Lanthanum permeability of tight junctions along the collecting duct of the rat

Lanthanum permeability of tight junctions along the collecting duct of the rat. The permeability of the tight junctions (zonulae occludentes) was evaluated along the entire length of the collecting duct of the rat using a lanthanum tracer technique. Nine rats with hereditary hypothalamic diabetes insipidus were studied using standard micropuncture and clearance techniques. Glomerular filtration rate (GFR) estimated from inulin clearance, urine and plasma osmolality (U/Posm) and urine flow rate (V) were determined in eight of nine animals. During either sustained diuresis (five animals) or vasopressin-induced antidiuresis (four animals), individual surface convolutions of distal convoluted tubules or early cortical collecting ducts were preserved for ultrastructural examination by intraluminal microperfusion with a glutaraldehyde-formaldehyde fixative followed by a second microperfusion with a lanthanum tracer. Mean GFR during diuresis was 6.31 ± SE 0.63 ml/min/kg of body wt and V = 797 ± SE 108 µl/min/kg or 13.6 ± SE 2.2% of the filtered load of water. After administration of exogenous vasopressin, V fell to 311 ± 157 µl/min/kg or 5.2 ± SE 3.8% of the filtered load of water and U/Posm rose from 0.658 ± SE 0.043 to 2.124 ± 0.454. Tight junctions of cortical and outer medullary segments of the collecting duct resisted lanthanum penetration. Tight junctions of the inner medullary and papillary segments of the collecting duct were freely permeable to lanthanum suggesting the presence of a paracellular shunt pathway for solute and water movement. The results were independent of the presence or absence of vasopressin. Physiological studies have previously demonstrated that cortical and outer medullary segments of the collecting duct have a low urea permeability while inner medullary and papillary segments of the collecting duct have a relatively high urea permeability. The possibility is suggested that urea movement across the inner medullary and papillary segments of the collecting duct may occur, at least in part, via a paracellular pathway formed by the nonoccluding tight junction and the lateral intercellular space.Perméabilité au lanthane des jonctions serrées le long du canal collecteur du rat. La perméabilité des jonctions serrées (zonulae occludentes) a été evaluée tout au long du canal collecteur du rat, au moyen d'une technique utilisant le lanthane comme traceur. Neuf rats atteints de diabète insipide hypothalamique héréditaire ont été étudiés au moyen des techniques habituelles de microponction et de clearance. La filtration glomérulaire (GFR) mesurée par la clearance de l'inuline, le rapport des osmolarités urine/plasma (U/Posm) et le débit urinaire (V) ont été obtenus chez huit des neuf animaux. Au cours de la diurèse entretenue (cinq animaux) ou de l'antidiurèse induite par la vasopressine (quatre animaux) des convolutions superficielles de tubes contournés distaux ou des canaux collecteurs précoces ont été préservés aux fins d'étude ultrastructurale par la microperfusion intraluminale d'un fixateur glutaraldéhyde-formaldéhyde suivie d'une deuxième microperfusion de lanthane. Le GFR moyen au cours de la diurèse était de 6,31 ± SE 0,63 ml/min/kg poids corporel et V = 797 ± SE 108 µl/min/kg ou 13,6 ± SE 2,2% de la charge d'eau filtrée. Après l'administration de vasopressine, V a diminué à 311 ± 157 µl/min/kg ou 5,2 ± SE 3,8 % de la charge d'eau filtrée et U/Posm est passé de 0,658 ± SE 0,043 à 2,124 ± 0,454. Les jonctions serrées des segments corticaux et médullaire externe des canaux collecteurs ont résisté à la pénétration de lanthane. Les jonctions serrées des segments médullaire interne et papillaire des canaux collecteurs ont été librement perméables au lanthane ce qui suggère une voie de shunt paracellulaire pour les mouvements d'ezu et de substances dissoutes. Les résultats sont indépendants de la présence ou de l'absence de vasopressine. Les études physiologiques antérieures ont montré que les segments corticaux et médullaires externes des canaux collecteurs ont une perméabilité faible pour l'urée, cependant que les segments médullaires internes et papillaires ont une perméabilité élevée. Il est suggéré que des mouvements d'urée à travers les segments médullaires internes et papillaires des canaux collecteurs puissent avoir lieu, au moins en partie, par une voie paracellulaire formée par les jonctions serrées non occlusives et l'espace intercellulaire latéral

The initial education of high school teachers : a critical review of major issues and trends

This paper draws on major research findings in international literature in order to provide a critical review of a number of key issues and trends in the initial education of high school teachers. Firstly, this paper contextualizes the prevalent discourse surrounding the field of initial teacher education (ITE) and explores the effect that this discourse has on the conceptualization of teachers’ work. Secondly, this paper focuses on the debates regarding the most propitious site for the teacher education enterprise, the programme structure for ITE, the field placement or practicum, the relationship between subject study and pedagogy, and the overall effectiveness of teacher education. The paper concludes by considering the new challenges that the field of initial teacher education must confront and the implications of such challenges for the ITE curriculum.peer-reviewe

Effects of oral powder electrolyte administration on packed cell volume, plasma chemistry parameters, and incidence of colic in horses participating in a 6-day 162-km trail ride.

This study evaluated the effects of administering oral powder electrolytes on packed cell volume (PCV), plasma chemistry parameters, and incidence of colic in horses participating on a 6-day 162-km trail ride in which water was not offered ad libitum. Twenty-three horses received grain with powder electrolytes daily while 19 control horses received grain only. Horses were ridden approximately 32 km a day at a walk or trot. Packed cell volume and plasma chemistry parameters were analyzed daily. Episodes of colic were diagnosed and treated by a veterinarian unaware of treatment group allocation. Blood parameters and incidence of colic were compared between treatment groups. Electrolyte administration did not alter PCV or plasma chemistry parameters compared to controls. The incidence of colic was significantly higher in treated horses (P = 0.05). Oral powder electrolytes did not enhance hydration status or electrolyte homeostasis and may be associated with colic in horses participating on long distance trail rides similar to this model