Calcitriol, parathyroid hormone, and accumulation of aluminum in bone in dogs with renal failure.

Accumulation of aluminum in bone is a frequent finding in patients requiring chronic dialysis and is associated with considerable morbidity and/or mortality. Until now, evidence seemed to point to relatively low circulating levels of parathyroid hormone as a contributing factor, but because levels of parathyroid hormone and calcitriol are interrelated, calcitriol might be also involved. In this study we employed an animal model to evaluate the single and combined effects of parathyroid hormone and calcitriol on bone aluminum accumulation. The results show significantly less aluminum accumulation in calcitriol-replete dogs independent of the presence or absence of parathyroid hormone. These results indicate that low levels of calcitriol may play a role in the development of aluminum related bone disease. Further studies are needed to demonstrate whether administration of calcitriol in patients with renal insufficiency will prevent development of aluminum-related bone disease.

Studies on mechanisms of hypocalcemia of magnesium depletion.

Studies were carried out to evaluate the mechanism of hypocalcemia in magnesium depletion. Day old chicks fed a magnesium deficient diet developed marked hypocalcemia, with a direct relation between serum calcium (y) and magnesium (x): y = 2.68 x + 4.24, r = 0.84 (both in mg/100 ml). Injections of parathyroid extract that increased serum calcium 2-3 mg/100 ml in normals had no effect in Mg-depleted birds. Very large dietary supplements of calcium or vitamin D(3) increased mean serum calcium only from 5.3 to 7.7 and 7.8 mg/100 ml, respectively, while a normal magnesium diet for 3 days increased calcium from 5.3 to 9.9 mg/100 ml despite absence of dietary calcium. Intestinal calcium transport, studied in vitro, and the calcium concentration of the carcass was significantly increased in magnesium-depleted chicks, making it unlikely that reduced intestinal absorption of calcium caused the hypocalcemia. In magnesium-deficient chicks, the bone content of magnesium was decreased by 74%, the calcium content was unchanged, and the cortical thickness of bone was markedly increased. After 3 days of magnesium-repletion, cortical thickness was reduced with increased endosteal resorption. There was an increase in unmineralized osteoid tissue in the magnesium-depleted chicks. Parathyroid gland size and histology did not differ in magnesium-depleted and control birds. The results suggest that hypocalcemia develops due to altered equilibrium of calcium between extracellular fluid and bone, favoring increased net movement into the latter. Failure of parathyroid gland function could also exist, and unresponsiveness to parathyroid hormone (PTH) may also contribute to the hypocalcemia. However, failure of PTH action is probably due to the presence of excess osteoid tissue rather than a primary event leading to hypocalcemia.

Bone changes occurring early after cessation of ovarian function in beagle dogs: a histomorphometric study employing sequential biopsies.

The beagle dog model has been established by our laboratory as a useful animal model to study bone loss after cessation of ovarian function. Previously we demonstrated bone loss associated with an osteoblastic insufficiency at 4 months after ovariohysterectomy (OHX). This study was designed to evaluate by four sequential monthly bone biopsies the development and course of the histologic bone abnormalities after OHX. We found cancellous bone volume, trabecular density, and wall thickness to be decreased (p less than 0.05) and trabecular separation increased (p less than 0.05) as early as 1 month after OHX. After 2 months, there was a decrease in mineralizing surface and mineral apposition rate (p less than 0.05). Volume and surface of osteoid were increased after 3 months (p less than 0.05), and there was an increase in the number of osteoblasts (p less than 0.01). No histologic signs of increased resorption were observed during the experiment. However, the findings of low bone volume with decreased trabecular density and increased separation without a change in trabecular plate thickness 4 weeks after OHX suggest that a dramatic increase in resorption must have taken place soon after OHX. These results point to an early phase of initiation of bone loss related to hyperresorption followed by a maintenance phase of low bone mass ascriblastic insufficiency. The events that stimulate the early initiating phase after cessation of ovarian function, the factors contributing to it, and the direct demonstration of hyperresorption await further studies.

A new bisphosphonate, BM 21.0955, prevents bone loss associated with cessation of ovarian function in experimental dogs.

We previously found that bone loss occurs as soon as 1 month after ovariohysterectomy (OHX) in beagle dogs. Indirect evidence pointed to an early dramatic increase in bone resorption. To verify this hypothesis and evaluate the effects of a newly developed bisphosphonate, BM 21.0955 (Boehringer Mannheim), 36 beagle dogs were subjected to OHX and 12 dogs were sham operated (Sham). OHX dogs were divided into six groups (n = 6 each) and received subcutaneous injections of vehicle or BM 21.0955 at various doses (0.1, 0.3, 1, 10, and 100 micrograms/kg/day) for 1 month. Sham dogs were given vehicle (n = 6) or BM 21.0955 (1 microgram/kg/day, n = 6). Iliac crest biopsies and blood drawings were done at baseline and at month 1. OHX dogs given vehicle exhibited a decrease in cancellous bone volume associated with an increase in erosion depth and a decrease in serum levels of 1,25-dihydroxyvitamin D. BM 21.0955 prevented the bone loss at a dose > or = 1 microgram/kg and the increase in erosion depth and the decrease in serum levels of 1,25-(OH)2D at a dose > or = 0.3 microgram/kg. No osteomalacia was observed at any dose of BM 21.0955. Bone turnover was reduced only when BM 21.0955 was administered at doses of 10 or 100 micrograms/kg. There were no changes in body weight or serum levels of calcium, phosphorus, creatinine, parathyroid hormone, or osteocalcin in all groups. The increase in erosion depth in OHX dogs given vehicle proves that the early rapid bone loss after cessation of ovarian function is related to an increase in osteoclastic activity. The antiosteoclastic activity of BM 21.0955 at a dose > or = 1 microgram/kg prevents this increase and preserves bone volume. The absence of any signs of osteomalacia at any dose confers a relatively wide therapeutic margin to BM 21.0955. BM 21.0955 at a dose > or = 10 micrograms/kg also acts as an inhibitor of bone turnover. This is not observed at a dose of 1 microgram/kg, at least after 1 month of administration.

Sites of clearance of endogenous parathyroid hormone in the vitamin D-deficient dog.

The sites of clearance of endogenous parathyroid hormone (PTH) were studied in dogs who developed secondary hyperparathyroidism on a vitamin D-deficient diet. Simultaneous blood samples were obtained from the femoral artery, and the hepatic, renal, portal, and femoral veins. Radioimmunoassay of canine immunoreactive PTH (iPTH) in a heterologous bovine PTH (bPTH) system indicated that the kidney and liver extracted 49% and 46%, respectively, of the iPTH circulating through these organs. Characterization of the circulating iPTH in these animals was carried out by gel filtration and radioimmunoassay of the eluant fractions utilizing specific amino- and carboxyl-terminal antisera. The hormone in the peripheral circulation co-eluted with [125I]-iodobPTH and no fragments of iPTH were detected. Immunochemical differences between bovine and canine PTH were detected in the carboxyl-terminal region of the molecule. The results indicate that endogenous PTH in the vitamin D-deficient, hypocalcemic dog is cleared by the kidney and liver and that the predominant form of the hormone in this animal is similar to glandular bPTH 1-84.

1,25-dihydroxyvitamin D3 corrects bone loss but suppresses bone remodeling in ovariohysterectomized beagle dogs.

The decrease in intestinal calcium absorption and lower blood levels of 1,25-dihydroxyvitamin D [1,25-(OH)2D3] have been implicated in the pathogenesis of postmenopausal osteoporosis. This study evaluates the effects on bone of 1,25-(OH)2D3 therapy using the ovariohysterectomized dog model. The cessation of ovarian function was ascertained by an increase in serum LH levels 4 weeks after ovariohysterectomy, and significant bone loss was revealed four months after ovariohysterectomy. The bone loss was associated with an increase in the number of bone-forming cells and a decrease in the activity of these cells. The administration of 1,25-(OH)2D3 increased the activity of the bone cells and resulted in a reversal of all abnormalities in structural parameters of bone, including cancellous bone mass, trabecular wall thickness, trabecular plate separation, trabecular plate density, and trabecular plate thickness (which increased above normal). However, 1,25-(OH)2D3 therapy was also associated with a significant decrease in the number of bone-forming cells, resulting in lower bone formation at the tissue level. The results of this study indicate that 1,25-(OH)2D3 therapy can reverse the bone loss and osteoblastic insufficiency responsible for the maintenance of negative bone balance after the cessation of ovarian function. However, this therapy has a suppressive effect on bone cell number and bone turnover. This undesired side-effect of 1,25-(OH)2D3 therapy renders a chronic therapeutic regimen inefficient for the long term management of patients with osteoporosis. Intermittent 1,25-(OH)2D3 therapy or a sequential therapy using 1,25-(OH)2D3 along with substances known to increase the number of bone-forming cells is strongly suggested by these results.

1,25-Dihydroxyvitamin D maintains bone cell activity, and parathyroid hormone modulates bone cell number in dogs.

The singular and combined effects of 1,25-dihydroxyvitamin D [1,25-(OH)2D] and PTH on bone were evaluated in a long term in vivo study in dogs. Dogs were rendered deficient in 1,25-(OH)2D and PTH by five sixths nephrectomy and parathyroidectomy. A control group was sham operated. Various combinations in status of 1,25-(OH)2D and PTH were produced by daily sc injections of 1,25-(OH)2D (1.25) and/or continuous infusion of 1-34 bovine PTH. These were 1.25+/PTH+, 1.25-/PTH-, 1.25+/PTH-, 1.25-/PTH+. Serum calcium levels were kept in the normal range by the administration of one or two of the hormones or by oral supplementation of calcium lactate. Histomorphometric evaluation of static and dynamic parameters of bone after 8 months of experimental observation revealed that deficiency in 1,25-(OH)2D and PTH resulted in decreased number and activity of bone-forming and resorbing cells. Administration of 1,25-(OH)2D increased the activity but not the number of bone cells. In contrast, administration of PTH increased the number but not the activity of bone cells. Tissue level activity was decreased when one or both hormones were deficient, and normal tissue level activity was found only when both hormones were given. These data are relevant for understanding and management of diseases with perturbations in vitamin D and/or PTH.

Hyponatremia and hypernatremia.

The concentration of serum sodium is determined by the external balance of water. Hyponatremia occurs when total body water is in excess of sodium, and hypernatremia develops when body water is relatively decreased in relation to sodium. Both disorders may be present in patients with various disease states in which total body sodium is either decreased, normal or increased. The symptomatology in both disorders is related to the disturbance in central nervous system due to brain edema in patients with hyponatremia and brain dehydration, and cerebrovascular hemorrhages in patients with hypernatremia. The treatment of hypo and hypernatremia is achieved by correcting the abnormalities in body water content.

Renal action of cholera toxin: II. Effects on adenylate cyclase-cyclic AMP system.

The effects of cholera toxin (CT) on the adenylate cyclase-adenosine 3',5'-cyclic monophosphate (cAMP) system(s) in renal cortex were examined using the isolated renal cortical tubules of rat. Unlike parathyroid hormone, catecholamines or prostaglandins, CT had no immediate effects on cAMP production by the tubules or on adenylate cyclase activity. However, after 30 min of incubation at 37 C, cAMP production by the tubules started to rise and reached a plateau between 60 and 90 min. This rise in cAMP production was not abolished by protein synthesis inhibitors (actinomycin D and cycloheximide) nor by the inhibitors of prostaglandin synthesis (acetyl-salicylate and indomethacin). Repeated washings of the tubules exposed to the toxin for five minutes at 0 or 37 C did not abolish the effect of CT to stimulate cAMP production. Assays of adenylate cyclase activity using homogenates prepared from isolated tubules which were incubated for 60 min with CT revealed an increase in the basal adenylate cyclase activity without any change in NaF-sensitive enzyme activity. It is concluded that CT binds to renal tubule cells rapidly, possibly through energy-independent process. CT stimulates adenylate cyclase activity and increases cAMP production by the renal tubule cells after a latent period of 30 min. The stimulatory effects of CT are not due to new protein synthesis or prostaglandin formation.

Effect of phosphate depletion on renal tubular reabsorption of glucose.

The renal handling of glucose was evaluated in six dogs before and after chronic phosphate depletion of 51 to 112 days' duration. Phosphate depletion was produced by feeding the dogs with phosphate-restricted diet and administration of Al(OH)3. With phosphate depletion, serum phosphate fell from 3.5 +/- 0.36 (S.E.) to 0.5 +/- 0.19 mg./dl. Both TmG and TmG/GFR fell significantly during phosphate depletion. TmG/GFR was 4.0 +/- 0.41 mg./ml. before phosphate depletion and 2.9 +/- 0.19 mg./ml. after phosphate depletion. In four phosphate-depleted dogs TmG/GFR fell precipitously after glucose loading for more than 200 minutes, and practically all of the filtered glucose was excreted in the urine. It is possible that the levels of cytosolic adenine nucleotides which are required for energy of the renal cell may be reduced during phosphate depletion. This may underlie the fall in TmG and may explain the precipitous fall in TmG when the transport mechanism for glucose was operating at maximum capacity for a prolonged period of time.

Acute renal failure due to nontraumatic rhabdomyolysis.

Twenty-one patients developed acute renal failure in association with nontraumatic rhabdomyolysis and myoglobinuria. The illness followed an overdose of ethanol, heroin, or other depressant drug in 18 patients. Lethargy or coma was present in 17 patients and muscle swelling in 11. Evidence of rhabdomyolysis included markedly elevated creatine phosphokinase, myoglobinuria, and aldolase in blood. Initial biochemical findings were similar to those of acute renal failure due to other causes, but the abnormalities were exaggerated. There was a disproportionate rise in serum creatinine concentration in relation to serum urea nitrogen concentration. Profound hyperuricemia was present in most patients. Transient hypercalcemia developed during the diuretic phase in 5 patients. One patient died. We conclude that nontraumatic myoglobinuria with acute renal failure is not infrequent and may occur after an overdose of ethanol or heroin. The disease has good prognosis despite severe hypercatbolism and untreated profound hyperuricemia.

Role of cyclic AMP in the natriuresis of extracellular fluid volume expansion in the dog.

1. The effect of extracellular volume expansion (ECVE) on renal production of cyclic AMP was evaluated in 19 thyroparathyroidectomized dogs. ECVE was produced by the infusion of Ringer bicarbonate solution at a rate of 2 ml min-1 kg-1 body weight; cyclic AMP was measured in plasma obtained from the aorta and renal vein and in the urine. 2. During the natriuresis of ECVE urinary excretion of cyclic AMP, the clearance of cyclic AMP, net nephrogenous cyclic AMP added both to urine and to the renal vein and hence total nephrogenous cyclic AMP increased significantly. 3. This rise in net production of cyclic AMP and a significant natriuresis by the kidney persisted for 60--90 min after discontinuation of active ECVE and return of renal plasma flow to normal. 4. The results support the notion that an increase in the production of cyclic AMP by the kidney may play a role in the natriuresis of ECVE.

Renal action of cholera toxin: I. Effects on urinary excretion of electrolytes and cyclic AMP.

The infusion of cholera toxin (CT), 4 mug/min, into one renal artery of normal and thyroparathyroidectomized (T-PTX) dogs produced ipsilateral increments in the excretion of Na, K, Ca, Mg and Cl. Phosphate excretion increased from both kidneys, but more from the infused kidney in intact dogs. Unilateral phosphaturia occurred in T-PTX dogs studied five or more days after T-PTX. The changes in electrolyte excretion appeared 40 to 80 min after initiation of CT infusion and the maximal effects were noted after 100 to 140 min. The effects of CT on electrolyte excretion could not be accounted for by changes in glomerular filtration rate or renal plasma flow. Urinary cyclic adenosine monophosphate (AMP) increased from both kidneys but slightly more from the infused kidney. Adenylate cyclase activity of cortex and outer medulla of the infused kidney was 109 to 142% higher than that of the control kidney. The results indicate that CT decreases the net transport of various electrolytes by the renal tubule. This effect is probably mediated by the activation of renal adenylate cyclase(s) sensitive to the enterotoxin.

Relationship between the kidney and parathyroid hormone.

This report reviews the interrelationship between the activity of the parathyroid glands and renal function. Among the topics discussed are: effects of parathyroid hormone on various aspects of renal function such as: (1) glomerular filtration rate and renal blood flow; (2) renal handling of phosphorus, calcium, magnesium, sodium and potassium; (3) renal production of 1,25-dihydroxycholecalciferol; (4) renal handling of bicarbonate and acid-base metabolism, and (5) mechanism of action of parathyroid hormone on the renal cell. Further topics include: renal metabolism of parathyroid hormone; the kidney in hyperparathyroidism, and effects of renal failure on structure and function of the parathyroid glands.