17A04: Exam Report

Describe the endocrine functions of the kidney.

39% of candidates passed this question.

It was expected that candidates would discuss the major hormones produced (or activated) by the kidney. These included erythropoeitin, renin and calcitriol. Good answers included the following: the area where the hormone is produced or modified; stimuli for release; factors which inhibit release; and the subsequent actions / effects. Marks were not awarded for hormones that
act on the kidney.

H1iv / 17A04: Describe the endocrine functions of the kidney

  • Hormone = a substance produced by cells & released into blood that usually has a distant site of action
  • 5 hormones produced by kidneys:
    1. RENIN
    2. EPO
    3. CALCITRIOL
    4. PGIs
    5. KALLEKREINS

Renin

A component of the RAS proteolytic cascade which produces potent peptide AII

  • Produced: GRANULAR CELLS of juxtaglomerular apparatus
    • Synthesise, stores + secretes renin
    • App is a distant part of a nephron where Thick Asc. LoH passes through aff. & eff. arteriole
Renin
  • Effects: as above
  • Stimulates release: 4 independent factors
    1. Renal stretch receptors
      • Renal perfusion pressure detected by a transluminal P of aff. arteriole
      • ↓RPP = most powerful stimulus to ↑RENIN
    2. Na delivery to macula densa
      • ↓NaCl delivery = ↑RENIN release
    3. Symp. Stimulation
      • Juxtaglom cells are innervated by symp nerves
      • Β-adrenergic stimulation = ↑RENIN
      • This is how fast RENIN secreted in stress + posture ∆
    4. Humoral
        • PGE2, prostacyclin (PGI2) = ↑RENIN
  • Inhibition of release
            • ↑Renal perfusion P
            • ↑NaCl delivery to macula densa
            • Humoral: AII via negative feedback ANP

Erythropoeitin

A peptide hormone which regulates erythropoiesis

  • Produced: interstitial cells of renal cortex
  • Effects:
    • Binds to EPO receptors on RBC precursors
    • ↓apoptosis of these cells
    • ∴progenitor cells mature & develop into RBC
  • Stimulus release: ↓pO2

Calcitriol

AKA 1,25-dihydroxycholecalciferol is a hormone synthesised from Vitamin D which regulates Ca2+ absorption

  • Produced:

D3 (UVB) → no metabolic activity; must be modified to influence target cells

 D3 (ingested)

                ↓ (hydroxylation at position 25 by liver)

         25 (OH) D

                ↓ (hydroxylated at position 1 by PCT cells)

        1,25 (OH)2 D

                ↓

Active Vit D = exerts action on target cells

 

  • Effects: GUT = ↑Ca2+ & PO42- active absorption

KIDNEY = ↑tubular reabsorption of Ca2+ & phosphate

  • Stimulates release
    • Hydroxylation of 25(OH)D → 1,25 (OHD) is stimulated by the enzyme 25(OH)D-1-alpha-hydroxylase
    • Activity of this enzyme is stimulated by PTH
    • Prolactin (stimulated lactogenesis) also ↑this enzyme activity due to ↑Ca2+ requirements

PGIs

Steroid hormones produced from Arach Acid that exert their effects at the site of their production

  • Produced/effects
    • Cortex arteries/arterioles → maintain BF & glomerular filtration
  • Stimulates release: VC AII, NA, Vasopressin → to prevent overshoot of their actions

Kallekreins

Peptide hormone of the kallikrein-kinin family

  • Produced: Renal kallikrein is a tissue kallikrein
    • Mostly in distal nephron
  • Effects: regulate renal microcirculation, H2O & Na+ excretion
    • Tissue kallikrein synth by distal nephron
    • Cleaves kininogens to BK
    • BK → VD of aff. & eff. arterioles
      • ↑RBF without ∆GFR
      • Natriuresis + diuresis
      • Inhibit vasopressin stimulated ↑H2O
      • Permeability of collecting duct
    • Regulated: +/- PGIs