Nephrogenous cyclic AMP and plasma parathyroid hormone in hypercalcaemia: the influence of renal function

Immunoreactive parathyroid hormone (PTH) levels and nephrogenous cyclic adenosine monophosphate (cAMP) have been reported to be useful parameters in the diagnosis of hyperparathyroidism. Measurements in hyperparathyroid patients usually give values above the normal range when PTH is measured with a carboxyterminal radioimmunoassay and when nephrogenous cAMP is related to glomerular filtration rate. We tested these two parameters in two groups of hypercalcaemic patients (twelve cases of primary hyperparathyroidism and fourteen cases of hypercalcaemia of non-parathyroid origin) and in two groups of normocalcaemic subjects (twenty-one young healthy volunteers and fourteen elderly subjects without parathyroid disease). Slight impairment of renal function caused elevated values of immunoreactive parathyroid hormone in a carboxyterminal radioimmunoassay and also of nephrogenous cAMP when related to glomerular filtration rate. We found that elevated nephrogenous cAMP without parathyroid disease could be attributed to renal insufficiency and to the mode of expression generally used for the nephrogenous cAMP.     

Urinary cyclic 3',5'-adenosine monophosphate responses to exogenous and endogenous parathyroid hormone in familial benign hypercalcemia and primary hyperparathyroidism

FBH is characterized by symptomless hypercalcemia, low urinary calcium excretion, normal iPTH values, generally normal parathyroid histology, and failure of subtotal parathyroidectomy is normalize serum calcium. We studies six patients with FBH from three kindreds, six patients with sporadic 1 omicron HPT, and six healthy volunteers. To characterize the renal response to PTH, 14 of the subjects had infusions of bovine PTE (300 U intravenously over 15 min) and, separately, stimulation of endogenous PTH release by infusion of disodium EDTA (50 mg/kg over 2 hr). PTE induced striking increases of UcAMP (nM/100 ml of GF) that were indistinguishable between controls and subjects having FBH. However, the rise of UcAMP in 1 omicron HPT was significantly reduced (p < 0.001) compared to controls or the FBH group. EDTA-induced hypocalcemia raised serum iPTH and UcAMP in all three groups; the increases of iPTH (two assays of differing specificity) were greatest in 1 omicron HPT and least in FBH. In contrast, increases of UcAMP were greatest in FBH and 1 omicron HPT and indistinguishable from one another. The increase of UcAMP considered as a function of the increase in PTH showed significantly greater UcAMP responses in FBH than in the other groups. These results are consistent with primary or secondary alterations of renal responsiveness to PTH in both FBH and 1 omicron HPT, which may in part explain the different renal tubular calcium handling in the two conditions.     

Inhibition of carbonic anhydrase by parathyroid hormone and cyclic AMP in rat renal cortex in vitro.

It has been demonstrated that parathyroid hormone (PTH) inhibits the proximal tubular reabsorption of bicarbonate, and increases the urinary excretion of that ion. There is also a qualitative similarity between the alterations of the proximal tubular reabsorption of phosphate, sodium, and water after PTH administration and after acetazolamide administration. These findings suggest that the renal effect of PTH is possibly mediated through the inhibition of carbonic anhydrase in proximal tubules. Therefore, a possible inhibitory effect of PTH on carbonic anhydrase was evaluated in the homogenate of rat renal cortex by an indicator titration method. Incubation of cortical homogenates with PTH for 10 min at 37degreesC inhibited carbonic anhydrase activity. The inhibitory effect of PTH was ATP-, Mg++-, and K+-dependent and temperature-dependent; inactivation of PTH by heating at 100degreesC abolished the effect of PTH both to activate adenylate cyclase and to inhibit carbonic anhydrase. Calcium 5 mM also partially abolished effects of PTH to activate adenylate cyclase and to inhibit carbonic anhydrase. The inhibitory effect of PTH on carbonic anhydrase was specific to renal cortex. Cyclic AMP, the intracellular messenger substance for PTH, also inhibited carbonic anhydrase in renal cortex. The cyclic AMP-induced inhibition was also Mg++ dependent and temperature dependent, and required preincubation at 37degreesC. But 5'-AMP, a metabolic derivative of cyclic AMP without its biological effect, had no inhibitory effect on carbonic anhydrase. All the above results are consistent with the hypothesis that PTH inhibits proximal tubular reabsorption of bicarbonate and phosphate through the inhibition of carbonic anhydrase, and that inhibitory effect is mediated through the cyclic AMP system.     

Early effects of synthetic bovine parathyroid hormone and synthetic salmon calcitonin on urinary excretion of cyclic AMP, phosphate and calcium in man.

Bovine synthetic parathyroid hormone infused intravenously in man increased both the urinary excretion of cyclic AMP and the urinary excretion of phosphate whereas a Salmon synthetic calcitonin infusion increased the urinary excretion of phosphate without change in urinary excretion of cyclic AMP. These data are consistent with the hypothesis that different renal mechanisms are involved in the response to each hormone.     

Effects of physical exercise on serum calcium and parathyroid hormone

The effects of physical exercise on plasma ionized calcium, total serum calcium and parathyroid hormone (PTH) concentrations were evaluated in healthy subjects submitted to work on an ergometer bicycle. When the workload was increased stepwise there was a significant increase (P less than 0.001) in the calcium concentrations (ionized calcium from 1.13 +/- 0.03 (SD) to 1.24 +/- 0.03 mmol 1(-1) and total calcium from 2.35 +/- 0.07 to 2.48 +/- 0.07 mmol 1(-1] when the workload exceeded approximately 65% of the estimated maximum--i.e. a load that caused accumulation in blood of lactic acid. The rise in plasma ionized calcium was, therefore, presumably largely attributed to the acidosis but reduction of plasma volume and influx from extracellular sources might also have contributed. Beta blockade (with oral intake of propranolol) reduced physical capacity, shortened the duration of work and caused less acidosis. These factors were probably responsible for a smaller rise in ionized calcium during beta blockade (7 +/- 4%) than in control studies (21 +/- 5%) without medication in subjects examined during short-term maximal exercise. Long-term (1 h) steady-state work which caused fatigue without producing lactic acidosis did not affect the calcium concentrations. Despite the effects of work on calcium levels there was no discernible suppression of the PTH concentrations. This might have been due to a concomitant stimulation of PTH secretion by work.     

Urinary cyclic AMP and post-traumatic acute renal failure.

Consecutive daily urinary excretion of cyclic AMP has been investigated in 16 patients with severe trauma or illness, five of whom developed acute renal failure (ARF). Fluctuations in the nucleotide excretion exceeded the range found in 20 healthy volunteers (1.26-14.74, mean 7.13+/-1.18 vs. 2.04-10.10, mean 5.07+/-2.21 micronmol/24 h). This resulted in a 41% increase of cAMP excretion in the group with normal renal function (P less than 0.003) with the highest individual increase of 87%. The excretion usually reached its peak by 24 h after trauma and its lowest value by the third day, (first day vs. third day; 7.82+/-4.23 vs. 3.96+/-2.58 micronmol/24 h, P less than 0.05 for a group of 11 patients), while creatinine clearance remained normal. In four patients with severe ARF, the mean urine volume was above control value but the cAMP excretion was reduced to 3.9 to 14.4% and in one patient with a mild ARF to 60.6%. Creatinine excretion of the group was less reduced than that of cAMP (41.2% vs. 19.6%, resp.). cAMP excretion declined proportionally with diminishing creatinine clearance. In the category of 33-65 ml/min it decreased by 33.4% to 3.39 micronmol+/-1.16 micronmol/24 h. cAMP/creatinine ratio proved to be a less sensitive indicator than cAMP/24 h. Daily output of cAMP and creatinine correlated highly with diuresis in ARF patients, controls (always P less than 0.001) and less in patients with normal renal function (P less than 0.02). Urinary cAMP appears to be a very sensitive and early indicator of the onset of ARF and subsequent recovery. This warrants its further study.     

The influence of serum calcium and parathyroid hormone upon glucose metabolism in uremia.

The alterations in carbohydrate metabolism which attend the uremic syndrome have been recognized for some time. Recently, an interaction between hyperparathyroidism and these alterations in intermediary metabolism has been postulated. To further define any such interaction, 6 stable dialysis patients with significant secondary hyperparathyroidism were studied prior to and after subtotal parathyroidectomy. Glucose utilization and insulin secretion were estimated by use of a standard intravenous glucose tolerance test and the resistance of peripheral tissues to exogenous insulin was evaluated by insulin tolerance testing. All of peripheral tissues to exogenous insulin was evaluated by insulin tolerance testing. All patients were studied under baseline conditions, as well as induced hyper- and hypocalcemia, prior to and at least 2 months after surgery. Parathyroidectomy, per se, had no significant effect upon glucose utilization, insulin secretion, or the resistance of peripheral tissues to the action of exogenous insulin. Both induced hyper- and hypocalcemia, on the other hand, significantly diminished glucose utilization as judged by a reduced glucose disappearance rate during intravenous glucose tolerance testing. Hypocalcemia was associated with a markedly reduced insulin secretory response and normal tissue insulin sensitivity, while hypercalcemia was associated with a normal insulin response but reduced tissue sensitivity. The data suggest that calcium ion concentration may affect both glucose utilization and insulin secretion. As such, it must be adequately controlled in furture metabolic studies.     

Improved diagnosis of primary hyperparathyroidism by defining the inverse relationship between serum immunoreactive parathyroid hormone and calcium

Serum concentrations of immunoreactive parathyroid hormone (iPTH) measured with a mid-region specific radioimmunoassay and total calcium were correlated in 300 healthy subjects and 158 patients with surgically verified primary hyperparathyroidism (HPT). All the healthy individuals could be separated from the patients by a monoexponential declining curve in which iPTH at concentrations of 0.60 micrograms/l and 0.33 micrograms/l corresponded to calcium concentrations of 2.20 mmol/l and 2.60 mmol/l, respectively. In 22 patients more than one sample was analysed and serum iPTH and calcium were inversely correlated. In contrast, three patients with parathyroid carcinoma showed no reciprocal fluctuations between serum iPTH and calcium. Of 75 patients with hypercalcaemia due to malignant diseases (metastatic mammary carcinoma, bronchial carcinoma, renal carcinoma, myelomatosis), 62 had a normal iPTH/calcium relationship. Two patients with myelomatosis had a temporary elevation of serum iPTH and calcium due to renal impairment. One patient with bronchial carcinoma probably had ectopic production of iPTH. The remaining 10 patients (six mammary carcinomas and four bronchial carcinomas) were found in the pathological iPTH/calcium range. In conclusion, we have demonstrated that an inverse relationship exists between serum iPTH and calcium in patients with non-malignant, primary HPT. Evaluation of iPTH and calcium in the same serum sample gave a correct diagnosis in more than 90% of patients with primary HPT.     

Bone resorption induced by parathyroid hormone and dibutyryl cyclic AMP: role of carbonic anhydrase

The role of carbonic anhydrase in bone resorption induced by parathyroid hormone (PTH) and dibutyryl cyclic AMP (DBcAMP) was studied using an in vitro neonatal mouse half-calvarial culture system. Both PTH (16.7 nM) and DBcAMP (0.3 mM) were effective in stimulating bone resorption, as assessed by measuring changes in media calcium concentrations. Bones treated with PTH or DBcAMP for 96 hr contained significantly greater carbonic anhydrase activity than control bones [PTH Treated/Control (T/C) = 2.44; DBcAMP T/C = 2.34]. Both PTH and DBcAMP significantly enhanced calvarial acid phosphatase activity relative to control values [PTH T/C = 1.48; DBcAMP T/C = 1.30]. Neither PTH nor DBcAMP significantly altered calvarial alkaline phosphatase activity. Bone resorption induced by PTH and DBcAMP was inhibited by the carbonic anhydrase inhibitor acetazolamide, but not by the acetazolamide analog CL 13,850 (N-t-butylacetazolamide), which does not inhibit carbonic anhydrase. These results support the concepts that PTH-induced bone resorption requires the action of osteoclastic carbonic anhydrase and that the action of PTH on bone is mediated, in part, by the action of cyclic AMP.     

Responsiveness of urinary cyclic AMP and phosphate to parathyroid extract in patients with parathyroid disorders.

Responsiveness of urinary cyclic AMP and phosphate to 200 units of parathyroid extract was evaluated in 5 normal subjects, 2 patients with idiopathic hypoparathyroidism, 4 patients with pseudohypoparathyroidism and 3 patients with primary hyperparathyroidism. Among them, 3 patients with pseudohypoparathyroidism were examined prior to and during therapy with vitamin D. Two patients with primary hyperparathyroidism were examined before and after removal of adenomas. In control subjects, percent increase in cyclic AMP after parathyroid extract administration was 7265 plus or minus 3312%, and in phosphate 290 plus or minus 72%. It was found that in idiopathic hypoparathyroidism the response of cyclic AMP was in the normal range, though that of phosphate was higher than normal. In pseudohypoparathyroidism, as distinguished from what Drezner et al. called pseudohypoparathyroidism type II, the response of cyclic AMP was uniformly low, while that of phosphate was variable. Similar results were obtained during treatment with vitamin D. In primary hyperparathyroidism, the responses of both cyclic AMP and phosphate were lower than normal. After removal of adenomas, the response of phosphate became norma., but the response of cyclic AMP rose to a subnormal level in one patient, and remained low in the other. For the diagnosis of pseudohypoparathyroidism, the response in cyclic AMP was considered to be a more reliable index than that in phosphate whether the patient was being treated with vitamin D or not.     

Urinary excretion of cyclic AMP in bacterial infections

Summary Urinary cyclic AMP excretion was found to be increased in patients with severe bacterial infections and normal renal function. The observed changes appeared due to a combination of an increased filtered load plus augmented nephrogenous production in some patients; while in others, only an increase in the apparent nephrogenous production of cAMP could be found to account for the elevation in the total urinary excretion. Since total serum calcium was found to be low in most of these patients, increased PTH secretion to reduce urinary excretion of calcium may have been responsible for an increase in renal parenchymal production, and subsequent excretion of cyclic AMP. Although speculative, this theory is tenable in that ionized hypocalcemia exists in septic patients.Abbreviations cAMP cyclic 3, 5-adenosine monophosphate - PTH parathormone - ADH andidiuretic hormone - GFR glomerular filtration rate     

Urinary cyclic AMP in spot urine of healthy children

We present reference values for the excretion of cAMP in spot urine collected between 09.00 and 12.00 hours in 143 healthy children aged 2-200 months. The excretion of cAMP was creatinine-corrected and expressed as a substance concentration ratio (UcAMP/crea)U due to a positive significant correlation between the excretion of cAMP and creatinine (r = 0.68, p less than 0.001). The mean value (95% significance limits) for (UcAMP/crea)U was 748 mumol/mol (254-2206 mumol/mol). A logarithmic transformation of the ratio was used, since preliminary analysis showed uneven distribution; when the logarithmic transformation was used, the data appeared evenly distributed. There was no significant difference between the results for girls and boys. The value of lg(UcAMP/crea)U was related to the age or body surface area with decreasing values at higher age or body surface area. (r = -0.55 and r = -0.57, p less than 0.001). Spot urine for measurement of urinary cAMP instead of a 24 h collection appears preferable due to the practicability of the test in children.     

Urinary excretion patterns of cyclic AMP and cyclic GMP following thermal injury

Measurement of 24-h urinary cyclic AMP and cyclic GMP levels in 19 patients for up to nine days after thermal injury has revealed differences in cyclic nucleotide excretion patterns between "severe" and "mild to moderate" groups of burned patients, classified according to a predictive index of burn mortality. Cyclic AMP excretion fell significantly from a high initial level in the "severe" group, but showed no significant change in the "mild to moderate" group. Differences in cyclic AMP excretion between the two groups of patients were only significant on the second day following burn injury. There was no correlation between cyclic AMP output averaged for each patient over the first nine days and percentage body surface area of the burn or the predictive index. In contrast to the results for cyclic AMP excretion, cyclic GMP output rose significantly over the 9-day period in severely burned patients, while levels reached a plateau on days four to six for the moderate to mild group. When cyclic GMP excretion was averaged over the first nine days for each patient, a significant correlation with both percent body surface area of burn and the predictive index of burn mortality was found. The tissue source or sources which are responsible for increased cyclic GMP excretion are as yet unknown.     

Antibodies to parathyroid hormone-related protein lower serum calcium in athymic mouse models of malignancy-associated hypercalcemia due to human tumors.

A parathyroid hormone-related protein (PTHrP) has recently been isolated from tumors associated with hypercalcemia. In the present study, we tested the effects of neutralizing antisera to the PTHrP on serum calcium and urine cAMP in two animal models of malignancy-associated hypercalcemia. The animal models consisted of (a) a human squamous cell lung cancer and (b) a human laryngeal cancer, both serially carried in athymic mice. The antisera specifically reduced the elevated serum calcium and urinary cAMP levels in the tumor-bearing animals. We conclude that PTHrP plays a major role in the pathogenesis of malignancy-associated hypercalcemia.     

Influence of pertussis toxin on parathyroid hormone stimulated cyclic AMP production and phosphate transport in opossum kidney cells

There is evidence that guanine nucleotide-sensitive (G) proteins intervene in the activation of adenylate cyclase by parathyroid hormone (PTH). Furthermore, recent studies suggest that G proteins may be involved in the activation by PTH of phospholipase C, with subsequent elevation of diacylglycerol, inositol trisphosphate, and intracellular calcium. Since G proteins may be involved in both transduction systems postulated to mediate the actions of PTH, the present studies were performed to evaluate the influence of pertussis toxin, which prevents receptor-mediated activation of G proteins, on the effects of PTH in opossum kidney (OK) cells. In OK cell membranes, pertussis toxin catalyzed the adenosine diphosphate (ADP) ribosylation of a protein with a molecular weight of 41 kd on SDS-PAGE. Cholera toxin catalyzed the ribosylation of two proteins of molecular weight 52 and 45 kd. Pretreatment of the cells with pertussis toxin abolished the labelling of this 41 kd protein, confirming the access of the toxin into the cells and the presence of pertussis toxin-sensitive substrates. The ribosylation of the cholera toxin substrates was unaffected by pertussis toxin pretreatment of the cells. Treatment of OK cells with pertussis toxin did not change the basal levels of cyclic AMP, but increased the levels of cyclic AMP in response to bPTH 1-34 from 355 +/- 17 to 449 +/- 20 pmoles cyclic AMP per 5 minutes per culture. These results were consistent with the inactivation of an inhibitory G protein. Furthermore, PTH-stimulated cyclic AMP generation was inhibited by norepinephrine from 362 +/- 10 to 228 +/- 18 pmole cyclic AMP per 5 minutes per culture.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypocalcemia increases and hypercalcemia decreases the steady-state level of parathyroid hormone messenger RNA in the rat.

To examine the effects of serum calcium concentrations on PTH biosynthesis, rats were made hyper- (serum total calcium, approximately 3.5 mM) or hypocalcemic (approximately 1.25 mM) and steady-state levels of PTH mRNA in parathyroid cells were measured by the primer extension method using a 32P-labeled synthetic oligomer. PTH mRNA levels increased about twofold in the rats made slightly hypocalcemic by infusion of calcium-free solution and decreased slightly in those made hypercalcemic by CaCl2 infusion (120-150 mumol/h) compared with the levels present in nonfasting control rats. Infusion of calcitonin (0.5 U/h) or EGTA (90 mumol/h) with calcium-free solution increased PTH mRNA levels further (two- to sevenfold) above the levels present in animals infused with calcium-free solution alone. These changes in PTH mRNA levels were observed after 48- but not 24-h infusion, and there was an inverse correlation between PTH mRNA levels and serum calcium concentrations. The results suggest that changes in serum calcium concentrations in the near physiological range regulate the biosynthesis of PTH by affecting steady-state levels of PTH mRNA when hypercalcemia or hypocalcemia continues for a relatively long period.