Povider:Dr.nourabadi. Provider: Davood Nourabadi Ph.D of medical physiology-IUMS...

Povider:Dr.nourabadi


Renal Handling of Ca2+

Provider: Davood Nourabadi

Ph.D of medical physiology-IUMS


Refrence1. Brenner & Rector’s the kidney ,Maarten W. Taal …,9th ed,2012.

2. Bern&Levy physiology,6thed,2008.

3. Medical Physiology, Boron& Boulpaep, 2th ed, 2012.

4. Seldin.and.Giebisch's.The.Kidney.5th ed,2013.


Ca2+ Homeostasis Tight control of blood calcium levels Many vital physiologic functions:• Muscle contraction• Signaling • Neuronal excitation • Bone formation • Coagulation• Cell division and growth • Apoptosis• Neurotransmitter release


Contd.. Disturbances in the Ca2+ balance:• Seizures • Rickets • Heart failure

Collaborative action for calcium balance:• Intestine (regulation of absorption in duodenum&colon)• Bone (regulation of metabolism)• Kidney (regulation of excretion)


Distribution of Ca2+ 99%is stored in bone. Approximately 1% in ICF 0.1% in the ECF• Total [Ca2+] in plasma is 10 mg/dL (2.5 mM or 5 mEq/L).

Distribution of Calcium in Normal Human Plasma: Diffusible (Filterable) • Ionized (Ca2+)• Complexed to HCO3

–, citrate, sulfate, phosphate.

Nondiffusible • Bound to albumin• Bound to globulin


Role of renal in Ca2+ homeostasis

►8 g/daily of Ca2+ is filtered at the glomerulus►Less than 2% is excreted into the urine ►Passive paracellular and active transcellular reabsorption

in the epithelial cell layers


HANDLING OF Ca2+ ALONG NEPHRON


Proximal Tubule(PTs)◙ Including the proximal convoluted tubule (PCT) and

proximal straight tubule (PST)◙ 65% of the filtered Ca2+ is reabsorbed◙ Transport is passive and follows the local Na+ and water

reabsorption (solvent drag)◙ Not provide an independent regulation of Ca2+ reabsorption


Loop of Henle◙ Thin descending and thin ascending limbs of the loop

of Henle virtually no Ca2+ is reabsorbed.

◙ Thick ascending limb of the loop of Henle (TAL) is permeable to Ca2+


Thick Ascending Limb (TAL) of Henle

◙ Accounts for approximately 20% of the total Ca2+ reabsorption.

50%paracellular-50%transcellular◙ Claudin-16 (paracellin-1) and claudin-19 together form

paracellular pores and determine the Ca2+ and Mg2+ selectivity of the paracellular junctions.


Distal Convoluted Tubule (DCT) and Connecting Tubule (CNT)

◙ Ca2+ reabsorption takes place against its electrochemical gradient=transport is active

◙ Tight junctions are nearly impermeable to Ca2+

◙ Relative contribution of the initial (DCT1) and later (DCT2) segments of the DCT and of the CNT to active Ca2+ reabsorption is not entirely clear.

◙ DCT2&CNT expressed:• Na+-K+-ATPase• NCX1• PMCA1b• TRPV5• Calbindin-D28K


Contd...

◙ Transepithelial transport of Ca2+ is a 3 step procedure:1) Apical Entry of Ca2+ via TRPV5

2) Intracellular Carrier Calbindin-D28K

3) Basolateral Extrusion Systems: NCX1 and PMCA1b (Energy-consuming step).

Povider:Dr.nourabadiModel of active Ca2+ reabsorption in DCT2 & CNT


Collecting Duct◙ Cortical part of the collecting duct (CCD) accounts for

small amount (3%) of Ca2+ reabsorption.◙ Net transport occurs against the electrochemical gradient

for Ca2+ (transport is active)


Passive Ca2+ Reabsorption Paracellular component of epithelial Ca2+ transport. Takes place in the PTs and TAL.


Active Ca2+ Reabsorption Transcellular component of epithelial Ca2+ transport. Is the main target site for specific regulation of Ca2+

(re)absorption by various calciotropic hormones. Takes place in principal cells of the DCT and CNT


Characteristics of Ca2+ Channels in the Kidney

TRPV4 is candidate for a molecular sensor that confers osmosensitivity


TRPV5(ECaC1)• A member of the TRP channel superfamily• Intracellular N and C-terminal tails• Homotetramers • Six transmembrane segments (S1-S6)• Pore region=S5-S6• N-terminal tail contains multiple ankyrin repeats • First extracellular loop between S1-S2 contains an asparagine

(complex glycosylation for regulation of channel activity)• C-terminal tail contains three potential PKC,PKA,PKG sites• Gatekeeper function of TRPV5 in active Ca2+ reabsorption • Increased [Ca2+]i to inhibit the activity of TRPV5

• TRPV6 (ECaC2) a homologous channel (75%) in proximal intestine


Epithelial Ca2+ channel TRPV5


Characteristics of TRPV5 Constitutive activity at low intracellular Ca2+ concentrations and

physiologic membrane potentials Shows strong inward rectification its 100 times larger selectivity for Ca2+ than for Na+

Mg2+ competes with Ca2+ for entry into the cytoplasm A constitutively open channel, because no stimulus or ligand was

required Localized in intracellular vesicles Reinternalized via dynamin- and clathrin-dependent processes In contact with the extracellular environment, TRPV5 channels exhibit

closed and open states (open probability) Extracellular H+ blocks channel activity


Contd...►Protein-Protein Interactions of TRPV5:

S100A10–Annexin 2 facilitates the translocation of TRPV5 toward the plasma membrane

80K-H acts as novel Ca2+ sensor controlling TRPV5 channel activity

Rab11a direct role is in the trafficking of the Ca2+ channel toward the plasma membrane


Calbindin-D28K In principal cells of the DCT2 and CNT Three pairs of EF hands that form the structural basis of

its high Ca2+ affinity binding capacity Slow Ca2+-binding kinetics of calbindin-D28K


PMCA(1b) High-affinity Ca2+ efflux pump Maintain the resting Ca2+ concentration in virtually all cells Highest Ca2+-ATPase activity in kidney was reported in the

DCT Capacity of PMCA in the CNT seems to be insufficient to

keep pace with the absorptive flux of Ca2+, because it can transport only approximately 30% of the total Ca2+ efflux


NCX1 Prerequisite for transepithelial Ca2+ transport Widely expressed protein that can be found in several

tissues:• Heart• Brain• Skeletal muscle• Kidney(distal part of the nephron-CNT)

70% of Ca2+ efflux in CNT


Regulation of Active Ca2+ Reabsorption

By Calciotropic Factors Vitamin D Parathyroid Hormone Estrogens Dietary Ca2+ Intake Acidosis and Alkalosis Klotho Tissue Kallikrein

By Immunosuppressives and Diuretics Calcineurin Inhibitors Furosemide Thiazide


Vitamin D Synthesized in the PTs by 1α-hydroxylase Genomic transcriptional mechanisms with a nuclear vitamin D

receptor (VDR) VDR-RXR complex Intestine and kidney are the main target organs Effects (↑): TRPV5 Calbindins (9K&28K) NCX1 [Ca2+]p

Bone formation


PTH A key role in maintaining Ca2+ in ECF by CaSR

(most important regulator) Stimulates the activity of 1α-hydroxylase in the PT PTH receptor mRNA in rat kidney (NOT thin limb of the

loop of Henle&CD) Directly stimulates active Ca2+ reabsorption in the distal

part of the nephron In the TAL, PTH increases the transepithelial driving force

for Ca2+ reabsorption, enhancing paracellular Ca2+ transport.


PTH(contd..)

►Mechanisms of PTH action for the effect in the DCT: Membrane insertion of apical Ca2+ channels Opening of basolateral CLC (cellular hyperpolarization) Modulation of PMCA activity


Estrogens Estrogen deficiency after menopause results in bone loss=

increase in plasma and urinary Ca2+

Estrogen receptors (ER) also reside in proximal and distal tubules within the nephron

Upregulates the expression of TRPV5 in kidney (independent of Vit-D)


Dietary Ca2+ Intake Dietary Ca2+ can affect active Ca2+ (re)absorption via

vitamin D–independent modulation of the expression of Ca2+ transport proteins.

Hypercalcemia also activates the CaSR in the TAL, which subsequently inhibits the activity of Na+-K+-2Cl– cotransporter 2 (NKCC2) and therefore passive paracellular Ca2+ transport


Acidosis and Alkalosis Chronic metabolic acidosis is associated with increased

renal Ca2+ excretion Extracellular protons inhibit TRPV5 channel activity• pH sensitivity is mainly mediated by glutamate at

position 522 and may act as the “pH sensor” of TRPV5.


Klotho Antiaging hormone Secreted and activated by cleavage of the N-terminal

extracellular domain Exhibits β-glucuronidase activity Expressed in the DCT/CNT and the parathyroid gland Klotho may work by affecting the extracellular

glycosylation status of the TRPV5 Increased expression of Na+-K+-ATPase


Tissue Kallikrein A serine protease that is expressed mainly in the DCT and

CNT Involved in the regulation of water, sodium, and potassium

metabolism Enhances active Ca2+ reabsorption through an increased

TRPV5 plasma membrane expression via the BK-activated phospholipase/DAG/PKC pathway


Calcineurin Inhibitors Tacrolimus • Reduced calbindin-D28K levels • Downregulation of the renal mRNA and protein

expression of TRPV5 Cyclosporine • High bone turnover • Osteopenia• Hypercalciuria • Suppresses the expression of calbindin-D28K


Furosemide Inhibit renal Ca2+ reabsorption Inhibits the NKCC2 transporter of the TAL • Reduction in NaCl reabsorption and K+ recycling across

the apical membrane Increase the expression level of TRPV5 and calbindin-

D28K


Thiazide Decreasing Na+ reabsorption while increasing Ca2+

reabsorption Two hypotheses exist with respect to the Ca2+-sparing

effect of thiazides

1) Renal salt and water loss=> contraction of ECFV => triggers a compensatory increase in proximal Na+ reabsorption

2) Stimulates Ca2+ reabsorption in the DCT

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Povider:Dr.nourabadi. Provider: Davood Nourabadi Ph.D of medical physiology-IUMS...

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