Urinary Cyclic Adenosine 3′,5′-Monophosphate Response in McCune-Albright Syndrome: Clinical Evidence for Altered Renal Adenylate Cyclase Activity

The recent finding of an activating mutation in the G_sα protein, the protein that couples receptors to stimulation of adenylate cyclase, from endocrine and nonendocrine tissues of patients with McCune-Albright syndrome (MAS) suggests that alterations in adenylate cyclase activity may account for the clinical abnormalities in these patients. Many patients with MAS have hypophosphatemia. This may result from the presence of the activating G_sα mutation in proximal renal tubules or the elaboration of a phosphaturic factor from fibrous dysplasia. We, therefore, sought to characterize renal cAMP generation and phosphate handling in MAS patients. Intravenous infusion of PTH is a classic clinical test used to evaluate hormonal responsiveness of renal proximal tubule adenylate cyclase and examine PTH-dependent phosphate clearance. We performed PTH infusion in 6 MAS patients, 10 normal subjects, and 6 patients with pseudohypoparathyroidism (PHP). The basal urinary cAMP (UcAMP) level in the MAS group [5.5 ± 2.6 nmol/dL glomerular filtration (GF)] was elevated (P <0.05) compared to those in both normal subjects (3.2 ± 1.2 nmol/dL GF) and patients with PHP (1.9 ± 0.6 nmol/dL GF). However, PTH-stimulated peak UcAMP (15.0 ± 7.0 nmol/dL GF) and the peak/basal UcAMP ratio (3.1 ± 1.7) in MAS were significantly lower than the respective values in normal subjects (30.8 ± 16.9 nmol/dL GF and 9.3 ± 2.9; P <0.05 for both) and were statistically similar to the blunted levels in PHP (respectively, 3.1 ± 1.5 nmol/dL GF and 2.0 ± 1.7). By contrast, the PTH-induced phosphaturic response in MAS patients was similar to that in the normal subjects. Our study provides clinical evidence that MAS patients have altered renal adenylate cyclase activity, manifested by an elevated basal UcAMP, but a blunted UcAMP response to PTH stimulation. These observations are presumably due to a mutation in the G_sα protein in the renal tubules. Despite the blunted UcAMP excretion, the phosphaturic response to PTH in MAS patients is intact.