Zinc, insulin and diabetes.

The relationship between diabetes, insulin and zinc (Zn) is complex with no clear cause and effect relationships. In Type 1 diabetes there is a lack of insulin production, in Type 2 diabetes resistance to the effects of insulin are predominant. Both Type 1 and Type 2 have the same long-term complications. Diabetes effects zinc homeostasis in many ways, although it is most probably the hyperglycemia, rather than any primary lesion related to diabetes, which is responsible for the increased urinary loss and decreases in total body zinc. The role of Zn deficiency, which could, at least potentially, exacerbate the cytokine-induced damage in the autoimmune attack which destroys the islet cell in Type 1 diabetes, is unclear. Since Zn plays a clear role in the synthesis, storage and secretion of insulin as well as conformational integrity of insulin in the hexameric form, the decreased Zn, which affects the ability of the islet cell to produce and secrete insulin, might then compound the problem, particularly in Type 2 diabetes. Several of the complications of diabetes may be related to increased intracellular oxidants and free radicals associated with decreases in intracellular Zn and in Zn dependent antioxidant enzymes. There appears to be a complex interrelationship between Zn and both Type 1 and Type 2 diabetes. The role of Zn in the clinical management of diabetes, its complications, or in its prevention is, at best, unclear.

Effect of mitogenic and hormonal stimulation on zinc transport in mixed lymphocyte cultures.

Human mixed lymphocyte cultures (hMLC) were used to examine the relationship between mitogenic stimuli (MITO), synthetic human calcitonin (hCT), and human insulin (hINS) on zinc (Zn) transport kinetics. Lymphocytes were isolated using a Ficoll density gradient. The hMLCs were labeled by incubation with 65Zn in either control or MITO-containing media. 65Zn release to equilibrium was then measured in unstimulated and mitogen-stimulated cells treated with hCT and hINS. hCT and hINS were added only during this final incubation due to the rapid response to peptide hormones. Bidirectional transmembrane flux coefficients were calculated using a closed two-compartment model. The hMLCs subjected to MITO stimulation demonstrated a 25% decrease in the fractional efflux coefficient (Kcm) and a 69% increase in the fractional influx coefficient (Kmc) compared with controls. Acute exposure to hINS resulted in a marked increase in Kmc with no significant change in Kcm. Acute exposure to hCT had effects qualitatively similar to those of MITO alone. Neither hormone significantly altered the transport of 65Zn when compared with stimulation with MITO alone.

Calcitonin, zinc, and testicular function.

Recent studies demonstrating decreases in transport kinetics of zinc (Zn) in testis in response to calcitonin (CT) and the presence of CT receptors on Leydig cells has suggested a physiological interrelationship between CT and cellular Zn metabolism in the testis. The present studies were undertaken to evaluate the acute effects of human synthetic calcitonin (hCT) on testosterone (T) synthesis and on transmembrane Zn transport as measured in a closed two-compartment model system in Leydig cells isolated from intact and thyroparathyroldectomized (TPTX) rats. Leydig cells acutely exposed to 1 ng/mL equine luteinizing hormone (LH) in vitro had a fivefold increase in medium T concentration. Calcitonin at 42 micrograms/mL had no effect on the basal T synthesis and did not affect the increase seen after LH administration. Lower doses of LH demonstrated a dose response for T production, but no alteration in the pattern of response. In TPTX rats pretreated with 167 micrograms/d (25 MRC U/d) hCT subcutaneously (sc) for three days before they were killed, a reduction to 73% of the control value was observed in the in vitro Leydig cell fractional influx coefficient for Zn transport (P less than .02). No difference was observed in the fractional efflux coefficient. Fractional flux coefficients from intact rats demonstrated qualitatively similar, but more variable, changes. These data demonstrate that there is no acute effect of CT on T synthesis in the isolated Leydig cell. There does appear, nevertheless, to be a role for CT in the modulation of transmembrane Zn transport. Clinically important Zn-dependent alterations of T synthesis may require long-term changes in Zn metabolism before they become manifest.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic evidence for a calcitonin receptor on testicular Leydig cells.

Previous studies have indicated that there is a relation between testicular function and adequate concentrations of zinc in testicular cells, and that calcitonin alters cellular zinc transfer in the testis. The present studies provide autoradiographic evidence that calcitonin binds in vivo to the cell membrane of testicular Leydig cells. The data thus confirm the presence of the testicular cell membrane calcitonin receptors that were previously demonstrated indirectly by Scatchard analysis of data collected from binding studies.

Hyperphosphatemic tumoral calcinosis: association with elevation of serum 1,25-dihydroxycholecalciferol concentrations.

Seven siblings with hyperphosphatemic tumoral calcinosis were studied using metabolic measures. Serum phosphorus and 1, 25-dihydroxycholecalciferol concentrations were significantly increased and serum parathyroid hormone and 25-hydroxycholecalciferol concentrations were significantly decreased in these subjects. Metabolic balance studies done in three of the siblings showed positive calcium and phosphorus balances, reflected by increased gastrointestinal absorption and decreased renal excretion. These data suggest that a hereditary abnormality of vitamin D metabolism may be present in patients with hyperphosphatemic tumoral calcinosis. Failure of the normal feedback mechanism regulating the 25-hydroxy-1-alpha-hydroxylase enzyme is suggested as the major cause. Although this defect could lead to many of the metabolic abnormalities seen in these patients, the overall contribution of altered vitamin D metabolism to the pathogenesis of tumoral calcinosis is not fully understood.

The effect of sodium intake on zinc excretion in patients with sickle cell anemia.

Urinary zinc excretion is known to be elevated in subjects with sickle cell anemia. Sodium intake has been suggested to influence zinc excretion in normal subjects. In order to assess the effect of sodium on zinc excretion in subjects with sickle cell anemia, urinary zinc excretion was measured in thirteen children and adolescents with sickle cell anemia on both a high (140 mEq/day) and low (20 mEq/day) sodium intake. Urinary zinc excretion was elevated on both diets. The mean urinary zinc excretion on the high sodium diet (775 +/- 238 micrograms/24 h) was significantly lower (P less than .005) than that on the low sodium diet (947 +/- 344 micrograms/24 h). The zinc excretion did not correlate with calcium or magnesium excretion or aldosterone secretion rates or plasma renin activity. Although elevated, the urinary zinc excretion in patients with sickle cell anemia is still significantly lowered by increasing sodium intake.

Tumoral calcinosis: scintigraphic studies of an affected family.

Tumoral calcinosis is a rare, familial ectopic calcification syndrome associated with hyperphosphataemia. A family in which seven of 13 siblings had demonstrable, clinical, radiological and pathological findings of tumoral calcinosis was evaluated. The purposes were to compare the efficacy of bone scintiscans with serum phosphorus determination in detecting subclinical disease early in asymptomatic siblings and to assess therapeutic results in affected family members following initiation of phosphate depletion therapy. History, physical examination, serum calcium, serum phosphorus and bone scintiscans were performed in 12 of 13 siblings. All the affected siblings had markedly elevated serum phosphorus levels and abnormal bone scintiscans while the unaffected siblings had normal serum phosphorus levels and normal bone scintiscans. All the siblings, affected and unaffected, were normocalcaemic. After initiation of phosphate depletion therapy, gross changes in the appearance of lesions were detected on bone scintiscans. Serum phosphorus levels likewise showed a modest decline, although still remaining in the hyperphosphataemic range. In conclusion, bone scintiscans and serum phosphorus determinations are equally sensitive in detecting subclinical disease. However, the scintiscans are helpful in assessing not only the extent of the disease, but also whole-body and regional changes following any therapeutic interventions.

Influence of parathyroid hormone and calcitonin on tissue zinc homeostasis in the rat.

Tissue zinc accumulation kinetics and compartmental analysis models were evaluated in thyroparathyroidectomized (TPTX) rats treated with parathyroid extract or calcitonin. The animals were injected with 65Zn and studied at multiple time intervals. SAAM-25 was utilized to generate models from the plasma specific activity decay curves. Calcitonin had a marked effect to decrease the fractional influx and efflux transfer coefficients in the model and inhibit tissue zinc accumulation in several, but not all, tissues. Parathyroid extract increased accumulation in all tissues studied, but had minimal effects on the model. It is concluded that parathyroid hormone acts nonspecifically and that calcitonin appears to have specific influences on zinc homeostasis.

Influence of cholecalciferol on tissue zinc homeostasis in the rat.

A role for calciferol in zinc homeostasis was investigated by radioisotope uptake and compartmental analysis techniques. cholecalciferol was injected into rats 4-5 days prior to radioisotopic study of rapidly exchangeable pools (< 4 h). Kidney, liver, and duodenum had significant increases in zinc uptake rates. Bone and skeletal muscle had significant decreases. No other tissues had significant differences. Compartmental analysis models generated by the SAAM-25 digital computer program suggested a decrease in the fractional tissue to plasma coefficient to be the mechanism for the observed changes in the serum zinc concentration and tissue zinc accumulation rates. It is not clear whether this is a specific effect of calciferol on zinc homeostasis or is a nonspecific response which may reflect some role for zinc in calciferol metabolism.

Cutaneous manifestations of tumoral calcinosis.

We have followed up a large family in which seven members have tumoral calcinosis. One girl had the skin lesions of localized calcinosis cutis apart from the typical subcutaneous deposits of calcium. Like most persons with tumoral calcinosis, our patient had normal serum calcium concentrations; however, the serum phosphorus levels were greatly elevated. The familial occurrence and elevated serum phosphorus levels suggest the possibility of some as yet undefined, heritable metabolic defect as the underlying cause. The occurrence of tumoral calcinosis with localized calcinosis cutis is a rare association, and there has been only one other reported case to our knowledge. This report describes our patient and offers a brief discussion of tumoral calcinosis. The therapeutic response to the phosphate depletion regimen and topical steroids was disappointing in our case.