Tubular Function I 03/24/15

List the percentages of sodium and reabsorbed by each nephron segment.

>99% of the filtered Na+ load is reabsorbed. Proximal convoluted tubule: reabsorbs 67% of sodium Loop of Henle (thick ascending limb): reabsorbs 25% of sodium Distal convoluted tubule: reabsorbs 5% of sodium Collecting duct: reabsorbs <3% of sodium • Proximal tubule: high capacity Na+ reabsorption (67%) • Loop of Henle: intermediate capacity Na+ reabsorption • Distal Nephron (distal convoluted tubule & collecting duct): low capacity Na+ reabsorption. Provides fine-tuning of Na+ excretion.

Electrochemical Equivalence

Equivalence (Eq/L)= valence × molar concentration - For monovalent ions, 1 mole = 1 Eq (and 1 mmol = 1 mEq) - For divalent ions, 1 mole = 2 Eq (and 1 mmol = 2 mEq)

Quantifying Reabsorption

Excreted = Filtered - Reabsorbed + Secreted For a substance handled by filtration and reabsorption (no secretion), the following applies: Quantity reabsorbed / min = quantity filtered / min - quantity excreted / min Standard Abbreviations in Renal Physiology: The concentration of a substance (Y) in plasma is abbreviated PY The concentration of that substance in urine is abbreviated UY Urine flow (volume per min) is abbreviated V Quantity filtered / min = "filtered load" = GFR × PY Quantity excreted / min = V × UY So, the quantity of substance Y reabsorbed / min = ( GFR × PY ) - ( V × UY )

PAH (para-aminohippurate) Titration Curve

Filtered Load — the amount of a substance filtered each minute. Transport Maximum (TmPAH) — the maximum transport rate for a substance. In the case of PAH, this is the maximum secretory rate. The Tm for a secreted substance is reached when the (limited number of) carriers are saturated and, therefore, transporting the substance at the maximal rate. Because secretion adds PAH to the tubular urine, excretion of PAH occurs even at very low filtered loads. Once the TmPAH is exceeded for all nephrons, PAH excretion increases in proportion to increases in filtered load Threshold — the plasma concentration of a substance above which the substance is excreted. For an avidly secreted substance such as PAH, the threshold = 0.

Glucose Titration Curve

Filtered Load — the amount of a substance filtered each minute. For a freely filterable substance (i.e. glucose), filtered load = GFR × plasma concentration of that substance. Transport Maximum (TmG) — the maximum transport rate for a substance. In the case of glucose, this is the maximum reabsorptive rate. The Tm for a reabsorbed substance is reached when the (limited number of) carriers are saturated and transporting glucose at the maximal rate. Glucose excretion occurs if the filtered load of glucose exceeds the TmG for any nephron. Threshold — the plasma concentration of a substance above which the substance is excreted. (Or, the minimum plasma concentration of a substance that is necessary to result in excretion of the substance.) Splay — the rounding of the reabsorption curve. It reflects the fact that all nephrons do not have identical filtering and reabsorptive capacities.

Describe the tubular reabsorption of glucose. Explain how diabetes mellitus or defects in glucose transport can result in excretion of glucose.

In the early proximal tubule, glucose is reabsorbed by the SGLT2 Na+, glucose cotransporter. The transporter has a high capacity, but low affinity for glucose. 98% of glucose is reabsorbed by these channels. Basolateral GLUT2 channels facilitate diffusion to the interstitial space. • Apical SGLT2 - low affinity, high capacity Na+-glucose cotransport; driven by electrochemical gradient for Na+ (established by basolateral Na+-K+-ATPase) • Basolateral GLUT2 - facilitated diffusion In the late proximal tubule, the apical SGLT1 2Na+, glucose cotransporter acts to reabsorb glucose. The channel has a low capacity but high affinity for glucose. The remaining 2% of glucose is reabsorbed by these transporters. Basolateral GLUT1 channels facilitate diffusion to the interstitial space. • Apical SGLT1 - high affinity, low capacity 2 Na+-glucose cotransport; driven by electrochemical gradient for Na+ (established by basolateral Na+-K+-ATPase) • Basolateral GLUT1 - facilitated diffusion

Coupling of Solute and Water Transport

Na+ reabsorption (with accompanying anions) establishes an osmotic gradient that provides the driving force for water reabsorption. • In some nephron segments, water transport is tightly coupled to solute transport. • In some segments, salt and water transport are dissociated. This distinction involves differences in water permeability between segments. • The water permeability (hydraulic conductivity) of a tubular segment primarily reflects the expression of aquaporins (AQP; "water channels") in apical and basolateral membranes

Osmolarity

Osmolarity (Osm/L) = # of particles into which a substance dissociates in solution × molar concentration - 1 mole of glucose = 1 osmole of solute - 1 mole of NaCl = 2 osmoles of solute

PAH (para-aminohippurate) Handling by the Kidney

Para-aminohippuric acid (PAH) is a diagnostic agent used to measure renal plasma flow. PAH is useful as a measurement of renal plasma flow because in addition to being freely filtered it is also secreted by the renal tubules, not reabsorbed. The renal clearance of PAH is useful in calculation of renal plasma flow (RPF).

List some compounds secreted by the proximal tubule.

Penicillin Furosemide Creatinine Histamine Phenol red Norepinephrine Quinine Organic Cations Histamine Cimetidine Cisplatin Metformin Creatinine Organic Anions Lasix Penicillin PAH Creatinine

Describe the reabsorption of sodium in the proximal tubule, loop of Henle and distal nephron (distal tubule and collecting duct).

Proximal tubule In the proximal tubule, Na+ moves across the apical membrane of the tubule down its concentration gradient, mostly via the Na+- glucose symporter. There is also the Na+/H+ antiporter, and a Na+-phosphate symporter (important during bone development). On the basolateral membrane, the Na+,K+-ATPase pumps Na+ into the interstitial space so it can be reabsorbed by the postglomerular capillaries. Water follows the movement of Na+, as does Cl-. Loop of Henle In the thin descending limb, only water can move out of the tubule. The TDL is not permeable to Na+. In the thick ascending limb of the loop of Henle, the Na+,K+,2Cltransporter (NKCC2) brings sodium, potassium, and chloride through the apical membrane; this is where most Na+ enters. The Na+,K+-ATPase in the basolateral membrane then pumps out Na+ into the interstitial space. The Na+/H+ exchanger also brings in some Na+ here. - Called the diluting segment; uses a lot of energy to pump out Na+ on the basolateral side. - Diuretics like furosemide and bumetanide block NKCC2, preventing sodium and water reabsorption. They also cause increased distal sodium reabsorption (with potassium loss), so are potassium wasting. They block the TGF mechanism, preventing a decrease in GFR. - Loss of function mutations of NKCC2 causes Bartter's syndrome, with salt wasting, hypokalemia, alkalosis, and hypercalciuria. Distal tubule Sodium is brought in by the sodium chloride cotransporter through the apical membrane. Na+,K+-ATPase pumps Na+ into the interstitial space through the basolateral membrane. Chloride follows passively through the basolateral membrane. - The NCC channel is sensitive to thiazide diuretics. Prevents reabsorption of Na+ and water. These diuretics are potassium wasting as well, because the collecting duct will see more Na+ and water, and will compensate with more Na+ channels (ENaC). As a result, the Na+,K+-ATPase will have to increase its activity, bringing more K+ into the cell in the process, which has the opportunity to diffuse out into the duct via a K+ channel (thus wasting). - Loss of function mutation causes Gitelman's syndrome, with salt wasting, hypokalemia, alkalosis, and hypocalciuria. Collecting duct The ENaC (epithelial Na+ channel) brings sodium through the apical membrane. The Na+,K+-ATPase pumps Na+ through the basolateral membrane to the interstitial space where it can be reabsorbed by the postglomerular capillaries. - Amiloride blocks the ENaC channel, causing more Na+ and water excretion. This diuretic is potassium sparing, since the collecting duct will not have the opportunity to detect and upregulate the ENaC channel in a way that will affect potassium excretion significantly. - The ENaC is located in principal cells and are sensitive to aldosterone; in response to binding of aldosterone to the aldosterone receptor, the ENaC channel will be produced and shuttled to the apical surface and to allow reabsorption of Na+. *In Liddle's syndrome, a mutation causes the channel to always be open. This leads to salt retention and severe hypertension. *In Pseudohypoaldosteronism, a mutation causes loss of function of the channel. This leads to salt wasting, hyperkalemia, acidosis, and hypotension. Summary: Apical Na+ re-absorption mechanisms vary between segments: • Proximal tubule: several Na+ co-transporters (SGLT1/2, NaPi2) and the Na+/H+ exchanger (NHE3). • Thick ascending limb: Na+-K+-2Cl- co-transporter (NKCC2/BSC1) • Distal convoluted tubule: Na+-Cl- co-transporter (NCC/TSC1) • Collecting duct: Epithelial Na+ channel (ENaC) Sodium is kept low inside all renal epithelial cells by the action of the Na+-K+-ATPase in the basolateral membrane. Na+ is moved from tubular epithelial cells to peritubular interstitium and K+is moved into the cells.

Qualitative Evidence for Reabsorption

Salt is not produced or consumed by the body so balance is maintained by regulating the amounts excreted in body fluids (urine, sweat, stool) such that they equal the amounts ingested (ingestion = excretion) Kidneys maintain water and salt balance in the body by regulating output of both in the urine • Glucose (a very small molecule that is uncharged and not bound to plasma protein) is freely filtered across the glomerular capillary wall. However, glucose is virtually absent from the final urine. Therefore, glucose must be reabsorbed! • Glomerular filtration rate is normally 125 ml/min. However, the volume of final urine excreted per minute is much smaller. Therefore, water must be reabsorbed!

Passive Reabsorption of Substances along the Nephron

Urea, chloride • Passive reabsorption by diffusion (urea and chloride) • Relies on transtubular electrochemical gradients and the permeability of the tubular epithelium to the substance.

Secretion

addition of substances to the urine (out of the body)

Transepithelial Solute Transport

basolateral to apical (movement of solutes from blood to lumen)

Absorption

removal of substances from the urine. Movement of solutes or water from lumen to blood v Transcellular (across both cell membranes) v Paracellular (between cell membranes)

Causes of Glucosuria

• Diabetes Mellitus Normal PGlu = 80 mg/dl Glucose threshold = 200 mg/dl In diabetes mellitus, PGlu can easily exceed the threshold (reaching levels perhaps as high as 500 mg/dl). Assuming a normal GFR, the filtered load of glucose (GFR × PGlu) in diabetes mellitus exceeds the TmG, and glucose spills into the urine • Mutations of Na+-Glucose Cotransporters - Familial renal glucosuria Mutation of SGLT2 decreases the transport capacity of this carrier reduces TmG glucosuria. Can lead to a reduction in plasma glucose concentration. - Glucose-galactose malabsorption syndrome Autosomal recessive 1 bp mutation of SGLT1 decreases the transport capacity of this carrier reduces TmG mild glucosuria. (Recall that this carrier reabsorbs only that portion of filtered glucose that reaches the last proximal tubule.) Because this transporter is also found in the intestine, the mutation also causes intestinal malabsorption of glucose and leads to severe (sometimes fatal) diarrhea in neonates.