Posterior Pituitary Flashcards
The serum here is not lipemic (aka lactisent)- when blood, TAGs, and choelsterol are all very elevated the serum and plasma can look like milk and this is important in patients like this with low sodium because high levels of TAGs and glucose can falsely depress sodium. Here, since the TAG and glucose levels are normal, you know the reduced Na is real
C.
Overview of production of the posterior pituitary hormones
These hormones are made by the magnocellular neurons in the hypothalamus and transported to the posterior piutitary along with their respective carrier neurophysins (I= Oxytocin, II= AVP) in vesicles until stimulation promotes release into the venous bloodstream
How are AVP and oxytocin structurally different?
Different at residues 3 and 8
AVP binds to Gs proteins and causes upregulation of aquaporin channels mainly in the distal tubules of the kidneys. Describe the structure of aquaporin channels
Aquaporin proteins are made up of 6 transmembrane a-helices arranged in a right-handed bundle, with the amino and carboxyl termini located on the cytoplasmic surface of the membrane and 5 loops connecting the transmembrane segments
AQP2 levels are increased in what pathological states?
Obviously lack of ECF but also in hyper-volumetric states such as CHF, pregnancy, and SIADH
Note that in situations where levels of ADH and oxytocin are very high, they are so structurally similar that they can cross-react and recognize each other’s receptors
Low Na+ can cause what? High Na+?
Low Na+ causes the brain to swell and high Na+ will make it shrink
Hyponatremia falls under what three categories?
- Hypovolemic
- Hypervolemic
- Euvolemic
What causes hypovolemic hyponatremia?
High urine output and Na+ excretion and increases in ANP
What causes hypervolemic hyponatremia?
water retention states like nephrotic syndrome, CHF, cirrhosis
use physical exam to diagnose
How does hyponatremia occur acutely (less than 24 hrs)?
A very acute hyponatremia will be assoicated with more noticable symptoms including headache, N/V and the severity will depend on the relative hyponatremia. Severe hyponatremia can lead to seizure, coma, or respiratory death
How does SIADH present in labs and clinically?
Hyponatremia with low Posm
Uosm > Posm
Urine sodium > 20mmol/l
Clinically: No edema, renal or endocrine disease
What tumors have been known to cause SIADH?
Small Cell Lung Carcinomas can make ADH paraneoplastically
(and Thymoma, Lymphoma, Leukemia, Sarcoma, and Mesothelioma)
Other causes of SIADH?
CNS disorders such as meningitis, head trauma, stroke, Guillain-Barre
Respiratory disorders such as pneumonia, TB, and asthma
Drugs that can cause SIADH?
ADH, Oxytocin
Vincristine and vinblastine
Thiazides
MAOIs
Nicotine
Carbamazaprine and Phenothiazines
T or F. SIADH will respond to fluid restriction
T.
How is SIADH treated?
Fluid restriction (preferred)
Lithium, Vaptans (V2 antagonists), and the ABX Demeclocycline (inhibits AQP2 expression transiently)
Hypertonic saline (dont give normal)
What are some causes of hypernatremia (Plasma Na 145+ mEq/L)?
- Pure water depletion (DI)
- Salt poisoning
- Water depletion exceeding Na loss (Diarrhea, vomiting)
- Drugs (lithium, cisplatin, cyclophosphamide)
How does hypernatremia present?
Water moves from the ICF to the ECF causing cellular dehydration leading to thirst, lethargy, irritability, seizures, fever, oliguria, etc.
Why would DI be common in pregnant women?
An enzyme called vasopressinase is upregulated
Rules of a water deprivation test
Complete fluid restriction with no cig smoking and the test measures plasma osmolality when Uosm plateaus both before and after subQ AVP injection to see if AVP is working
