Potassium, Calcium, Phosphate Regulation

The kidney regulates plasma $K^+$ .
Most $K^+$ is intracellular, so cellular shifts are important.
Changes in $[K^+]<em>{plasma}$ reflect changes in $[K^+]</em>{body}$ and shifts of $K^+$ into or out of cells.
Insulin promotes intracellular accumulation of $K^+$ .
Epinephrine stimulates cellular uptake via $Na^+K^+ATPase$ .
Acidemia stimulates potassium efflux.
Small changes in $[K^+]_{plasma}$ can have profound effects, especially on the heart.
Hypokalemia: Hyperpolarization.
Hyperkalemia: Depolarization.
$K^+$ excretion can exceed $K^+$ filtration, implying tubular secretion.
Proximal tubule reabsorption is relatively fixed.
Distal tubule and collecting duct: Principal cells secrete $K^+$ under aldosterone control.

Calcium is important for intercellular adhesion, blood coagulation, neuronal excitability, muscle contraction, and mineralization of bones and teeth.
Calcium is stored in bone and sequestered in organelles.
Regulated by gut absorption, release from bone, entering bone, and urinary excretion.
Free $Ca^{2+}$ is physiologically important and regulated by the kidney.
Acidosis increases free $[Ca^{2+}]$ , while alkalosis decreases it.
Daily calcium balance: Intake = Urinary + Fecal loss (output).
Regulation of calcium and phosphate is coordinated because almost all of the body’s calcium is in bone with phosphate.
High levels of intracellular calcium are toxic.
Calcium and phosphate are reciprocally regulated.
Calcitonin decreases plasma $Ca^{2+}$ .
PTH increases plasma $Ca^{2+}$ .
PTH increases $Ca^{2+}$ reabsorption and decreases phosphate reabsorption. It also promotes the production of bioactive Vitamin D.
Active form of Vitamin D (Calcitriol) promotes $Ca^{2+}$ absorption from the intestine.