Transgenic Nicotiana plumbaginifolia plants that express either a 5-fold increase or a 20-fold decrease in nitrate reductase (NR) activity were used to study the relationships between carbon and nitrogen metabolism in leaves. Under saturating irradiance the maximum rate of photosynthesis, per unit surface area, was decreased in the low NR expressors but was relatively unchanged in the high NR expressors compared with the wild-type controls.However, when photosynthesis was expressed on a chlorophyll (Chl) basis the low NR plants had comparable or even higher values than the wild-type plants. Surprisingly, the high NR expressors showed very similar rates of photosynthesis and respiration to the wild-type plants and contained identical amounts of leaf Chl, carbohydrate, and protein. These plants were provided with a saturating supply of nitrate plus a basal level of ammonium during all phases of growth. Under these conditions overexpression of NR had little impact on leaf metabolism and did not stimulate growth or biomass production. Large differences in photochemical quenching and nonphotochemical quenching components of Chl a fluorescence, as well as the ratio of variable to maximum fluorescence, (F,/FM), were apparent in the low NR expressors in comparison with the wild-type controls. Light intensity-dependent increases in nonphotochemical quenching and decreases in F,/FM were greatest in the low NR expressors, whereas photochemical quenching decreased uniformly with increasing irradiance in all plant types. Nonphotochemical quenching was increased at all except the lowest irradiances in the low NR expressors, allowing photosystem II to remain oxidized on its acceptor side. l h e relative contributions of photochemical and nonphotochemical quenching of Chl a fluorescence with changing irradiance were virtually identical in the high NR expressors and the wild-type controls. Zeaxanthin was present in all leaves at high irradiances; however, at high irradiance leaves from the low NR expressors contained considerably more zeaxanthin and less violaxanthin than wild-type controls or high NR expressors. l h e leaves of the low NR expressors contained less Chl, protein, and amino acids than controls but retained more carbohydrate (starch and sucrose) than the wild type or high NR expressors. Sucrose phosphate synthase activities were remarkably similar in all plant types regardless of the NR activity. In contrast phosphoenolpyruvate carboxylase activities were increased on a Chl or protein basis in the low NR expressors compared with the wild-type controls or high NR expressors. We conclude that large decreases in NR have profound repercussions for photosynthesis * Corresponding author; fax 33-1-30-83-30-96. 171and carbon partitioning within the leaf but that increases in NR have negligible effects.The hypothesis that the pathways of carbon and N assimilation in leaves compete for energy and carbon skeletons in higher plants is widely accepted, but very little evidence is available to support this view (Robinson, 1988)....

Infants of diabetic mothers are at increased risk of a number of problems at birth. Among these problems are increased risks of respiratory distress syndrome and transient tachypnea of the newborn. Because surfactant synthesis, surfactant secretion, and lung fluid resorption are all mediated in part by beta-adrenergic responses, we asked if excess insulin interferes with the beta-adrenergic response cascade in fetal lung. Lungs from fetal rabbits (26 day) were grown in explant culture in hormone-supplemented culture medium. The explants were harvested after 48 h exposure to hormones and processed for determination of beta-adrenergic receptor concentration, guanine nucleotide regulatory proteins (Gs, Gi), beta-agonist stimulated adenosine 3',5'-cyclic monophosphate (cAMP) generation, cAMP-dependent phosphodiesterase activity, and choline incorporation into phosphatidylcholine. Although insulin did not change the concentration of beta-adrenergic receptors, it decreased the ability of isoproterenol to stimulate cAMP generation. Increase in stimulation over basal was similar in explants treated with dexamethasone and dexamethasone plus insulin, but absolute levels of isoproterenol-stimulated cAMP were less in the explants treated with dexamethasone plus insulin. We speculate that insulin inhibition of cAMP generation may be important in the pathogenesis of the respiratory problems of infants of diabetic mothers.

Background -Suitable models for the study of lung development are needed. The suitability of the guinea pig for studying the role of the P adrenergic response cascade in fetal lung development has been evaluated. Methods -Radioligand binding assays with iodine-125 labelled iodopindolol were performed to identify and characterise the adrenergic receptors. To demonstrate that these receptors were functional, isoprenaline and forskolin stimulated generation of cyclic AMP (cAMP) in the lung tissue was quantitated by radioimmunoassay. would be a more suitable animal model for several reasons. Firstly, the guinea pig has a longer gestation (67-69 days) than rabbits and rats and is also relatively mature at the time of delivery.4 Secondly, the proportion of time spent in the different stages of morphological maturation of the lung and the timing of these stages in relation to delivery in the guinea pig5 are more similar to those in the human than those in the other small mammals. For example, the saccular stage of lung development begins by 50 days (75% of gestation) in the guinea pig5 and by 28 weeks (70% of gestation) in the human fetus,6 whereas the rabbit and rat only reach the saccular stage by 90-95% of gestation.7 In addition, in both the guinea pig8 and the human6 alveolarisation of the lung is well established before term, and surfactant synthesis9 and induction of antioxidant enzymes'0 occur relatively late in gestation in both. The purpose of this study was to characterise 1 adrenergic responses in fetal guinea pig lung in order to assess the appropriateness of this model to study the involvement of the ,B adrenergic response cascade in lung development. Results