The hormone itself is the first messenger, binding to specific membrane receptors of hormone-responsive cells. This leads to the activation of an enzyme located at the inner side of the cell membrane, called adenylate cyclase, which catalyzes the synthesis of cAMP from ATP.
The increased cellular level of cAMP initiates the activation of protein kinases.
The determination of cAMP in urine or plasma has become more and more important in the clinical assessment of parathyroid functions. The parathyroid hormone (PTH) stimulates the synthesis of cAMP in the renal cortex and an increased secretion of cAMP.
Cyclic AMP is a specific marker of circulating PTH: 90% of patients with hyperparathyroidism show increased levels of nephrogenic cAMP. The determination of cAMP is also significant for the differential diagnosis of hypercalcemia