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Physiology

Endocrine

Question 106 of 180

Which of the following acts to inhibit antidiuretic hormone (ADH) release from the posterior pituitary:

Answer:

ADH release is inhibited by low plasma osmolality, alcohol, caffeine, glucocorticoids and atrial natriuretic peptide (ANP).

Extracellular fluid osmolality must be closely regulated as alterations cause the swelling or shrinking of cells and can lead to cell death. Osmoreceptors in the anterior hypothalamus control water intake by altering thirst and control renal water excretion by altering antidiuretic hormone (ADH) release. A rise in osmolality triggers ADH release and stimulates thirst; a fall has the opposite effect.

Synthesis

Antidiuretic hormone is synthesised in the hypothalamus and transported to the posterior pituitary within nerve fibres where it is stored in secretory granules.

Action

ADH binds V2 receptors on renal principal cells in the late distal tubule and collecting ducts, raising cAMP levels and causing intracellular vesicles to fuse with the apical membrane. In their membrane these vesicles have water channels called aquaporins, which increase the water permeability allowing greater water reabsorption and concentration of urine.

ADH also binds to V1 receptor receptors on vascular smooth muscle, causing vasoconstriction and enhancing the effect of aldosterone on sodium reabsorption in the distal tubule.

Metabolism

ADH is rapidly removed from plasma, falling by about 50% in about 10 minutes, mainly due to metabolism in the liver and kidneys.

Stimulating Factors

ADH release is stimulated primarily by raised plasma osmolality detected by osmoreceptors in the anterior hypothalamus. Other factors that increase ADH release include: extracellular fluid volume depletion, angiotensin II, nausea, pain, stress, exercise, emotion, hypoglycaemia.

Inhibiting Factors

ADH release is inhibited by low plasma osmolality, alcohol, caffeine, glucocorticoids and atrial natriuretic peptide (ANP).

Clinical Implications

ADH deficiency (or an inadequate response to ADH) results in diabetes insipidus. Excess levels of ADH results in syndrome of inappropriate ADH secretion (SIADH).

By OpenStax College [CC BY 3.0 , via Wikimedia Commons

Function of Antidiuretic Hormone. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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