L37: Potassium, Calcium, Phosphorus, and Magnesium

Potassium

major intracellular cation important for maintaining RMP

Calcium

major mineral tested in chemistry panels along with phosphates and Mg, assessed GFR, nephron function, bone growth, PTH

Phosphorus

inorganic form of phosphates

Phosphates

ingested in meats & plants and are regulated by the kidney and PTH, PO4- = body phosphates

Body phosphates (PO4-) and Pi concentrations will increase in the blood when

glomerular filtration is decreased

Magnesium

major intracellular cation involve in activation of many enzymes & regulated by the kidney & PTH

Excretion of K+ in kidneys

increase aldosterone secretion = increased renal loss of K+

In vito hemolysis in species or breeds with RBC K+ > plasma K+

horses, cattle, some Akita & Shiba dogs

Major factors that determine serum [Pi]

1. intestinal absorption

2. renal excretion

3. shifting

For free calcium (fCa2+) concentration, which sample is preferred?

anaerobic sample, heparinized whole blood or plasma

- if air added: pH will increase (less H+) → more Ca2+ bound

What analytes have 3 fractions with 40% being bound to albumin and 50% being free?

calcium, magnesium

What analyte measures phosphates and is excreted under the influence of PTH?

phosphorus

What analytes are released from RBC in a hemolyzed sample?

potassium & phosphorus

What analyte is excreted by the kidney under the influence of aldosterone?

potassium

What analyte is affected by blood pH?

calcium

What acid-base disorder leads to hyperkalemia due to shifting of K+ out of the cells in exchange for H+ ions?

inorganic metabolic acidosis

3 major regulatory processes of K, Ca, P, Mg

1. intake through diet

2. renal excretion

3. shifting

Pathogenesis of Hyperkalemia

increased total body K+ due to:

- increased intake

- decreased excretion

- shift from cells

Pathogenesis of Hyperkalemia: increased intake

K+ rich fluids (fluid therapy)

Pathogenesis of Hyperkalemia: decreased renal excretion

inorganic metabolic acidosis

oliguric states:

- renal failure

- urinary obstruction

- uroperitoneum

hypoadrenocorticism

Pathogenesis of Hyperkalemia: shift from cells

K+ from ICF → ECF (out of cells)

H+ from ECF → ICF (into cells)

Pathogenesis of Hyperkalemia: decreased renal excretion due to inorganic metabolic acidosis

inorganic metabolic acidosis progresses to Acidemia → raises K+

- increased anion gap due to phosphates & nephron damage (kidney not removing metabolic waste)

Pathogenesis of Hyperkalemia: decreased renal excretion due to uroperitoneum

K+ moves to blood down concentration gradient

Pathogenesis of Hyperkalemia: decreased renal excretion due to hypoadrenocorticism (Addison's)

- decreased aldosterone leads to decreased Na/K/ATPase pump function

- decreased K+ secretion in distal nephron

- decreased renal excretion in K+

- hyperkalemia

A male castrated cat presents with frequent painful urination during the last two days. He goes to the litter box but very little urine is eliminated. The cat has not eaten since yesterday. What is the proper description of the chemistry results?

Na+: normal

K+: high

Cl-: low

HCO3-: low

Anion gap: high

Na:K ratio: low

normonatremia, hyperkalemia, hypochloremia, metabolic acidosis with increased unmeasured anions

Besides hypoadrenocorticism (Addison's), what other disorders can cause a Sodium:Potassium ratio of <19?

renal failure

hemorrhagic diarrhea (whipworms)

urinary tract obstruction or uroperitoneum

diabetes mellitus with ketonuria

3rd space loss of Na+

Causes of hypokalemia

- anorexia

- shifting into cells (metabolic alkalosis, insulin stimulated)

- K+ loss disorders (horse sweat, intestine, polyuria)

- K+ loss with anions

Pathogenesis of hypokalemia: decreased intake

anorexia

Pathogenesis of hypokalemia: shifting into cells

in exchange for H+ or insulin

- less K+ in ECF = hypokalemia

Pathogenesis of hypokalemia: increased K+ loss

renal, intestinal (diarrhea), cutaneous (sweating)

Pathogenesis of hypokalemia: increased K+ loss from renal (polyuria)

- increased tubular fluid flow rate

- increased K+ secretion = hypokalemia

- ketonuria, lactaturia: anions obligate excretion of cations (K+) = hypokalemia)

Normokalemia in acidotic animal

acidemia causes shift of K+ from ICF to ECF

- correction of the metabolic acidosis causes hypokalemia

- normokalemia in organic acidosis causes ↓ K+

A male castrated cat presents with frequent painful urination during the last two days. He goes to the litter box but very little urine is eliminated. The cat has not eaten since yesterday. What is the most likely cause of the potassium result?

Na+: normal

K+: high

Cl-: low

HCO3-: low

Anion gap: high

Na:K ratio: low

hyperkalemia due to decreased renal excretion of K+ & shifting out of cells

A 10-day-old calf presents for acute onset of diarrhea (2 days). The calf has not eaten for the past 2 days. The veterinarian provides oral electrolytes and subcutaneous fluids. Evaluate the chemistry data. What is the most likely cause of the sodium, chloride, and potassium findings?

Na+: high

K+: high

Cl-: high

HCO3-: low

Anion gap: high

administration of oral and fluid rich electrolytes

An 8-year-old female spayed dog is presented with vomiting, diarrhea, and inappetence for 4 days. Evaluate the chemistry data. What is the most likely cause of the potassium result?

Na+: low

K+: high

Cl-: low

HCO3-: OK

Anion gap: OK

Na:K ratio: less than 19

hyperkalemia due to adrenal gland hypoplasia (hypoadrenocorticism)

An 11-year-old male dog is presented with polyuria and USG of 1.014. Evaluate the chemistry data. What is the most likely cause of the potassium result?

Na+: OK

K+: low

Cl-: OK

HCO3-: OK

Anion gap: OK

Na:K ratio: high

hypokalemia due to increased tubular fluid flow (polyuria)

The kidneys reabsorb Ca & Pi in the

proximal tubule, dependent on GFR

fCa & fMG are reabsorbed in the

ascending loop of Henle, dependent on PTH

fCa is reabsorbed in the

distal nephron, dependent on PTH

Hypercalcemia caused by Primary parathyroidism

- increased PTH production by parathyroid glands

- increased Ca reabsorption = hypercalcemia

Hypercalcemia caused by humoral hypercalcemia of malignancy (HHM)

- tumors produce PTHrp

- decreased urinary excretion of Ca

Hypercalcemia caused by hypervitaminosis D

vitamin D and PTH cause increased Ca+ reabsorption

Hypercalcemia caused by equine renal failure

- decreased GFR

- decreased clearance of Ca

- increased fCa in blood

- increased tCa = hypercalcemia

relatively common in horses (if high Ca in diet/alfalfa)

Hypercalcemia caused by hypoadrenocorticism (Addison's)

- increased activation of RAAS

- increased angiotensin causes increased proximal tubule reabsorption of Na & Ca

- increased fCa & tCa = hypercalcemia

relatively uncommon

Hypocalcemia caused by decreased protein bound Ca fraction

hypoalbuminemia

Hypocalcemia caused by pregnancy, parturient, lactational cause

Ca from bone < Ca to fetus or milk

Hypocalcemia caused by urinary excretion

caused by anions, furosemide, urinary tract obstruction

Hypocalcemia caused by hypovitaminosis D

decreased vitamin D due to chronic kidney disease

- decreased total fCa & Pi increases PTH

What percentage of Pi is bound to cations and is free?

50% bound, 50% free

Major factors that determine serum Pi

- intestinal absorption

- renal excretion

- shifting out of RBCs (hemolysis or muscle damage)

- bone growth in young animals

Hyperphosphatemia due to decreased urinary PO4 excretion

decreased GFR - decreased PO4 clearance - increased Pi

Hypophosphatemia pathogenesis

- increased urinary PO4 excretion: prolonged diuresis, increased PTH/PTHrp activity

- PO4 from ECF - ICF: hyperinsulinism, glucose infusion

- equine renal disease (pathogenesis unknown)

Hypermagnesemia causes

decreased urinary excretion

- decreased GFR:

- prerenal due to dehydration (high USG)

- renal (low USG)

- post renal (urinary obstruction)

Hypomagnesemia causes

hypoproteinemia

prolonged anorexia

grass tetany

A 3-year-old female spayed cat presents with straining to urinate with very little urine output. Based on the chemistry data, what is the most likely cause of the phosphorus result?

Glucose: OK

Na+: OK

K+: high

tCa: OK

Pi: high

USG: OK

hyperphosphatemia due to decreased GFR (post renal cause)