Cervical ripening and parturition in cows are driven by a cascade of pro-inflammatory cytokines.

The final stages of cervical ripening and parturition resemble an inflammatory process. Although the role of cytokines in both spontaneous and experimentally induced parturitions has been described in several small laboratory animals and humans, the involvement of pro-inflammatory and regulatory cytokines in physiologic parturition in cows has not been determined. In this study, the cytokine expression profiles were assessed in bovine cervical tissue at several stages of pregnancy and at parturition. Serial biopsy samples of the cervix were obtained from 10 cows on day 185 and day 275 of pregnancy (which was on average 5.4 days before parturition) and at parturition. Messenger RNA expression levels of interleukin (IL)-1beta, IL-6, IL-8, IL-10 and tumour necrosis factor (TNF)alpha were determined using real-time polymerase chain reaction and the number of neutrophils and eosinophils was estimated by Luna and Sirius Red staining. At parturition, IL-8 expression had increased 430-fold (p < 0.001) when compared with that of the day 185 of pregnancy, large numbers of neutrophils had invaded the cervix while eosinophils remained scarce, IL-1beta had increased eightfold (p < 0.05) and IL-6 had not changed significantly. Additionally, IL-10 was increased by 10-fold (p < 0.001) and TNFalpha decreased by 57% (p < 0.05) when compared with that of the day 185 of pregnancy. The large increase in expression of IL-8, enabling the influx of neutrophils, is indicative of its important role in the final stage of cervical ripening and at parturition. As previous studies have shown that neutrophils excrete matrix metalloproteinases (MMP), this might contribute to softening of the cervix. In contrast, the only slightly increased levels of IL-1, steady concentrations of IL-6 and decreased TNFalpha, the potential consequences of increased IL-10 expression, indicate that final cervical of cows in ripening at term parturition is an inflammatory process influenced by regulatory cytokines.

Smooth muscle cells of the bovine cervical stroma may have a secretory, rather than a contractile function during parturition.

The bovine cervix contains a large amount of smooth muscle cells distributed over an outer muscular layer and within a stromal layer. The stromal layer exhibits no electromyographic (EMG) activity at parturition. This leads to the question whether the stromal smooth muscle cells of the bovine cervix are prepared to contract with parturition, or whether they have another function. To this end, cervical biopsies were repeatedly taken from 10 pregnant cows at day-185 and -275 of gestation, at spontaneous, uncomplicated calving and at 30 days after calving. The smooth muscle bundles of the stroma were immunohistochemically analysed (n = 5) with regard to their integrity and cellular density, and the degree of staining for connexin-43, smooth muscle actin alpha (SMA), desmin and vimentin. Additionally, the mRNA expression for connexin-43, SMA, desmin and vimentin was determined with RT-PCR (n = 5). The smooth muscle tissue was arranged in bundles, also at parturition. However, the cellular density of these bundles and the SMA mRNA expression were decreased at parturition. Additionally, the SMA staining and connexin-43 expression and staining remained constant during pregnancy and at parturition. This might indicate that stromal smooth muscle cells are not prepared to contract with parturition, in contrast to the myometrial smooth muscle cells. The smooth muscle cells, stained for SMA, also expressed vimentin, and the proportion of co-expression was increased at day-275 of pregnancy. This suggests that the stromal smooth muscle cells predominantly have a secretory function in cows.

MMP-2 expression precedes the final ripening process of the bovine cervix.

Collagen is denatured in the gradual cervical ripening process during late pregnancy, already before the onset of final cervical ripening at parturition. Matrix Metallo Proteinases (MMPs) might be responsible for this process. To investigate the presence and potential function of MMPs at the different stages of the ripening process, serial cervical biopsies were obtained from 10 cows at Days 185 and 275 of pregnancy (approximately 5 days before calving), at parturition and at 30 days after parturition. The mRNA and protein expression of MMP-1, MMP-2, and MMP-9 and of the tissue inhibitors of MMPs (TIMP)-1 and TIMP-2 were semi-quantitatively determined using RT-PCR, respectively, zymography, Westernblot, and ELISA techniques and the localization of MMP-2 protein and presence of granulocytes by immunohistochemistry and Luna staining. At parturition compared to 185 days pregnancy the MMP-1 protein expression and the numbers of granulocytes were significantly increased by 3 and 26-fold respectively. MMP-2 mRNA and protein expression had already increased 2.5 (P < 0.05) and twofold (P < 0.05) at 5 days before parturition, prior to final ripening. At that time, MMP-2 was present in smooth muscle cells and extra cellular matrix. TIMP-1 mRNA expression was significantly increased at parturition and TIMP-2 mRNA expression peaked at 5 days before parturition. The increased expression of MMP-2 at 5 days before parturition, suggests that in the cow MMP-2 is responsible for collagen denaturation in the last part of gradual cervical ripening, while MMP-1 and MMP-9 are only active during the final cervical ripening process at parturition.

Cervical diameter in relation to uterine and cervical EMG activity in early postpartum dairy cows with retained placentas after PGF2alpha induced calving.

The cervix must regain its normal diameter after parturition. Until now, little has been known about the pattern of cervical closure and the possible influences of myometrial and cervical contractions in this process. We continuously measured the cervical diameter with ultrasound cervimetry during the first 48h after calving in six cows with retained fetal membranes, while uterine (n=6) and cervical outer muscular layer (n=4) electromyographic (EMG) activity was measured with bipolar EMG electrodes. We found that the cervical diameter which was 6.2cm (+/-0.7) at 1.4h after calving, initially increased to 9.0cm (+/-1.0) during the first 14.8h (+/-2.8) postpartum. After this time, the diameter decreased gradually to 5.3cm (+/-1.0) at 48h after calving. The overall EMG activity after parturition decreased by 59% (+/-6) and 35% (+/-17) for the uterus and cervix, respectively. The decrease in EMG activity was due to a 50% (+/-7) decrease in EMG amplitudes of the myometrium; the EMG amplitudes of the cervix decreased by only 8% (+/-21) (P>0.05). At the same time in the cervix, burst frequency decreased by 69% (+/-17), while the decrease in burst frequency of the myometrium was only 11% (+/-5) (P>0.05). Uterine myometrial and cervical EMG activity after parturition showed burst patterns. These contractions of the uterus and cervix were accompanied by and correlated with transient dilatations of the caudal cervix. This could have functional relevance in the evacuation of the uterus.

Treadmill but not wheel running improves fatigue resistance of isolated extensor digitorum longus muscle in mice.

AIM: The present study is the first to compare the physiological impact of either forced treadmill or voluntary wheel running exercise on hindlimb muscle in mice. METHODS: Male C57BL/6 mice were subjected to either 6 weeks of forced treadmill or voluntary wheel running exercise. Mice in the treadmill running exercise group (TRE; n = 8) ran 1.9 km day(-1) at a speed of 16 m min(-1) against an uphill incline of 11 degrees. In the running wheel exercise group (RWE; n = 8) animals ran 8.8 +/- 0.2 km per day (average speed 42 +/- 2 m min(-1)). After the experimental period, animals were killed and mechanical performance and oxygen consumption of isolated extensor digitorum longus (EDL) muscle were determined during serial electrical stimulation at 0.5, 1 and 2 Hz. RESULTS: Steady-state half-width time (HWT) of twitch contraction at 0.5 Hz was significantly shorter in TRE and RWE than controls (CON) (41.3 +/- 0.2, 41.3 +/- 0.1 and 44.3 +/- 0.1 s respectively; P < 0.05). The rate of fatigue development and HWT lengthening at 2 Hz was the same in RWE and CON but lower in TRE (1.2-fold and twofold respectively; P < 0.05). EDL oxygen consumption, mitochondrial content and myosin heavy chain (MyHC) composition were not different between the groups. CONCLUSION: These results indicate that both exercise modalities have an effect on a hindlimb fast-twitch muscle in mice, with the greatest impact seen with forced treadmill running.

Na(+), K(+)-ATPase content in skeletal muscle of dogs with pituitary-dependent hyperadrenocorticism.

Several hormones regulate Na(+), K(+)-ATPase content in the muscle cell membrane, which is essential for maintaining muscle cell excitability. Chronic glucocorticoid excess is associated with muscle weakness and reduced endurance. We hypothesized that chronic glucocorticoid excess affects Na(+), K(+)-ATPase content in canine skeletal muscle, and contributes to reduced endurance and muscle weakness associated with pituitary-dependent hyperadrenocorticism (PDH) in dogs. Therefore, Na(+), K(+)-ATPase content in skeletal muscle was evaluated before and after hypophysectomy and hormone replacement (cortisone and l-thyroxin) in dogs with PDH (n=13), and in healthy controls (n=6). In addition, baseline and exercise-induced changes in plasma electrolyte concentrations and acid-base balance were evaluated before and after hypophysectomy in dogs with PDH. Na(+), K(+)-ATPase content of gluteal muscle in dogs with PDH was significantly lower than in control dogs (201+/-13pmol/g versus 260+/-8pmol/g wet weight; P<0.01). Similar differences were found in palatine muscle. After hypophysectomy and on hormone replacement, Na(+), K(+)-ATPase was increased (234+/-7pmol/g wet weight). Both plasma pH and base excess in dogs with PDH (7.44+/-0.01; 1.7+/-0.6mmol/l, respectively) were significantly higher (P<0.05) than after hypophysectomy and hormone replacement (7.41+/-0.01; -0.2+/-0.4mmol/l, respectively). Exercise induced respiratory alkalosis, but did not result in hyperkalemia in dogs with PDH. In conclusion, chronic glucocorticoid excess in dogs with PDH is associated with decreased Na(+), K(+)-ATPase content in skeletal muscle. This may contribute to reduce endurance in canine PDH, although dogs with PDH did not exhibit exercise-induced hyperkalemia. Na(+), K(+)-ATPase content normalized to values statistically not different from healthy controls after hypophysectomy and hormone replacement.

Myosin heavy chain fibre type composition in foals: analyses at the mRNA and protein level.

REASONS FOR PERFORMING STUDY: An optimal developed musculoskeletal system is vital for the performance of the horse. Previously, we showed that in the m. gluteus medius from adult untrained horses, identical mRNA and protein expression patterns were found in the majority of fibres. However, co-expression of IIa and IId/x myosin heavy chain (MyHC) was substantially more common at the protein than at the mRNA level, suggesting a transcriptionally controlled fine-tuning of these 2 genes. OBJECTIVE: To analyse the MyHC transcripts and proteins (including the cardiac alpha isoform) in the same muscle during post natal development when the muscle is adapting to movement and load. METHODS: Biopsies were taken from the m. gluteus medius of 2 Dutch Warmblood foals at 0, 2, 4, 22 and 48 weeks of age. mRNA was compared to protein expression on a fibre-to-fibre basis using in situ hybridisation and immunofluorescence. The MyHC slow (I), alpha, IIa and IId/x isoforms were analysed. RESULTS: At all ages the expression of the mRNA and protein MyHC isoforms was almost identical. Surprisingly, coexpression of the IIad isoform was also detected at the mRNA level especially early in life. The transcript of the alpha isoform was only detectable at young age, indicating silencing of the gene around birth. CONCLUSION: During the first year of life, MyHCs are continuously adapting at the mRNA and protein level. Additionally, the regulation of hybrid fibres is different from that in adult fibres. POTENTIAL RELEVANCE: We postulate that interfering in this process by e.g. early training will be levelled out by the maturation of the muscle.

Muscle characteristics of dutch warmblood foals with different genetic background at ages 6 and 12 months.

UNLABELLED: REASONS OF PERFORMING STUDY: To obtain broader insight into the muscle of foals with different genetic background, muscle fibre composition, its post natal development and citrate synthase (CS) activity of the gluteus medius were investigated. HYPOTHESIS: Because muscle properties are influenced genetically and related directly to performace ability, muscle characteristics should be affected by selection and correspond with the requirement of ths sports selected for. METHODS: The foals were divided into Group A, (n = 16), considered an average of the population, and Group B (n = 36), selected for showjumping. Biopsies were taken deeply from the gluteus medius muscle. Fibre type identification was performed immunohistochemically at ages 6 and 12 months. Citrate synthase activity was measured at age 6 months. RESULTS: age 6 months statistically significant differences were found between Groups A and B in the proportion of type I, Had and IId fibres. Oxidative fibres (I and IIad) were more abundant in Group B, while fast-glycolytic fibres (IId) were more abundant in Group A. Except for IIa, the fibre type composition at age 12 months had become equal in the 2 groups. The difference in the sum of oxidative fibres (I + IIad + IIad) between Groups A and B at age 6 months was similar to the difference in CS activity at that age, although the latter was not significant. CONCLUSIONS: At age 6 months the 2 groups had different proportions of fibre types I and IId, but these differences had disappeared at age 12 months. POTENTIAL RELEVANCE: The muscular response to training at an early age can be interpreted with knowledge of muscle characteristics and its post natal development in foals selected for showjumping.

Effects of training on Na, K-ATPase contents in skeletal muscle and K homeostasis of African draught bulls and cows.

In semiarid parts of Africa animal traction is still one of the most reliable sources for rural work power. However, draught animals have to produce most of their work power at an unfavourable moment of the year that is at the end of the dry season when feedlot is scare. To improve their condition prior to the planting season, a short training could help. The effect of training can be expressed by the changes in contents of Na(+), K(+)-pumps in the muscle cell membrane. After a training period of 15 days all cattle showed a mean increase in Na(+), K(+)-ATPase of 24% (P < 0.01) in the semitendinosus muscle of the hind leg, whereas the control group showed no change. Bulls demonstrated already after 8 days of training an increase of 20% (P < 0.05). The principal factor responsible for this up-regulation of the Na(+), K(+)-pumps is most probably the excitation of muscles during exercise. In the course of the 15 days training period, the surge of plasma K(+) in during exercise showed a tendency to decrease, but this was not significant. Nevertheless, the reduced elevations of plasma [K(+)] may delay the moment of fatigue and so improve endurance. In conclusion, a training period of 8-15 days improves the contents of Na(+), K(+)-pumps and so the possible work output of draught cattle.

Activities of UDP-glucuronyltransferase, beta-glucuronidase and deiodinase types I and II in hyper- and hypothyroid rats.

We have investigated the hypothesis that uridine 5'-diphosphate (UDP)-glucuronyltransferases (UGTs) and beta-glucuronidase are jointly involved in a mechanism for the storage and mobilization of iodothyronine metabolites in liver, kidney, heart and brain. Specifically, we predicted UGT activities to decrease and increase respectively, and beta-glucuronidase activity to increase and decrease respectively in hypo- and hyperthyroidism. To this end we have studied the effects of thyroid status on the activities of different enzymes involved in thyroid hormone metabolism in liver, kidney, heart and brain from adult rats with experimentally induced hypo- and hyperthyroidism. We used whole organ homogenates to determine the specific enzyme activities of phenol- and androsteron-UGT, beta-glucuronidase, as well as iodothyronine deiodinase types I and II. Deiodinase type I activities in liver and kidney were decreased in hypothyroid animals and, in liver only, increased in hyperthyroidism. Deiodinase type II activity was increased in hyperthyroid rat kidney only. Interestingly, in the heart, deiodinase type I-specific activity was increased fourfold, although the increase was not statistically significant. Cardiac deiodinase type I activity was detectable but not sensitive to thyroid status. Hepatic phenol-UGT as well as androsteron-UGT activities were decreased in hypothyroid rats, with specific androsteron-UGT activities two to three orders of magnitude lower than phenol-UGT activities. Both UGT isozymes were well above detection limits in heart, but appeared to be insensitive to thyroid status. In contrast, cardiac beta-glucuronidase activity decreased in hypothyroid tissue, whereas the activity of this enzyme in the other organs investigated did not change significantly. In summary, cardiac beta-glucuronidase, albeit in low levels, and hepatic phenol-UGT activities were responsive only to experimental hypothyroidism. Although a high basal activity of the pleiotropic beta-glucuronidase masking subtle activity changes in response to thyroid status cannot be ruled out, we conclude that hepatic, renal and cardiac UGT and beta-glucuronidase activities are not regulated reciprocally with thyroid status.

Ca2+ ATPase in Dutch warmblood foals compared with Na+, K+ ATPase: intermuscular differences and the effect of exercise.

We studied the effects of exercise without or with a subsequent period on pasture on Ca2+ ATPase concentration in foal skeletal muscle, and compared the results with those previously reported on Na+, K+ ATPase. Ca2+ ATPase was measured in homogenates as Ca2+-dependent steady-state phosphorylation from [gamma-32P]ATP. From day 7 after birth, 24 foals were divided into three groups: (i) staying in a box stall (Box); (ii) staying in a box stall with an exercise programme of an increasing number of sprints per day (Exercise); and (iii) staying on pasture (Pasture). Half of the foals (12 with four in each treatment group) were killed after 5 months. The remaining foals stayed on pasture until 11 months. In the 5-month Pasture group, Ca2+ ATPase concentration was 29.4 +/- 4.3 nmol/g wet weight (wt) (n = 4) in gluteus medius muscle, 25.2 +/- 3.3 nmol/g wet wt (n = 4) in semitendinosus muscle (both mixed fibre type), and 4.1 +/- 1.7 nmol/g wet wt (n = 3) in the slow masseter muscle. These values were not altered by exercise or by box rest. This was in contrast to the Na+, K+ ATPase concentration which was not different between the three muscles, but showed a 20% rise in gluteus medius and semitendinosus muscle after exercise. In the period from 5 to 11 months on pasture, there was no change in Ca2+ ATPase in any group. In conclusion, the Ca2+ ATPase concentration in foal muscle is around 6-fold higher in mixed fibres than in slow fibres. Furthermore, the enzyme is not up- or down-regulated by sprint exercise or subsequent rest.

Uptake of tri-iodothyronine and thyroxine in myoblasts and myotubes of the embryonic heart cell line H9c2(2-1).

Uptake of tri-iodothyronine (T(3)) was compared with that of thyroxine (T(4)) in the embryonic heart cell line H9c2 (2-1). These cells propagate as myoblasts and form differentiated myotubes upon reduction of the serum concentration, as indicated by a 31-fold increase in creatine kinase activity. Protein and DNA content per well were around 2-fold higher in myotubes than in myoblasts. When expressed per well, T(3) and T(4) uptake were, compared with myoblasts, 1.9- to 2-fold and 3.1- to 4-fold higher in myotubes respectively. On the other hand, the characteristics of T(3) and T(4) uptake were similar in myoblasts and myotubes. At any time-point, T(4) uptake was 2-fold higher than that of T(3), and both uptakes were energy but not Na(+) dependent. T(3) and T(4) uptake exhibited mutual inhibition in myoblasts and myotubes: 10 microM unlabeled T(3) reduced T(4) uptake by 51-60% (P<0.001), while 10 microM T(4) inhibited T(3) uptake by 48-51% (P<0.001). Furthermore, T(3) and T(4) uptake in myoblasts was dose-dependently inhibited by tryptophan (maximum inhibition around 70%; P<0.001). Exposure of the cells to T(3) or T(4) during differentiation significantly increased the fusion index (35 and 40%; P < 0.01). Finally, both myoblasts and myotubes showed a small deiodinase type I activity, while deiodinase type II activity was undetectable. In conclusion, T(3) and T(4) share a common energy-dependent transport system in H9c2(2-1) cells, that may be important for the availability of thyroid hormone during differentiation.

Preliminary studies on the concentration of Na+,K(+)-ATPase in skeletal muscle of draught cattle in Mozambique: effect of sex, age and training.

The effect of training on the potential for work in draught cattle was assessed by measuring the Na+,K(+)-ATPase in the muscle cell membrane and the elevation in the concentration of K+ in plasma during exercise. Biopsies of the semitendinosus muscle and venous blood samples were taken from the cattle used for draught work in Mozambique. No differences were found in the plasma ion or Na+,K(+)-ATPase concentrations in samples taken from Nguni, Africander and Angoni breeds. There were no significant differences in plasma ions (Na+,K+ and Cl-) or muscle Na+,K(+)-ATPase concentrations between the Angoni males and females, although the males showed an increase in Na+,K(+)-ATPase with age, while the females showed a decrease. The increase in males might be attributed to their higher level of activity in the herds than that of females. After a training period of 15 days, a significant increase in Na+,K(+)-ATPase concentration in semitendinosus muscle was found in Angoni cattle. In females, this was significant after 8 days of training (about 30%); in males after 15 days of training (about 16%). On day 15, there was a reduction in the elevation of plasma K+ during the 2 h of draught work, indicating an increased capacity of the Na+,K+ pumps to maintain the extracellular K+ concentration in working muscles and a possible delay in the moment of fatigue.

Metabolism of thyroid hormones in cultured cardiac fibroblasts of neonatal rats.

We have investigated the potential role of fibroblasts in local thyroid hormone metabolism in neonatal rat heart. Incubation of cardiac fibroblasts with thyroxine (T4) or 3,5,3'-tri-iodothyronine (T3) resulted in the appearance of water-soluble metabolites, whereas incubation of cardiomyocytes under the same conditions did not or did so to a much lesser extent. Time-course studies showed that production is already evident after 1-5 h of exposure and that the process equilibrates after 24-48 h. Analysis of the products revealed both the T4 and the T3 metabolites to be glucuronides. These results were corroborated by the detection of uridine diphosphate (UDP)-glucuronyltransferase activity in cardiac fibroblasts. We found no indication for outer ring deiodination in fibroblasts, cardiomyocytes or heart homogenates. From these results we have concluded that cardiac fibroblasts, but not cardiomyocytes, are able to glucuronidate T4 and T3 and secrete the conjugates. This could play a role in local metabolism, e.g. to protect the heart tissue from high levels of thyroid hormones.

Uptake of triiodothyronine and triiodothyroacetic acid in neonatal rat cardiomyocytes: effects of metabolites and analogs.

Cellular and nuclear uptake of [125I]tri-iodothyronine (T3) and [125I]triiodothyroacetic acid (Triac) were compared in cardiomyocytes of 2-3 day old rats, and the effect of thyroid hormone analogs on cellular T(3) uptake was measured. Cells (5-10 x 10(5) per well) were cultured in DMEM-M199 with 5% horse serum and 5% FCS. Incubations were performed for from 15 min to 24 h at 37 degrees C in the same medium, 0.5% BSA and [125I]T3 (100 pM), or [125I]Triac (240 pM). Expressed as % dose, T(3) uptake was five times Triac uptake, but expressed as fmol/pM free hormone, Triac uptake was at least 30% (P<0.001) greater than T3 uptake, whereas the relative nuclear binding of the two tracers was comparable. The 15 min uptake of [125I]T3 was competitively inhibited by 10 microM unlabeled T3 (45-52%; P<0.001) or 3,3'- diiodothyronine (T2) (52%; P<0.001), and to a smaller extent by thyroxine (T(4)) (27%; 0.05

Exercise-induced hyperkalemia in hypothyroid dogs.

We investigated the effect of hypothyroidism in dogs on (1) the Na+-, K+ -ATPase concentration in skeletal muscle, and (2) potassium (K+) homeostasis at rest and during exercise. Prior to and 1 year after induction of hypothyroidism by surgery and subsequent radiothyroidectomy, the Na+-, K+ -ATPase concentrations were quantified in biopsies of sternothyroid muscles of seven Beagle dogs by measuring [3H]ouabain binding capacity. In addition, plasma K+ concentrations were measured at rest and after treadmill exercise in six hypothyroid and seven euthyroid Beagle dogs. During hypothyroidism, the mean Na+ -, K+ -ATPase concentration in muscle biopsies was 41% lower than during euthyroidism. The mean resting plasma K+ value of the hypothyroid dogs was significantly (14%) higher than that of the euthyroid dogs. In the hypothyroid dogs, plasma K+ concentration increased significantly during exercise, whereas there was no rise in the euthyroid dogs. The rise in plasma K+ concentration could not be ascribed to muscle damage, as plasma creatine kinase concentrations remained within reference range. Also renal K+ retention was an unlikely explanation, as plasma aldosterone concentration and plasma renin activity rather increased than decreased during exercise. In conclusion, hypothyroid dogs tend to develop hyperkalemia during exercise, which for a large part can be explained by the severe reduction of the Na+ -, K+ -ATPase capacity in the skeletal muscle pool.

Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes.

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.

Characterization of uptake and compartmentalization of 3,5,3'-tri-iodothyronine in cultured neonatal rat cardiomyocytes.

The uptake of tri-iodothyronine (T(3)) in cultured neonatal rat cardiomyocytes was investigated and compared with the uptake of reverse T(3 )(rT(3)) and thyroxine (T(4)). Cellular compartmentalization of T(3) was studied by distinguishing T(3) activity associated with the plasma membrane from that in the cytosol or incorporated in the cell nucleus. T(3) and T(4) uptake displayed similar temperature dependencies which, in magnitude, differed from that of rT(3) uptake. T(3) uptake was Na(+ )independent, and sensitive to oligomycin and monodansylcadaverine (42-49% and 25% inhibition of 15-min cellular uptake respectively). Furthermore, T(3) uptake could be inhibited by tryptophan (20%) and tyrosine (12%), while 2-aminobicyclo[2,2,1]heptane-carboxylic acid had no effect. Co-incubation with tryptophan and oligomycin resulted in an additive inhibition of T(3) uptake (77%). We therefore conclude that (i) T(3) uptake is energy dependent, (ii) receptor-mediated endocytosis may be involved and (iii) the aromatic amino acid transport system T may play a role, while system L is not involved in T(3) transport in cardiomyocytes. Co-incubation with unlabeled iodothyronines showed that 3,3'-di-iodothyronine and T(3) itself were the most effective inhibitors of T(3) uptake (30% and 36% inhibition of 15-min cellular uptake respectively). At 15-min incubation time, 38% of the total cell-associated T(3) was present in the cytosol and nucleus, and 62% remained associated to the plasma membrane. Unidirectional uptake rates did not saturate over a free T(3) concentration range up to 3.9 microM. We have concluded that T(3) uptake in neonatal rat cardiomyocytes occurs by an energy- and temperature-dependent mechanism that may include endocytosis and amino acid transport system T, and is not sensitive to the Na(+) gradient. Elucidation of the molecular basis for the T(3) transporter is the subject of current investigation.

Prolonged food restriction and mild exercise in Shetland ponies: effects on weight gain, thyroid hormone concentrations and muscle Na(+),K(+)-ATPase.

We determined the effects of food supply and low-intensity training on growth, serum thyroid hormone levels and the Na(+),K(+)-pump concentration in equine skeletal muscle. Twenty-two Shetland ponies were subjected to two different feeding regimes for 2(1/2) years (11 ponies per group): food restriction (body condition score kept at 2) or ad libitum fed (body condition score kept at 8). Five ponies in each group underwent low-intensity training. Gluteus medius muscle and serum samples were obtained in April 1998. Subsequently, all ponies were fed ad libitum and the training programme was stopped. Muscle biopsies and serum samples were collected again in November 1998. Food restriction was associated with a 30-50% reduction of body weight gain. While the total thyroxine (T(4)) level was increased, the free T(4) remained at the control level. The serum total tri-iodothyronine (T(3)) and free T(3) were reduced by 30% and 49% respectively. After 6 months of refeeding there were no differences in any of the hormone levels between the ad libitum fed and the food-restricted groups. Food restriction produced a minor, but not significant, decrease in the Na(+),K(+)-pump concentration in the gluteus medius muscle of the Shetland ponies. Low-intensity training reduced weight gain of the ad libitum fed group by 25%, but had no detectable effect on the concentration of the Na(+), K(+)-pumps. We conclude that prolonged food restriction in Shetland ponies results in a weight gain reduction of 30-50%, and is associated with similar decreases in serum total and free T(3). The reduction in serum T(3) only slightly influenced the Na(+), K(+)-ATPase concentration in skeletal muscle, indicating that muscle tissue of different species may respond differently to changes in circulating thyroid hormones.

Thyroid hormone uptake in cultured rat anterior pituitary cells: effects of energy status and bilirubin.

Transport of thyroxine (T(4)) into the liver is inhibited in fasting and by bilirubin, a compound often accumulating in the serum of critically ill patients. We tested the effects of chronic and acute energy deprivation, bilirubin and its precursor biliverdin on the 15-min uptake of [(125)I]tri-iodothyronine ([(125)I]T(3)) and [(125)I]T(4) and on TSH release in rat anterior pituitary cells maintained in primary culture for 3 days. When cells were cultured and incubated in medium without glucose and glutamine to induce chronic energy deprivation, the ATP content was reduced by 45% (P<0. 05) and [(125)I]T(3) uptake by 13% (NS), but TSH release was unaltered. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced ATP content by 51% (P<0.05) and 53% (P<0. 05) under energy-rich and energy-poor culture conditions respectively; [(125)I]T(3) uptake was reduced by 66% (P<0.05) and 64% (P<0.05). Neither bilirubin nor biliverdin (both 1-200 microM) affected uptake of [(125)I]T(3) or [(125)I]T(4). Bilirubin (1-50 microM) did not alter basal or TRH-induced TSH release. In conclusion, the absence of inhibitory effects of chronic energy deprivation and bilirubin on thyroid hormone uptake by pituitary cells supports the view that the transport is regulated differently than that in the liver.