DEVELOPMENT OF THE DIMORPHIC CHLOROPLASTS OF SUGAR CANE

The vascular bundle sheath cells of sugar cane contain starch-storing chloroplasts lacking grana, whereas the adjacent mesophyll cells contain chloroplasts which store very little starch and possess abundant grana. This study was undertaken to determine the ontogeny of these dimorphic chloroplasts. Proplastids in the two cell types in the meristematic region of light-grown leaves cannot be distinguished morphologically. Bundle sheath cell chloroplasts in tissue with 50% of its future chlorophyll possess grana consisting of 2-8 thylakoids/granum. Mesophyll cell chloroplasts of the same age have better developed grana and large, well structured prolamellar bodies. A few grana are still present in bundle sheath cell chloroplasts when the leaf tissue has 75% of its eventual chlorophyll, and prolamellar bodies are also found in mesophyll cell chloroplasts at this stage. The two cell layers in mature dark-grown leaves contain morphologically distinct etio-plasts. The response of these two plastids to light treatment also differs. Plastids in tissue treated with light for short periods exhibit protrusions resembling mitochondria. Plastids in bundle sheath cells of dark-grown leaves do not go through a grana-forming stage. It is concluded that the structure of the specialized chloroplasts in bundle sheath cells of sugar cane is a result of reduction, and that the development of chloroplast dimorphism is related in some way to leaf cell differentiation.     

Nuclear membranous whorls

Summary Membranous whorls have been seen in the nuclei of peritoneal and testicular cells which had been subjected to various experimental manoeuvres. It seems likely that this is an early manifestation of cell degeneration which is demonstrated readily only by glutaraldehyde fixation, and to that extent can be regarded as a glutaraldehyde artifact. Acknowledgements. This work was supported by grants from the Medical Research Council, and the University of Sheffield Tuberculosis Research Fund, and by a grant to the Department from Unilever Ltd.I am grateful to Professor R. Barer for his advice and criticism, to Dr. G. A. Meek for guidance on electron microscopy, to Dr. E. J. Clegg for permission to use material from joint experiments. Technical and photographic assistance was provided by Messrs. P. GarLick and L. Murgatroyd and by Miss M. Tune.     

A fluorescence depolarization study of the orientational distribution of crossbridges in muscle fibres

A fluorescence depolarization study of the orientational distribution of crossbridges in dye-labelled muscle fibres is presented. The characterization of this distribution is important since the rotation of crossbridges is a key element in the theory of muscle contraction. In this study we exploited the advantages of angle-resolved experiments to characterize the principal features of the orientational distribution of the crossbridges in the muscle fibre. The directions of the transition dipole moments in the frame of the dye and the orientation and motion of the dye relative to the crossbridge determined previously were explicitly incorporated into the analysis of the experimental data. This afforded the unequivocal determination of all the second and fourth rank order parameters. Moreover, this additional information provided discrimination between different models for the orientational behaviour of the crossbridges. Our results indicate that no change of orientation takes place upon a transition from rigor to relaxation. The experiments, however, do no rule out a conformational change of the myosin S 1 during the transition. Correspondence to: Y. K. Levine     

[3H] CGP 39653 Binding to the Agonist Site of the N-Methyl- D-Aspartate Receptor Is Modulated by Mg2+ and Polyamines Independently of the Arcaine-Sensitive Polyamine Site

Abstract: This study investigated the binding of [³H] CGP 39653, a novel high-affinity antagonist of the N-methyl-D- aspartate (NMDA) recognition site of the NMDA receptor complex. [³H] CGP 39653 bound to the NMDA receptor in well washed rat brain membranes with an affinity of about 15 nM. Other NMDA site drugs inhibited [³H] CGP 39653 binding with the following order of potency: DL-(tetrazol-5- yl)glycine > glutamate > CGS 19755 > DL-2-amino-5- phosphonovalerate (DL-AP5) > NMDA. Glycine and 5, 7- dichlorokynurenate partially inhibited binding. The poly-amines spermine and spermidine increased [³H] CGP 39653 binding (EC₅₀ values of 10 and 22 μM, respectively). This effect was mimicked by arcaine, 1, 5-diethylaminopiperidine, diaminodecane, diethylenetriamine, and Mg²⁺. The increase in [3H] CGP 39653 was a result of an increased affinity of the binding site for the ligand with very little effect on binding site density. Spermine and Mg²⁺also increased the affinity of the antagonists DL-AP5 and CGS 19755, but had only minor effects on the affinity of glutamate and NMDA. Arcaine did not reverse the enhancement of [3H] CGP 39653 binding by spermine, spermidine, or Mg²⁺. Channel-blocking dissociative anesthetics, including dizocilpine and ketamine, did not alter basal or Mg²⁺-stimulated [3H] CGP 39653 binding. Spermine did not alter either the enhancement of [³H]- dizocilpine by glutamate or the inhibition of [³H]dizocilpine by DL-AP5 or CGS 19755. These studies show that poly-amines and divalent cations selectively enhance the affinity of antagonists for the agonist binding site on the NMDA receptor complex. However, this effect is mediated by a site independent of the primary polyamine site defined using [³H] dizocilpine binding.