Characteristics of glucose transport across the microvillous membranes of human term placenta.

Transport characteristics of D-glucose were studied in the microvillous vesicles isolated from the human term placenta. Transport occurred by selective and rapid facilitated diffusion system which was inhibitable by phloretin and HgCl2. The transport was dependent on a transmembrane. Na+-gradient indicating a "secondary active transport" system operating. The transport influx was saturable and the kinetic analysis based on Hanes-Woolf plot produced a kt and Jmax value of 1.2 mM and 34 nmoles.mgprotein(-1).min(-1), respectively. The efflux of D-glucose from the membrane vesicles in a pre-equilibrated assay conditions showed a distinct biphasic pattern differing significantly in the half time efflux. The t1/2 of the fast and slow components was found to be 15 sec and 660 sec, respectively. The transport showed distinct sensitivity to temperature and the Ea values both below and above the transition temperature of 37 degrees C, as calculated from the Arrhenius plot were found to be 7600 and 5472 kCal.mol(-1), respectively. Inhibition studies with a number of sugars for hexose transport pathway showed that the glucose epimers, phosphorylated sugars, and even the disaccharides and the pentose sugars competed effectively with D-glucose. The influx was also inhibited by a number of steroids such as progesterone, 17alpha-hydroxyprogesterone, testosterone and estrogen. Insulin was found to increase glucose transport in a dose- dependent fashion at a concentration of 0.2-1 unit.ml(-1). Ouabain, dinitrophenol and nicotine strongly inhibited D-glucose uptake in the membrane vesicles.

Characteristics of glucose transport across the microvillous membranes of human term placenta / Características del transporte de glucosa a través de las membranas con microvellosidades de la placenta humana a término

Transport characteristics of D-glucose were studied in the microvillous vesicles isolated from the human term placenta. Transport occurred by selective and rapid facilitated diffusion system which was inhibitable by phloretin and HgCl2. The transport was dependent on a transmembrane. Na+-gradient indicating a "secondary active transport" system operating. The transport influx was saturable and the kinetic analysis based on Hanes-Woolf plot produced a kt and Jmax value of 1.2 mM and 34 nmoles. mgprotein-1.min-1, respectively. The efflux of D-glucose from the membrane vesicles in a pre-equilibrated assay conditions showed a distinct biphasic pattern differing significantly in the half time efflux. The t1/2 of the fast and slow components was found to be 15 sec and 660 sec, respectively. The transport showed distinct sensitivity to temperature and the Ea values both below and above the transition temperature of 37 ºC, as calculated from the Arrhenius plot were found to be 7600 and 5472 kCa1.mol-1, respectively. Inhibition studies with a number of sugars for hexose transport pathway showed that the glucose epimers, phosphorylated sugars, and even the disaccharides and the pentose sugars competed effectively with D-glucose. The influx was also inhibited by a number of steroids such as progesterone, 17α-hydroxyprogesterone, testosterone and estrogen. Insulin was found to increase glucose transport in a dose- dependent fashion at a concentration of 0.2-1 unit.ml-1. Ouabain, dinitrophenoi and nicotine strongly inhibited D-glucose uptake in the membrane vesicles (AU)

Se estudiaron las características del transporte de la D-glucosa en las vesículas con microvellosidades aisladas de la placenta humana a término. El transporte ocurría por un sistema de difusión selectiva y facilitada rápida que podía inhibirse por floretina y por HgCl2. El transporte dependía de un gradiente de Na+ transmembrana,indicativo de un sistema operativo de "transporte activo secundario". El flujo de entrada del transporte era saturable y el análisis cinético basado en el gráfico de Hanes- Woolf produjo una kt y una Jmax de 1,2 mM y 34 nmoles• mg de proteína4•min-1, respectivamente. El flujo de salida de la D-glucosa desde las vesículas de membrana en condiciones de ensayo de pre-equilibrio mostró un patrón bifásico distintivo que difería significativamente en la mitad del flujo de salida. Se halló que la t1/2 de los componentes rápido y lento 15 seg. y 660 seg., respectivamente. El transporte mostró una sensibilidad distintiva a la temperatura, y se encontró que los valores de Ea, tanto por encima como por debajo de la temperatura de transición de 37º C, calculada por el gráfico de Arrhenius, fueron de 7.600 y 5.472 kCa1.mol-1, respectivamente. Los estudios de inhibición con una serie de azúcares para la ruta del transporte de la hexosa mostraron que los epímeros de la glucosa, los azúcares fosforilados, e incluso los disacáridos y los azúcares pentosa, competían de forma eficaz con la D-glucosa. El flujo de entrada también se inhibió por una serie de esteroides como la progesterona, la 17α-hidroxiprogesterona, la testosterona y los estrógenos. Se encontró que la insulina aumentaba el transporte de glucosa de manera dependiente de la dosis a una concentración de 0,2-1 unidades•ml-1. La ouabaína, el dinitrofenol y la nicotina inhibieron fuertemente la captación de D-glucosa por las vesículas de membrana (AU)