outer-ring deiodination of T3.

Animals↗
PubMed1992

The acute influence of ingested thyroid hormones on hepatic deiodination pathways in the rainbow trout, Oncorhynchus mykiss.

Juvenile rainbow trout were fed once daily with trout pellets supplemented with L-thyroxine (T4) or 3,5,3'-triiodo-L-thyronine (T3) and the effects on plasma T4 and T3 levels and hepatic 5'-monodeiodinase (5'D) activity determined after 1, 2, or 3 days. In all cases T3 (12 ppm) elevated plasma T3 and caused a significant reduction in the functional level (Vmax) of 5'D with no change in enzyme affinity (Km). After 3 daily meals (3 ppm T3), 5'D activity fell to 52% of control levels. In most instances plasma T4 was increased. In contrast, ingestion of T4 for 3 days at levels up to 48 ppm did not modify hepatic 5'D. This may reflect either insensitivity of the 5'D system to T4 or poor T4 uptake from the gut, as plasma T4 levels were influenced to a small extent by T4 ingestion. HPLC analyses showed that dietary T3 supplements (0, 3, 6, or 12 ppm) for 3 days acted in a dose-dependent manner, not only to suppress T3 formation from T4 by outer-ring deiodination, but also to promote inner-ring deiodination of T4 to 3,3',5'-triiodo-L-thyronine (reverse T3) and outer-ring deiodination of T3. In conclusion, the present data indicate that in the face of a T3 challenge there is a rapidly responding hepatic autoregulation of T3 production, achieved by a complex coordinated regulation of several different iodothyronine deiodination pathways.

Analysis of Variance↗
PubMed1992

Properties of T4 5'-deiodinating systems in various tissues of the rainbow trout, Oncorhynchus mykiss.

L-Thyroxine (T4) 5'-monodeiodinase (5'D) activity was examined in the microsomal fractions of liver, kidney, gill, white skeletal muscle, and red blood cells (RBC) of fed rainbow trout held in freshwater at 12 degrees. Two distinct 5'D systems were established and were examined at low (0.08-1.3 nM) or high (1.6-25 nM) T4 substrate ranges. The low substrate 5'D occurred in liver, gill, and muscle, but not in kidney or RBC. The pH optimum was 7.0 and the optimum dithiothreitol (DTT) level ranged from 7 to 10 mM. The Km values (nM) were liver, 0.098; muscle, 0.198; and gill, 0.168. The Vmax values (pmol.hr-1.mg protein-1) were liver, 3.74; muscle, 0.79; and gill, 0.62. DTT affected both the Vmax and the Km, and propylthiouracil (PTU) inhibited the Vmax. These data suggest a ping-pong type mechanism. In contrast, the high substrate 5'D occurred only in liver (pH 7 optimum, DTT optimum 15 mM) and in kidney (pH optima 6 and 8, DTT optimum 15 mM). The Km values (nM) were liver, 10.0; and kidney, 14.7; the Vmax values (pmol.hr-1.mg protein-1) were liver, 8.21; and kidney, 5.76. DTT affected the Vmax but not the Km and PTU did not inhibit, indicating a sequential type mechanism. In conclusion, in rainbow trout there are at least two types of 5'D which differ in their tissue distribution, T4 substrate affinity, and enzyme mechanism, and which do not resemble in their combined properties the 5'D forms established in higher vertebrate taxa.

Animals↗
PubMed1992

Thyroxine 5'-monodeiodinase activity in microsomes from isolated hepatocytes of rainbow trout: effects of growth hormone and 3,5,3'-triiodo-L-thyronine.

Rainbow trout hepatocytes isolated by collagenase perfusion were suspended in primary culture for up to 72 hr at 11 degrees and then the microsomal L-thyroxine (T4) 5'-monodeiodinase (5'D) activity was evaluated by 125I- generation from [125I]T4. The 5'D activity and Vmax (level of functional enzyme) and Km (Michaelis-Menten constant) values for microsomes obtained from incubated hepatocytes corresponded to those for microsomes obtained directly from intact livers. HPLC analysis revealed 3,5-[125I]3'-triiodo-L-thyronine (T3) as the only significant 125I-labeled organic product. Hepatocyte survival ( > 90%) and 5'D activity were unaltered by insulin (10(-9) M) in the incubate, but 5'D activity was inhibited by 10% fetal calf serum. Human growth hormone (hGH) at concentrations of 5-250 ng/ml did not increase 5'D activity. These results do not support previous in vivo studies demonstrating hGH-enhanced hepatic 5'D function in trout and indicate that either hGH acts indirectly on the liver to enhance 5'D activity or incubated hepatocytes lose GH responsiveness. However, coincubation of hepatocytes with T3 (15 or 30 nM) for 24 hr inhibited 5'D activity in a dose-dependent manner and induced the production of 3-[125I]3',5'-triiodo-L-thyronine (reverse T3). These data support previous in vivo studies in showing that T3 autoregulates its own hepatic production and show that T3 does so by acting directly on the hepatocyte to modify deiodination pathways.

Analysis of Variance↗
PubMed1992

Stimulation of hepatic thyroxine 5'-deiodinase activity in rainbow trout (Oncorhynchus mykiss) by Pacific salmon growth hormone.

1. Growth hormone extracted from Pacific salmon pituitaries (sGH) was injected intra-peritoneally into rainbow trout to determine sGH effects on plasma levels of thyroxine (T4), 3,5,3'-triiodothyronine (T3) and properties of the hepatic 5'-deiodinase enzyme (5'-D) responsible for T4 to T3 conversion. 2. After 24 hr, sGH (0.1 or 0.5 microgram/g) did not alter the plasma T4 level or 5'-D, Km, but elevated plasma T3 and especially 5'-D Vmax. 3. Thus sGH, like the previously tested human GH, acutely enhances the potential for extra-thyroidal T3 production in trout by increasing the functional level of hepatic 5'-D, but without changing the plasma T4 level.

Animals↗
PubMed1991

Effects of cortisol on aspects of 3,5,3'-triiodo-L-thyronine metabolism in rainbow trout (Oncorhynchus mykiss).

Aspects of 3,5,3'-triiodo-L-thyronine (T3) metabolism were studied in fed rainbow trout (Oncorhynchus mykiss) held at 11.5-14 degrees and intraperitoneally implanted with hydrogenated corn oil (controls) or oil containing cortisol. Cortisol implants caused dose-related plasma cortisol elevations within the physiological range for 2-3 weeks, loss in body weight, and depression in plasma T3 and free T3 index with no consistent change in plasma thyroxine (T4) or free T4 index. Plasma T3 clearance rate and plasma T3 appearance rate were both increased by cortisol, with no change in hepatic microsomal T4 5'-monodeiodinase activity (Km or Vmax), but with a significant decrease in muscle T3 concentration. It is concluded that chronic physiologic cortisol treatment enhances plasma T3 clearance without change in hepatic T4 to T3 conversion, resulting in a decline in T3 concentration in both plasma and tissue (muscle) compartments.

Animals↗
PubMed1991

The acute effects of alteration in the dietary concentrations of carbohydrate, protein, and lipid on plasma T4, T3, and glucose levels in rainbow trout, Oncorhynchus mykiss.

The acute (4 hr) postprandial effects of a single isocaloric meal varying in the proportions of either carbohydrate (C)/lipid (L), C/protein (P), or L/P on plasma levels of glucose, T4 (L-thyroxine) and T3 (3,5,3'-triiodo-L-thyronine) were examined in rainbow trout starved for 3 days. Relative to starved controls, plasma T3 was generally uninfluenced by feeding but was increased by diets containing the highest C/L and P/L ratios. Plasma T4 was elevated only in instances where there was sufficient available dietary C to raise plasma glucose to at least 126 mg/100 ml. High dietary P or L levels in combination with low C levels and a postprandial plasma glucose level below 126 mg/100 ml did not elevate plasma T4. For fish fed an acaloric alpha-cellulose diet, plasma T4 was unchanged indicating that gastric filling alone does not contribute significantly to the T4 surge. It is concluded that the previously demonstrated postprandial elevation in plasma T4 is determined mainly by the level of dietary C and the available glucose, and not by P, L, total caloric content, or bulk properties of the ingesta.

Animals↗
PubMed1990

Growth hormone stimulates hepatic thyroxine 5'-monodeiodinase activity and 3,5,3'-triiodothyronine levels in rainbow trout (Salmo gairdneri).

Intraperitoneal injection of rainbow trout (Salmo gairdneri) with 0.4 microgram/g of human growth hormone (hGH) increased plasma levels of 3,5,3'-triiodo-L-thyronine (T3) and Vmax of the hepatic microsomal 5'-monodeiodinase enzyme (5'D) that converts thyroxine (T4) to T3, with no effect on Km or plasma T4 levels. A dose of 0.4 microgram hGH/g increased both plasma T3 and Vmax as early as 8 hr postinjection (pi). Maximal levels for both parameters occurred at 24 hr pi and significant stimulation was sustained to 48 hr pi. In trout injected with hGH over a dose range of 0.1-1.0 microgram/g and sampled at 24 hr pi, both Vmax and plasma T3 increased in a dose-dependent manner. It is concluded that the increase in the level of functional hepatic 5'D may contribute to the rapid hGH-induced elevation in plasma T3.

Animals↗
PubMed1990

The acute effects of food and glucose challenge on plasma thyroxine and triiodothyronine levels in previously starved rainbow trout (Oncorhynchus mykiss).

The acute effects of a single meal on plasma L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) levels were examined in rainbow trout starved for 3 days. Plasma T4 increased within 2 hr of food intake and remained elevated to 8 hr. Plasma T3 was not altered consistently. Feeding-induced elevations in plasma T4 were present only in trout weighing less than 250 g and if they consumed a ration exceeding 0.38% of body weight. Postprandial elevations in plasma glucose paralleled those in plasma T4, suggesting a possible relationship between glucose intake and food-induced alterations in plasma T4. In trout intraperitoneally (ip) injected 4 hr earlier with 0.7% NaCl containing 0.2 or 2.0 g/kg D-glucose, plasma T4 increased relative to that in saline-injected controls. In starved trout cannulated in the dorsal aorta to permit serial blood removal, ip injection of glucose (0.85 g/kg) increased plasma glucose at 1 hr and plasma T4 at 2 hr, but did not alter plasma T3. It is concluded that enhanced glucose availability associated with feeding starved trout contributes to the postprandial elevation in plasma T4.

Animals↗
PubMed1990

Influence of dietary lipid and carbohydrate levels and chronic 3,5,3'-triiodo-L-thyronine treatment on thyroid function in immature rainbow trout, Oncorhynchus mykiss.

The influence of varying dietary levels of nonprotein energy sources (lipid, L; carbohydrate, C) and 3,5,3'-triiodo-L-thyronine (T3) on thyroid function in immature rainbow trout was studied. Three diets of equivalent available energy content and identical nutrient composition, except for dissimilar concentrations of L and C (diet 1, L = 7%, C = 28.3%; diet 2, L = 13%, C = 14.9%; diet 3, L = 19%, C = 1.5%), were each supplemented with 0, 4, 8, or 12 ppm T3 and fed to satiation to trout at 6.5 +/- 0.5 degrees on a 12-hr photoperiod for 12 weeks. Dietary L and C concentrations did not influence plasma total L-thyroxine (T4) or T3 levels, indices of free T4 or free T3 levels, hepatic T4 5'-monodeiodinase (5'D) activity, capacity or affinity of hepatic nuclear T3 receptors, or thyroid follicle epithelial cell height. T3 treatment elevated total and free T3 levels and decreased 5'D activity (Vmax) in approximate proportion to T3 dose, and without effect on plasma total or free T4 levels or T3 receptor properties. However, thyroid follicle epithelial cell height was depressed at 8 or 12 ppm dietary T3. In trout reverted for 20 days to a T3-free diet from a T3 (12 ppm) diet, plasma total T3 levels fell to 30% of those of control trout (0 ppm T3 throughout). It was concluded that, under our experimental conditions, (i) trout thyroid function was refractory to dietary concentrations of L and C, (ii) the primary response to T3 supplementation was suppressed hepatic 5'D level and T3 production, which was sustained for at least 20 days after T3 treatment ceased, and (iii) despite causing a probable indirect decrease in thyroidal secretion, T3 did not modify the set point of the hypothalamo-hypophyseal-thyroid axis based on plasma total or free T4 levels.

Animals↗
PubMed1990

Saturable triiodothyronine-binding sites in the pituitary nuclei of salmonid teleost fish.

High-affinity, limited-capacity, 3,5,3'-triiodo-L-thyronine (T3)-binding sites were established by in vitro saturation analysis in cell nuclei of the pituitary gland of arctic charr. The sites were extracted from the purified nuclei using 0.4 M NaCl and incubated with [125I]T3 in the presence of 0.2 M NaCl. T3 saturable binding attained equilibrium after 18-24 hr of incubation at 4 degrees. The association constant ranged from 6.7 to 20.1 liters.mol-1 x 10(9), indicating a T3 affinity greater than that for T3-binding sites in rainbow trout liver. The maximal binding capacity ranged from 0.93 to 2.05 10(-13) mol.mg DNA-1, representing a mean site abundance corresponding to 60% of that for nuclei from trout liver. Thyroxine (T4) completely displaced [125I]T3 in the pituitary nuclei of arctic charr and T3 completely displaced [125I]T4 in the pituitary nuclei of rainbow trout, suggesting that in salmonids both T4 and T3 bind to the same single class of sites. However, the site affinity for T4 was approximately 20-50x less than that for T3. The possible roles of these sites in pituitary function as well as their relationship to other nuclear T3-binding sites in salmonid fish are discussed.

Animals↗
PubMed1988

Seasonal patterns in serum levels of thyroid hormones and sex steroids in relation to photoperiod-induced changes in spawning time in rainbow trout, Salmo gairdneri.

Serum levels of thyroid hormones (T4 = L-thyroxine and T3 = 3,5,3'-triiodo-L-thyronine) were measured and correlated with previously published levels of 17 beta-estradiol, testosterone (T), and Ca2+ (index of vitellogenin) in four groups of female trout held for 1-3 years at 8.5-9.0 degrees on a ration of 0.5% of body weight day-1 under different constant photoperiod regimes. In group 1, trout under a regimen of 18L:6D became sexually mature (SM) in April/May and then SM again in September/October; in group 2, trout under a regimen of 6L:18D became SM in January/February; in group 3, trout under a regimen of 6L:18D became SM in March/April; in group 4 trout under a regimen of 18L:6D followed by 10L:14D became SM in September/October. In all groups, regardless of the photoperiod-induced changes in spawning time, serum T3 exceeded T4 and both serum T3 and T4 patterns showed a consistent relationship with the sequence of steroid hormone changes and spawning time. Thyroid hormone levels were high during previtellogenesis but fell as sex steroids and serum Ca2+ increased. T3 and T4 were lowest before spawning when sex steroids were at their peak and then increased sharply following spawning as sex steroid levels declined. Peak serum T coincided with ovulation and usually preceded the postreproductive surge in serum T3 and T4. The hypothesis is discussed that T3 enhances early ovarian development, but as energy-demanding vitellogenesis proceeds T3 formation is suppressed, thereby curtailing growth and favoring energy partition to the ova.

Animals↗
PubMed1988

The influence of short-term 17 beta-estradiol treatment on plasma T3 levels and in vitro hepatic T4 5'-monodeiodinase activity in immature rainbow trout, Salmo gairdneri.

To determine the effects of 17 beta-estradiol (E2) on aspects of thyroid function, immature rainbow trout were intraperitoneally injected with estradiol benzoate (0.5 mg/100 g) on Days 0 and 3 and sampled on Days 7 and 12. This protocol created plasma E2 concentrations during the first 7 days comparable to those during naturally induced vitellogenesis. Control trout received peanut oil alone. Plasma levels of 3,5,3'-triiodothyronine (T3) were significantly depressed on Day 7 but returned to levels by Day 12. Plasma thyroxine (T4) levels were not altered consistently by E2 treatment. Hepatic microsomal T4 5'-monodeiodinase (5'D) activity responsible for conversion of T4 to T3 was significantly depressed on Day 7 but returned to control levels by Day 12. Lineweaver-Burke plots showed that the lower hepatic 5'D resulted from a 10-fold decrease in Vmax, indicating a lower enzyme concentration. A slight reduction in Km was also observed. These results confirm that high E2 levels, comparable to those in vitellogenesis, depress plasma T3 levels in trout and show that, at least in part, this effect is the result of a decrease in the amount of functional hepatic 5'D.

Animals↗
PubMed1988

High-affinity, limited-capacity triiodothyronine-binding sites in nuclei from various tissues of the rainbow trout (Salmo gairdneri).

High-affinity, limited-capacity 3,5,3'-triiodo-L-thyronine (T3)-binding sites were detected by in vitro saturation analysis in cell nuclei from liver, gill, kidney, brain, and erythrocytes (RBC), but not spleen. The sites were extracted from the purified nuclei using 0.4 M NaCl and incubated with [125]T3 in the presence of 0.2 M NaCl. In all tissues T3 binding approached equilibrium after 18 to 48 hr of incubation at 4 degrees and was reversible upon addition of excess unlabeled T3. The T3 association and dissociation rate constants (k+ and k-) were measured from the initial (4 hr) [125I]T3 association and dissociation rates for liver (k+ = 8.9 x 10(9) liters.mol-1.hr-1; k- = 0.067 hr-1) and for RBC (k+ = 1.9 x 10(8) liters.mol-1.hr-1; k- = 0.11 hr-1). The association constants (Ka) determined by saturation analysis were similar in all tissues investigated (average Ka = 2.8 x 10(9) liters.mol-1), except in RBC (Ka = 1.2 x 10(10) liters.mol-1). The Ka values calculated from the k+/k- ratio (1.4 x 10(11) liters.mol-1 and 1.8 x 10(9) liters.mol-1 for liver and RBC, respectively) differed substantially from those determined by saturation analysis. This discrepancy is likely due to nonsaturable T3 binding by coextracted nuclear proteins in the assay medium, altering the estimated k+. The maximal binding capacity of the nuclear sites varied widely between tissues (liver, 250; gill, 130; kidney, 63; brain, 30; and RBC, 10 fmol.(mg DNA)-1; spleen, below detection).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

In vitro effects of thyroid hormones on gonadotropin-induced estradiol-17 beta secretion by ovarian follicles of rainbow trout, Salmo gairdneri.

Ovarian follicles isolated from rainbow trout during early exogenous vitellogenesis were used to study the in vitro effects of thyroid hormones on salmon gonadotropin (GtH)-induced estradiol-17 beta (E2) secretion. Triiodothyronine (T3) alone did not alter E2 secretion but T3 in the presence of GtH (0.5 micrograms/ml) modified E2 secretion according to a biphasic dose-response curve. Maximum E2 secretion occurred at 1.9 x 10(-8) M T3; a concentration of 3.0 x 10(-7) M was inhibitory. T3 was more potent in stimulating GtH-induced E2 secretion than thyroxine. The stimulatory and inhibitory effects of T3 were consistent over a range of GtH concentrations (0.1-1.0 micrograms/ml). Cycloheximide (0.1-10 microM) decreased E2 secretion by GtH-treated follicles in a dose-dependent manner, but failed to overcome all the stimulatory effects of T3. Time course studies with follicles incubated with GtH, GtH + T3, GtH + cycloheximide, or GtH + T3 + cycloheximide indicated that T3 stimulation of GtH-induced E2 secretion occurs within 6 hr. It is concluded that thyroid hormones amplify the effects of GtH on E2 secretion by isolated ovarian follicles; at least a part of this effect does not require de novo protein synthesis.

Animals↗