Hormonal and cardiovascular response to low-intensity exercise with atropine administration.

The hormonal and cardiovascular responses to atropine and low-intensity exercise were examined in 7 young men. Subjects completed 3 trials in a single blind crossover design. During the first trial (T1), subjects received 2.0 mg of atropine intramuscularly at rest. Subsequently in trial 2 (T2), subjects received a saline placebo before 90 minutes of intermittent exercise, and during trial 3 (T3), they received atropine before 90 min of intermittent exercise [3 x (25-minute cycle/5-minute rest) @ 40% VO2 peak]. Venous blood samples and physiological data were collected before, during, and post exercise. Growth hormone (GH) was significantly increased in T2 but unchanged in T1 and T3. Cortisol (CORT) was unchanged in T1 and T2, but in T3 significantly increased (p <0.05) from 45 to 90 minutes compared to T2. Plasma luteinizing hormone (LH) was unaffected in all trials. Plasma prolactin (PRO) significantly increased in T3 from 45 to 90 minutes in comparison to T2. Norepinephrine (NE) was unaffected in T1, but significantly increased in both T2 and T3 (5 to 90 minutes). NE in T3 was also significantly higher compared to T2 (30 to 90 minutes). The heart rate (HR) and rate pressure product (RPP) significantly increased in all trials (15 to 90 minutes) and T3 was significantly elevated in comparison to T2. The administration of atropine before 90 minutes of low-intensity exercise significantly increased cortisol, prolactin, and norepinephrine, decreased growth hormone, and significantly increased cardiovascular stress.

Tungiasis in North America: a report of 2 cases in internationally adopted children.

Tungiasis is a cutaneous infestation caused by the gravid female sand flea, Tunga penetrans. We describe 2 cases of tungiasis that occurred in siblings who recently emigrated as international adoptees from Liberia to the United States. Both patients had infectious complications as a result of the infestation with T penetrans.

Myelodysplastic syndrome presenting as cutaneous purpura.

Myelodysplastic syndrome is a preleukemic, clonal disorder of the hematopoetic stem cell. Cutaneous manifestations include infections, vasculitis, neutrophilic dermatoses, and leukemia cutis. Senile purpura is a common condition occurring in elderly individuals who lack other systemic or hematologic diseases. We present a case of myelodysplastic syndrome in which cutaneous lesions resembling senile purpura were the initial clinical manifestation.

Applications of holographic interferometryto structural and dynamic analysis of anadvanced graphite-epoxy compositecomponent.

It was undertaken to apply holographic techniques to char- acterize the dynamic behavior and structure of an advanced graphite-epoxy composite part and its ancillary mounting geometry. Holograms of the vibrational modes of the structure are used to accurately map the nodes, maxima, minima, and geometry of the induced motion. Holograms of the displacement patterns of mechanical and thermally induced stress in the structure are also used to map the location and extent of nonuniformities, discontinuities, and micro-structural defects in the volume and mounting of the composite material. Holographic data was imaged by a photo-thermoplastic based holocamera system configured for off-axis holograms and coupled to high resolution video capture for subsequent image analysis.

Genocide by Attrition 1939­1993: The Warsaw Ghetto, Cambodia, and Sudan.

Genocide by attrition occurs when a group is stripped of its human rights, political, civil and economic. This leads to deprivation of conditions essential for maintaining health, thereby producing mass death. Genocide by attrition is epitomized by the Warsaw Ghetto (1939­43), Democratic Kampuchea (1975­79), and Sudan (1983­93). Potentialities of response are considered, as well as state and international interests in overlooking genocide, and the inadequacy and misuses of humanitarian aid. Lastly, guidelines are offered for future policy to prevent genocide by attrition, involving governments, health professionals, and aid workers.

Thyrotropin-releasing hormone stimulates c-jun and c-fos messenger ribonucleic acid levels: implications for calcium mobilization and protein kinase-C activation.

The hypothalamic neuropeptide TRH, through G-protein-coupled transmembrane pituitary receptors, rapidly stimulates intracellular signaling events that, in turn, stimulate gene transcription. Our previous studies in transfected pituitary tumor cells indicated that TRH stimulation of thyrotropin beta-subunit (TSH beta) gene expression involves both calcium mobilization and protein kinase-C activation. To characterize the gene-proximal elements of the intracellular signaling pathways involved, we examined the effects of TRH, ionomycin, and phorbol ester (TPA) on cellular protooncogenes (c-jun and c-fos) known to be responsive to calcium mobilization and protein kinase-C activation. TRH stimulated a 3-fold increase in both c-jun and c-fos mRNA levels within 1 h, followed by a rapid decline in steady state mRNA levels. A secondary response to the single administration was noted, culminating in a 5-fold stimulation at 20 h. The increase in c-jun and c-fos mRNA levels occurred before the increased steady state mRNA levels of both PRL and TSH beta chimera in transfected pituitary GH3 cells. Furthermore, we examined the role of calcium in these effects using the ionophore ionomycin to elevate and TMB-8 to decrease intracellular calcium. We used the phorbol ester TPA to investigate the effects of increased protein kinase-C activity and H7 or pretreatment with TPA to monitor the decreased kinase activity. Our data indicate that calcium mobilization and protein kinase activation represent distinct components of the signaling events initiated by TRH resulting in increased c-jun and c-fos mRNA levels. Only c-fos mRNA is increased by all three factors, suggesting that c-fos may be a key element in mediating the intracellular processes reflecting TRH action.

Usefulness of antimicrosomal antibody titers in the diagnosis and treatment of postpartum thyroiditis.

BACKGROUND: Postpartum thyroiditis is a common but frequently unrecognized disorder, affecting approximately 5% of women during the first 12 months after delivery. We investigated whether the antimicrosomal antibody titer could be used to determine which women with positive titers postpartum (1) might develop symptomatic or biochemical abnormalities within the first postpartum year (early disease), (2) might require therapy with thyroid hormone, and (3) might have persistent abnormalities (late disease). METHODS: Women (n = 55) who had positive antimicrosomal antibody titers at delivery were prospectively followed for 11 to 45 months. Titers were evaluated again at 6 to 10 weeks postpartum and approximately every 8 weeks for the first year. RESULTS: Early disease occurred in 40 of 55 (73%) women, late disease occurred in 29 of 55 (53%) women, and treatment was required by 21 of 55 (38%) women. The occurrence of early disease was associated with the occurrence of late disease (P < .05). The chances of developing early disease were 6 to 1 (P = .01) when serum titers of antimicrosomal antibodies were > or = 400 at delivery, and 5 to 1 (P = .02) when titers were > or = 1600 at 6 to 10 weeks postpartum. The chances of being given thyroid hormone therapy were 23 to 1 (P = .006) when titers at delivery were > or = 6400, and 6 to 1 when titers at 6 to 10 weeks postpartum were > or = 6400 (P = .004). Titers were not useful in estimating who would have late disease. CONCLUSIONS: Screening for postpartum thyroid dysfunction after delivery using antimicrosomal antibody titers is highly useful. The titer value can help guide the physician in the care of patients with postpartum thyroiditis whose disease may not be self-limiting and who will probably require thyroid hormone therapy.

A cryptic peptide (160-169) of thyrotropin-releasing hormone prohormone demonstrates biological activity in vivo and in vitro.

TRH is synthesized as a precursor peptide containing five copies of the sequence Gln-His-Pro-Gly, QHPG, flanked by paired basic amino acids, and linked by other peptides. We tested one cryptic peptide, PPT (160-169, SFPWMESDVT), as a possible physiological regulator of pituitary activity in vivo. Male rats were cannulated (jugular) and received a single dose of either PPT or TRH (10(-8)-10(-6) M). PPT caused no consistent effects on either TSH or PRL secretion, while TRH stimulated the secretion of both hormones. However, PPT stimulated a dose-dependent increase in both pituitary TSH beta and PRL mRNA content at 240 min similar to TRH. In primary cultures of rat pituitaries, PPT stimulated a maximum 4-fold increase in TSH beta mRNA and a 2-fold increase in PRL mRNA in 4 h, while TRH increased both TSH beta and PRL mRNA approximately 3-fold. Again, PPT had no significant effect on TSH or PRL secretion into the medium. Thus, PPT appears to be a physiological regulator of both TSH and PRL synthesis, but, unlike TRH, does not act as a secretagogue.

U-73122, an aminosteroid phospholipase C antagonist, noncompetitively inhibits thyrotropin-releasing hormone effects in GH3 rat pituitary cells.

TRH increases cytosolic-free calcium ([Ca2+]i) by activating phospholipase C(PL-C), which induces phosphoinositol hydrolysis, leading to Ca2+ mobilization from inositol trisphosphate (IP3) sensitive stores, and by increasing Ca2+ influx. Increases in [Ca2+]i stimulate PRL secretion. We investigated the effects of U-73122, an aminosteroid inhibitor of PL-C dependent processes, on TRH-stimulated second messenger pathways and on PRL secretion in GH3 rat pituitary cells. [Ca2+]i was monitored by Indo-1 fluorescence, and IP3 and metabolites separated on ion exchange columns. In Ca(2+)-free buffer, [Ca2+]i was 96 +/- 6 nM and increased to 323 +/- 23 nM (P less than 0.001) after TRH (100 nM). U-73122 dose dependently inhibited the TRH effect (IC50 = 967 nM; complete inhibition at 3-5 microM). Subsequent addition of monensin (100 microM) increased [Ca2+]i from 107 +/- 4 to 142 +/- 4 nM (P < 0.001), confirming our previous findings of a non-TRH regulated Ca2+ pool in GH3 cells. Pretreatment (15 sec) with U-73122 partly inhibited the TRH effect on [Ca2+]i; complete suppression occurred with 70 sec of pretreatment. An inactive analog (U-73343) had no inhibitory effect at 5 microM. U-73122 acted noncompetitively, as the mean maximum velocity (expressed as percent increase in [Ca2+]i after TRH) was reduced from 225 to 91 while the Michaelis-Menten constant for TRH was unchanged (15.4 vs. 13.8 nM, n = 3). Of note, U-73122, at 3-5 microM, increased basal [Ca2+]i from 109 +/- 5 to 120 +/- 5 nM (P less than 0.001). In 1.3 mM Ca2+ buffer containing nifedipine (1 microM) and verapamil (50 microM), similar effects of U-73122 (5 microM) were observed on basal and TRH-stimulated [Ca2+]i. IP3, IP2, and IP1 increased to 241 +/- 12%, 148 +/- 23%, and 167 +/- 39% of control, 30 sec after TRH (100 nM); these responses were prevented by 1 microM U-73122. At 5 microM, U-73122 also significantly increased IP3 levels. TRH (100 nM) increased 4-h PRL secretion from 16.3 +/- 1.4 to 27.6 +/- 3.2 ng/well (P less than 0.05). U-73122 (5 microM) increased basal PRL secretion to 35.9 +/- 3.2 ng/well (P less than 0.05), but abolished the TRH effect. In contrast, U-73343 (with Ca2+ channel blockers) did not inhibit the TRH effect on PRL (control: 24.3 +/- 2.1; TRH: 51.0 +/- 6.3 ng/well).(ABSTRACT TRUNCATED AT 400 WORDS)

Ricin and lentil lectin-affinity chromatography reveals oligosaccharide heterogeneity of thyrotropin secreted by 12 human pituitary tumors.

Some patients with thyrotropin (TSH)-producing pituitary tumors are more hyperthyroid than others despite similar TSH levels in serum, suggesting that qualitatively different TSH molecules with differing bioactivities may be secreted by different tumors. We used ricin and lentil lectin-affinity chromatography to test whether the TSH oligosaccharides varied among 12 patients with TSH-producing tumors. We found that each tumor secreted heterogeneous isoforms of TSH that differed in their extents of exposed galactose (Gal) residues, and their degrees of sialylation and core fucosylation. These biochemical parameters also varied markedly for TSH secreted by different tumors. Isoforms appeared to reflect poor sialyltransferase activity in two tumors and efficient sialyltransferase in the remainder. TSH secreted by tumors was more fucosylated than TSH secreted by control euthyroid persons. There was an inverse relationship between the sialylation and fucosylation of tumor TSH. No simple relationship between TSH oligosaccharide structures and bioactivity was evident, although mixtures of isoforms having the least and most sialylated TSH seemed to be the most bioactive clinically. In three patients from whom serum and medium TSH were both available, TSH in serum was more sialylated than TSH secreted by the tumor in vitro, perhaps reflecting slow clearance of sialylated isoforms from the circulation. Core fucosylation of serum TSH was less than that of medium TSH. These data prove that human tumors secrete TSH with heterogeneous oligosaccharide structures.

Generalized thyroid hormone resistance: identification of an arginine to cystine mutation in codon 315 of the c-erb A beta thyroid hormone receptor.

The present report studies a large kindred (WR) with generalized thyroid hormone resistance that has varying degrees of neuropsychological dysfunction, hyperactivity, poor attention span, decreased IQ and/or abnormalities in spatial perception. In this kindred, there has been found tight linkage of the syndrome with the c-erb A beta gene. The present study was performed to identify the presence of a possible gene mutation as a cause for this syndrome. DNA from peripheral leukocytes was isolated from 15 unaffected and 8 affected individuals from the kindred. Primers encompassing exons 9 (nucleotides 1171-1429) and 10 (nucleotides 1430-1698) were synthesized and used in PCR reactions to amplify these exons. Direct sequencing revealed a consistent substitution in each affected subject, but in none of the unaffected individuals, of a C to T change in one allele from nucleotide 1243, resulting in an arg to cys change in codon 315. The mutant and wild-type human beta 1 receptors were prepared and their translated proteins were analyzed for T3 binding. The WR T3 receptor from affected patients had reduced T3 binding affinity, with values approximately 2.5 x 10(10) M-1 compared to about 5 x 10(10) M-1 in normals. In summary, we have: i) identified a consistent and reproducible mutation of a C to T change in nucleotide 1243 in each of the affected but in none of the unaffected individuals of a large well characterized kindred with generalized thyroid hormone resistance; and ii) noted that the WR allele causes an approximate 50% decrease in the T3 binding affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudomalabsorption of levothyroxine.

OBJECTIVE: --The issue of patient compliance with pharmacological therapy vs malabsorption of medication was explored in the context of persistent hypothyroidism despite the administration of large doses of levothyroxine sodium. DESIGN: --Retrospective case series. SETTING: --Referred care in two large tertiary care centers. PATIENTS: --Four patients, seen within two decades, with clinical and biochemical hypothyroidism while receiving levothyroxine, were evaluated for selective malabsorption of this hormone. INTERVENTIONS: --Studies included serial measurements of thyroid hormone levels after a loading dose of levothyroxine or liothyronine sodium or evaluation with a double-labeled thyroxine tracer technique. Results were compared with studies of levothyroxine malabsorption in the medical literature. RESULTS: --All patients were ultimately found to have normal (82% to 100%) absorption of oral levothyroxine. There was no evidence that malabsorption of levothyroxine can occur as an isolated abnormality. CONCLUSIONS: --Some patients exhibit a factitious disorder suggesting malabsorption of levothyroxine. When treating hypothyroidism, psychiatric issues may result in noncompliance with levothyroxine therapy.

Diisopropylfluorophosphate (DFP) reduces serum prolactin, thyrotropin, luteinizing hormone, and growth hormone and increases adrenocorticotropin and corticosterone in rats: involvement of dopaminergic and somatostatinergic as well as cholinergic pathways.

Cholinergic mechanisms have been implicated in the regulation of anterior pituitary hormone secretion. The present study was designed to determine the effect of a single injection of an organophosphate acetylcholinesterase inhibitor, diisopropylfluorophosphate (DFP), on anterior pituitary function in male rats. DFP increased serum ACTH (2.7-fold) and corticosterone (9.1-fold), while suppressing TSH, PRL, LH, and GH by up to 95%. The earliest response was at 1 hr, with a duration of at least 18 hr for TSH and LH. Responses were similar in adrenalectomized animals. After DFP, responses to hypothalamic releasing factors were normal for TSH, GH, and ACTH, but significantly blunted for PRL and LH. TSH suppression was partially prevented by combined therapy with a nicotinic (mecamylamine) and a muscarinic (atropine) antagonist. TSH suppression was partially reversed by immunoneutralization with somatostatin antibody, and PRL suppression was completely prevented by a dopamine antagonist (haloperidol). Atropine alone prevented the effects on corticosterone. TSH pituitary content and TSH-beta mRNA were reduced by 37 and 22%, respectively, by DFP. In contrast, PRL mRNA was unchanged but PRL content was increased 3-fold. We conclude that cholinesterase inhibition evokes a multiplicity of effects on anterior pituitary function. There is a hierarchy of responses, with corticosterone being the most and TSH the least sensitive. There is evidence for inhibition at both the hypothalamic and pituitary levels, involving both nicotinic and muscarinic receptors. Although cholinesterase inhibition is the proximate event, other neurotransmitter pathways involved in TSH and PRL suppression are somatostatin and dopamine, respectively.

Thyrotropin-releasing hormone regulation of thyrotropin beta-subunit gene expression involves intracellular calcium and protein kinase C.

Our previous studies demonstrated TRH stimulation of TSH beta gene expression in rat pituitary cell cultures and GH3 tumor cells in a transient expression assay. To begin to characterize the gene-proximal elements of the pathways involved in TRH stimulation of TSH beta gene transcription, we examined the effects of factors that increase intracellular calcium concentration, [Ca2+]i, or activate protein kinase C on TSH beta promoter activity in transfected GH3 cells. TPA, a tumor-promoting phorbol ester, stimulated a dose-dependent increase in TSH beta promoter activity at 8 h similar to TRH (2-3-fold). TPA did stimulate protein kinase C activation without [Ca2+] mobilization. The calcium ionophore ionomycin increased cytoplasmic free [Ca2+] by stimulating both calcium influx and release from internal stores without affecting protein kinase C. Ionomycin also stimulated a dose-dependent increase (2-fold) in TSH beta promoter activity at 8 h. However, the voltage-dependent Ca2+ channel agonist Bay K 8644, which increased influx of extracellular calcium, had little or no effect on TSH beta gene expression until 48 h (5-fold). Similar effects on prolactin/mRNA levels were observed in these cells. Effects of these factors were not additive, suggesting a common pathway(s) to stimulate gene expression. Inhibition of intracellular calcium mobilization by treatment with 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8) inhibited ionomycin effects on gene expression without affecting phorbol ester activity, and, conversely, inhibition of protein kinase C activity by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride (H-7) or TPA desensitization blocked TPA effects without affecting ionomycin activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Tight linkage of the human c-erbA beta gene with the syndrome of generalized thyroid hormone resistance is present in multiple kindreds.

Generalized thyroid hormone resistance recently was reported to map in a single kindred to the same chromosomal region as the c-erbA beta gene, which codes for a putative thyroid hormone receptor. Restriction fragment length polymorphisms (RFLPs) of c-erbA beta were linked with GTHR in three kindreds with variable neuropsychologic dysfunction; two unrelated kindreds have been reported to possess different single base mutations in the T3 binding domain of c-erbA beta. In order to ascertain if tight linkage with c-erbA beta could be generalized to other families with this syndrome, we performed RFLP analysis in a separate laboratory on an unrelated family with GTHR which lacks the neuropsychologic defects or short stature often associated with GTHR (Kindred WR). RFLPs were identified after Bam Hl and Eco RV digestion of DNA from leukocytes from 14 family members. The Bam Hl RFLPs were 2.8 and 5.3 kb bands, and the Eco RV RFLPs were 1.6 and 3.3 kb bands. These RFLPs cosegregated with the GTHR phenotype and 11 family members were informative when both RFLPs were employed. The logarithm of the odds ratio between GTHR and c-erbA beta was 3.67, and therefore GTHR mapped to the c-erbA beta locus in this kindred. Allelic-specific hybridization with a probe constructed to identify the C to A mutation at nucleotide position 1643 (previously identified in one other kindred) suggested that our family has a different c-erbA beta abnormality. Although GTHR appears to be commonly associated with alterations in the human c-erbA beta gene, different kindreds may inherit different genetic defects.

Effects of soman on neuroendocrine and immune function.

We have previously reported that plasma growth hormone (GH) and prolactin levels were markedly decreased in rats two weeks following a single dose (100 micrograms/kg, SC) of soman. We have now conducted additional experiments to attempt to determine whether neuroendocrine responses to physiological or pharmacological challenge are altered in rat survivors of soman exposure, and whether immune function, which can be affected by circulating hormones, is altered in the soman-exposed rats. In the present study, basal prolactin levels were not significantly lower in the soman-treated rats although prolactin increases in response to physiological or pharmacological challenge were attenuated. Also, basal growth hormone levels in soman survivors were similar to control levels in 2 of 3 experiments in the present report. In the third experiment, growth hormone levels were lower in soman-treated animals. Endocrine abnormalities appeared to be related to the severity of soman insult as assessed by changes in body weight following exposure. Both ACTH and prolactin responses to stress were impaired in a severely affected subpopulation of soman survivors. The thymus, an important immune organ, was decreased in weight in severely affected soman survivors, but other tests of immune function did not show differences between control and soman-exposed rats.

Na(+)-ionophore, monensin-induced rise in cytoplasmic free calcium depends on the presence of extracellular calcium in FRTL-5 rat thyroid cells.

Calcium is an important regulator of cell function, and may be influenced by the intracellular sodium content. In the present study, the Na(+)-ionophore, monensin, was used to investigate the interrelationship between changes in intracellular Na+ concentration ([Na+]i) and elevation of cytosolic Ca2+ concentration ([Ca2+]i) in FRTL-5 thyroid cells. Cytoplasmic Ca2+ levels were measured using the fluorescent dye, indo-1. Monensin induced a dose-dependent increase in [Ca2+]i in FRTL-5 cells. Inhibitors of intracellular Ca2+ release, TMB-8 and ryanodine, were unable to prevent the monensin effect on [Ca2+]i. The alpha 1-receptor antagonist, prazosin, did not block the monensin-stimulated increase in [Ca2+]i. In the absence of extracellular calcium there was a marked diminution in the monensin effect on [Ca2+]i, yet calcium channel antagonists (nifedipine, diltiazem and verapamil) did not inhibit the response. Replacement of Na+ by choline chloride in the medium depressed the monensin-evoked rise in [Ca2+]i by up to 84%. Furthermore, addition of the Na(+)-channel agonist, veratridine, elicited an increase in [Ca2+]i, even though less dramatic than that caused by monensin. Ouabain increased the resting cytosolic Ca2+ concentration as well as the magnitude of the monensin effect on [Ca2+]i. The absence of any effect on the Na(+)-ionophore evoked increase in [Ca2+]i upon addition of tetrodotoxin (TTX) excluded a possible involvement of TTX-sensitive Na+ channels. These data show that the rise in [Ca2+]i induced by increasing [Na+]i is largely dependent on both external Na+ and Ca2+. Calcium entry appears not to involve voltage-dependent or alpha 1-receptor sensitive Ca2+ channels, but may result from activation of an Na(+)-Ca2+ exchange system.

Effects of irradiation and semistarvation on rat thyrotropin beta subunit messenger ribonucleic acid, pituitary thyrotropin content, and thyroid hormone levels.

The effect of radiation-induced anorexia on serum thyrotropin (TSH), pituitary TSH-beta mRNA, pituitary TSH content, serum thyroxine (T4), and serum 3,5,3'-triiodothyronine (T3) was investigated using feed-matched controls. Rats received 10 Gy gamma whole-body irradiation and were examined 1-3 days postirradiation. Feed-matched and untreated controls were also studied. The average food intake of the irradiated and feed-matched groups was approximately 18% of the untreated controls. Over the three day period both the irradiated and feed-matched groups lost a significant amount of body weight. The serum T4 levels of both the irradiated and feed-matched groups were not significantly different from each other, but were significantly depressed when compared to the untreated control group. The serum TSH and T3 were, however, significantly greater in the irradiated than the feed-matched groups at day 3 posttreatment. To determine if the difference in the serum TSH level between the two groups was due to a pretranslational alteration in TSH production, we measured the TSH-beta mRNA using an RNA blot hybridization assay. We found that the TSH-beta mRNA level was the same in the irradiated and feed-matched groups, suggesting that the mechanism responsible for the radiation-induced increase in the serum TSH level is posttranscriptional. Pituitary TSH content in the irradiated rats was significantly less than in pair-fed controls, suggesting that irradiation may permit enhanced secretion of stored hormone.

Alterations in rat cardiac myosin isozymes induced by whole-body irradiation are prevented by 3,5,3'-L-triiodothyronine.

Changes in cardiac myosin isozymes and serum thyroid hormone levels were investigated in rats following 10 Gy whole-body gamma irradiation. The percent beta-myosin heavy chain increased from 21.3 +/- 1.8 to 28.1 +/- 6.8 (NS) at 3-day postirradiation, 37.7 +/- 1.9 (P less than .001) at 6-day postirradiation, and 43.8 +/- 3.3 (P less than .001) at 9-day postirradiation. Along with the change in myosin isozymes was a significant 53% decrease (P less than .001) in the serum thyroxine (T4) level by day 3 postirradiation, remaining depressed through day 9 postirradiation. The serum 3,5,3'-triiodothyronine (T3) level, however, was normal until day 9, when significant depression was also observed. In contrast, the thyroid-stimulating hormone (TSH) level was significantly increased by fourfold at day 3, returning to near normal values by day 9 postirradiation. Daily injections of physiological doses of T3 (0.3 microgram/100 g body weight) prevented the change in the myosin isozymes following whole-body irradiation. Daily pharmacological injections of T3 (3.0 micrograms/100 g body weight) to the irradiated rats produced a further decrease in the percent beta-myosin heavy chain (below control values) indicating tissue hyperthyroidism. Thus, this study suggests that the change in myosin isozymes following whole-body irradiation is caused by an alteration in thyroid hormone activity.