The effect of corticotrophin on liver-type lipase activity in adrenals, liver and high-density lipoprotein subfractions in the rat.

Hypercortisolism was induced in rats by the administration of a corticotrophin analogue (Synacthen depot). The effect of this treatment during different periods was studied in normally fed and overnight-fasted rats. The activity of liver-type lipases, i.e., of lipases similar to the heparin-releasable lipase of rat liver (liver lipase), was determined in the adrenal gland and in the liver. Short-term (16 h) treatment had no effect on the lipase activity in the adrenal gland. During prolonged treatment, however, the lipase activity rose to 600-700% of control values in 10 days and from then on remained constant. The effect was similar in fed and overnight-fasted rats. The lipase activity in the liver decreased upon Synacthen administration. In the fed rats a decrease of 25% of the initial value was found after 16 h, 40% after 3 days and 50% after 20 days of treatment. In overnight-fasted rats the lowering of the lipase activity was less marked than in fasted controls. Serum lipid levels and high-density lipoprotein (HDL) subclass concentrations were also measured. The cholesterol concentration in the lipoproteins with a density greater than 1.050 g/ml (HDL) was elevated in rats treated for 3-20 days. If the rats were treated for longer than 10 days, overnight fasting led to a normalization of the HDL-cholesterol levels. After separation of the HDL into two subfractions, a relatively 'light' apolipoprotein E-rich fraction and a more 'heavy' apolipoprotein A-I-rich fraction, in fed and fasted animals treated with Synacthen for 3 days both HDL subfractions were elevated. After 10 days treatment only the apolipoprotein A-I-rich HDL fraction was still enhanced in both fed and fasted rats.

Prolactin and delayed pseudopregnancy in the rat.

In (RXU)F1 hybrid rats delayed pseudopregnancy was induced in three different ways: 1) by removal of all recent corporl lutea on day 2 of pseudopregnancy, 2) by removal of the in situ ovaries from ovarian graft-bearing animals on day 0 of pseudopregnancy, 3) by administration of 1 mg of ergocornine hydrogenmaleinate (ECO) on day 1 of pseudopregnancy. These procedures ended pseudopregnancy and in 50-60% of the animals a delayed pseudopregnancy with a duration of 6-15 days was observed after the experimental cycle. After sterile copulation two daily prolactin peaks were observed. Removal of the ovaries in situ (from ovarian graft-bearing animals) or of recently formed corpora lutea (from non-grafted animals) caused the disappearance of the prolactin peak at 19.00 h, without affecting the occurrence of the 03.00 h peak. The administration of ECO caused the disappearance of both prolactin peaks. Until the day of estrus prior to delayed pseudopregnancy there were no differences in prolactin concentrations at 03.00 or 19.00 h between animals which became delayed pseudopregnant and those which remained cyclic. In the latter animals the 03.00 h surges decreased slowly in the luteectomized and the ovariectomized animals and were no longer present 7-9 days after copulation. In animals becoming delayed pseudopregnant both prolactin peaks were present from day 0 of delayed pseudopregnancy onwards. Progesterone cencentrations during delayed pseudopregnancy were relatively low when delayed pseudopregnancy had a duration of 6-9 days. When delayed pseudopregnancy lasted 10 days or more normal progesterone values were found.

Serum prolactin concentrations during hormonally induced pseudopregnancy in the rat.

Treatment of 5-day cyclic rats with 10 or 100 microgram estradiol benzoate on the day of estrus induced a luteal phase in all animals studied. On the other hand, an injection with 1 microgram estradiol benzoate given on the same day failed to induce pseudopregnancy. When 10 mg progesterone were injected on the day of estrus, about 50% of the rats became pseudopregnant, whereas most of the remaining rats had a 6-day cycle. The injection of 10 or 100 microgram estradiol benzoate or 10 mg progesterone induced a period of increased PRL secretion which lasted for 2--4 days, followed by twice daily surges of PRL at the end of the dark and light periods, respectively. It is argued that pseudopregnancy induction by estradiol benzoate or progesterone is primarily a result of the induction of a period of increased PRL secretion. In this way, progesterone secretion by the recently formed corpora lutea is induced, and the elevated levels of progesterone in turn generate diurnal surges of PRL.

Dopamine levels in hypophysial stalk plasma of the rat during surges of prolactin secretion induced by cervical stimulation.

Tonic hypothalamic inhibition of PRL release is partially explainable by dopamine secretion into hypophysial portal blood. However, the probable existence of other PRL-inhibiting factors as well as PRL-releasing factors opens to question the role of dopamine in the dynamic regulation of PRL secretion. We investigated this question in the present study by measuring dopamine concentrations in hypophysial stalk blood of the rat during the surgess of PRL secretion induced by cervical stimulation. Urethane anesthesia, necessary for the surgery attendant to stalk blood collection, did not suppress the surge of PRL secretion induced by cervical stimulation 16-24 h previously. Increases in plasma PRL levels during such surges were 4- to 5-fold above baseline. Dopamine concentrations in hypophysial stalk plasma were 36% lower in cervically stimulated than in control rats during the diurnal and nocturnal PRL surges. However, dopamine levels were not different during the interval between the surges, a time at which PRL levels are similar in stimulated and control rats. To determine if the observed 36% decrease in dopamine levels might account for the associated 4- to 5-fold rise in PRL levels during surges, we treated rats with alpha-methyl-p-tyrosine to block endogenous dopamine secretion and then infused dopamine at various rates to achieve plasma dopamine concentrations throughout the physiological range. These dopamine levels significantly but incompletely suppressed PRL levels, and a 36% decrease in administered dopamine was associated with only an approximate 1.5-fold increase in plasma PRL levels. Thus, it is unlikely that changes in dopamine secretion alone can account for the increased release of PRL engendered by cervical stimulation.

Effect of hysterectomy on serum luteinizing hormone concentrations and on corpus luteum function in the rat.

Serum progesterone, follicle stimulating hormone (FSH) and luteinizing hormone (LH) concentrations were estimated in intact and hysterectomized pseudopregnant rats. Progesterone concentrations increased significantly from day 2 of pseudopregnancy (PSP) onwards, and a plateau was reached on days 4 and 5. No significant differences in progesterone concentrations between intact and long-term hysterectomized animals were observed until day 10 of PSP. In the intact rats, progesterone concentrations decreased after day 8, but in the hysterectomized rats they remained elevated until days 18-20 of PSP. No differences between FSH concentrations in intact and hysterectomized rats were found. In hysterectomized rats, however, LH concentrations were significantly lower than those in the intact rats on days 2-7 of PSP. When hysterectomy was performed on day 2 of PSP (acute hysterectomy) lower LH concentrations were also found in the hysterectomized animals compared with sham-operated animals. No differences in progesterone concentrations were found between sham-operated controls and acutely hysterectomized animals on days 3-8 of PSP. It was found that LH concentrations in ovariectomized long-term hysterectomized rats were lower than those in ovariectomized, but otherwise intact, animals. In contrast to its effect on tonic LH secretion, long-term hysterectomy had no influence on the surge of LH during the afternoon of proestrus. In both intact and long-term hysterectomized rats the duration of PSP was not significantly altered after transplantation of an ovary under the kidney capsule and the removal of the ovaries in situ. It is concluded that hysterectomy leads to a decreased LH secretion by the pituitary gland, but has no influence on the maximal progesterone concentrations during PSP. The significance of these findings, particularly with regard to the prolongation of PSP after hysterectomy, is discussed.

Diurnal prolactin surges and sexual differentiation of the rat hypothalamus.

Neonatal exposure to testicular androgens interferes with the ability in adulthood to release an ovulatory amount of LH, with the display of female sexual behavior, and also with control mechanisms of the activity of the corpora lutea. It is likely that effects on luteal activity involve, in part, effects on the control of PRL secretion. Therefore, serum concentrations of PRL were studied in ovarian graft-bearing male rats, castrated as neonates (NC-males) or as adults (AC-males), during a luteal phase induced and temporarily sustained by an ectopic pituitary graft. Before the removal of the pituitary graft, serum PRL and progesterone levels were high at all times when measured during the day in both AC-males and NC-males. Removal of the pituitary graft, 6-7 days after ovulation led to immediate cessation of luteal activity in AC-males but not in NC-males. In the latter animals, luteal activity was maintained for at least 6 days after pituitary graft removal by intermittently high levels of PRL, i.e. around 0400 and 1900 h. However, in AC-males, PRL levels were generally low after pituitary graft removal. It is concluded that the absence of neonatal exposure to testicular secretion contributes to the ability of the adult rat to release PRL in a diurnal surge-like manner. The inability to release PRL in this way may explain the failure of AC-males to maintain luteal activity.

Effects of adrenocorticotropin and corticosterone on the negative feedback action of testosterone in the adult male rat.

Injections of ACTH (Synacthen depot) to intact male rats resulted in high serum levels of corticosterone and progesterone, and decreased levels of LH, FSH, and testosterone. In gonadectomized rats with low serum testosterone levels (approximately 1 ng/ml) induced by a small testosterone-filled silicone elastomer capsule, ACTH inhibited the postcastration rise in LH and FSH and reduced the wts of the prostate and seminal vesicles. After adrenalectomy the inhibitory effects of ACTH on serum gonadotropins and organ wts were almost totally absent. Administration of corticosterone acetate (10 mg/day) to gonadectomized and adrenalectomized male rats resulted in high serum levels of corticosterone (approximately 400 ng/ml) which were about 3 times higher than those measured in intact control animals. Nevertheless, in these rats the serum levels of LH and FSH were as high as those measured in gonadectomized and adrenalectomized oil-treated rats. However, when in addition to the injections of corticosterone acetate, a small testosterone-filled capsule was implanted, the postcastration rise in FSH was fully inhibited, whereas the serum levels of LH were below the level of detection. Significant inhibition of the postcastration rise in LH and FSH also occurred when smaller quantities of corticosterone acetate were given. Since in gonadectomized and adrenalectomized male rats similar testosterone-filled capsules did not prevent the postcastration rise in LH and FSH, it is concluded that a high serum level of corticosterone increases the sensitivity to the negative feedback effects of testosterone.

Hypothalamo-hypophysial-thyroid axis in streptozotocin-induced diabetes.

Diabetes mellitus is frequently associated with reduced levels of TSH, PRL, GH, and gonadotropins. In this study we have wanted to determine whether chemically induced diabetes mellitus is associated with a decreased hypothalamic release of TRH. Male rats were made diabetic with streptozotocin (STZ; 65 mg/kg), whereas controls received vehicle. After 2 weeks, STZ diabetic rats had 25% lower body weights, 3.5-fold higher blood glucose, and 40-60% lower plasma TSH, T3, and T4 levels than controls. The plasma T4 dialyzable fraction had increased 2.5-fold in STZ diabetic rats, and the plasma free T4 concentration was similar to that in controls. Thus, treatment with STZ results in decreased plasma TSH and T4 levels, but does not reduce free T4 concentrations. The content of TRH in hypothalami of 2-week STZ diabetic rats was similar to that in controls, but in vitro these hypothalami released less TRH than those of control rats. In 2-week STZ diabetic rats, TRH in hypophysial stalk blood was 30% lower than that in control rats. The in vitro TRH secretion from hypothalami of untreated rats was dependent on the glucose concentrations in the incubation medium; increasing the glucose concentration from 10 to 30 mM did not alter TRH secretion, but basal TRH release increased in the absence of glucose. In conclusion, STZ-induced diabetes in the rat is associated with reduced hypothalamic secretion of TRH, which, in turn, may be responsible for the reduced plasma TSH and thyroid hormone levels. Furthermore, it is suggested that the inhibitory effect of STZ-induced diabetes on TRH secretion is probably not due to hyperglycemia.

Thyrotropin-releasing hormone gene expression by anterior pituitary cells in long-term cultures is influenced by the culture conditions and cell-to-cell interactions.

It has been suggested that TRH, synthesized by anterior pituitary (AP) cells in long-term monolayer cultures, may act as a paracrine or autocrine regulator. Because local control through messenger molecules depends on the cellular microenvironment, we were interested in studying the synthesis of TRH by AP cells in different culture systems and under various conditions. When AP cells were cultured as monolayers in medium containing 10% FCS for long periods of time (up to 3 weeks), a considerable increase in TRH content and prepro-TRHmessengerRNA (preproTRHmRNA) levels could be demonstrated by RIA and Northern blot analysis, whereas the cellular content of the TRH-like peptide pyroGlu-Glu-Pro-NH2 decreased with time in culture to undetectable levels. The release of TRH could be stimulated by depolarizing concentrations of K+ (55 mM), by the Ca++ ionophore A23187, and by GnRH, but not by CRH or GRF, indicating that TRH is stored in gonadotropes. Moreover, a combined in situ hybridization and immunocytochemical analysis demonstrated colocalization of LH in preproTRHmRNA-positive AP cells. When AP cells were cultured as reaggregates in the same (FCS-containing) medium, only a marginal increase in TRH content and preproTRHmRNA levels was observed. Irrespective of the culture systems and the culture conditions used, TRH gene expression was not observed when FCS was omitted. These results indicate that TRH gene expression more likely reflects derepression, rather than induction, of the TRH gene.

Characterization of iodothyronine sulfotransferase activity in rat liver.

Sulfation is an important pathway in the metabolism of thyroid hormone because it strongly facilitates the degradation of the hormone by the type I iodothyronine deiodinase. However, little is known about the properties and possible regulation of the sulfotransferase(s) involved in the sulfation of thyroid hormone. We have developed a convenient method for the analysis of iodothyronine sulfotransferase activity in tissue cytosolic fractions, using radioiodinated 3,3'-diiodothyronine (3,3'-T2) as the preferred substrate, unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor, and Sephadex LH-20 minicolomns for separation of the products. We found that iodothyronine sulfotransferase activity in rat liver cytosol is 1) higher in male than in female rats; 2) optimal at pH 8.0; 3) characterized (at 50 microM PAPS and pH 7.2) by apparent Michaelis-Menton (Km) values for 3,3'-T2 of 1.77 and 4.19 microM, and Vmax values of 1.94 and 1.45 nmol/min per mg protein in male and female rats, respectively; 4) characterized (at 1 microM 3,3'-T2 and pH 7.2) by apparent Km values for PAPS of 4.92 and 3.80 microM and Vmax values of 0.72 and 0.31 nmol/min per mg protein, in males and females, respectively; 5) little affected by hyperthyroidism in both male and female rats, but significantly decreased by hypothyroidism in males but not in females; and 6) not affected by short-term (3 days) fasting in both male and female rats, but significantly decreased by long-term (3 weeks) food restriction to one-third of normal intake in males but not in females. It is suggested that the higher hepatic iodothyronine sulfotransferase activity in male vs. female rats, as well as the decreases induced in males by hypothyroidism and long-term food restriction, represents differences in the expression of the male-dominant isoenzyme rSULT1C1.

Suckling stimulus, lactation, and suppression of ovulation in the rat.

Lactation pseudopregnancy in rats suckling a 5-pup litter lasted 22.0 +/- 0.4 days (mean +/- SEM; n = 11). By day 13 of lactation (day 1 of lactation = day of parturition), the continuation of lactation pseudopregnancy was dependent on the suckling stimulus, as litter removal on day 13 resulted consistently in ovulation on day 16. Measurements of various hormones before and after litter removal revealed high concentrations of progesterone and PRL during lactation and a rapid drop of both hormone concentrations after litter removal. Lactation pseudopregnancy in rats suckling a 10-pup litter lasted 26.1 +/- 0.9 days (n = 16). After litter removal on day 13 of lactation, the lactation pseudopregnancy continued for a further 7- to 11-day period, as evidenced by daily vaginal smears which remained mucified during that period. Measurements of hormone concentrations revealed continuously high concentrations of PRL before litter removal and a pattern of PRL secretion characterized by at least two diurnal peaks during the first days after litter removal. Progesterone concentrations decreased by 50% after litter removal, but the levels then remained constant and well above those found after the removal of 5-pup litters. It is argued that the different response to litter removal on day 13 of lactation between rats suckling 5 or 10 pups is due to the initiation of PRL peaks in rats with 10-pup litters: these PRL peaks are able to maintain luteal function for some period. It is further argued that the initiation of PRL peaks in rats with 10-pup litters is due to the high blood concentrations of progesterone at the time of litter removal compared to those of rats with a 5-pup litter.

Effects of hypothyroidism on hypothalamic release of thyrotropin-releasing hormone in rats.

The aim of this study was to investigate whether the severity and duration of primary hypothyroidism influence hypothalamic TRH release. Hypothyroidism was induced in male Wistar rats by treatment with different thyrostatic drugs or by thyroidectomy. Serum TSH in rats treated for up to 3 weeks with methimazole (MMI; 0.05% in drinking water) increased 20-fold, but TRH release into hypophyseal portal blood (HPB) did not change. Treatment with propylthiouracil (PTU; 0.1% in drinking water), which inhibits thyroidal T4 production and peripheral conversion of T4 to T3, resulted in a more rapid reduction in serum T3 levels and increase in serum TSH than those in rats treated with 0.1% MMI. Although these differences were no longer observed after 3 weeks of treatment, TRH release into HPB of rats treated with PTU was 34-49% higher than that in MMI-treated rats. Combined treatment with MMI (0.05-0.1% in drinking water) and iopanoic acid (IOP; 4 mg/100 g BW.day, ip), an inhibitor of both peripheral and central T4 to T3 conversion, also tended to produce a more rapid decrease in serum T3 and increase in serum TSH. After 3 weeks of treatment, serum T4, T3, and TSH were not different in the two groups, but TRH release into HPB was 48-65% increased by MMI plus IOP vs. MMI alone. Three to 10 weeks after thyroidectomy, TRH release into HPB was 58-72% higher than that in untreated controls. In vitro incubation of hypothalami isolated from rats treated for 3 weeks with MMI, MMI plus IOP, or PTU, as described above, showed that basal and 56 mM K(+)-induced TRH release were not influenced by the different drugs. Also, the total hypothalamic TRH content was not changed by any of these treatments. However, in rats treated for 1 or 2 weeks with MMI or PTU, the TRH content of the median eminence was decreased by 17-25%. These findings indicate that, depending on severity and duration, experimental hypothyroidism may cause a significant increase in hypothalamic TRH release in rats. The magnitude of these changes compared with the much larger increases in serum TSH suggests that the feedback of thyroid hormone on TSH secretion is mainly exerted at the pituitary level.

Hypothyroidism may account for reduced prolactin secretion in lactating rats bearing paraventricular area lesions.

Lesions in the paraventricular area (PVA) of lactating rats have been found to inhibit PRL release. We have examined whether this reduced PRL release is due to hypothyroidism resulting from destruction of the PVA. Rats were made hypothyroid by thyroidectomy on day 15 of pregnancy or by methimazole treatment from the day of parturition. Electrolytic lesions were placed bilaterally in the PVA on day 15 of pregnancy. The following variables were studied: weight gain of the pups, nursing behavior, thyroid status, and release of PRL. The treatments did not affect the time the mothers spent with the pups but reduced the daily weight gain of the pups. Rats with PVA lesions had reduced PRL and TSH levels during lactation compared with controls. Suckling-induced PRL release after 6 h of separation of mothers and pups was less in PVA-lesioned rats than in controls, but T4-treatment did overcome this blunted response in rats with lesions. Levels of T3 and T4 in PVA-lesioned rats were lower than those in controls. In rats made hypothyroid by thyroidectomy or treatment with methimazole, PRL levels were lower and TSH levels higher than those in euthyroid mothers on days 8, 15, and 22 of lactation. Suckling after 6 h of separation of pups and mothers raised PRL levels both in control and methimazole-treated rats, but in the latter animals the response was blunted. It is suggested that the reduced PRL release in lactating rats with PVA lesions could be due to hypothyroidism resulting from these lesions.

In vivo hypothalamic release of thyrotropin-releasing hormone after electrical stimulation of the paraventricular area: comparison between push-pull perfusion technique and collection of hypophysial portal blood.

Unilateral electrical stimulation for 15 min of the paraventricular area of anesthetized rats induced a 2- to 3- fold increase in plasma TSH levels and caused an increased release of TRH into hypophysial stalk blood from 217 +/- 25 to 530 +/- 90 pg/15 min (n = 6). This experimental model was then used to determine the in vivo hypothalamic release of TRH by push-pull perfusion of either the mediobasal hypothalamus (MBH) or anterior pituitary (AP). Before stimulation, TRH release per 15 min was 4.2 +/- 0.7 pg from the MBH (n = 18) and 3.5 +/- 0.3 pg from the AP (n = 13). Unilateral electrical stimulation of the paraventricular area led to higher plasma TSH levels in 27 of 31 rats, and levels during stimulation increased from 0.89 +/- 0.04 to 1.86 +/- 0.10 ng/ml (n = 31). No significant increase in TRH in the perfusates was observed when push-pull perfusion was done in the MBH contralateral to the site of stimulation (n = 6). However, TRH release increased 2- to 3-fold during the perfusion of the MBH ipsilateral to the site of stimulation (15.4 +/- 4.3 pg/15 min; n = 13). In conclusion, push-pull perfusion of the MBH or AP can be used to estimate hypothalamic TRH release. However, the output of TRH by push-pull perfusion is low and varies considerably between individual rats. Thus, the practical value of push-pull perfusion for measurement of in vivo TRH release seems limited.

Control of prolactin release induced by suckling.

In the present study, the role of dopamine and TRH in suckling-induced PRL release was investigated. Bupropion, a dopamine reuptake blocker, increased hypophysial stalk dopamine levels and inhibited suckling-induced PRL release. A short period of suckling, thought to induce a transient decrease in hypothalamic dopamine release, led to higher PRL levels following an iv injection of TRH than in rats which had not nursed their young for a short period after 4- to 6-h separation. These results, in combination with previous data, suggest that a decrease in hypothalamic dopamine release is important for suckling-induced PRL release. Increased PRL release may be in part due to an augmented hypothalamic release of TRH. Since serotonergic mechanisms seem involved in TRH release, lactating rats were treated with drugs acting on serotonergic pathways. Parachlorophenylalanine and pizotifen did not alter suckling-induced PRL release. Methysergide, a serotonin receptor blocker, prevented this PRL release when administered ip but not when injected into the lateral brain ventricle. Since methysergide is converted peripherally into metabolite(s) with dopamine agonistic activity, its effect on suckling-induced PRL release may be due to this action, rather than to its action on serotonin receptors. Thus, these data do not indicate that serotonergic mechanisms are important for suckling-induced PRL release. Passive immunization against TRH inhibited suckling-induced PRL release, indicating that TRH is a hypophysiotropic mediator of this PRL release.

Effect of thyroid status and paraventricular area lesions on the release of thyrotropin-releasing hormone and catecholamines into hypophysial portal blood.

TRH is a potent stimulator of pituitary TSH release, but its function in the physiological regulation of thyroid activity is still controversial. The purpose of the present study was to investigate TRH and catecholamine secretion into hypophysial portal blood of hypothyroid and hyperthyroid rats, and in rats bearing paraventricular area lesions. Male rats were made hypothyroid with methimazole (0.05% in drinking water) or hyperthyroid by daily injections with T4 (10 micrograms/100 g BW). Untreated male rats served as euthyroid controls. On day 8 of treatment they were anesthetized to collect peripheral and hypophysial stalk blood. In euthyroid, hypothyroid and hyperthyroid rats plasma T3 was 1.21 +/- 0.04, 0.60 +/- 0.04, and 7.54 +/- 0.33 nmol/liter, plasma T4 50 +/- 3, 16 +/- 2, and 609 +/- 74 nmol/liter, and plasma TSH 1.58 +/- 0.29, 8.79 +/- 1.30, and 0.44 +/- 0.03 ng RP-2/ml, respectively. Compared with controls, hyperthyroidism reduced hypothalamic TRH release (0.8 +/- 0.1 vs. 1.5 +/- 0.2 ng/h) but was without effect on catecholamine release. Hypothyroidism did not alter TRH release, but the release of dopamine increased 2-fold and that of noradrenaline decreased by 20%. Hypothalamic TRH content was not affected by the thyroid status, but dopamine content in the hypothalamus decreased by 25% in hypothyroid rats. Twelve days after placement of bilateral electrolytic lesions in the paraventricular area plasma thyroid hormones and TSH levels were lower than in control rats (T3: 0.82 +/- 0.05 vs. 1.49 +/- 0.07 nmol/liter; T4: 32 +/- 4 vs. 66 +/- 3 nmol/liter; TSH: 1.08 +/- 0.17 vs. 3.31 +/- 0.82 ng/ml). TRH release in stalk blood in rats with lesions was 15% of that of controls, whereas dopamine and adrenaline release had increased by 50% and 40%, respectively. These results suggest that part of the feedback action of thyroid hormones is exerted at the level of the hypothalamus. Furthermore, TRH seems an important drive for normal TSH secretion by the anterior pituitary gland, and thyroid hormones seem to affect the hypothalamic release of catecholamines.

Different effects of continuous infusion of interleukin-1 and interleukin-6 on the hypothalamic-hypophysial-thyroid axis.

The cytokines interleukin-1 (IL-1) and IL-6 are thought to be important mediators in the suppression of thyroid function during nonthyroidal illness. In this study we compared the effects of IL-1 and IL-6 infusion on the hypothalamus-pituitary-thyroid axis in rats. Cytokines were administered by continuous ip infusion of 4 micrograms IL-1 alpha/day for 1, 2, or 7 days or of 15 micrograms IL-6/day for 7 days. Body weight and temperature, food and water intake, and plasma TSH, T4, free T4 (FT4), T3, and corticosterone levels were measured daily, and hypothalamic pro-TRH messenger RNA (mRNA) and hypophysial TSH beta mRNA were determined after termination of the experiments. Compared with saline-treated controls, infusion of IL-1, but not of IL-6, produced a transient decrease in food and water intake, a transient increase in body temperature, and a prolonged decrease in body weight. Both cytokines caused transient decreases in plasma TSH and T4, which were greater and more prolonged with IL-1 than with IL-6, whereas they effected similar transient increases in the plasma FT4 fraction. Infusion with IL-1, but not IL-6, also induced transient decreases in plasma FT4 and T3 and a transient increase in plasma corticosterone. Hypothalamic pro-TRH mRNA was significantly decreased (-73%) after 7 days, but not after 1 or 2 days, of IL-1 infusion and was unaffected by IL-6 infusion. Hypophysial TSH beta mRNA was significantly decreased after 2 (-62%) and 7 (-62%) days, but not after 1 day, of IL-1 infusion and was unaffected by IL-6 infusion. These results are in agreement with previous findings that IL-1, more so than IL-6, directly inhibits thyroid hormone production. They also indicate that IL-1 and IL-6 both decrease plasma T4 binding. Furthermore, both cytokines induce an acute and dramatic decrease in plasma TSH before (IL-1) or even without (IL-6) a decrease in hypothalamic pro-TRH mRNA or hypophysial TSH beta mRNA, suggesting that the acute decrease in TSH secretion is not caused by decreased pro-TRH and TSH beta gene expression. The TSH-suppressive effect of IL-6, either administered as such or induced by IL-1 infusion, may be due to a direct effect on the thyrotroph, whereas additional effects of IL-1 may involve changes in the hypothalamic release of somatostatin or TRH.(ABSTRACT TRUNCATED AT 400 WORDS)

High serum levels of the thyrotropin-releasing hormone-like peptide pyroglutamyl-glutamyl-prolineamide in patients with carcinoid tumors.

The TRH-like peptide pGlu-Glu-Pro-NH2 ( < EEP-NH2) has been identified in the prostate, the anterior pituitary, and a human neuroblastoma cell line. We here report the determination of TRH-like immunoreactivity (TRH-LI) in serum of control subjects and patients with carcinoid tumors. TRH-LI was distinguished from authentic TRH (pGlu-His-Pro-NH2) in RIAs using the nonspecific antiserum 4319, which recognizes most peptides with the structure pGlu-X-Pro-NH2, or the TRH-specific antiserum 8880. TRH levels were undetectable ( < 25 pg/mL) in unextracted serum from healthy subjects, whereas the TRH-LI level was 42 +/- 22 pg/mL (mean +/- SD; n = 175). TRH levels were also undetectable in unextracted serum from 60 patients with carcinoid tumors, whereas TRH-LI levels ranged from less than 10 to 2540 pg/mL, being elevated in 27 of 60 (45%) patients. Serum TRH-LI was significantly correlated with 1) the number of tumor localizations (tumor load), 2) the presence of liver metastases, and 3) urinary 5-hydroxyindoleacetic acid excretion. Serum TRH-LI was completely extracted with methanol, and its identity was analyzed in serum extracts from 3 patients with carcinoid tumors by QAE-Sephadex A-25 anion exchange chromatography and reverse phase high performance liquid chromatography. With both techniques, TRH-LI predominantly coeluted with synthetic < EEP-NH2, suggesting secretion of this peptide by carcinoid tumors. The mechanism of < EEP-NH2 production by these tumors and its possible biological function remain to be determined.

Renal clearance of the thyrotropin-releasing hormone-like peptide pyroglutamyl-glutamyl-prolineamide in humans.

TRH-like peptides have been identified that differ from TRH (pGlu-His-ProNH2) in the middle amino acid. We have estimated TRH-like immunoreactivity (TRH-LI) in human serum and urine by RIA with TRH-specific antiserum 8880 or with antiserum 4319, which binds most peptides with the structure pGlu-X-ProNH2. TRH was undetectable in serum (< 25 pg/mL), but TRH-LI was detected with antiserum 4319 in serum of 27 normal subjects, 21 control patients, and 12 patients with carcinoid tumors (range 17-45, 5-79, and 18-16,600 pg/mL, respectively). Because serum was kept for at least 2 h at room temperature, which causes degradation of TRH, pGlu-Phe-ProNH2, and pGlu-Tyr-ProNH2, serum TRH-LI is not caused by these peptides. On high-performance liquid chromatography, serum TRH-LI coeluted with pGlu-Glu-ProNH2 (< EEP-NH2), a peptide produced in, among others, the prostate. Urine of normals and control patients also contained TRH-LI (range 1.14-4.97 and 0.24-5.51 ng/mL, respectively), with similar levels in males and females. TRH represented only 2% of urinary TRH-LI, and anion-exchange chromatography and high-performance liquid chromatography revealed that most TRH-LI in urine was < EEP-NH2. In patients with carcinoid tumors, increased urinary TRH-LI levels were noted (range 1.35-962.4 ng/mL). Urinary TRH-LI correlated positively with urinary creatinine, and the urinary clearance rate of TRH-LI was similar to the glomerular filtration rate. In addition, serum TRH-LI was increased in 17 hemodialysis patients (43-373 pg/mL). This suggests that serum < EEP-NH2 is cleared by glomerular filtration with little tubular resorption. The possible role of the prostate as a source of urinary TRH-LI was evaluated in 11 men with prostate cancer, showing a 25% decrease in urinary TRH-LI excretion after prostatectomy (0.19 +/- 0.02 vs. 0.15 +/- 0.01 ng/mumol creatinine, mean +/- SEM). However, TRH-LI was similar in spontaneously voided urine and in urine obtained through a nephrostomy cannula from 16 patients with unilateral urinary tract obstruction (0.15 +/- 0.01 vs. 0.14 +/- 0.01 ng/mumol creatinine). These data indicate that: 1) TRH-LI in human serum represents largely < EEP-NH2, which is cleared by renal excretion; 2) part of urinary < EEP-NH2 is derived from prostatic secretion into the blood and not directly into urine; and 3) urinary < EEP-NH2 can be used as marker for carcinoid tumors.

Different thyroid hormone-deiodinating enzymes in tilapia (Oreochromis niloticus) liver and kidney.

Enzymes catalyzing the outer ring deiodination (ORD) of iodothyronines are important for the regulation of thyroid hormone bioactivity. We have studied ORD of thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) in liver and kidney microsomes of fish, i.e. tilapia (Oreochromis niloticus). Tilapia kidney contains an enzyme which resembles the mammalian selenoenzyme type I iodothyronine deiodinase (ID-I) with respect to substrate preference (rT3 > T4) and high (approximately microM) Km values, but is much less sensitive to selenocysteine (Sec)-targeted inhibitors, including 6-propyl-2-thiouracil (PTU). In contrast, tilapia liver contains an enzyme very similar to mammalian type II deiodinase (ID-II) with respect to substrate preference (T4 > rT3), low (approximately nM) Km values, and lack of sensitivity to Sec inhibitors.