Transporte electrogénico en el colon de ratas privadas de sodio : bloqueo de canales epiteliales versus inhibición de la NA, K-ATPASA

La privación dietaria de sodio estimula la secreción de aldosterona. En el colon de rata, la aldosterona elevada aumenta la absorción de Na+, pero además torna electrogénico el mecanismo de absorción (normalmente electroneutro). Dicho transporte electrogénico puede suprimirse mediante el bloqueo de los canales epiteliales de Na+ en la membrana apical o la inhibición de la Na, KATPasa de la membrana basolateral. La absorción electrogénica de sodio está estrechamente acoplada al metabolismo aerobio, pero se desconoce si el bloqueo de los canales de Na+ reduce el consumo de oxígeno en igual medida que la inhibición de la Na, K-ATPasa. Se obtuvieron preparados de mucosa aislada del colon distal de ratas alimentadas con una dieta hiposódica por 10 días. Se determinó simultáneamente la corriente de cortocircuito y el consumo de oxígeno en condición basal y luego del bloqueo de canales de Na+ con amilorida (n=12) o de la Na, K-ATPasa con uabaína (n=12). Ambos tratamientos redujeron la corriente de cortocircuito en igual medida (>80%), pero la reducción en el consumo de oxígeno fue mayor con uabaína que con amilorida (p<0.03). Esto se debe probablemente a que la Na, KATPasa cumple otras funciones, además del transporte transepitelial de Na+, que son suprimidas por la uabaína pero no por la amilorida.

Benthic foraminifera in surface sediments of the Gulf of Lions

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Euphausiid respiration model revamped, link to model results

Euphausiids constitute major biomass component in shelf ecosystems and play a fundamental role in the rapid vertical transport of carbon from the ocean surface to the deeper layers during their daily vertical migration (DVM). DVM depth and migration patterns depend on oceanographic conditions with respect to temperature, light and oxygen availability at depth, factors that are highly dependent on season in most marine regions. Changes in the abiotic conditions also shape Euphausiid metabolism including aerobic and anaerobic energy production. Here we introduce a global krill respiration model which includes the effect of latitude (LAT), the day of the year of interest (DoY), and the number of daylight hours on the day of interest (DLh), in addition to the basal variables that determine ectothermal oxygen consumption (temperature, body mass and depth) in the ANN model (Artificial Neural Networks). The newly implemented parameters link space and time in terms of season and photoperiod to krill respiration. The ANN model showed a better fit (r**2=0.780) when DLh and LAT were included, indicating a decrease in respiration with increasing LAT and decreasing DLh. We therefore propose DLh as a potential variable to consider when building physiological models for both hemispheres. We also tested for seasonality the standard respiration rate of the most common species that were investigated until now in a large range of DLh and DoY with Multiple Linear Regression (MLR) or General Additive model (GAM). GAM successfully integrated DLh (r**2= 0.563) and DoY (r**2= 0.572) effects on respiration rates of the Antarctic krill, Euphausia superba, yielding the minimum metabolic activity in mid-June and the maximum at the end of December. Neither the MLR nor the GAM approach worked for the North Pacific krill Euphausia pacifica, and MLR for the North Atlantic krill Meganyctiphanes norvegica remained inconclusive because of insufficient seasonal data coverage. We strongly encourage comparative respiration measurements of worldwide Euphausiid key species at different seasons to improve accuracy in ecosystem modelling.