Pheochromocytoma Physiology

Hypertension Exercise Program

High Blood Pressure Natural Treatments

Get Instant Access

Pheochromocytomas synthesize catecholamines at a high rate, up to 27 times the normal adrenal medulla.9 Pheochromocytoma cells ordinarily contain more norepinephrine than epinephrine, the opposite of the normal adrenal medulla. In adults, about 90% of pheochromocytomas arise from the adrenal medulla. Pheochromocytomas that secrete epinephrine are even more likely to be located in the adrenal medulla. Paragangliomas rarely secrete epinephrine.

The serum concentration of catecholamines does not correlate well with the size of the tumor. This is attributable to several factors, including the fast production and secretion of catecholamines by small tumors and the slower secretion of cate-cholamines by larger tumors. Additionally, larger tumors tend to undergo necrosis and cystic degeneration (Figure 9-3).

The persistent oversecretion of catecholamines by most pheochromocytomas is likely owing to the lack of feedback inhibition on tyrosine kinase. Cat-

Figure 9-3. The abdominal computed tomographic scan shows a recurrent cystic right adrenal pheochromocytoma (arrow), medial to the inferior vena cava, in a 27-year-old woman with multiple endocrine neoplasia type IIB 1 year after bilateral adrenalectomy for pheochromocytoma. This cystic adrenal tumor was reresected successfully. Courtesy of Quan-Yang Duh, MD.

Figure 9-3. The abdominal computed tomographic scan shows a recurrent cystic right adrenal pheochromocytoma (arrow), medial to the inferior vena cava, in a 27-year-old woman with multiple endocrine neoplasia type IIB 1 year after bilateral adrenalectomy for pheochromocytoma. This cystic adrenal tumor was reresected successfully. Courtesy of Quan-Yang Duh, MD.

echolamines are produced in quantities that exceed the vesicular storage capacity. Catecholamines outside the storage vesicles are subject to intracellular metabolism; the excess catecholamines and their metabolites diffuse out of the pheochromocytoma cell into the circulation.

Paroxysms of severe hypertension occur in most patients with pheochromocytoma. Exocytosis of catecholamines from the pheochromocytoma appears to play a small role in such paroxysms, as evidenced by the minimal sympathetic innervation of pheochromocytomas. Instead, hypertensive crises can result from spontaneous hemorrhages within the tumor or from the release of cate-cholamine-rich blood from sinusoids within the tumor, caused by physical stimuli such as bending or twisting or micturition in patients with bladder paragangliomas and, of course, manipulation of such tumors during surgery.

Additionally, in the presence of the persistently high circulating levels of catecholamines in most patients with pheochromocytomas, the normal sympathetic nervous system may become saturated with catecholamines, owing to normal neuronal reuptake mechanisms. This may explain the paroxysms that are triggered by pain, emotional upset, intubation, anesthesia, or surgical skin incision. It may also account for the intermittent elevation in serum and urine catecholamines for 10 days or longer after the successful surgical resection of a pheochromocytoma.

Pheochromocytomas also secrete neuropeptide Y (NPY), a 36-amino acid oligopeptide that is a potent nonadrenergic vasoconstrictor and vascular growth factor. It is a constituent of neurosecretory granules and is cosecreted with norepinephrine. About 59% of adrenal pheochromocytomas secrete NPY during surgical resection. NPY appears to contribute to hypertension in most patients with pheochromocy-toma.1011 However, few patients with paraganglioma secrete NPY.12

Was this article helpful?

0 0
Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

Get My Free Ebook


Post a comment