Thyroid Hormone Physiology in Galliformes: Age and Strain Related Changes in Physiological Control

SYNOPSIS. Considering thyroid hormone levels in the chicken,we should keep in mind that these concentrations are the integratedresult of production and release on one hand and elimination,tissue utilization or peripheral interconversion on the other.Moreover all these processes can be and are probably influencedby the relative amounts of thyroxine (T₄), triiodothyronine(T₄) and reverse-triiodothyronine (rT₄) as well as by the ratioof free/bound thyroid hormones. Additionally, other hormoneswill act upon the thyroid axis and its peripheral metabolismand vice versa. Environmental factors directly or indirectlymodify or modulate this already complicated picture. In this review, we therefore will first mention the circulatingthyroid hormone levels and the thyroid content of iodohormones.In the second section, the control by the central axis of thyroxineproduction by the thyroid gland as well as some aspects of peripheralmetabolism, with the attention focussed on thyroxine monodeiodinationis discussed. Processes which are known to be of importancein thyroid physiology such as recompartementalization or bileexcretion are hardly studied in avian species. Many questionsabout the control of thyroid functioning remain unanswered becauseof the lack of a homologous TSH radioimmunoassay. It is temptingto assume that the mammalian model can give us the answers;however, too many exceptions in regard to thyroid hormone physiologyas it is known in mammals are described, thereby underscoringthat thyroid physiology in avians should be investigated withouta bias towards research done in mammals.     

Is Lamprey Metamorphosis Regulated by Thyroid Hormones?

Lampreys are one of just a few fishes which have a true (firstor first type) of metamorphosis in their life cycle. In thesea lamprey (Petromyzon marinus), spontaneous metamorphosisis initiated when the size (length and weight), condition factor,and lipid stores reach appropriate levels and coincide withthe postwinter rise in water temperature. The serum levels ofthe thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃),drop dramatically at the onset of metamorphosis and metamorphosiscan be induced with treatment of animals with the goitrogen,KCIO₄, which also results in a decline in serum levels of thyroidhormones. The fact that thyroid hormone treatment can blockspontaneous and induced metamorphosis is support for the viewthat thyroid hormones, particularly T₃, operates like a juvenilehormone in lamprey metamorphosis; this view is counter to therole of thyroid hormones in metamorphosis of other vertebrates.The monodeiodinase pathways, whereby T₄ is converted to T₃ orto the biologically inactive reverse T₃, and even further degradationof T₃, may be a significant mechanism directing metamorphicchange. Lamprey metamorphosis is facultative in that it is initiatedor inhibited depending upon the coordination of a complex integrationof environmental, metabolic and hormonal cues. Thyroid hormonesdo not regulate lamprey metamorphosis in the sense observedin other vertebrate metamorphoses but they are important tothe developmental process. Some of the features of the involvementof thyroid hormones in lamprey metamorphosis may be relatedto the presence of the endostyle in larvae which in turn reflectsthe ancient origins of this vertebrate and perhaps the conservationof an ancient method of induction of metamorphosis. Some cluefor other factors which initiate lamprey metamorphosis may comethrough the examination of inducers of metamorphosis in lowerchordates     

Cold-induced changes in thyroid function in a poikilothermic mammal, the naked mole-rat

Cold acclimation induces very divergent responses in thyroid function in reptiles and mammals reflective of their different thermoregulatory modes. Naked mole-rats, unlike other small mammals, are unable to effectively employ endothermy and are operatively poikilotherms. We therefore investigated changes in their thyroid status with chronic cold exposure. Under simulated burrow conditions, free thyroxine (T(4); 0.39 +/- 0.09 ng/dl) and thyroid stimulating hormone (TSH; 1.12 +/- 0.56 microIU/ml) levels fell within the reptilian range, one order of magnitude lower than mammalian levels. However, cold induced typical mammalian responses: free T(4) levels (0.55 +/- 0.09 ng/dl) and thyroid follicular cell height were significantly greater. Although TSH levels (1.28 +/- 0.83 microIU/ml) were not significantly elevated, thyrotrophs exhibited ultrastructural signs of increased secretory activity. Low thyroid hormone concentrations may contribute substantially to the unusual thermoregulatory mode exhibited by naked mole-rats.     

Thyroid hormone modifies mitochondrial phenotype by increasing protein import without altering degradation

The mitochondrial phenotype within cardiac muscle cells isdramatically altered by thyroid hormone. We report here that this canbe accounted for, in part, by modifications in the rate of mitochondrial protein import. The import of matrix-localized precursor proteins malate dehydrogenase (MDH) and ornithine carbamoyltransferase was augmented, whereas the insertion of the outer membrane protein Bcl-2 was unaffected by thyroid hormone treatment. Coincident withincreases in the import of these matrix-localized precursors werethyroid hormone-induced elevations in the outer membrane receptor Tom20and the matrix heat-shock protein mthsp70. The phospholipid cardiolipinwas not involved in mediating the thyroid hormone-induced increase inimport, as judged from adriamycin inhibition studies. When the importreaction was supplemented with rat heart cytosol, we found that1) MDH import was stimulated, butBcl-2 import was inhibited and 2)thyroid hormone did not influence the effect of the cytosol on importrates. Thus distinct requirements exist for the mitochondrial import ofprecursor proteins, destined for different organellar compartments.Although import of these matrix-localized proteins was augmented bythyroid hormone treatment, the proteolysis of matrix proteins wasunaffected as indicated by the degradation ofcytob₂(167)^RIC-dihydrofolatereductase, a chimeric protein missorted to the matrix. Thus our dataindicate that at least some thyroid hormone-induced modifications ofthe mitochondrial phenotype occur due to the compartment-specificupregulation of precursor protein import rates, likely mediated viachanges in the expression of protein import machinery components.

     

Interaction between Hormones, Light, and Nutrition on Extension of Lateral Buds in Phaseolus vulgaris L.

Gibberellic acid (GA₃), kinetin, and indol-3yl-acetic acid (IAA)contained in lanolin were applied in various combinations andconcentrations to decapitated stems and petioles and to budsof Phaseolus vulgaris L. GA₃ applied alone usually promotedgrowth of main stems and laterals but this was by no means consistentand occasionally it acted in the opposite way. IAA applied alonereduced lateral bud extension slightly, but not consistently;however, when applied with GA₃ or GA₃ plus kinetin, it oftenmarkedly inhibited the promotion caused by these compounds.On occasions, however, GA₃ and IAA acted synergistically topromote and sometimes to inhibit lateral shoot growth. Kinetinalone showed few significant effects on lateral shoot growthbut applied with GA₃ it often dramatically increased GA₃-inducedgrowth of main stems and laterals. The diversity of these results,which parallels that found in the literature, was shown to bepartly dependent on the point of hormone application and ageof the plant or bud, on concentration of hormone and on lightintensity or nutrition. However, no meaningful relationshipswere found and it is concluded that growth of laterals and mainstems is dependent on a hormone balance which can be criticallymodified by a wide range of internal and external factors thenature of which is still to be determined.     

Evidence from knockout mice against physiologically significant aquaporin 8-facilitated ammonia transport

Aquaporin (AQP)8-facilitated transport of NH₃ has been suggested recently by increased NH₃ permeability in Xenopus oocytes and yeast expressing human or rat AQP8. We tested the proposed roles of AQP8-facilitated NH₃ transport in mammalian physiology by comparative phenotype studies in wild-type vs. AQP8-null mice. AQP8-facilitated NH₃ transport was confirmed in mammalian cell cultures expressing rat or mouse AQP8, in which the fluorescence of a pH-sensing yellow fluorescent protein was measured in response to ammonia (NH₃/NH₄⁺) gradients. Relative AQP8 single-channel NH₃-to-water permeability was 0.03. AQP8-facilitated NH₃ and water permeability in a native tissue was confirmed in membrane vesicles isolated from testes of wild-type vs. AQP8-null mice, in which BCECF was used as an intravesicular pH indicator. A series of in vivo studies were done in mice, including 1) serum ammonia measurements before and after ammonia infusion, 2) renal ammonia clearance, 3) colonic ammonia absorption, and 4) liver ammonia accumulation and renal ammonia excretion after acute and chronic ammonia loading. Except for a small reduction in hepatic ammonia accumulation and increase in ammonia excretion in AQP8-null mice loaded with large amounts of ammonia, there were no significant differences in wild-type vs. AQP8-null mice. Our results support the conclusion that AQP8 can facilitate NH₃ transport but provide evidence against physiologically significant AQP8-facilitated NH₃ transport in mice. water transport; transgenic mouse; liver     

Hormonal and metabolic responses to exercise across time of day and menstrual cycle phase

Galliven, E. A., A. Singh, D. Michelson, S. Bina, P. W. Gold, and P. A. Deuster. Hormonal and metabolic responses to exercise across time of day and menstrual cycle phase.J. Appl. Physiol. 83(6):1822-1831, 1997.Two studies, each utilizing short-term treadmillexercise of a different intensity, assessed the metabolic and hormonalresponses of women to exercise in the morning (AM) and late afternoon(PM). In study 1, plasmaconcentrations of growth hormone, arginine vasopressin, catecholamines,adrenocorticotropic hormone, cortisol, lactate, and glucose weremeasured before, during, and after high-intensity exercise (90%maximal O₂ uptake) in the AM andPM. In study 2, plasma concentrationsof adrenocorticotropic hormone, cortisol, lactate, andglucose were measured before, during, and aftermoderate-intensity exercise (70% maximalO₂ uptake) in the AM and PM in thefollicular (days 3-9), midcycle (days 10-16), and luteal(days 18-26) phases of themenstrual cycle. The results of studies1 and 2 revealed nosignificant diurnal differences in the magnitude of responses for anymeasured variable. In addition, study2 revealed a significant time-by-phase interaction forglucose (P = 0.014). However, netintegrated responses were similar across cycle phases. These datasuggest that metabolic and hormonal responses to short-term,high-intensity exercise can be assessed with equal reliability in theAM and PM and that there are subtle differences in blood glucoseresponses to moderate-intensity exercise across menstrual cycle phase.

     

Protein synthesis-dependent potentiation by thyroxine of antiviral activity of interferon-gamma

We have studiedthe prenuclear signal transduction pathway by which thyroid hormonepotentiates the antiviral activity of human interferon- (IFN-) inHeLa cells, which are deficient in thyroid hormone receptor (TR). Theaction of thyroid hormone was compared with that of milrinone, whichhas structural homologies with thyroid hormone.L-Thyroxine(T₄),3,5,3'-L-triiodothyronine (T₃), and milrinone enhanced theantiviral activity of IFN- up to 100-fold, a potentiation blocked bycycloheximide. The 5'-deiodinase inhibitor6-n-propyl-2-thiouracil did not blockthe T₄ effect. 3,3',5,5'-Tetraiodothyroacetic acid prevented the effect ofT₄ but not of milrinone. Theeffects of T₄ and milrinone wereblocked by inhibitors of protein kinases C (PKC) and A (PKA) andrestored by PKC and PKA agonists; only the effect ofT₄ was blocked by genistein, atyrosine kinase inhibitor. In separate models, milrinone was shown notto interact with nuclear TR-.T₄ potentiation of the antiviralactivity of IFN- requires PKC, PKA, and tyrosine kinase activitiesbut not traditional TR.

     

LIGHT-INDUCED OXYGEN EVOLUTION CAUSED BY HYDROGEN PEROXIDE IN CATALASE-INHIBITED CHLORELLA CELLS

1. Chlorella cells and spinach chioroplasts, whose catalase activityhad been more than 90% inhibited by 10^–5 M azide, werefound to decompose H₂O₂ photochemically to liberate oxygen,indicating that H₂O₂ was used as an oxidant of the HILL reaction.
2. That, however, the observed phenomena cannot be fully accountedfor in terms of the HILL reaction with H₂O₂ was revealed bythe observation that an extract of Chiorella cells, which hadbeen completely freed from chlorophyll, also showed a light-acceleratedO2 evolution from H₂O₂ in the presence of 10^–5 M azide.This extract contained a large quantity of catalase, which seemedto have been, in some way, involved in the reaction in question.
3. The catalatic H₂O₂ decomposition caused by crystalline catalaseof mammalian liver (in the presence of 10^–5 M azide) wasnot accelerated by the effect of light.

¹ Present address: Department of Biology, Faculty of Science,Niigata University, Niigata. (Received June 4, 1961; )     

Characterization of ganglioside associated with the thyrotrophin receptor

The receptor protein for thyrotrophin (thyroid-stimulating hormone;TSH) is associated with a glycosphingolipid moiety. The proteinbelongs to the family of receptors that couple to guanine nucleotidebinding proteins; the glycosphingolipid contains sialic acidand belongs to the family of gangliosides. This report definesthe structure of the receptor ganglioside in the Fisher ratthyroid cell line (FRTL-5). Receptor protein was purified byTSH affinity chromatography from FRTL-5 cells, biosyntheticallylabelled with [³H]galactose and [³H]glucosamine, and resolvedby SDS-PAGE. A single radiolabelled band of M_r     

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains CO₂-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to CO₂. Cells isolated from the dorsal subesophageal ganglia retained CO₂ chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel (I_KA and I_KDR). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that I_KA was affected by extracellular or intracellular pH, but the activity of I_KDR was modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel (I_KCa) in neurons in situ. The results of these studies support the hypothesis that I_KA initiates the chemosensory response, and I_KDR and I_KCa prolong the period of activation of CO₂-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates CO₂ chemosensory responses in H. aspersa. We did not find a single "chemosensory channel," and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein "machinery" of CO₂ chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory CO₂ chemosensor probably depends on the resting membrane potential and synaptic connectivity. carbon dioxide     

Whole Plant Carbon Gain of an Endangered Herbaceous Species Aster kantoensis and the Influence of Shading by an Alien Grass Eragrostis curvula in its Gravelly Floodplain Habitat

Aster kantoensis, an endangered plant species, is endemic togravelly floodplains of a few large rivers in central Japan.In recent years, competitive exclusion by alien perennial grassesin its natural habitat has been suspected to be one of the majorfactors threatening this species. In the River Kinu, increasedshading by the perennial alien grass Eragrostis curvula reduceslight availability for A. kantoensis. To reveal the influenceof shading on the establishment and growth of A. kantoensisrosettes, the potential carbon gain of A. kantoensis in itsnatural habitat was estimated using microenvironmental dataand whole plant photosynthetic and respiratory responses tolight and temperature. Whole plant CO₂exchange responses weremeasured with a specifically designed ‘double chamber’,which enabled measurement of the CO₂gas exchange rates of thefoliage (F) and the culm (C; stem and roots) separately. Itwas demonstrated that in a plant with average C/F ratio, positivecarbon gain could be maintained only in the microsites wherethe relative PPFD (photosynthetically active photon flux density)was above 15 or 30% of unshaded conditions in early- or mid-summer,respectively. Increasing C/F ratio, caused by an increase inroot biomass as an adaptive response to drought, resulted ina large reduction in the carbon gain irrespective of microsite,weather and season. The high light requirement of A. kantoensisis interpreted as a cost of the morphological responses necessaryto avoid stresses characteristic of this gravelly floodplainhabitat. Copyright 2000 Annals of Botany Company Aster kantoensis Kitam., Eragrostis curvula Nees, biomass distribution, carbon gain, gravelly floodplain, respiration, shading, whole plant photosynthesis     

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a dryingsoil a non-hydraulic (chemical) signal is produced in the rootand transported to the shoot, causing stomatal closure and growthretardation. This study was designed to reveal genetic diversityin wheat response to soil conditions which elicit a root signal,as the first step in the investigation of the genetic controlof the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were establishedin the growth chamber in soil-filled polyvinyl chloride tubes,120 cm long and of an internal diameter of 10·2 cm. Soilwas well fertilized and wet to field capacity at emergence whentwo treatments were imposed: (1) tubes were watered from thetop as needed to eliminate stress (control); and (2) tubes hada constant water table at a soil depth of 100 to 120 cm, withno applied water. Measurements were performed on five dateson leaf water status and stomatal diffusive resistance. Above-groundbiomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soilconditions which were previously indicated to elicit a possibleroot signal. Under these experimental conditions, cultivarsdiffered in their leaf water status, stomatal diffusive resistance(R_s) and plant production. In the control treatment, R_s of cultivarsincreased with reductions in their relative water content (RWC)and leaf water potential (LWP), indicating the expected controlof R_s by leaf water status. Under conditions of a drying topsoil, relative water content (RWC) and leaf water potential(LWP) increased in cultivars that had a higher R_s, indicatingthat stomatal activity was controlling leaf water status. Itwas therefore suggested that the drying top soil elicited aroot signal which caused stomatal closure and reduced plantproduction. Under such conditions, two cultivars (Bethlehemand V748) consistently maintained relatively low R_s and highplant production, despite their relatively lower RWC and LWP,as compared with cvs C97, V747 and V652. Limited observationssuggest that in these two cultivars relatively fewer roots mayhave been exposed to the drying top soil, as compared with theother three cultivars. Key words: Triticum aestivum, cultivars, soil moistrue, drought stress, root, root signal, stomata, relative water content, leaf water potential, biomass, yield     

Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity ([Os]_e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (I_L) and T-type (I_T) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]_e) and strong (31-32% decrease in [Os]_e). Exposure to moderate hyposmotic stimuli resulted in three response types in I_L (a decrease, a biphasic effect, and an increase in I_L) and in increase in I_T. Exposure to strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Thus it appears that the main effect of decrease in [Os]_e is increase in calcium channel currents. This increase was differential (I_L were more sensitive than I_T) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of I_L and I_T may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed. L-type channels; mechanosensitivity; somatotrophs; lactotrophs     

The escape behavior of marine copepods in response to a quantifiable fluid mechanical disturbance

The threshold shear values needed to elicit the escape reactionto a quantifiable fluid mechanical disturbance were comparedbetween five free-swimming oceanic copepod species. The resultsindicate a significant difference in the threshold for differentspecies of copepods and between different age groups withina single species. In general, animals captured from more energeticregimes required a higher threshold than those captured frommore pacific locations. Labidocera madurae required the highestshear values with 51.5 s^–1 for 50% of the animals testedto elicit an escape reaction (S₅₀). Acartia tonsa and Euchaetarimana, in contrast, were behaviorally the most sensitive requiringan S₅₀ of only 1.5 and 4.1 s^–1, respectively, to initiatean escape reaction. Pleuromamma xiphias and Oithona requiredintermediate shear values with an S₅₀ of 7.2 and 8.1 s^–1.When compared to literature values, the threshold needed toelicit an escape reaction was consistently higher than averageenvironmental shear values. Threshold shear values also variedsignificantly with developmental stage. Naupliar stages of A.tonsarequired greater than six times the S₅₀ value required by adultsof the same species. This suggests that the higher vulnerabilityto predation of naupliar stages of copepods may not only reflectinferior escape strength, but may also result from the higherthreshold needed to elicit an escape reaction. This study supportsthe hypothesis that selective feeding patterns exhibited bypredators of copepods may be the result of the differentialbehavioral sensitivities of different species and developmentalstages of copepods.     

Studies on Plant Growth Hormones: III. APPLICATION OF ENZYME REACTION KINETICS TO CELL ELONGATION IN THE AVENA COLEOPTILE

1. Previous reports that Michaelis enzyme kinetics may be appliedto the system controlling cell elongation in Avena coleoptilesare confirmed.
2. Substrate (applied hormone) and enzyme arenot in direct contactwith each other. Active transport of theapplied hormone incoleoptile sections, and permeability ofcells to hormone, regulatethe intracellular substrate concentration,thereby markedlyinfluencing the value of K₅, a parameter givinga measure ofthe affinity of substrate for enzyme.
3. The effectof permeability and transport factors on calculatingK₁ valuesof competitive hormone inhibitors is considered. Dataof otherworkers are used to show that observed K₁ values arenot necessarilyindependent of the hormone used.
4. Auxin-induced inhibitionof cell elongation results primarilyfrom a toxic action ofhormones on cell protoplasm leading finallyto cellular disorganization,shrinkage, and death. The datado not decisively support thehypothesis of inhibition resultingfrom steric hindrance totwo-point attachment between substrateand enzyme.

     

Developmental Exposure to Polychlorinated Biphenyls or Dioxin: Do Changes in Thyroid Function Mediate Effects on Spatial Learning?

Polychlorinated biphenyls (PCBs) and dioxins are widespreadenvironmental contaminants that are suspected of causing cognitivedeficits in children exposed in utero. Very little is knownabout which of the PCB and dioxin congeners present in the environmentare responsible for the changes in cognitive function or aboutthe mechanisms through which these chemicals affect the centralnervous system (CNS). Because both PCBs and dioxins reduce circulatingthyroid hormone levels, it has been proposed that these chemicalsmay affect CNS function indirectly by reducing the availabilityof thyroid hormone to the brain during development. Thyroidhormones play a critical role in brain development, and spatiallearning and memory is one of the behavioral functions mostseverely affected by neonatal hypothyroidism. In the studiesreviewed here, we investigated the effects of three ortho-substitutedPCBs, two coplanar PCBs and 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD) on spatial learning and memory in the rat. Pups exposedto PCBs or TCDD in utero and during lactation were tested whenthey reached adulthood. All three ortho-substituted PCBs causeddeficits in spatial learning, but only two of the three reducedcirculating thyroid hormone concentrations. Furthermore, thelearning deficit was observed only in females, but thyroid hormoneswere equally depressed in males and females. Thus, the PCB-induceddeficit in spatial learning did not appear to be mediated bythe decreased thyroid hormone levels. TCDD and coplanar PCBscaused moderate reductions in thyroid hormone, but facilitatedrather than impaired spatial learning, an effect that has beenreported in neonatally hyperthyroid rats. The possibility thatTCDD and coplanar PCBs may act as "mixed" thyroid agonists/antagonistscausing hypothyroid-like effects in some tissues and hyperthyroid-likeeffects in other tissues is discussed.     

The Influence of Nutritional State on Thyroid Function in Various Vertebrates

SYNOPSIS. The thyroid system in endothermic birds and mammalsas well as ectothermic teleost fish is profoundly sensitiveat several levels to acute and chronic alterations in the qualityand quantity of ingested food. Predominant amongst the thyroidchanges is a consistent alteration in peripheral T₃, productiondue to regulation of hepatic T₄ 5'-monodeiodinase activity,which causes decreased T₃ blood levels in caloric- (mainly carbohydrate)deficient states. The essentially similar response of endothermicand ectothermic (teleost) thyroid systems to caloric intakeprovides a unifying theme, previously lacking in comparativethyroid function, and emphasizes the need to consider nutritionaltatus in thyroid hormone studies.     

The action of cyclic-AMP on GA3 controlled responses IV. Characteristics of the promotion of seed germination in Lactuca sative variety 'Spartan Lake' by gibberellic acid and cyclic 3,5'-adenosine monophosphate

GA₃ (1 mM) promotes the germination of dark grown seeds of Lactucasativa variety "Spartan Lake" above levels obtained when theseseeds are germinated in the light for 18 hr. Cyclic-AMP alsopromotes the germination of these seeds in the dark but to amuch smaller extent than that obtainable with 1 mM GA₃ and equivalentto levels obtained with 1 µM GA₃. Combinations of lowconcentrations of GA₃ and cyclic-AMP significantly increasedgermination over levels obtained with either compound alone,promoting germination at times to near optimum levels. GA₃ isrequired for less than half the incubation period to bring aboutmaximum germination. ABA was found to inhibit the GA₃ and cyclic-AMP induced responseas well as the response obtained with combinations of the nucleotideand hormone. Theophylline and caffeine in combination with lowGA₃ concentrations promoted germination over seeds treated withthe hormone alone. Of a variety of adenine containing compoundstested only ADP and ATP gave responses similar to those obtainedwith cyclic-AMP. Some of the implications of these results arediscussed. (Received January 16, 1973; )     

FRET-based voltage probes for confocal imaging: membrane potential oscillations throughout pancreatic islets

Insulin secretion is dependent on coordinated pancreatic islet physiology. In the present study, we found a way to overcome the limitations of cellular electrophysiology to optically determine cell membrane potential (V_m) throughout an islet by using a fast voltage optical dye pair. Using laser scanning confocal microscopy (LSCM), we observed fluorescence (Förster) resonance energy transfer (FRET) with the fluorescent donor N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidyl-ethanolamine and the acceptor bis-(1,3-diethylthiobarbiturate) trimethine oxonol in the plasma membrane of essentially every cell within an islet. The FRET signal was approximately linear from V_m –70 to +50 mV with a 2.5-fold change in amplitude. We evaluated the responses of islet cells to glucose and tetraethylammonium. Essentially, every responding cell in a mouse islet displayed similar time-dependent changes in V_m. When V_m was measured simultaneously with intracellular Ca²⁺, all active cells showed tight coupling of V_m to islet cell Ca²⁺ changes. Our findings indicate that FRET-based, voltage-sensitive dyes used in conjunction with LSCM imaging could be extremely useful in studies of excitation-secretion coupling in intact islets of Langerhans. pancreatic -cell; optical electrophysiology; islet electrical coupling