Control of Ca2+ in rod outer segment disks by light and cyclic GMP

Photons absorbed in vertebrate rods and cones probably cause electrochemical changes at the photoreceptor plasma membrane by changing the cytoplasmic concentration of a diffusible transmitter substance, reducing the Na+ current flowing into the outer segment of the cell in the dark, to produce the observed membrane hyperpolarization that is the initial excitatory response. Cyclic GMP has been proposed as the transmitter because a light-activated cyclic GMP phosphodiesterase (PDE) has been found in rod disk membranes and because intracellularly injected cyclic GMP reduces rod membrane potentials. Free Ca2+ has also been proposed because increasing external [Ca2+] quickly and reversibly reduces the dark current and divalent cationophores increase the Ca2+ sensitivity. Ca2+ efflux from rod outer segments (ROS) of intact retinas occurs simultaneously with light responses. Vesicles prepared from ROS disk membranes become more permeable on illumination, releasing trapped ions or molecules, but intact outer segment disks have not previously been found to store sufficient Ca2+ in darkness and to release enough in light to meet the theoretical requirements for control of the dark current by varying cytoplasmic Ca2+ (refs 14-18). We now report experiments that show the required Ca2+ storage and release from rod disk membranes suspended in media containing high-energy phosphate esters and electrolytes approximating the cytoplasmic composition of live rod cells. Cyclic GMP stimulates Ca2+ uptake by ROS disks in such media.     

Bombesin-like immunoreactivity in the pancreas of man and other mammalian species

Summary Bombesin-like immunoreactivity has been measured in pancreatic tissues of man (12.4±1.2 pmol/g), pig (15.8±3.2), calf (4.3±0.9), rat (8.5±1.2) and guinea-pig (2.8±0.6) by a specific radioimmunoassay. Gel filtration of the pancreatic extracts revealed 2 major immunoreactive peaks: the earlier peak was eluted in the position of porcine gastrin-releasing peptide, and the later peak was eluted just after the amphibian bombesin standard. Immunocytochemistry demonstrated the presence of bombesin-like immunoreactivity in nerves in the rat pancreas, particularly in the exocrine pancreas, and occasionally in the peri-insular spaces. Isolated rat pancreatic islets were found to contain small quantities of bombesin-like immunoreactivity (0.037±0.003 fmol/islet) suggesting that mammalian bombesin-like peptides may be invovled in the regulation of endocrine as well as exocrine pancreatic secretion.Acknowledgments. We are grateful to Dr. A. V. Edwards, Physiological Laboratory, Cambridge, U. K. for providing the calf tissues, and the British Diabetic Association, U. K. for support.To whom reprint requests should be addressed.     

Photochemistry of riboflavin

Résumé Le spectre d'absorption, les caractéristiques luminescentes et photochimiques de la riboflavine sont présentés. L'interprétation des données expérimentales d'autres auteurs sur la décomposition photochimique de l'eau sensibilisée par la riboflavine est critiquée. Il faut distinguer deux cas: dans les réactions photochimiques qui ont lieu en l'absence de donneurs d'électrons ajoutés, la portion ribosique de la riboflavine se comporte en donneuse d'électrons et elle est détruité. Par contre, en présence de donneurs d'électrons adjoutés, la riboflavine agit comme véritable photosensibilisateur et n'est pas consumée dans l'ensemble de la réaction photochimique.     

The generation of random,binary unordered trees

Several techniques are given for the uniform generation of trees for use in Monte Carlo studies of clustering and tree representations. First, general strategies are reviewed for random selection from a set of combinatorial objects with special emphasis on two that use random mapping operations. Theorems are given on how the number of such objects in the set (e.g., whether the number is prime) affects which strategies can be used. Based on these results, methods are presented for the random generation of six types of binary unordered trees. Three types of labeling and both rooted and unrooted forms are considered. Presentation of each method includes the theory of the method, the generation algorithm, an analysis of its computational complexity and comments on the distribution of trees over which it samples. Formal proofs and detailed algorithms are in appendices.     

Evolution of the extracellular space in immature nervous tissue

Resumen En el cerebelo de rata recién nacida existen espacios extracelulares mayores que 1000 Å los cuales progresivamente se reducen hasta que en la tercera semana, como en el adulto, las celulas y fibras se hallan separadas por endiduras de 150–200 Å. Estas observaciones indican que el reducido espacio usualmente hallado en el adulto no es un artificio tecnico como ha sido sugerido por algunos autores.     

An extant cichlid fish radiation emerged in an extinct Pleistocene lake

The haplochromine cichlid fish of the East African Great Lakes represent some of the fastest and most species-rich adaptive radiations known, but rivers in most of Africa accommodate only a few morphologically similar species of haplochromine cichlid fish. This has been explained by the wealth of ecological opportunity in large lakes compared with rivers. It is therefore surprising that the rivers of southern Africa harbour many, ecologically diverse haplochromines. Here we present genetic, morphological and biogeographical evidence suggesting that these riverine cichlids are products of a recent adaptive radiation in a large lake that dried up in the Holocene. Haplochromine species richness peaks steeply in an area for which geological data reveal the historical existence of Lake palaeo-Makgadikgadi. The centre of this extinct lake is now a saltpan north of the Kalahari Desert, but it once hosted a rapidly evolving fish species radiation, comparable in morphological diversity to that in the extant African Great Lakes. Importantly, this lake seeded all major river systems of southern Africa with ecologically diverse cichlids. This discovery reveals how local evolutionary processes operating during a short window of ecological opportunity can have a major and lasting effect on biodiversity on a continental scale.     

Biochemistry: a cadmium enzyme from a marine diatom

The ocean biota contains a vast reservoir of genomic diversity. Here we present the sequence and preliminary characterization of a protein that is a cadmium-containing carbonic anhydrase from the marine diatom Thalassiosira weissflogii. The existence of a cadmium enzyme in marine phytoplankton may indicate that there is a unique selection pressure for metalloenzymes in the marine environment, and our discovery provides a long-awaited explanation for the nutrient-like behaviour of cadmium in the oceans.     

Escherichia coli swim on the right-hand side

The motion of peritrichously flagellated bacteria close to surfaces is relevant to understanding the early stages of biofilm formation and of pathogenic infection. This motion differs from the random-walk trajectories of cells in free solution. Individual Escherichia coli cells swim in clockwise, circular trajectories near planar glass surfaces. On a semi-solid agar substrate, cells differentiate into an elongated, hyperflagellated phenotype and migrate cooperatively over the surface, a phenomenon called swarming. We have developed a technique for observing isolated E. coli swarmer cells moving on an agar substrate and confined in shallow, oxidized poly(dimethylsiloxane) (PDMS) microchannels. Here we show that cells in these microchannels preferentially 'drive on the right', swimming preferentially along the right wall of the microchannel (viewed from behind the moving cell, with the agar on the bottom). We propose that when cells are confined between two interfaces--one an agar gel and the second PDMS--they swim closer to the agar surface than to the PDMS surface (and for much longer periods of time), leading to the preferential movement on the right of the microchannel. Thus, the choice of materials guides the motion of cells in microchannels.