Rapid thyroxine to 3,5,3'-triiodothyronine conversion and nuclear 3,5,3'-triiodothyronine binding in rat cerebral cortex and cerebellum.

Thyroxine (T(4)) to 3,5,3'-triiodothyronine (T(3)) conversion was evaluated in vivo in cerebral cortex, cerebellum, and anterior pituitary of male euthyroid Sprague-Dawley rats. Tracer quantities of (125)I-T(4) and (131)I-T(3) were injected into controls and iopanoic acid-pretreated rats 3 h before isolation of nuclei from these tissues. Specifically-bound nuclear (131)I-T(3), denoted T(3)(T(3)); (125)I-T(3), denoted T(3)(T(4)); and (125)I-T(4) were extracted and identified by chromatography. Plasma iodothyronines were similarly quantitated. In control rats, nuclear T(3)(T(3)) (percent dose per milligram DNA x 10(-4)) was 174+/-31 in cerebral cortex, 50+/-9 in cerebellum, and 932+/-158 in pituitary (all values, mean+/-SEM). Nuclear T(3)(T(4)) (percent dose per milligram DNA x 10(-4)) was 23.3+/-3.3 in cortex, 3.5+/-0.6 in cerebellum, and 39.4+/-6.9 in pituitary. Two-thirds of nuclear T(3)(T(4)) derived from local T(4) to T(3) conversion. Nuclear T(3)(T(4)) in all tissues was reduced to less than 15% of its control value by iopanoic acid treatment and all of the residual nuclear T(3)(T(4)) could be accounted for by plasma T(3)(T(4)). Nuclear T(3)(T(3)) binding was not inhibited by iopanoic acid. These results indicate there is rapid local T(4) to T(3) conversion in rat brain and nuclear binding of the T(3) produced. We have previously found that local T(3)(T(4)) production is the source of approximately 50% of the T(3) in rat anterior pituitary. The present observations that the ratio of locally derived nuclear T(3)(T(4)) to nuclear T(3)(T(3)) is much higher in cerebral cortex (0.1) and cerebellum (0.04) than in anterior pituitary (0.015) suggest that this locally produced T(3)(T(4)) is the predominant source of intracellular T(3) in these portions of rat brain.

Evidence for two tissue-specific pathways for in vivo thyroxine 5'-deiodination in the rat.

Propylthiouracil (PTU) is a well known inhibitor of thyroxine (T(4)) to triiodothyronine (T(3)) conversion as evidenced by its effect in several in vitro systems and by the decrease in serum T(3) caused by this drug in either rats or man receiving T(4) replacement. However, the failure of PTU to decrease the intrapituitary T(3) concentration and to completely blunt the serum T(3) concentration in T(4)-replaced athyreotic rats suggest that there may be a PTU-insensitive pathway of T(4) to T(3) conversion in some tissues. To address this question, we have studied the in vivo effect of PTU treatment on the generation of [(125)I]T(3) from [(125)I]T(4) in the serum and cerebral cortex (Cx), cerebellum (Cm), liver (L), and anterior pituitary (P) of euthyroid rats. Whereas PTU decreased the concentration of [(125)I]T(3) in the serum, L homogenates, and L nuclei after [(125)I]T(4), it did not affect the concentration of [(125)I]T(3) in homogenates or nuclei of Cx, Cm, or P. Iopanoic acid pretreatment significantly reduced the [(125)I]T(3) concentration in serum, homogenates, and cell nuclei of all these organs. Neither agent affected the metabolism or tissue distribution of simultaneously injected [(131)I]T(3). The presence of PTU in these tissues was evaluated by in vitro assessment of iodothyronine 5'-deiodinating activity using both [(125)I]rT(3) and [(125)I]T(4) as substrates. In agreement with the in vivo findings, generation of [(125)I]T(3) from T(4) in vitro was not affected by PTU in Cx, Cm, P but it was inhibited by 76% in L. However, rT(3) 5'-deiodination, known to be sensitive to PTU in these tissues, was inhibited in all four indicating that the PTU given in vivo was present in significant amounts. These results demonstrate that in rat Cx, Cm, and P unlike liver, PTU does not inhibit T(4) to T(3) conversion in vivo despite the presence of the drug in the tissues in amounts that significantly inhibit reverse T(3) 5'-deiodination. These results show that in vivo 5'-deiodination of T(4) proceeds via a PTU-insensitive pathway in the central nervous system and pituitary, while this pathway is not quantitatively important in the L. This mechanism accounts for the "locally generated" T(3) in central nervous system and pituitary and could also provide the approximately one-third of extrathyroidally produced T(3) not blocked by PTU administration in athyreotic T(4)-replaced rat.

The effect of keto-analogues of essential amino acids in severe chronic uremia.

Alpha keto-analogues of valine, leucine, isoleucine, methionine, phenylalanine, and (in one instance) tryptophan and histidine, along with the remaining essential amino acids, were administered orally to 10 patients with severe chronic uremia fed a diet low in protein but adequate in calories. Ketoacid dosage varied from 6 to 14 g daily, as sodium or calcium salts. Net nitrogen intake, calculated as intake minus urinary protein nitrogen, averaged 1.8 g/day. The urea space was either estimated or measured with [(14)C]urea and daily changes in the body urea pool were calculated. Urea appearance was measured as the sum of urea excretion and the change in urea pool. If these ketoacids were converted to amino acids and utilized for protein synthesis, a fall in urea nitrogen appearance should occur. In five subjects, ketoacids were given for 15-18 days and then withdrawn. Urea nitrogen appearance increased 1.55 g/day on withdrawing ketoacids, and corrected nitrogen balance decreased by 1.73 g/day. In two other subjects ketoacid administration was followed, on two occasions each, by a period of administration of nine essential amino acids. In three of these four instances, urea appearance rose significantly with amino acids. In four patients studied at high blood urea levels, ketoacid treatment was relatively ineffective; two of these patients responded more favorably when studied again after peritoneal dialysis. One of these improved enough clinically to be managed as an out-patient for short intervals, despite virtual anuria. No accumulation of ketoacids in plasma or urine could be detected, and no toxicity was identified.

Qualitative and quantitative differences in the pathways of extrathyroidal triiodothyronine generation between euthyroid and hypothyroid rats.

Propylthiouracil (PTU) in maximally inhibitory doses for liver and kidney iodothyronine 5'-deiodinase activity (5'D-I), reduces extrathyroidal T4 to T3 conversion by only 60-70% in euthyroid rats. A second pathway of T4 to T3 conversion (5'D-II) has been found in pituitary, central nervous system, and brown adipose tissue. 5'D-II is insensitive to PTU and increases in hypothyroidism, whereas 5'D-I decreases in hypothyroid rats. Thyroxine (T4) and triiodothyronine (T3) kinetics were assessed in euthyroid and thyroidectomized rats by noncompartmental analysis after injecting [125I]T4 and [131I]T3. Neither the volume of distribution nor the rate of fractional removal of plasma T4 was affected by the thyroid status, but the fractional removal rate of T3 was approximately 50% reduced in hypothyroid rats (P less than 0.001). Fractional T4 to T3 conversion was 22% in euthyroid and 26% in hypothyroid rats. In euthyroid rats, sufficient PTU to inhibit liver and kidney 5'D-I greater than 90% reduced serum [125I]T3 after [125I]T4 (results given as percent dose per milliliter X 10(-3) +/- SEM): 4 h, control 16 +/- 2 vs. PTU 4 +/- 1, P less than 0.005, and 22 h, control 6.4 +/- 0.4 vs. PTU 3.6 +/- 0.7, P less than 0.025. In thyroidectomized rats, the same dose of PTU also inhibited 5'D-I in liver and kidney, but had no effect on the generation of serum [125I]T3 from [125I]T4. Similarly, after 1 microgram T4/100 g bw was given to thyroidectomized rats, serum T3 (radioimmunoassay) increased by 0.30 +/- 0.6 ng/ml in controls and 0.31 +/- 0.09 ng/ml in PTU-treated rats. However, when the dose of T4 was increased to 2-10 micrograms/100 g bw, PTU pretreatment significantly reduced the increment in serum T3. T3 clearance was not affected by PTU in hypothyroid rats. The 5'D-II in brain, pituitary, and brown adipose tissue was reduced to less than or equal to 60% of control by 30 micrograms/100 g bw reverse T3 (rT3), an effect that lasted for at least 3 h after rT3 had been cleared. In rT3-pretreated thyroidectomized rats, the generation of [125I]T3 from tracer [125I]T4 was reduced in the serum: 6 +/- 1 vs. 12 +/- 1 X 10(-3)% dose/ml, P less than 0.01, during this 3-h period. We conclude that virtually all the T3 produced from low doses of exogenous T4 given to hypothyroid rats is generated via a PTU-insensitive pathway, presumably catalyzed by the 5'D-II. This is a consequence of the enhanced activity of this low Km enzyme together with the concomitant decrease in the hepatic and renal 5'D-I characteristic of the hypothyroid state. The results indicate that in some circumstances, 5D-II activity may contribute to the extracellular, as well as intracellular, T3 pool.

Differences in response to the peptidyldipeptide hydrolase inhibitors SQ 20,881 and SQ 14,225 in normal-renin essential hypertension.

We compared vascular and hormonal responses to teprotide (SQ 20,881) and captopril (SQ 14,225) in patients with normal renin essential hypertension given a 10 mEq sodium diet. In 10 patients receiving SQ 20,881, significant changes occurred in diastolic blood pressure (DBP, -13 +/- 2.5 mm Hg), angiotensin II (-7.1 +/- 2.1 pg/ml), and plasma renin activity (PRA, +6.6 +/- 1.9 ng/ml/hr) (p < 0.01 in all cases). Twenty-one patients receiving SQ 14,225 had significant changes in mean DBP (-18 +/- 1.5 mm Hg), angiotensin II (-6.6 +/- 1.5 pg/ml), and PRA (+7.8 +/- 2.4 ng/ml/hr) (all p values < 0.01). In spite of a significantly greater hypotensive response (p < 0.02), patients receiving SQ 14,225 showed increments in PRA and decrements in angiotensin II that did not differ significantly from those seen after SQ 20,881. Moreover, there was a significant change in plasma kinins in patients receiving SQ 20,881 (+2.0 +/- 0.9 ng/ml, p < 0.01) but no change in kinins in patients receiving SQ 14,225 (0.0 +/- 0.1, ns). We conclude that there are important differences in the mechanism mediating the hypotensive response to SQ 20,881 and SQ 14,225 in normal renin essential hypertension.

Contribution of prostaglandins to the antihypertensive action of captopril in essential hypertension.

To determine whether prostaglandins contribute to the depressor response to the converting enzyme inhibitor, captopril, we measured the plasma prostaglandin levels by radioimmunoassy before and after captopril administration, and then examined the effect of prostaglandin synthetase inhibition on captopril's antihypertensive effect. When a single oral captopril dose (25-100 mg) was given to 31 sodium-restricted patients with essential hypertension, the levels of the stable transformation product of prostacyclin remained unmeasurable and that of thromboxane A2 did not change, while the metabolite of PGE2 (PGE-M) increased by 53% (34 +/- 4pg/ml pre-captopril, 52 +/- 5 pg/ml after; p less than 0.001). As expected, blood pressure (BP) and angiotension II (AII levels fell, and kinin levels rose (all changes p less than 0.001). We then blocked prostaglandin synthesis in 18 of these subjects for 24 hours with either indomethacin (n = 10) or aspirin (n = 8) before repeating the captopril dose, to assess the importance of these PGE-M increments. The PGE-M responses to captopril were effectively blocked in nine of 10 subjects receiving indomethacin and four of eight receiving aspirin. In these 13 patients, the depressor response to captopril was significantly blunted (-20 +/- 3mm Hg pre-synthetase inhibition vs - 13 +/- 2 mm Hg post; p less than 0.05). When these agents did not block the PGE-M response to captopril, the BP response was also unchanged (-15 +/- 4mm Hg pre, -18 +/- 5mm Hg post). Neither indomethacin nor aspirin changed the AII or kinin responses to captopril. We conclude that the prostaglandins may be important mediators of captopril's antihypertensive effect in the sodium-restricted state.

Prevalence of abnormal thyroid function test results in patients with acute medical illnesses.

We measured serum total and free thyroxine (T4) and triiodothyronine (T3) concentrations, free T4 and T3 indexes, thyroid-stimulating hormone (TSH), thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) concentrations in 98 patients hospitalized for acute medical illnesses. The free thyroxine index (FT4I) or TSH level was abnormal in 16 percent, but only 3 percent had thyroid disease. Serum fre T4 measurements by equilibrium dialysis were abnormal in 25 percent, but no additional patients who initially had abnormal concentrations of serum free T4 were subsequently proved to have thyroid disease. Patients with supranormal serum free T4 concentrations (21 percent) ahd higher serum T4, lower serum T3, and higher serum reverse T3 (rT3) concentrations than other patients, but the measured changes in serum T4, TBG and TBPA levels could only partly account for the magnitude of the free T4 elevation. In these acutely ill patients, an accurate diagnosis of thyroid disease could be achieved by determination of FT4I and TSH level and a history of medication usage. We conclude that other tests are rarely necessary for this purpose in a patient population such as this.

Cortisol suppression test in patients with elevated adrenocorticotropic hormone levels.

Increased adrenocorticotropic hormone (ACTH) levels after bilateral adrenalectomy could be secondary to a pituitary tumor, under replacement with cortisol, or an abnormality in the hypothalamic-pituitary-adrenal feedback loop. To distinguish between these possibilities, ACTH levels were measured before and after cortisol infusion (20 mg/h for 4 hours) in five groups: normal volunteers; patients with idiopathic adrenal insufficiency; and with bilateral adrenalectomy for Cushing's syndrome with no roentgenographic evidence of pituitary tumor, with pituitary tumors, and with equivocal roentgenographic studies (suspect pituitary tumors). Control ACTH levels in all groups of patients were higher than in normal volunteers but there was overlapping. Cortisol infusion suppressed ACTH in all subjects but the reductions in the last two groups were less than in the first three. The cortisol suppression test appears to be useful in determining whether increased ACTH level after adrenalectomy is due to a pituitary tumor.

Increase in prostaglandins during converting enzyme inhibition.

1. Because changes in the plasma concentration of angiotensin II and bradykinin appear inadequate to account completely for the hypotensive response to captopril, we measured changes in plasma prostaglandins in response to increasing doses of captopril in nine supine normal male subjects studied on both a high (200 mol/l) and low (10 mol/l) sodium intake. 2. On both the high and low sodium diets, captopril induced significant (P < 0.01) increments in the 13,14-dihydro-15-keto metabolite of the vasodilatory prostaglandin E2, which correlated significantly with the fall in blood pressure (P < 0.0001). 3. No significant changes were noted in the plasma levels of 6-keto-prostaglandins F1 alpha or thromboxane B2, the stable products of prostacyclin and thromboxane A2 respectively.