Nephrogenous cyclic AMP (NcAMP), total cyclic AMP excretion (UcAMP), and plasma immunoreactive parathyroid hormone (iPTH), determined with a multivalent antiserum, were prospectively measured in 55 control subjects, 57 patients with primary hyperparathyroidism (1 degrees HPT), and 10 patients with chronic hypoparathyroidism. In the group with 1 degrees HPT, NcAMP was elevated in 52 patients (91%), and similar elevations were noted in subgroups of 26 patients with mild (serum calcium </=10.7 mg/dl) or intermittent hypercalcemia, 19 patients with mild renal insufficiency (mean glomerular filtration rate, 64 ml/min), and 10 patients with moderate renal insufficiency (mean glomerular filtration rate, 43 ml/min). Plasma iPTH was increased in 41 patients (73%). The development of a parametric expression for UcAMP was found to be critically important in the clinical interpretation of results for total cAMP excretion. Because of renal impairment in a large number of patients, the absolute excretion rate of cAMP correlated poorly with the hyperparathyroid state. Expressed as a function of creatinine excretion, UcAMP was elevated in 81% of patients with 1 degrees HPT, but the nonparametric nature of the expression led to a number of interpretive difficulties. The expression of cAMP excretion as a function of glomerular filtration rate was developed on the basis of the unique features of cAMP clearance in man, and this expression, which provided elevated values in 51 (89%) of the patients with 1 degrees HPT, avoided entirely the inadequacies of alternative expressions. Results for NcAMP and UcAMP in nonazotemic and azotemic patients with hypoparathyroidism confirmed the validity of the measurements and the expressions employed.

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Nephrogenous Cyclic Adenosine Monophosphate as a Parathyroid Function Test

The calcium-sensing receptor (CASR) is a plasma membrane G protein coupled receptor that is expressed in the parathyroid hormone (PTH) producing chief cells of the parathyroid gland and the cells lining the kidney tubule. By virtue of its ability to sense small changes in circulating calcium concentration ([Ca(2+)](o)) and to couple this information to intracellular signaling pathways that modify PTH secretion or renal cation handling, the CASR plays an essential role in maintaining mineral ion homeostasis. Inherited abnormalities of the CASR gene located on chromosome 3p13.3-21 can cause either hypercalcemia or hypocalcemia depending upon whether they are inactivating or activating, respectively. Heterozygous loss-of-function mutations give rise to familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong hypercalcemia is asymptomatic. The homozygous condition manifests itself as neonatal severe hyperparathyroidism (NSHPT), a rare disorder characterized by extreme hypercalcemia and the bony changes of hyperparathyroidism which occur in infancy. The disorder autosomal dominant hypocalcemia (ADH) is due to gain-of-function mutations in the CASR gene. ADH may be asymptomatic or present with neonatal or childhood seizures. A common polymorphism in the intracellular tail of the CASR, Ala to Ser at position 986, has a modest effect on the serum calcium concentration in healthy individuals.

Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia.

The discovery of a G protein-coupled, calcium-sensing receptor (CaR) a decade ago and of diseases caused by CaR mutations provided unquestionable evidence of the CaR's critical role in the maintenance of systemic calcium homeostasis. On the cell membrane of the chief cells of the parathyroid glands, the CaR "senses" the extracellular calcium concentration and, subsequently, alters the release of parathyroid hormone (PTH). The CaR is likewise functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Intracellular signal pathways to which the CaR couples via its associated G proteins include phospholipase C (PLC), protein kinase B (AKT); and mitogen-activated protein kinases (MAPKs). The receptor is widely expressed in various tissues and regulates important cellular functions in addition to its role in maintaining systemic calcium homeostasis, i.e., protection against apoptosis, cellular proliferation, and membrane voltage. Functionally significant mutations in the receptor have been shown to induce diseases of calcium homeostasis owing to changes in the set point for calcium-regulated PTH release as well as alterations in the renal handling of calcium. Gain-of-function mutations cause hypocalcemia, whereas loss-of-function mutations produce hypercalcemia. Recent studies have shown that the latter clinical presentation can also be caused by inactivating autoantibodies directed against the CaR Newly discovered type II allosteric activators of the CaR have been found to be effective as a medical treatment for renal secondary hyperparathyroidism.

THE CALCIUM-SENSING RECEPTOR IN NORMAL PHYSIOLOGY AND PATHOPHYSIOLOGY: A Review

Familial benign hypercalcemia (hypocalciuric hypercalcemia) was diagnosed in 125 members of 21 families. The syndrome was generally characterized by autosomal dominant inheritance of symptomless, nonprogressive hypercalcemia with normal serum immunoreactive parathyroid hormone concentrations, parathyroid glands that had normal gross and histologic features, relatively low urinary excretion of calcium, and failure to achieve normocalcemia after subtotal parathyroidectomy. Affected persons had normal longevity and no discernible increase in other medical problems except gallstones. The parathyroid glands were not seen using high-resolution ultrasonography. Plasma calcitonin and calcitriol levels were normal or low. Skeletal mass was normal as assessed by photon absorptiometry of the radius and lumbar spine, and fractures were not more frequent. Familial benign hypercalcemia or hypocalciuric hypercalcemia is a distinctive heritable syndrome that should always be considered in the differential diagnosis of asymptomatic hypercalcemia.

Familial Benign Hypercalcemia (Hypocalciuric Hypercalcemia): Clinical and Pathogenetic Studies in 21 Families

In the 1950s and 1960s, several inherited disorders of fluid and electrolyte metabolism were described in which the principal disturbance appeared to be a specific functional defect in the renal tu...

Genetic disorders of renal electrolyte transport.

Familial benign hypercalcemia

Eleven patients wiht IGR of unknown cause were studied; 4 of 5 followed for 12–19 months on GH grew significantly. In 9 of 11 the bone age (BA) was significantly retarded. Pretreatment growth rates (GR) were less than expected for chronological age (CA). All secreted normal amounts of GH with arginine infusion and insulin hypoglycemia before and after therapy. The data is summarized below in 5 patients: conclusions: (1) Exogenous GH accelerates growth rates in some patients with IGR; (2) Higher doses are required to accelerate growth to the GR attained in patients with GH deficiency; (3) Some patients with IGR with skeletal retardation may be candidates for exogenous GH therapy when available in abundant supply.

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Thomas P. Foley

Papers

Familial benign hypercalcemia

Growth hormone (GH) as a therapeutic agent in patients with intrauterine growth retardation (IGR)