The usefulness of phentolamine mesylate in testing for pheochromocytoma is limited by false-positive blood pressure responses in other hypertensive states not characterized by elevated catecholamine levels. Since phentolamine can release the suppression of insulin secretion caused by catecholamines, changes in plasma insulin and glucose levels were measured following phentolamine administration in an attempt to enhance this test's specificity. Three patients with proven pheochromocytoma and positive reactions had a mean fall in glucose value of 52 mg/100 ml and a mean rise in insulin level of 39 microunits/ml. Six patients with false-positive reactions had a mean fall in glucose content of 3 mg/100 ml and a mean rise in insulin value of 7 microunits/ml. Ten patients with negative reactions had similar changes. Thus, these measurements during a modified phentolamine test may identify the falsepositive reactions and strengthen this procedure.
Spergel G, Levy LJ, Chowdhury FR, Rodman HM, Ertel NH, Bleicher SJ. A Modified Phentolamine Test for the Diagnosis of Pheochromocytoma. JAMA. 1970;211(2):266–269. doi:10.1001/jama.1970.03170020030006
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