Blue light activates potassium-efflux channels in flexor cells from Samanea saman motor organs via two mechanisms

Light-induced leaflet movement of Samanea saman depends on the regulation of membrane transporters in motor cells. Blue light (BL) stimulates leaflet opening by inducing K(+) release from the flexor motor cells. To elucidate the mechanism of K(+)-efflux (K(D))-channel regulation by light, flexor motor cell protoplasts were patch-clamped in a cell-attached configuration during varying illumination. Depolarization elicited outward currents through single open K(D) channels. Changes in cell membrane potential (E(M)) were estimated by applying voltage ramps and tracking the change of the apparent reversal potential of K(D)-channel current. BL shifted E(M) in a positive direction (i.e. depolarized the cell) by about 10 mV. Subsequent red light pulse followed by darkness shifted E(M) oppositely (i.e. hyperpolarized the cell). The BL-induced shifts of E(M) were not observed in cells pretreated with a hydrogen-pump inhibitor, suggesting a contribution by hydrogen-pump to the shift. BL also increased K(D)-channel activity in a voltage-independent manner as reflected in the increase of the mean net steady-state patch conductance at a depolarization of 40 mV relative to the apparent reversal potential (G(@40)). G(@40) increased by approximately 12 pS without a change of the single-channel conductance, possibly by increasing the probability of channel opening. Subsequent red-light and darkness reversed the change in G(@40). Thus, K(+) efflux, a determining factor for the cell-volume decrease of flexor cells, is regulated by BL in a dual manner via membrane potential and by an independent signaling pathway.     

Stress down regulates milk yield in cows by plasmin induced beta-casein product that blocks K+ channels on the apical membranes

Stress and stress related hormones such as glucocorticoids inhibit lactation in cows. In the present study we propose a novel mechanism connecting stress with plasminogen-plasmin system (PPS) (an enzymatic mechanism in milk, which leads to the breakdown of the major milk protein casein). We show that stress activates the PPS leading to an increase in plasmin activity, and that a distinct plasmin-induced beta-casein breakdown product (fraction 1-28) is a potent blocker of potassium channels in mammary epithelia apical membranes. The reduction in milk production due to dehydration stress or glucocorticoid (dexamethsone) was correlated with the activities of plasmin and channel blocking activity in the milk of the tested cows. The notion that the axis Stress-PPS-beta-casein fraction 1-28 is responsible for the reduction in milk yield is supported by the results of experiments showing that injecting solution composed of casein digest enriched with beta-casein fraction 1-28 to the udder lumen leads to a transient reduction in milk production. Furthermore, injecting a pure beta-casein fraction 1-28 to the udder lumen of goat's lead also to a transient reduction in milk production with kinetics that was similar to the kinetics observed in cows.     

Role of endogenous CCK in the inhibition of gastric emptying by peptone and Intralipid in rats

To assess the role of endogenous cholecystokinin in the control of gastric emptying of peptone solutions and Intralipid suspensions, we examined the ability of a dose range of the CCK-A antagonist, devazepide to accelerate the gastric emptying of various caloric concentrations of peptone and Intralipid in rats. In the absence of devazepide, both peptone and Intralipid emptying slowed with increasing concentration. Devazepide's effect on peptone gastric emptying diminished with increasing peptone concentration. The threshold dose for accelerating the emptying of 0.2 kcal/ml peptone was lower than the threshold dose for affecting 0.5 kcal/ml peptone and devazepide had no effect on the gastric emptying of 1.0 kcal/ml peptone. In contrast, devazepide affected Intralipid gastric emptying at all three Intralipid concentrations and the threshold dose decreased with increasing Intralipid concentration. However, the magnitude of the effect of devazepide on peptone or Intralipid gastric emptying was partial and did not increase as a function of concentration. These data demonstrate a role for endogenous CCK in the emptying of peptone and Intralipid but suggest that endogenous CCK does not account for the increased slowing of gastric emptying evident with increased caloric concentration.     

Long-term population studies and the development of an integrated management programme for control of Opuntia stricta in Kruger National Park, South Africa

1. A cactus, Opuntia stricta , has invaded almost 16 000 ha of conserved, natural habitatand has become a major weed problem in Kruger National Park (KNP), South Africa.
2. The main objectives in the control of O. stricta are to reduce the density of the weed and to curb long-range dispersal of seeds by preventing young plants from reaching the size (28 cladodes) at which they start to produce fruits.
3. Herbicides have failed to provide satisfactory control of O. stricta because the weed infestations are replenished from seeds in the soil and from small plants that are overlooked during spraying.
4. A phycitid moth, Cactoblastis cactorum , was released in KNP during 1988 in an attempt to control O. stricta biologically.
5. Population counts of the biological control agent and of the weed over a 5-year period showed that, even though C. cactorum has not provided complete control of O. stricta in KNP, the moderate levels of larval damage have stunted the growth of O. stricta and have considerably extended the time that the young plants take to reach sexual maturity.
6. Comparisons of modelled (i.e. with no C. cactorum ) and actual populations of O. stricta showed that C. cactorum is making a substantial contribution to the control of O. stricta in residual infestations of the weed that have been treated with herbicides.
7. The need for long-term evaluation studies in biological weed control is demon strated by the development of an integrated management programme for effective control of O. stricta .     

Edaphic Distribution of Some Species of the Fern Genus Adiantum in Western Amazonia1

The distribution patterns of Adiantum species were inventoried at twelve noninundated rain forest (tierra firme) sites in Amazonian Peru and Ecuador. Six species belonging to the genus were found, and four of these were abundant at some of the sites. At six of the sites none of the species was particularly abundant. Both the occurrence and the abundance of the species varied among sites with regard to soil texture and concentration of exchangeable bases and within sites with regard to topography. Adiantum tomentosum) Klotzsch was restricted to loamy soils with intermediate to low content of extractable bases and was locally most abundant in the well drained parts of slopes and ridge tops. Adiantum terminatum Kunze ex Miq. and A. humile Kunze occurred at sites with intermediate to high content of extractable bases, but their relative abundances differed among and within sites. Adiantum pulverulentum L. was restricted to soils with relatively high base concentration. Ecological differences among the species obviously explained many of the differences in their distribution patterns at the local and regional scales, and may be important for the understanding of such patterns at biogeographical scales as well.     

Cytokinesis and Midzone Microtubule Organization in Caenorhabditis elegans Require the Kinesin-like Protein ZEN-4

Members of the MKLP1 subfamily of kinesin motor proteins localize to the equatorial region of the spindle midzone and are capable of bundling antiparallel microtubules in vitro. Despite these intriguing characteristics, it is unclear what role these kinesins play in dividing cells, particularly within the context of a developing embryo. Here, we report the identification of a null allele of zen-4, an MKLP1 homologue in the nematode Caenorhabditis elegans, and demonstrate that ZEN-4 is essential for cytokinesis. Embryos deprived of ZEN-4 form multinucleate single-celled embryos as they continue to cycle through mitosis but fail to complete cell division. Initiation of the cytokinetic furrow occurs at the normal time and place, but furrow propagation halts prematurely. Time-lapse recordings and microtubule staining reveal that the cytokinesis defect is preceded by the dissociation of the midzone microtubules. We show that ZEN-4 protein localizes to the spindle midzone during anaphase and persists at the midbody region throughout cytokinesis. We propose that ZEN-4 directly cross-links the midzone microtubules and suggest that these microtubules are required for the completion of cytokinesis.     

The Middle Subunit of Replication Protein A Contacts Growing RNA-DNA Primers in Replicating Simian Virus 40 Chromosomes

The eukaryotic single-stranded DNA binding protein replication protein A (RPA) participates in major DNA transactions. RPA also interacts through its middle subunit (Rpa2) with regulators of the cell division cycle and of the response to DNA damage. A specific contact between Rpa2 and nascent simian virus 40 DNA was revealed by in situ UV cross-linking. The dynamic attributes of the cross-linked DNA, namely, its size distribution, RNA primer content, and replication fork polarity, were determined. These data suggest that Rpa2 contacts the early DNA chain intermediates synthesized by DNA polymerase α-primase (RNA-DNA primers) but not more advanced products. Possible signaling functions of Rpa2 are discussed, and current models of eukaryotic lagging-strand DNA synthesis are evaluated in view of our results.