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Two step process, the substance moves through the luminal membrane then though the basolateral membrane to the capillaries ii.Secretion Basic Transport Mechanisms i.Channels Simple diffusion driven by passive diffusion; the movement of a substance with its concentration gradient ii.Transportersa)UniportersPermits movement of a single molecule with its concentration gradient b)Symporters Permits movement of two molecules in the same direction across a membrane with its concentration gradient c)Antiporters 52
Permits the movement of two molecules in opposite directions across a membrane with itsconcentration gradient iii.Primary Active TransportRequires ATP to move molecules against their concentration gradient Regulation of Transporters and Channels of the Kidney A specific transporter or channel is changed in its function in response to a hormone Some channels/transporters are non-regulated, so transport occurs at a constant rate Different levels of transporter/channel regulation i.Regulation at the level of gene expression ii.Regulation at the level of cellular location iii.Regulation at the level of activity Transport Mechanisms in the Nephron Tubule Cell and Filtrate Ion Concentration Tubule cells have very similar ion concentrationsSodium is low inside the cell in comparison to the outside and potassium is high inside the cell in comparison to the outside Sodium always wants to leave the filtrate into the tubule cells – the Na+ gradient is a major diving force for the reabsorption of salts, glucose, amino acids and water Transport Properties of Specific Nephron Segments Proximal Tubule Channels/Transporters i.Na+/Amino Acid Symporter – Luminal membrane ii.Na+/Glucose Symporter – Luminal membrane iii.Na+/H+ Exchanger – Luminal membrane ** regulated iv.Na+/K+ ATPase – Basolateral membrane **regulated v.Water Channel (AQ1) – Luminal membrane vi.Water Channel (AQ1) – Basolateral membrane vii.Amino Acid Uniporter – Basolateral membrane viii.Glucose Uniporter – Basolateral membrane ix.Paracellular transport Descending Limb – Loop of Henle Overall theme: Reabsorption of water Reabsorbs only water – through aquaporins Epithelial cells are tightly packed – there is no paracellular transport No hormonal control Ascending Limb – Loop of HenleOverall theme: Reabsorption of ions No aquaporins – no water reabsorption No hormone regulation Not tight – some paracellular reabsorption Reabsorbs K+, Na+, Cl- Contains multiporters (ion multipliers) Distal Convoluted Tubule 53
Overall theme: Ion reabsorption Does no reabsorb water Epithelial cells are tightly packed – there is no paracellular transport Reabsorbs K+, Na+, Cl- & Ca+Ca+ reabsorption is hormonally regulated by parathyroid hormone Collecting DuctOverall theme: Fine tuning (H2O, Na+ Reabsorption)Channels/Transportersi.Water Channel (AQII) – luminal membrane **regulated by ADHii.Water Channel (AQII & IV) – basolateral membrane iii.Na+ Channel – luminal membrane ** regulated by aldosterone iv.