A labile,Ca2+-dependent cytoskeleton in rhabdomeral microvilli of blowflies

Summary Rhabdomeral microvilli of photoreceptors of the blowfly Lucilia are shown to contain a cytoskeleton. An axial filament ( 6–11 nm) in each microvillus is inserted into a terminal cap distally, and into a plug filling the narrow neck of the microvillus proximally. In some states, the axial filament projects beyond the neck; within the microvillus it is surrounded by amorphous material. Together, they form an axial complex, which supports side-arms linking it to the plasma membrane. Conventional fixation for examination with the electron microscope destroys the cytoskeleton. To preserve it, retinae are pre-treated with a Ringer's solution buffered with 20 mM imidazole and containing, minimally, the following components: (i) a protease inhibitor, usually phenylmethylsulphonyl-fluoride (PMSF); (ii) either the Ca²⁺-chelator EGTA, or the calmodulin-blocking agent trifluoperazine (TFP); and (iii) a source of divalent cations to preserve the side-arms. When EGTA is used, Mg²⁺, Sr²⁺, Ba²⁺, Mn²⁺ and Co²⁺ are effective, Ba²⁺ giving the most satisfactory contrast, and Mg²⁺ and Co²⁺ the best preservation. It is inferred that the cytoskeletal complex includes at least one Ca²⁺-activated protease, and possibly calmodulin. Microvilli are bonded together by intermicrovillar bridges with a periodicity of 11–17nm. The cytoskeleton is destroyed by pretreatment with 1 mM dithiothreitol (DTT), possibly by the activation of a thiol protease. It does not survive osmication unless treated with low molecular weight tannic acid (LMWT). The evidence does not discriminate between actin and intermediate filaments as the basis of the cytoskeleton. Attention is drawn to similarities and differences between the rhabdomeral cytoskeleton and that of vertebrate intestinal brush-borders. The extreme lability of the rhabdomeral cytoskeleton to conventional methods of fixation is attributed in part to the Ca²⁺ fluxes experienced by invertebrate photoreceptors, and in part to the effects of osmication.The authors thank Dr. Lindsay Barton-Brown and Tom Van Gerwen for supplying flies from CSIRO cultures: Smith Kline and French Laboratories Ltd., French's Forest, N.S.W. for a generous gift of trifluoperazine, and Mallinckrodt, Inc., St. Louis, Missouri, USA, for a gift of low molecular weight tannic acid. Many colleagues, especially Richard Payne, Steve Shaw and Gert De Couet helped by discussing the results. George Weston and the staff of the Electron Microscope Unit provided support and advice. Sandy Smith prepared Table 1     

Adrenergic regulation of pyruvate kinase and gluconeogenesis in hepatocytes from phosphorylase kinase-deficient (gsdgsd) rats

Hepatocytes were prepared from a strain of rats deficient in hepatic phosphorylase $\text{b}$ kinase and were used to assess the role of this enzyme in the adrenergic regulation of pyruvate kinase and gluconeogenesis. Epinephrine (10 μM) stimulated glucose output and gluconeogenesis from 1.8 mM lactate but did not significantly affect the concentration of hepatocyte glycogen. In addition epinephrine treatment led to an inhibition of pyruvate kinase. The stimulation of gluconeogenesis and the inhibition of pyruvate kinase by epinephrine were blocked by both α- and β-antagonists: similar effects with epinephrine were observed in cells from control animals. It is concluded that mechanisms for the adrenergic regulation of pyruvate kinase and gluconeogenesis are similar in hepatocytes from both phosphorylase kinase-deficient and normal rats.     

Systematics of Hanseniaspora Zikes and Kloeckera Janke

The physiological and morphological characteristics of eighty-two strains of Hanseniaspora and Kloeckera, representing twenty-nine described species, were examined. These results along with DNA base composition and DNA/DNA reassociation experiments revealed that the genus Hanseniaspora comprises six distinct species, viz. H. valbyensis, H. uvarum, H. guilliermondii, H. occidentalis, H. osmophila and H. vineae, with K. japonica, K. apiculata, K. apis, K. javanica, K. corticis and K. africana, respectively, as their imperfect states.     

Enhanced Text Spacing Improves Reading Performance in Individuals with Macular Disease

The search by many investigators for a solution to the reading problems encountered by individuals with no central vision has been long and, to date, not very fruitful. Most textual manipulations, including font size, have led to only modest gains in reading speed. Previous work on spatial integrative properties of peripheral retina suggests that ‘visual crowding’ may be a major factor contributing to inefficient reading. Crowding refers to the fact that juxtaposed targets viewed eccentrically may be difficult to identify. The purpose of this study was to assess the combined effects of line spacing and word spacing on the ability of individuals with age-related macular degeneration (ARMD) to read short passages of text that were printed with either high (87.5%) or low contrast (17.5%) letters. Low contrast text was used to avoid potential ceiling effects and to mimic a possible reduction in letter contrast with light scatter from media opacities. For both low and high contrast text, the fastest reading speeds we measured were for passages of text with double line and double word spacing. In comparison with standard single spacing, double word/line spacing increased reading speed by approximately 26% with high contrast text (p < 0.001), and by 46% with low contrast text (p < 0.001). In addition, double line/word spacing more than halved the number of reading errors obtained with single spaced text. We compare our results with previous reading studies on ARMD patients, and conclude that crowding is detrimental to reading and that its effects can be reduced with enhanced text spacing. Spacing is particularly important when the contrast of the text is reduced, as may occur with intraocular light scatter or poor viewing conditions. We recommend that macular disease patients should employ double line spacing and double-character word spacing to maximize their reading efficiency.     

Stable isotope analysis of vegetation history and land use change at Laguna Santa Elena in southern Pacific Costa Rica

Vegetation History and Archaeobotany - Laguna Santa Elena (8.9290° N, 82.9257° W, 1055&nbsp;m a.s.l.) is a small lake in the Diquís archaeological sub-region of southern Pacific...