Phototropism of rice (Oryza sativa L.) coleoptiles: fluence-response relationships, kinetics and photogravitropic equilibrium

Phototropism of rice (Oryza sativa L.) coleoptiles induced by unilateral blue light was characterized using red-light-grown seedlings. Phototropic fluence-response relationships, investigated mainly with submerged coleoptiles, revealed three response types previously identified in oat and maize coleoptiles: two pulse-induced positive phototropisms and a phototropism that depended on stimulation time. The effective ranges of fluences and fluence rates were comparable to those reported for maize. Compared with oats and maize, however, curvature responses in rice were much smaller and coleoptiles straightened faster after establishing the maximal curvature. When stimulated continuously, submerged coleoptiles developed curvature slowly over a period of 6 h, whereas air-grown coleoptiles, which showed smaller phototropic responsiveness, established a photogravitropic equilibrium from about 4 h of stimulation. The plot of the equilibrium angle against log fluence rates yielded a bell-shaped optimum curve that spanned over a relatively wide fluence-rate range; a maximal curvature of 25° occurred at a fluence rate of 1 μmol · m⁻² · s⁻¹. This optimum curve apparently reflects the light sensitivity of the steady-state phototropic response. Received: 28 June 1996 / Accepted: 30 July 1996     

Phytochrome modulation of blue-light-induced phototropism

Red light enhances hypocotyl phototropism toward unilateral blue light through a phytochrome‐mediated response. This study demonstrates how the phytochromes modulate blue‐light‐induced phototropism in the absence of a red light pre‐treatment. It was found that phytochromes A, B, and D have conditionally overlapping functions in the promotion of blue‐light‐induced phototropism. Under very low blue light intensities (0.01 µmol m^?2 s^?1) phyA activity is necessary for the progression of a normal phototropic response, whereas above 1.0 µmol m^?1 s^?2 phyB and phyD have functional redundancy with phyA to promote phototropism. PhyA also contributes to attenuation of phototropism under high fluence rates of unilateral blue light, which was previously shown to be dependent on the phototropins and cryptochromes. From these results, it appears that phytochromes are required to develop a robust phototropic response under low fluence rates, whereas under high irradiances where phototropism may be less important, phyA suppresses phototropism.     

Red-light-induced positive phototropism in Arabidopsis roots

The interaction between light and gravity is critical in determining the final form of a plant. For example, the competing activities of gravitropism and phototropism can determine the final orientation of a stem or root. The results reported here indicate that, in addition to the previously described blue-light-dependent negative phototropic response in roots, roots of Arabidopsis thaliana (L.) Heynh. display a previously unknown red-light-dependent positive phototropic response. Both phototropic responses in roots are considerably weaker than the graviresponse, which often masks phototropic curvature. However, through the use of mutant strains with impaired gravitropism, we were able to identify a red-light-dependent positive phototropic response in Arabidopsis roots. The red-induced positive phototropic response is considerably weaker than the blue-light response and is barely detectable in plants with a normal gravitropic response. Received: 22 May 2000 / Accepted: 3 July 2000     

Phytochrome-controlled phototropism of protonemata of the moss Ceratodon purpureus: physiology of the wild type and class 2 ptr–mutants

Phototropism and polarotropism in protonemata of the moss Ceratodon purpureus are controlled by the photoreceptor phytochrome. One class of phototropism mutants is characterised by growing randomly when kept for a prolonged time (5 d or longer) in unilateral red light. It was found that a subclass of these mutants grows faster than the wild type, the rate of cell division and the length of the cells being increased. This difference is found for light-grown and dark-grown filaments. It is therefore suggested that the mutant phenotype neither results from a defect in phytochrome photoconversion nor from a defect in phytochrome-gradient formation. Instead, it is possible that a factor which is involved in both signal transduction of phototropism and regulation of cell size and cell division is deregulated. If dark-grown mutant filaments are phototropically stimulated for 24 h, they show a weak phototropic response. Phototropism and polarotropism fluence-rate effect curves for mutants were flattened and shifted to higher fluence rates compared with those for the wild type. With wild-type filaments, a previously unreported response was observed. At a low fluence rate, half of the filaments grew positively phototropically, while the other half grew negatively phototropically. It seems that under these conditions, a phytochrome gradient with two maxima for the far-red-absorbing form of phytochrome (Pfr) within the cross-section of the cell is displayed by the response of the filaments. At higher fluence rates, all filaments of the wild type grew towards the light. These data and results from microbeam irradiation experiments and from phototropism studies with filaments growing within agar, indicate that light refraction plays an important role in the formation of the Pfr gradient in phototropism of Ceratodon. Received: 10 September 1998 / Accepted: 30 December 1998     

Catecholamine release in heat-stressed Antarctic fish causes proton extrusion by the red cells

Two species of Antarctic fish were stressed by moving them from seawater at −1 °C to seawater at 10 °C and holding them for a period of 10 min. The active cryopelagic species Pagothenia borchgrevinki maintained heart rate while in the benthic species Trematomus bernacchii there was an increase in heart rate. Blood pressure did not change in either species. Both species released catecholamines into the circulation as a consequence of the stress. P. borchgrevinki released the greater amounts, having mean plasma concentrations of 177 ± 54 nmol · l⁻¹ noradrenaline and 263 ± 131 nmol · l⁻¹ adrenaline at 10 min. Plasma noradrenaline concentrations rose to 47 ± 14 nmol · l⁻¹ and adrenaline to 73 ± 28 nmol · l⁻¹ in T. bernacchii. Blood from P. borchgrevinki was tonometered in the presence of isoprenaline. A fall in extracellular pH suggests the presence of a Na⁺/H⁺ antiporter on the red cell membrane, the first demonstration of this in an Antarctic fish. Treatment with the β-adrenergic antagonist drug sotalol inhibited swelling of red blood cells taken from temperature-stressed P. borchgrevinki, suggesting that the antiporter responds to endogenous catecholamines. Accepted: 22 January 1998     

Blue light- and genetically-reversed gravitropic response in protonemata of the moss Ceratodon purpureus

In darkness, protonemal filaments of Ceratodon purpureus (Brid.) grow negatively gravitropically (upwards). Red light induces a positive phototropic response mediated by the photoreceptor phytochrome. A red light treatment also has an inhibitory effect on the gravitropic response, an effect also mediated by phytochrome. In this study the effects of blue light on phototropism and on gravitropism were analysed. Unilateral blue light resulted in only a weak phototropic response, but markedly randomised growth direction. Blue light given together with a gravitropic stimulus reversed the gravitropism, changing it from negative to positive (filaments grow downward). The effect of blue light was also analysed with the mutant ptr116, which is defective in the biosynthesis of the phytochrome chromophore, and in a newly isolated mutant wwr2, which is positively gravitropic in darkness. Blue light induced the same reversal of gravitropism in ptr116 as in the wild type, indicating that phytochrome is not involved in this process. In wwr2 the direction of gravitropism was unaltered by the blue light treatment. Light also affects chlorophyll content and the size of plastids, potential statoliths for gravitropism. Red light induced an increase in plastid size and chlorophyll content in the wild type but not in ptr116. Blue light induced a similar change in wild type plastids. It seems as though light-induced alterations of gravitropism are not simply mediated by alterations in plastid properties, and that red light and blue light evoke fundamentally different responses. Received: 11 July 1997 / Accepted: 30 January 1998     

Blue Light-Induced Phototropic Response and the Intracellular Photoreceptive Site in Adiantum Protonemata

Blue light-induced phototropism in Adiantum protonemata wasinvestigated with microbeam irradiation. Brief irradiation withblue light effectively induced a phototropic response when itwas applied to a half-side of the apical 200d µm regionof a protonema. The phototropic response was partly reversedby the subsequent far-red light irradiation but the full reversalof the response was not observed even when the fluence of far-redlight was increased. In the far-red reversible part of the response,blue/far-red photoreversibility was repeatedly observed. Thus,both phytochrome and a blue light-absorbing pigment (other thanphytochrome) seem to be involved in the blue light-induced phototropicresponse in Adiantum protonemata. Furthermore, detailed studiesof the far-red light effect on the fluence-response curve forblue lightinduced phototropism revealed that the concomitantmediation by the two receptors was limited to the response inthe relatively higher fluence range of blue light and that theblue light-absorbing pigment alone was responsible in the lowerfluence range. In the higher fluence range, the response mediatedby the blue light-absorbing pigment became saturated and thephytochrome response became evident, indicating a differencein the sensitivities of the two receptor pigments to blue light. When various regions of half-sides of protonemata were irradiatedwith a blue microbeam of 10 µm width, irradiation at theapical 5–25 µm region was most effective both forphytochrome- and blue light-absorbing pigment-mediated response,indicating that the site of blue light perception is almostidentical for each response. (Received July 14, 1986; Accepted September 26, 1986)     

Negative phototropism in young gametophytes of the fern Schizaea pusilla

Schizaea pusilla is a rare fern that occurs in acidic bogs and is one of the few fern species that maintains a filamentous gametophyte throughout its development. To expand our knowledge of the physiology of this fern, phototropic responses were examined in young gametophytes. In contrast to germ filaments of other fern species, apical protonemata of young gametophytes are negatively phototropic in continuous white, red and blue light at all fluence rates tested. The expression of phototropic curvature is not limited by time since apical protonemata are also negatively phototropic when they are given brief exposures of light and then placed in the dark. In other lower plant groups such as mosses and some algae, the direction of phototropic curvature can change depending on light quality and intensity, but in young gametophytes of Schizaea, negative phototropic curvature was observed in all conditions studied. Blue light is the most effective in promoting the negative phototropic response in Schizaea.     

Blue light but not red light induces a calcium transient in the moss Physcomitrella patens (Hedw.) B., S. & G.

In caulonemal filaments of the moss, Physcomitrella patens, which had been incubated in darkness, 3 s irradiation with blue light (λ_max 450 nm) at fluence rates of 100 μmol m⁻² s⁻¹ and above caused a transient␣increase in cytosolic calcium ion concentration, [Ca²⁺]_cyt, which was both intensity- and time-dependent. Measurements of [Ca²⁺]_cyt were made using moss transformed with the cDNA for apoaequorin and reconstituting the Ca²⁺-dependent photoprotein aequorin in the cytosol by incubation in coelenterazine.␣In response to blue light at fluence rates of 100–1000 μmol photons m⁻² s⁻¹, [Ca²⁺]_cyt increased transiently from a basal level of approximately 50 nM to between 200 and 700 nM. Irradiation with red light did not evoke any measurable change in [Ca²⁺]_cyt. The presence of calcium in the incubating medium was not required for the increase in [Ca²⁺]_cyt to occur. A mutant strain, gad-139, was identified which required an irradiance of only 1 s to evoke a response. The kinetics showed a delay of approximately 6 s from the beginning of illumination before the beginning of the increase in [Ca²⁺]_cyt. The data suggest that the activation of a photoreceptor rather than the direct opening of calcium channels is involved in this blue-light response. Received: 4 December 1997 / Accepted: 4 May 1998     

Photolimitation and photoinhibition as factors determining optimal dilution rate to produce eicosapentaenoic acid from cultures of the microalga Isochrysis galbana

Eicosapentaenoic acid (EPA) productivity from continuous cultures of the marine microalga Isochrysis galbana was studied, taking into account the irradiance on the reactor surface, that is, the photolimitation/photoinhibition regime to which the cells are exposed. Experiments were conducted under a wide variety of operating conditions. The dilution rate ranged from 0.005 h⁻¹ to 0.040 h⁻¹ at five external irradiances (820, 1620, 2050, 2450 and 3270 μmol photons m⁻² s⁻¹) covering photolimited to photoinhibited growth. Under these conditions, the specific growth rate (μ) was found to be the main factor influencing EPA content (ranging from 2.35% to 5.23% dryweight) and productivity (up to 0.88 mg l⁻¹ h⁻¹). The fatty acid content was not significantly affected by the external irradiance, but was influenced by the state of growth of the microalga, depending on whether the light regime was photolimiting or photoinhibiting. It might be suggested that light should no longer be considered an isolated factor affecting EPA synthesis, but an indirect influence through the photolimitation/photoinhibition regime and growth rate. At a given dilution rate, EPA content and biomass concentration are lower under photoinhibiting external irradiances than those corresponding to photolimiting conditions, and consequently EPA productivity decays. Since the effect of photoinhibition is less marked at high biomass concentration, a strategy to optimize EPA productivity from microalgal cultures could consist of reducing the dilution rate when the external irradiance increases above the phoinhibition threshold. Received: 16 January 1998 / Revised revision: 27 March 1998 / Accepted: 27 March 1998     

Active calcium ion transport in Xenopuslaevis skin

The skin of intact, free-swimming Xenopus laevis transports Ca²⁺ inwardly in a manner that is proportional to the external [Ca²⁺] up to about 0.3 mmol · l⁻¹, saturates above 0.3 mmol · l⁻¹, and is opposed to the electrochemical gradient. Efflux is relatively constant at external concentrations between 0.016 and 0.6 mmol · l⁻¹; net flux which is negative below 0.125 mmol · l⁻¹ becomes positive above this external [Ca²⁺]. Allometric analysis suggests that both Ca²⁺ influx and efflux scale to the 2/3 power approximately like surface area. There were no significant differences in influx between summer and fall animals; however, efflux was greater in the fall and this resulted in a change from positive balance in the summer to negative balance in the fall. Isolated skins were shown to support a Ca²⁺ uptake rate of nearly 30 nmol · cm⁻² · h⁻¹. The phenylalkylamine verapamil in the apical bathing solution significantly inhibited this at 25 μmol · l⁻¹. The benzothiazepine diltiazem was also effective at 50 μmol · l⁻¹ while the dihydropyradine nifedipine was ineffective up to 100 μmol · l⁻¹. The inorganic ion La³⁺ was effective at blocking Ca²⁺ uptake at 300 μmol · l⁻¹; Ni²⁺ was also effective at 500 μmol · l⁻¹ but Co²⁺ was ineffective up to 500 μmol · l⁻¹. These results suggest that apical calcium channels in Xenopuslaevis skin have properties similar to mammalian L-channels and fish gill Ca²⁺ channels. Accepted: 23 January 1997     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

A positively gravitropic mutant mirrors the wild-type protonemal response in the moss Ceratodon purpureus

Wild-type Ceratodon purpureus (Hedw.) Brid. protonemata grow up in the dark by negative gravitropism. When upright wild-type protonemata are reoriented 90°, they temporarily grow down soon after reorientation (“initial reversal”) and also prior to cytokinesis (“mitotic reversal”). A positively gravitropic mutant designated wrong-way response (wwr-1) has been isolated by screening ultraviolet light-mutagenized Ceratodon protonemata. Protonemata of wwr-1 reoriented from the vertical to the horizontal grow down with kinetics comparable to those of the wild-type. Protonemata of wwr-1 also show initial and mitotic reversals where they temporarily grow up. Thus, the direction of gravitropism, initial reversal, and mitotic reversal are coordinated though each are opposite in wwr-1 compared to the wild-type. Normal plastid zonation is still maintained in dark-grown wwr-1 apical cells, but the plastids are more numerous and plastid sedimentation is more pronounced. In addition, wwr-1 apical cells are wider and the tips greener than in the wild-type. These data suggest that a functional WWR gene product is not necessary for the establishment of some gravitropic polarity, for gravitropism, or for the coordination of the reversals. Thus, the WWR protein may normally transduce information about cell orientation. Received: 4 November 1996 / Accepted: 26 November 1996     

Physiology of Movements in the Stems of Seedling Pisum sativum L. cv Alaska : III. Phototropism in Relation to Gravitropism, Nutation, and Growth

Phototropic response in etiolated pea (Pisum sativum L. cv Alaska) seedlings is poor. However, the curvature induced by unilateral blue light can be hastened and increased in magnitude by a previously administered red light pulse followed by several hours of darkness. Phytochrome is involved in the red light effect. Phototropic response was almost completely inhibited by removal of the apical bud and hook, but it was restored if exogenous indole-3-acetic acid was applied apically to the cut stump. Therefore, the stem contains both the phototropic photoreceptor and response mechanism. Perception of gravity and gravitropic response were also localized in the stem, but gravitropism was scarcely inhibited by decapitation. It was also observed that the kinetics and curvature pattern of gravitropism differed greatly from those of phototropism. Like phototropism, stem nutation required auxin and was promoted by red light. Unlike phototropism, photoenhanced nutational curvature required the apical hook and was propagated as a wave down the stem. Naphthylphthalamic acid inhibited, in order of decreasing effect, nutation, phototropism/gravitropism, and growth. Phototropism, gravitropism, and nutation appear to represent distinct forms of stem movement with fundamental differences in the mechanisms of curvature development.     

A polarographic, oxygen-selective, vibrating-microelectrode system for the spatial and temporal characterisation of transmembrane oxygen fluxes in plants

 A simple procedure is described for the fabrication of micrometer to nanometer-scale platinum electrodes to be used in a vibrating oxygen-selective system. The electrode was prepared by etching a fine platinum wire and insulating it with an electrophoretic paint. The dimensions allowed this electrode to be used with the “vibrating probe technique” in exploratory studies aimed at mapping and measuring the patterns of net influxes as well as effluxes of oxygen in Olea europaea L. leaves and roots with spatial and temporal resolutions of a few microns and a few seconds, respectively. The magnitude and spatial localisation of O₂ influxes in roots was characterised by two distinct peaks. The first, in the division zone, averaged 38 ± 5 nmol m⁻² s⁻¹; the second, in the elongation region, averaged 68 ± 6 nmol m⁻² s⁻¹. Long-term records of oxygen influx in the elongation region of the root showed an oscillatory regime characterised by a fast oscillation with periods of about 8–9 min. In leaves, the system allowed the measurement of real-time changes in O₂ evolution following changes in light. Furthermore, it was possible to obtain “topographical” images of the photosynthetically generated oxygen diffusing through different stomata from a region of the leaf of 120 μm × 120 μm. The combination of topographic and electrochemical information at the micrometer scale makes the system an efficient tool for studying biological phenomena involving oxygen diffusion. Received: 12 November 1999 / Accepted: 1 February 2000     

In vitro studies on calcium absorption from the gastrointestinal tract in␣small ruminants

Unidirectional flux rates of Ca²⁺ across gastrointestinal tissues from sheep and goats were measured in vitro by applying the Ussing-chamber technique. Except for the sheep duodenum, mucosal to serosal Ca²⁺ flux rates (J _ms) exceeded respective flux rates in the opposite direction (J _sm) in both species and in all segments of the intestinal tract. This resulted in net Ca²⁺ flux rates␣(J _net = J _ms − J _sm) ranging between −2 and 9 nmol · cm⁻² · h⁻¹ in sheep and between 10 and 15 nmol cm⁻² · h⁻¹ in goats. In sheep, only J _net in jejunum, and in goats, J _netin duodenum and jejunum were significantly different from zero. Using sheep rumen wall epithelia, significant J _net of Ca²⁺ of around 5 nmol · cm⁻² · h⁻¹ could be detected. Since the experiments were carried out in the absence of an electrochemical gradient, significant net Ca²⁺ absorption clearly indicates the presence of active mechanisms for Ca²⁺ transport. Dietary Ca depletion caused increased calcitriol plasma concentrations and induced significant stimulations of net Ca²⁺ absorption in goat rumen. J _net of Ca²⁺ across goat rumen epithelia was significantly reduced by 1 mmol · l ⁻¹ verapamil in the mucosal buffer solution. In conclusion, there is clear evidence for the rumen as a main site for active Ca²⁺ absorption in small ruminants. Stimulation of active Ca²⁺ absorption by increased plasma calcitriol levels and inhibition by mucosal verapamil suggest mechanistic and regulatory similarities to active Ca²⁺ transport as described for the upper small intestines of monogastric species. Accepted: 31 July 1996     

Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using ¹⁴C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of ³H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.     

Population and community responses of phytoplankton to fluctuating light

Light is a major resource in aquatic ecosystems and has a complex pattern of spatio-temporal variability, yet the effects of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light supply affects the structure and dynamics of phytoplankton communities. The effect of light fluctuations was tested at two average irradiances: low, 25 μmol quanta m⁻² s⁻¹ and high, 100 μmol quanta m⁻² s⁻¹ in 2- and 18-species communities of freshwater phytoplankton. Species diversity, and abundances of individual species and higher taxa, depended significantly on both the absolute level and the degree of variability in light supply, while total density, total biomass, and species richness responded only to light level. In the two-species assemblage, fluctuations increased diversity at both low and high average irradiances and in the multispecies community fluctuations increased diversity at high irradiance but decreased diversity at low average irradiance. Species richness was higher under low average irradiance and was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, especially at low average irradiance, where they became the dominant group, while cyanobacteria and green algae dominated low constant light and all high light treatments. Within each taxonomic group, however, there was no uniform pattern in species responses to light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of light supply had a significant effect on the growth rates of individual species grown in monocultures. Species responses to the regime of light supply in monocultures qualitatively agreed with their abundances in the community experiments. The results indicate that the temporal regime of light supply may influence structure of phytoplankton communities by differentially affecting growth rates and mediating species competition. Received: 24 September 1997 / Accepted: 8 July 1998     

Characteristics of dipeptide transport in pig jejunum in vitro

 Characteristics of dipeptide transport in pig jejunum were investigated in vitro by applying the Ussing-chamber technique and mucosal uptake studies. Addition of both glycyl-l-glutamine and glycyl-l-sarcosine (20 mmol · l⁻¹) to the mucosal buffer solution significantly increased the short-circuit current by 2.60 ± 0.15 and 1.57 ± 0.20 μeq · cm⁻² · h⁻¹, respectively. Concentration-dependent changes in short-circuit current followed Michaelis-Menten kinetics with similar affinity constants for both dipeptides. From unidirectional flux rates for radiolabelled glycyl-l-sarcosine, a net flux rate for glycyl-l-sarcosine of 49.8 ± 6.7 nmol · cm⁻² · h⁻¹ was calculated. In mucosal uptake experiments, the apical influx of ¹⁴C-labelled glycyl-l-sarcosine into isolated porcine mucosa was pH dependent and significantly inhibited by glycyl-l-glutamine. Moreover, RT-PCR studies with primers derived from rabbit PepT1 identified two PCR fragments of identical size to rabbit PepT1 from pig intestinal mRNA preparations. In conclusion, our studies revealed key features of mammalian intestinal peptide transporters and give evidence for a PepT1-like transporter in the pig jejunum that could significantly contribute to the overall amino acid absorption from the gut. Accepted: 30 June 1999     

Effects of ANG II and III and angiotensin receptor blockers on nasal salt gland secretion and arterial blood pressure in conscious Pekin ducks (Anas platyrhynchos)

The vertebrate renin-angiotensin system controls cardiovascular, renal and osmoregulatory functions. Angiotensin II (ANG II) is the most potent hormone of the RAS but in some vertebrate animals angiotensin III (Val⁴-ANG III) may be a hormone. We studied the effects of some angiotensins and mammalian ANG II receptor antagonists on nasal salt gland function and arterial blood pressure in conscious white Pekin ducks. Nasal salt gland fluid secretion (NFS) was induced by a 10 ml · kg⁻¹ bw i.v. injection of a NaCl solution (1000 mosmol · kg⁻¹ H₂O) and maintained by a continuous i.v. infusion of the same solution at a rate of 0.97 ml · min⁻¹. There was a positive linear correlation between nasal fluid [Na⁺] and osmolality, between [Na⁺] and [K⁺], and also between the rate of NFS and [Na⁺] and [K⁺]. [Asp¹,Val⁵]-ANG II (1 nmol · kg⁻¹ i.v.) inhibited NFS but did not change ionic concentrations. Val⁴-ANG III (1 or 5 nmol · kg⁻¹) and ANG I (1-7) (20 nmol · kg⁻¹) had no effect on NFS. [Sar¹, Ile⁸]-ANG II (SARILE) acted as an ANG II receptor agonist and resulted in a prolonged and complete inhibition of NFS. The AT₁ receptor antagonist, losartan (DuP 753) and the AT₂ receptor antagonist, PD 123319 both failed to block the inhibitory effect of [Asp¹, Val⁵]-ANG II on the nasal salt glands. [Asp¹,Val⁵]-ANG II (2 nmol · kg⁻¹ i.v.) increased mean arterial blood pressure (MABP), whereas the same dose of [Asn¹,Val⁵]-ANG II (teleost) had only 30% of the pressor potency of the avian ANG II. Neither 1 nor 5 nmol · kg⁻¹ of Val⁴-ANG III i.v. nor 20 nmol · kg⁻¹ of ANG I (1-7) had any measurable effect on MABP. SARILE blocked completely the pressor response to [Asp¹,Val⁵]-ANG II but the AT₁ antagonists losartan and CGP 48933 and the AT₂ antagonist PD 123319 all failed to block the pressor response to [Asp¹,Val⁵]-ANG II. These results have substantiated an important role of the nasal salt gland in potassium regulation and highlighted a pharmacological dimorphism of saralasin, namely agonist and antagonist to angiotensin II-mediated inhibition of nasal salt gland function and pressor response, respectively. Using specific nonpeptidergic angiotensin II receptor antagonists, we have confirmed the distinct pharmacology of the avian angiotensin II receptors in a nongallinaceous species and the absence of significant angiotensin I (1-7) and angiotensin II effects on the cardiovascular system and nasal salt gland. Accepted: 6 November 1997