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MECHANISM OF RENAL TRANSPORT OF SODIUM & GLUCOSE

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MECHANISM OF RENAL TRANSPORT OF SODIUM & GLUCOSE

MECHANISM OF RENAL TRANSPORT OF SODIUM & GLUCOSE

1. RENAL SODIUM TRANSPORT:

  • Occurs in all regions except thin descending tubule.

– Via, Active reabsorption – In all regions.

– Passive resorption – Only in thin ascending tubule

RENAL PART MECHANISM BEHIND SODIUM TRANSPORT
Proximal Convoluted Tubule (PCT)
  • Na2+reabsorption driven by Na2+-K+ATPase located on basolateral membrane.

– 60-70% filtered Na2+ reabsorbed.

  • Na2+entry coupled to H+ secretion via NHE3 antiporter.
Proximal Straight Tubule (PST)
  •  Cl– driven Na2+ transport (Paracellular pathway)
Descending Thin Limb (DTL)


  • Highly permeable to water & relatively permeable to solutes like sodium, chloride & urea.
  • Hence, no sodium resorption. 
Thick Ascending Limb (TAL)
  • 30% filtered Na2+ reabsorbed (60% reabsorbed in proximal tubule).
  • Major transporter: Na2+– K+-2Cl– symporter/co-transporter (NKCC).
  • Transports one Na2+& K+ & 2 Cl– ions.
Distal Convoluted Tubule (DCT)
  • Apical membrane contains Na+– Cl– symporter/Co-transporter (NCC).
  • Approx. 7% Na2+ reabsorbed.
 Collecting Tubule (CT)
  • Via apical sodium channels (ENaC)-P cell.
  • Aldosterone -Important for Na2+ & water resorption.

Factors affecting renal sodium transport:

Increasing sodium resorption:
  • Angiotensin II
  • Renin increase
  • Aldosterone
Decreasing sodium resorption:
  • Atrial Natriuretic Peptide (ANP) 
  • Brain Natriuretic Peptide (BNP)
  • Dopamine
2. RENAL GLUCOSE TRANSPORT:
  • Glucose is reabsorbed by secondary active transport.
  • Complete reabsorption in proximal tubule.
  • Via sodium-glucose cotransporter – 2 (SGLT 2) in brush border(luminal membrane)

Factors influencing resorption:

1. Transport maximum for glucose (TmG)-

  • i.e. Maximum rate of glucose absorption by tubule

– 375 mg/min in males

– 300 mg/min in females

2. Actual value of renal threshold – (much less than TmG)
  • Arterial blood – 200mg/dl.

– Is “Glucose Threshold value” in blood to activate urinary excretion

– Urinary glucose is mildly positive.

  • In venous blood – 180mg/dl.
SPLAY EFFECT:
  • Deviation in renal threshold (from calculated predicted value)

– Due to nephron heterogeneity.

– Not all nephrons have TmG of 375 mg/min

  • Once Tm for kidneys (375 mg/min) is reached, all nephrons reached maximal capacity to reabsorb glucose.

– Further, not all nephrons are maximally active simultaneously.

Exam Important

MECHANISM OF RENAL TRANSPORT OF SODIUM & GLUCOSE

1. RENAL SODIUM TRANSPORT:

RENAL PART MECHANISM BEHIND SODIUM TRANSPORT
Proximal Convoluted Tubule (PCT)
  • Driven by Na2+-K+ATPase located on basolateral membrane

– 60-70% filtered Na2+reabsorbed.

  • Na2+ entry coupled to H+  secretion, via NHE3 antiporter.

Descending Thin Limb (DTL)

  • Highly permeable to water & relatively permeable to solutes like sodium, chloride & urea.
  • Hence, no sodium resorption. 
Thick Ascending Limb (TAL)
  • 30% filtered Na2+reabsorbed.
  • Via Na2+– K+-2Cl– symporter/co-transporter (NKCC).
  • Transports one Na2+& K+ & 2 Cl– ions.

Distal Convoluted Tubule (DCT)

  • Apical membrane contains Na+– Cl– symporter/Co-transporter (NCC).
  • Approx. 7% filtered Na2+ reabsorbed.
 Collecting Tubule (CT)
  • Via apical sodium channels (ENaC)-P cell.
  • Aldosterone – Important for Na2+ & water resorption.
 
2. RENAL GLUCOSE TRANSPORT:
  • Glucose reabsorption by 2º active transport.
  • Complete reabsorption in proximal tubules.
  • Occurs by sodium-glucose cotransporter – 2 (SGLT-2) in brush border(luminal membrane)

Factors influencing resorption:

1. Transport maximum for glucose (TmG)-

  • 375 mg/min in males
  • 300 mg/min in females
2. Actual value of renal threshold.
  • In arterial blood – 200mg/dl.
  • “Glucose threshold value” in blood to activate urinary excretion.
  • When glucose filtered load – 250mg/min → urinary glucose is mildly positive.
SPLAY EFFECT:
  • Deviation in renal threshold from calculated predicted value
  • Overall Tm for kidneys (375 mg/min) is reached, all nephrons reached their maximal capacity for glucose reabsorption.
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