CASE FILES: BIOCHEMISTRY
protein kinase C
O pens Ca
Figure 51-1. The phosphoinositide cascade. Binding of hormone to the Gq-
coupled receptor leads to the activation of phospholipase C, which hydrolyzes
PIP2 in the membrane to release the two second messengers, DAG and IP3.
including the calcium ATPase. This spike in intracellular calcium concentra-
tion is restored to the resting level by the activity of calcium transporters
including the calcium ATPase and the sodium/calcium exchanger.
Parathyroid hormone (PTH) plays a crucial role in regulating concentra-
tions of calcium and phosphorus in the extracellular fluid. The major signal for
parathyroid hormone release is low extracellular levels of free calcium ion.
PTH acts on 3 major targets—small intestine, kidney, and bone—to restore
calcium ion concentrations in the extracellular fluid to the normal range if they
fall too low.