Effect of 1,25-dihydroxycholecalciferol on impaired calcium transport by the sarcoplasmic reticulum in experimental uremia

Higher serum 25-hydroxyvitamin D concentrations associate with a faster recovery of skeletal muscle strength after muscular injury

The primary purpose of this study was to identify if serum 25-hydroxyvitamin D (25(OH)D) concentrations predict muscular weakness after intense exercise. We hypothesized that pre-exercise serum 25(OH)D concentrations inversely predict exercise-induced muscular weakness. Fourteen recreationally active adults participated in this study. Each subject had one leg randomly assigned as a control. The other leg performed an intense exercise protocol. Single-leg peak isometric force and blood 25(OH)D, aspartate and alanine aminotransferases, albumin, interferon (IFN)-γ, and interleukin-4 were measured prior to and following intense exercise. Following exercise, serum 25(OH)D concentrations increased (p < 0.05) immediately, but within minutes, subsequently decreased (p < 0.05). Circulating albumin increases predicted (p < 0.005) serum 25(OH)D increases, while IFN-γ increases predicted (p < 0.001) serum 25(OH)D decreases. Muscular weakness persisted within the exercise leg (p < 0.05) and compared to the control leg (p < 0.05) after the exercise protocol. Serum 25(OH)D concentrations inversely predicted (p < 0.05) muscular weakness (i.e., control leg vs. exercise leg peak isometric force) immediately and days (i.e., 48-h and 72-h) after exercise, suggesting the attenuation of exercise-induced muscular weakness with increasing serum 25(OH)D prior to exercise. Based on these data, we conclude that pre-exercise serum 25(OH)D concentrations could influence the recovery of skeletal muscle strength after an acute bout of intense exercise.

Vitamin D and its role in skeletal muscle

This review discusses the clinical and laboratory studies that have examined a role of vitamin D in skeletal muscle. Many observational studies, mainly in older populations, indicate that vitamin D status is positively associated with muscle strength and physical performance and inversely associated with risk of falling. Clinical trials of vitamin D supplementation in older adults with low vitamin D status mostly report improvements in muscle performance and reductions in falls. The underlying mechanisms are probably both indirect via calcium and phosphate and direct via activation of the vitamin D receptor (VDR) on muscle cells by 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. VDR activation at the genomic level regulates transcription of genes involved in calcium handling and muscle cell differentiation and proliferation. A putative membrane-associated VDR activates intracellular signaling pathways also involved in calcium handling and signaling and myogenesis. Additional evidence comes from VDR knockout mouse models with abnormal muscle morphology and physical function, and VDR polymorphisms which are associated with differences in muscle strength. Recent identification of CYP27B1 bioactivity in skeletal muscle cells and in regenerating adult mouse muscle lends support to the direct action of both 25-hydroxyvitamin D and 1,25(OH)(2)D in muscle. Despite these research advances, many questions remain. Further research is needed to fully characterize molecular mechanisms of vitamin D action on muscle cells downstream of the VDR, describe the effects on muscle morphology and contractility, and determine whether these molecular and cellular effects translate into clinical improvements in physical function.

Effect of recombinant human erythropoietin on muscle energy metabolism in patients with end-stage renal disease: a 31P-nuclear magnetic resonance spectroscopic study

To evaluate the effects of recombinant human erythropoietin (r-HuEPO) on muscle energy metabolism in patients with end-stage renal disease (ESRD), 10 patients receiving maintenance hemodialysis were given r-HuEPO (3,000 U three times a week for 8 weeks). Intracellular phosphocreatine to inorganic phosphate ratios and pH were measured with 31P-nuclear magnetic resonance spectroscopy before and after all-out handgrip exercise, before treatment and at 4 and 8 weeks after r-HuEPO treatment. The same measurements, from 14 normal individuals also were studied for comparison. The hematocrit increased significantly with r-HuEPO treatment, although the dose of r-HuEPO did not correct it to the normal level. The exercise capacity improved significantly. Intracellular pH was not different between the ESRD patients and controls or between before and after r-HuEPO treatment. The phosphocreatine to inorganic phosphate ratio at the resting state improved significantly with r-HuEPO treatment, reaching the same level as the normal subjects even before the low hematocrit normalized. The measurements immediately after exercise were the lowest levels obtained (the nadir state) and were not different between groups. In the state of maximum recovery, a pattern was noted that was similar to that found in the resting state. These results showed that r-HuEPO treatment improved the phosphorylation potential during the resting state and the maximum phosphorylation potential during the postexercise recovery phase, and suggest that the treatment improved the rate of oxidative phosphorylation in ESRD patients receiving hemodialysis.

Regulation of Ca2+ uptake in skeletal muscle by 1,25-dihydroxyvitamin D3: role of phosphorylation and calmodulin

Experiments were carried out to obtain information about the mechanism underlying the fast action of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in skeletal muscle. N-2'-o-dibutyryladenosine-3',5'-cyclic monophosphate (dbcAMP), similarly as 1,25(OH)2D3 (5 x 10(-10) M), rapidly increased 45Ca uptake by soleus muscle from vitamin D-deficient chicks (+25% and +98% at 3 min and 10 min, respectively) in a dose-dependent manner. The effects of the cAMP analog (10 microM) and 1,25(OH)2D3 could be abolished by the Ca(2+)-channel blocker nifedipine and the calmodulin antagonist flufenazine. Calmodulin binding by two muscle microsomal proteins of 28 kDa and 30 kDa was stimulated within 1 min of exposure of the tissue to 1,25(OH)2D3. Direct effects of the sterol on membrane calmodulin binding were shown with isolated microsomes. The 1,25(OH)2D3-mediated rise of [125I]calmodulin binding to microsomal membranes was dependent on the presence of medium ATP. Forskolin (10 microM) and cAMP (10 microM) also increased [125I]calmodulin binding (+75% and +64%, respectively, with respect to controls). Pretreatment of microsomal membranes with cAMP-dependent protein kinase inhibitor (1 microgram/ml) or addition of alkaline phosphates (1 U/ml) after hormonal treatment caused complete inhibition of 1,25(OH)2D3-induced [125I]calmodulin binding to microsomal membrane proteins. These results imply modifications of membrane protein phosphorylation through the cAMP signal pathway and in turn of calmodulin binding in the mechanism by which 1,25(OH)2D3 rapidly stimulates skeletal muscle Ca2+ uptake.

Modulation of 1,25-dihydroxyvitamin D3-dependent Ca2+ uptake in skeletal muscle by protein kinase C

In vitro studies have shown that short exposure (1-10 min) of vitamin D-deficient chick soleus muscle to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] causes an acute stimulation of tissue 45Ca uptake through voltage-gated Ca2+ channels, with parallel increases in cyclic AMP levels, adenylate cyclase activity and membrane protein phosphorylation. We further investigated the involvement of protein kinases in the rapid effects of 1,25(OH)2D3 on skeletal muscle. The hormone was found to stimulate the protein kinase C (PKC) activity of muscle membranes. The PKC activator phorbol 12-myristate 13-acetate (PMA, 100 nM) was found to rapidly stimulate muscle 45Ca uptake, mimicking 1,25(OH)2D3. Increases of 68% and 46% were observed at 1 and 15 min of exposure to PMA respectively. The effects of PMA were dose-dependent (50-200 nM) and were specific, since the inactive analogue 4 alpha-phorbol was without effect. Analogously to the effects of the sterol, PMA-enhanced 45Ca uptake was abolished by the Ca2+ channel antagonists nifedipine (30 microM) and verapamil (50 microM). Staurosporine (10 nM), a PKC inhibitor, surprisingly potentiated 1,25(OH)2D3-dependent stimulation of 45Ca uptake. Exposure of skeletal muscle to PMA (100 nM) plus 1,25(OH)2D3 (1 nM) produced a less pronounced effect on 45Ca uptake than either agent alone. PMA also decreased muscle cyclic AMP levels. These results suggest a regulatory link between the two major transmembrane signalling systems in the mechanism of action of 1,25(OH)2D3 in skeletal muscle.

Stimulation of calmodulin synthesis in proliferating myoblasts by 1,25-dihydroxy-vitamin D3

Myoblasts contain a receptor specific for 1,25-dihydroxy-vitamin D3. Morphological data have indicated that the hormone stimulates both myoblast proliferation and fusion. The synthesis of myoblast proteins in response to the sterol was studied during the proliferating stage of the cells. Chick embryo myoblast primary cultures (precultured for 24 h in the presence of low levels of 1,25-dihydroxy-vitamin D3 after isolation) were used. Labelling (2 h) of cells incubated in the absence and presence of 1,25-dihydroxy-vitamin D3 (10(-10) M) for 1-12 h with [14C]leucine and [3H]leucine, respectively, followed by coelectrophoresis of double-labelled proteins on sodium dodecyl sulfate polyacrylamide gels showed that the sterol initially stimulates the synthesis of proteins of 60 kDa (1.2 h), 70 kDa (2.4 h) and 80 kDa (4 h). These changes were transient and between 6 and 12 h a protein of 19 kDa was induced. This protein was identified as calmodulin on the basis of its isoelectric point (pI 4.1), Ca2(+)-dependent electrophoretic mobility, ability to bind 45Ca and to interact with an immobilized phenothiazine in a Ca2(+)-dependent manner, and by means of immunoblotting with a specific anti-calmodulin antibody and 3',5'-cyclic AMP phosphodiesterase activation assays. In agreement with these results, hybridization analysis with a specific cDNA probe showed increased calmodulin mRNA levels in myoblasts treated for 4-12 h with 1,25-dihydroxy-vitamin D3. These changes were paralleled by a stimulation of [3H]thymidine incorporation into DNA suggesting that they may be involved in the mitogenic action of the hormone.

Effect of uremia and its treatment on pulmonary function

Alterations in respiratory drive, mechanics, muscle function, and gas exchange are frequent if not invariable consequences of uremia. Pulmonary dysfunction may be the direct result of circulating uremic toxins or may result indirectly from volume overload, anemia, immune suppression, extraosseous calcification, malnutrition, electrolyte disorders, and/or acidbase imbalances. The pulmonary system is unique because it is affected by the disease and its treatment. Acetate hemodialysis reduces alveolar ventilation and PaO2 due to extrapulmonic CO2 unloading. Peritoneal dialysis increases alveolar ventilation and intraperitoneal pressure. The latter leads to an elevated and lengthened diaphragm, a reduced functional residual capacity, basilar atelectasis, possible hypoxemia, and altered respiratory muscle function. In patients on chronic peritoneal dialysis, adaptations may occur that limit the reductions in lung volumes, PaO2, and respiratory muscle strength that are often observed during acute peritoneal dialysis. This review details how uremia and dialysis interact to alter pulmonary function.

Cyclic AMP-dependent membrane protein phosphorylation and calmodulin binding are involved in the rapid stimulation of muscle calcium uptake by 1,25-dihydroxyvitamin D3

Rapid in vivo effects of 1,25-dihydroxyvitamin D3 on muscle calcium metabolism have been reported. In vitro studies have shown that exposure of vitamin D-deficient chick soleus muscles to the sterol for 1-10 minutes causes a significant stimulation of tissue 45Ca uptake which can be suppressed by Ca channel blockers. A parallel increase in muscle membrane calmodulin content that could be mimicked by forskolin was observed. Experiments were carried out to obtain information about the mechanism underlying the fast action of 1,25-dihydroxyvitamin D3. Like the sterol, forskolin (10 microns) rapidly increased (+48% at 5 min) soleus muscle 45Ca uptake and its effect could be reversed by nifedipine (50 microns). In agreement with these observations, 1,25-dihydroxyvitamin D3 markedly elevated tissue cAMP levels within 45 seconds to 5 minutes of treatment in a dose-dependent manner (10(-11)-10(-7) M). Moreover, incubation of isolated muscle microsomes with 1,25-dihydroxyvitamin D3 increased adenylate cyclase activity and caused a similar profile of stimulation of protein phosphorylation with [gamma-32P]-ATP as forskolin. Major changes were detected in proteins whose calmodulin binding ability has been previously shown to be increased by 1,25-dihydroxyvitamin D3. In addition, the calmodulin antagonists fluphenazine and compound 48/80 abolished the increase in muscle Ca uptake and membrane calmodulin content produced by the sterol. The results suggest that 1,25-dihydroxyvitamin D3 activates muscle Ca channels through a direct membrane action which involves cAMP-dependent protein phosphorylation and calmodulin binding.

In vitro calcium transport properties of skeletal muscle mitochondria from vitamin D-deficient and 1,25-dihydroxy-vitamin D3-treated chicks

Previous work has shown that vitamin D3 or 1,25-dihydroxy-vitamin D3 affect calcium content and fluxes in mitochondria of chick skeletal muscle in situ. Studies were performed to investigate whether these effects are related to variations in the Ca2+ transport properties of mitochondrial membranes. Mitochondria isolated from skeletal muscle of vitamin D-deficient chicks and chicks dosed with 1,25(OH)2D3 for 3 or 7 days (50 ng/day) were employed. No changes in the rate and affinity for calcium of the Ruthenium Red-sensitive Ca2+ uptake system were detected after treatment with 1,25(OH)2D3. The metabolite did not cause either modifications in Ca2+ efflux from mitochondria preloaded with the cation induced by Na+ or blockage of mitochondria energy supply. Prior treatment of animals with vitamin D3 was also without effects. However, a significant stimulation of Ca2- uptake by intact muscle preparations from the same experimental animals was observed in response to treatment with 1,25(OH)2D3 in vivo (50 ng/day, 3 days) or in vitro (10(-10) M, 60 minutes). In addition, the Ca content of muscle mitochondria was markedly diminished in chicks treated with the sterol. It is suggested that the effects of 1,25(OH)2D3 on muscle mitochondrial Ca metabolism may be secondary to changes in cytoplasmic Ca2+.

Doppler echocardiographic analysis of diastolic left ventricular function in dialysis patients and its relation to intradialytic hypotension

Doppler echocardiography was used to evaluate left ventricular relaxation and filling in 20 patients on chronic maintenance hemodialysis. In comparison with 11 normal controls hemodialysed patients showed a marked prolongation of isovolumic relaxation period (83 +/- 23 ms vs 67 +/- 11 ms, P less than 0.01). Peak velocity of early diastolic filling was similar in both groups, but peak velocity of late ventricular filling due to atrial contraction was substantially increased in dialysis patients (66 +/- 23 cm/s vs 37 +/- 7 cm/s, P less than 0.01) and the ratio late to early peak velocity was significantly enlarged (0.97 +/- 0.35 vs 0.58 +/- 0.19, P less than 0.01). Although left ventricular mass index, as determined by Echo, was markedly increased in dialysis patients, no significant correlation was found between ventricular mass and indexes of diastolic function. When patients were divided into two groups on the basis of development of hypotension during dialysis clinical and echocardiographic characteristics were similar, although patients with dialysis hypotension (n = 9) were significantly older (53 +/- 9 years) than normotensive patients (n = 11, 42 +/- 14 years, P less than 0.05). Indexes of diastolic function showed a great overlap between the two groups, but ratio late to early peak velocity was significantly greater in patients with intradialytic hypotension (1.13 +/- 0.35 vs 0.83 +/- 0.32, P less than 0.05). It is concluded that dialysis patients exhibit significant alterations of left ventricular relaxation and diastolic filling as assessed by Doppler echocardiography which might be independent of left ventricular hypertrophy. Impaired diastolic function might contribute to intradialytic hypotension.

Induction of specific proteins in cultured skeletal muscle cells by 1,25-dihydroxyvitamin D-3

The presence in myoblasts of an intracellular receptor specific for 1,25-dihydroxyvitamin D-3 [1,25(OH)2D3) and 1,25(OH)2D3-dependent changes in myoblast Ca2+ transport and phospholipid metabolism which are suppressed by RNA and protein synthesis inhibitors have been shown. In agreement with these observations, incubation of chick embryo myoblasts, precultured for 24 h in a medium containing low levels of vitamin D-3 metabolites, with 1,25(OH)2D3 at conditions which induce maximum cell responses (10(-10) M, 24 h) markedly stimulated the incorporation of [3H]leucine into total cell proteins and this effect was abolished when sterol treatment was performed in the presence of cycloheximide or puromycin. To investigate whether 1,25(OH)2D3 selectively stimulates the de novo synthesis of muscle cell proteins, mixtures of myoblast proteins from control and sterol-treated cultures labelled with [14C]leucine and [3H]leucine, respectively, were separated by SDS-polyacrylamide gel electrophoresis and isoelectric focussing. Examination of 3H/14C ratios in gel fractions revealed that 1,25-(OH)2D3 stimulates the production of proteins of molecular masses (isoelectric points) of 9 kDa (4.1 and 8.5), 17 kDa (7.5), 30 kDa (7.2), 40 kDa (5.5), 55 kDa (4.5) and 100 kDa (8.6). Cell fractionation studies showed the following subcellular distribution: 9 kDa (85% cytosol, 15% microsomes); 17 and 100 kDa (100%, 1200 X g pellet); 30 kDa (65% cytosol, 35% mitochondria); 40 kDa (100% microsomes); 55 kDa (65% microsomes, 35% mitochondria). Marker enzyme data indicated that this distribution is not due to cross-contamination between fractions. Affinity chromatography of double-labelled myoblast proteins on an immobilized lectin showed that the 55 kDa protein contains carbohydrate. Labelling of myoblast proteins with 45CaCl2 after their separation on SDS-polyacrylamide gels showed in addition that the 1,25(OH)2D3-dependent proteins of 9, 17, 40 and 100 kDa are major Ca2+-binding components of the cells. Synthesis of these proteins may mediate the effects of the sterol on myoblast calcium metabolism.

1,25 Dihydroxyvitamin D3 affects calmodulin distribution among subcellular fractions of skeletal muscle

1,25 Dihydroxyvitamin D3 has been shown to stimulate calcium fluxes across skeletal muscle membranes. The involvement of calmodulin in the effects of the metabolite was investigated. Primary cultures of chick embryo skeletal muscle myoblasts and soleus muscles from vitamin D-deficient or 1,25 (OH)2D3-treated chicks were used. Culture of myoblasts and vitamin D-deficient soleus with 1,25 (OH)2D3 (0.05 ng/ml) for 24 and 1 hour, respectively, significantly increased 45Ca uptake by the preparations. In the presence of the calmodulin antagonists flufenazine or compound 48/80 in the uptake medium, no differences between control and treated cultures were observed. The calmodulin content of myoblasts and soleus homogenates and subcellular fractions derived therefrom was estimated by measuring their capacity to stimulate calmodulin-depleted cAMP phosphodiesterase. No changes in total calmodulin cellular content could be detected in response to 1,25(OH)2D3. However, the sterol produced an increase in calmodulin levels of microsomes, mitochondria, and crude myofibrillar fraction and a proportional decrease in cytosolic calmodulin concentration. The 1,25(OH)2D3-dependent changes in calmodulin distribution among subcellular fractions of soleus muscle were observed either in vivo or in vitro. The effects in vitro were already detectable after 5 minutes of treatment with the sterol and parallel 1,25(OH)2D3-dependent changes in tissue Ca uptake. The results suggest that changes in calmodulin intracellular distribution may underly part of the mechanism by which 1,25(OH)2D3 affects muscle calcium transport.

Influence of renal impairment, chemical form, and serum protein binding on intravenous and oral aluminum kinetics in the rabbit

The influence of renal impairment on the intravenous kinetics of aluminum (Al) lactate and the oral absorption of eight representative Al forms was determined. The serum protein binding of Al was assessed. Creatinine clearance in renally impaired rabbits was 23% of controls. Systemic clearance of Al was less in renally impaired rabbits (39 vs. 53 ml/hr/kg), as were the steady-state volume of distribution (516 vs. 1175 ml/kg), the half-life of elimination (14 vs. 27 hr), and the mean residence time of Al (14 vs. 25 hr). The shorter Al half-life and mean residence time in renally impaired rabbits were due to a diminished volume of Al distribution. Oral bioavailability of Al in renally intact rabbits ranged from 0.3 to 2.2% (Al borate less than glycinate less than hydroxide less than chloride less than sucralfate less than lactate less than nitrate less than citrate). Renal impairment had little influence on oral bioavailability of most Al forms, although it increased Al citrate absorption to 4.6%. In vitro and in vivo determination of Al ultrafilterability (less than 30,000 D) as an estimate of serum protein binding suggested a greater percentage of ultrafilterable Al species in renally impaired rabbit serum than in control rabbit serum. The increase in ultrafilterable Al species produced the less than expected reduction in Al clearance in renally impaired rabbits. The ultrafilterability of various Al concentrations was greater for citrate greater than lactate greater than nitrate greater than chloride, perhaps partially explaining the similar rank order of oral absorption of these Al forms. The physicochemistry of the eight Al forms was further characterized by determination of their octanol/water partitioning coefficients and their water solubility. There was a significant correlation between the percentage absorbed and the log of the octanol/water partition coefficient. Knowledge of the physicochemistry of Al aids in the understanding of Al pharmacokinetics.

The phospholipid and fatty acid composition of skeletal muscle cells during culture in the presence of vitamin D-3 metabolites

The phospholipid and fatty acid composition of primary cultures (24 h) of chick embryo skeletal muscle myoblasts treated for 4-24 h with physiological concentrations of 1,25-dihydroxyvitamin D-3 and 25-hydroxyvitamin D-3 were analyzed. 25-Hydroxyvitamin D-3 did not alter the relative amounts of individual muscle cell phospholipids whereas 1,25-dihydroxyvitamin D-3 significantly increased phosphatidylcholine content, mainly at the expense of a decrease in phosphatidylethanolamine concentration. The increase in phosphatidylcholine occurred at a faster rate during the first 8 h than in the subsequent 8-24 h treatment period. A similar time course in 1,25-dihydroxyvitamin D3-dependent changes in myoblast calcium uptake has been observe. In addition, this metabolite markedly increased (100%) the arachidonate content of myoblast phosphatidylcholine near the fusion stage of the cells (24 h of treatment). The levels of docosahexaenoate, a minor polyunsaturated fatty acid, in phosphatidylcholine and phosphatidylethanolamine were also substantially elevated by 1,25-dihydroxyvitamin D-3. No significant changes in fatty acid composition in response to 25-hydroxyvitamin D-3 were observed. Modifications in phospholipids and polyunsaturated fatty acids may play a role in the effects of 1,25-dihydroxyvitamin D-3 on muscle cell calcium transport and differentiation.

Changes in muscle lipid metabolism induced in vitro by 1,25-dihydroxy-vitamin D-3

1,25-Dihydroxy-vitamin D-3 has been shown to increase phosphatidylcholine and decrease phosphatidylethanolamine levels in skeletal muscle in vitro. To elucidate the metabolic pathway and mechanism involved in the effect of the sterol, chick embryo myoblast and vitamin D-deficient chick soleus muscle cultures were treated with 1,25-dihydroxy-vitamin D-3 (5.0 X 10(-10)-3.6 X 10(-11) M, total concentration) for 12-14 h and 1 h, respectively, in the absence and presence of protein and RNA synthesis inhibitors. Lipids were then labelled with [3H]glycerol and [14C]acetate. A significant increase in phosphatidylcholine and triacylglycerol labelling and a decrease in phosphatidylethanolamine labelling were observed in response to the hormone. Cycloheximide suppressed these changes in both types of preparations. Puromycin and actinomycin D were also effective blockers in cultured muscle cells. In addition, double-labelling of control and 1,25-dihydroxy-vitamin D-3-treated myoblasts with [3H]choline and [14C]ethanolamine were performed. The sterol did not affect [3H]choline labelling of total cell lipid extracts and phosphatidylcholine. However, the total lipid fraction of treated cells was labelled to a greater extent with [14C]ethanolamine. In addition, an increased incorporation of this precursor into phosphatidylcholine accompanied by a proportional decrease in phosphatidylethanolamine could be shown in cells pretreated with the metabolite. These changes were abolished by cycloheximide and actinomycin D. The results suggest that 1,25-dihydroxy-vitamin D-3 stimulates methylation of phosphatidylethanolamine into phosphatidylcholine in myoblasts by a nuclear mechanism. The data are consistent with the presence in the cells of a receptor specific for the hormone.

Adult hypophosphatemic osteomalacia: report of two cases

Two cases of late hypophosphatemic osteomalacia are described: a male aged 30 who had the disease since he was 22 and a woman of 23 who had the disease since she was 14. Both presented with myopathy and bone pain, and showed hypophosphatemia, hyperglycinuria, reduced tubular phosphate reabsorption (TPR), increased hydroxyprolinuria and normal iPTH and iCT values. Radiologically the male had no Looser's zones and the woman did. Bone biopsy confirmed hypophosphatemic osteomalacia. Both cases were treated with vitamin D and oral phosphate and no improvement was observed. When treatment with 25(OH)D3 was initiated, no improvement was seen and afterwards this was combined with treatment using 1.25(OH)2D3 and from this time on a clinical improvement of the myopathy became evident in both patients. In the woman, healing of the bone lesions occurred at the same time as that of the myopathy, whereas in the male the bone lesions became worse. Healing of the myopathy was only obtained when treatment with 1.25(OH)2D3 was begun. Both patients had reduced values of 2.3 erythrocytic DPG and low level of serum phosphorus when the myopathy was cured, which suggests a lack of effect of 2.3 DPG or serum phosphorus as a cause of the myopathy. Although this had been attributed to a deficiency in the function of 25(OH)D3, the response to 1.25(OH)2D3 and due to the effects of this metabolite on calcium transport in muscle, suggests that the myopathy which occurs in late hypophosphatemic osteomalacia is a result of deficiency or resistance to the muscular effect of this metabolite. We cannot explain the lack of bone healing in the man and further therapeutic studies are required.

Suppression of 1,25-dihydroxy-vitamin D3-dependent calcium transport by protein synthesis inhibitors and changes in phospholipids in skeletal muscle

The effects of protein synthesis inhibitors on 1,25-dihydroxy-vitamin D3-dependent Ca uptake were evaluated in vitamin D-deficient chick soleus muscle and chick embryo myoblast cultures in order to obtain information about the mechanism by which 1,25-dihydroxy-vitamin D3 affects muscle calcium transport. Puromycin (50 microM, 5 h) and cycloheximide (50 microM, 24 h) blocked the increase in Ca uptake induced by the metabolite in soleus muscle and myoblasts, respectively. Actinomycin D (1.6 microM, 12 h) was also effective in inhibiting 1,25-dihydroxy-vitamin D3-dependent Ca uptake in myoblasts. These results suggest that the effects of 1,25-dihydroxy-vitamin D3 on muscle Ca uptake are mediated by de novo protein and RNA synthesis. In addition, it could be observed that myoblasts treated with 1,25-dihydroxy-vitamin D3 had increased lipid phosphorus, phosphatidylcholine and sphingomyelin contents. These changes may be the consequence of the nuclear action of the sterol or, alternatively, represent an independent effect as has been proposed for intestine.

Presence of a 1,25-dihydroxy-vitamin D3 receptor in chick skeletal muscle myoblasts

The presence of a specific receptor for 1,25-dihydroxy-vitamin D3 was investigated in myoblasts released from chick embryo skeletal muscle by trypsin and collagenase treatment. Density gradient analysis of the cytosol obtained from these muscle cell preparations showed that 1,25-dihydroxy-vitamin D3 binds specifically to a 3.7 S macromolecule. Scatchard analysis yielded an equilibrium dissociation constant of 2.46 x 10(-10) M and a Nmax of 74 fmol/mg of cytosol protein. The data is in agreement with previous evidence which indicates that the action of the vitamin D metabolite on muscle Ca uptake is mediated by de novo protein and RNA synthesis, and supports the concept that muscle is a target organ for 1,25-dihydroxy-vitamin D3.

Effects of vitamin D3 metabolites on calcium fluxes in intact chicken skeletal muscle and myoblasts cultured in vitro

25-hydroxycholecalciferol (25OHD3) and 1,25-dihydroxycholecalciferol (1,25(OH)2D3) at physiological concentrations exerted direct effects on Ca fluxes in cultured vitamin D-deficient chick soleus muscle and myoblasts. Isotopic desaturation curves of soleus muscle prelabeled with 45Ca indicated that the action of 25OHD3 is localized in a slow-exchangeable Ca pool where it stimulates net Ca uptake. On the other hand, the predominant effects of 1,25(OH)2D3 consist in an increase of the rate constant of Ca efflux of this pool and in an increase of net Ca uptake in a fast-exchangeable pool. 24,25-dihydroxycholecalciferal proved to be inactive on both Ca uptake and efflux. In addition, 1,25(OH)2D3 significantly increased 45Ca labeling of cultured chick myoblasts. These effects were accompanied by changes in the growth and differentiation of the cultures. The results suggest a direct involvement in vivo of 25OHD3 and 1,25(OH)2D3 on muscle cellular calcium.

Effects of vitamin D-3 on phosphate and calcium transport across and composition of skeletal muscle plasma cell membranes

The effects of vitamin D-3 on calcium and phosphate transport in skeletal muscle plasma membranes were studied. Sarcolemma vesicles were isolated from vitamin D-deficient and vitamin D-treated (one week) chicks by sucrose density gradient centrifugation of a crude muscle plasma membrane fraction. Measurement of (Na+ + K+)-ATPase activity, cholesterol to phospholipid molar ratios and levels of intracellular marker enzymes showed a high degree of purification of the preparations. Administration of vitamin D-3 significantly increased active Ca2+ and phosphate uptake into the vesicles. The efflux of both ions from preloaded vesicles was only slightly altered by the sterol. Ca2+-ATPase activity was higher in sarcolemma from treated animals. This confirms that the effects of vitamin D-3 on calcium transport are related to the Ca2+ pump and not to the passive permeability properties of the membrane. No changes in the protein composition of vesicles from both experimental groups were observed. However, treatment with vitamin D-3 increased sphingomyelin and phosphatidylcholine concentrations. These changes in lipid structure may play a role in the effects of vitamin D-3 on transport characteristics of sarcolemma.

The effect of cholecalciferol in vivo on proteins and lipids of skeletal muscle from rachitic chicks

The protein and lipid constituents of skeletal muscle subcellular fractions isolated from chicks fed a vitamin D-deficient diet for 3 weeks and chicks replated with cholecalciferol (vitamin D3) were analyzed. Administration of the sterol markedly altered the protein composition of mitochondria. The changes were localized in the inner membranes and consisted of a modification of the relative amounts of proteins of approximate mol wt of 83,000, 58,000, 42,000, and 34,000. In addition, treatment with vitamin D3 modified the distribution pattern of components of the actomyosin contractile complex. An increase in actin and troponin C was particularly noticeable. No differences between rachitic and treated animals were detected in the protein composition of sarcoplasmic reticulum membranes and postmicrosomal soluble fraction. A significant increase in the phospholipid content of sarcoplasmic reticulum (P less than 0.05), and to a lesser extent of mitochondria, was observed in repleted chicks. The relative proportions of individual phospholipids, however, were not changed. Injection of an acute dose of cholecalciferol to chicks less severely depleted in vitamin D significantly stimulated the incorporation of 32PO4 in vivo to muscle homogenates, mitochondria, and sarcoplasmic reticulum (P less than 0.05). As the increases in specific activities of sarcoplasmic inorganic P and membrane lipid P were similar whereas that of serum remained unchanged, the results are compatible with the idea that vitamin D3 stimulates phosphate fluxes across muscle membranes. The sterol produced minor modifications in the fatty acid composition of sarcoplasmic reticulum (P less than 0.05).

Effects of vitamin D metabolites on protein catabolism of muscle from uremic rats

Epitrochlearis muscles obtained from normal male Sprague-Dawley rats used as controls (C) and rats with reduced renal mass (Nx) were incubated for 1 hr in Krebs-Ringer buffer containing 5 mM glucose with or without insulin, 25-hydroxycholecalciferol [25(OH)D3] or 1,25 dihydroxycholecalciferol [1,25(OH)2D3]. Plasma levels of 25(OH)D3 were unaffected by reduction in renal mass. Alanine (ALA), tyrosine (TYR), and phenylalanine (PHE) release rates from muscles of Nx rats were increased 40% above C values. Addition of 100 ng/ml of 25(OH)D3 to the incubating media reduced these release rates to C values within 1 hr of incubation. No additive effects with insulin were seen. Addition of 1 ng/ml of 1,25(OH)2D3 did not affect these results. Reduction of renal mass or the addition of insulin or 25(OH)D3 did not affect tissue concentrations of ATP or phosphocreatine. On the other hand, tissue levels of TYR and PHE were increased significantly (approximately equal to 20 to 25%) in muscles from Nx rats compared to C values and were reduced to control values by the addition of 25(OH)D3. The addition of insulin to the incubating media reduced the tissue levels of TYR and PHE in muscles of C rats by approximately equal to 20%, but reduced these levels in muscles of Nx rats by approximately equal to 55%. 25(OH)D3 did not affect tissue levels of cyclic AMP in muscles from either C or Nx rats. Protein synthetic rates were reduced significantly in muscles from Nx rats and returned to C values after 3 hr of incubation but were unaffected by 25(OH)D3. Muscle uptake of 3H,25(OH)D3 was reduced by approximately equal to 30% in muscles from Nx rats compared to C rats. These data suggest that increased muscle protein catabolism exists in rats with reduced renal mass which can be reduced to C values by 25(OH)D3 and does not appear to be mediated through stimulation of adenylate cyclase. 25(OH)D3 did not affect muscle protein synthetic rates. Reduced uptake of 3H,25(OH)D3 by muscles of Nx rats suggests that resistance to this vitamin metabolite may exist at the level of muscle in uremia.

Effect of 1,25-dihydroxycholecalciferol on sarcoplasmic reticulum calcium transport in strontium-fed chicks

Feeding of chicks with strontium, an inhibitor of 1,25-dihydroxycholecalciferol synthesis in kidney, during 7 days, significantly depressed the initial rate of calcium uptake and calcium storing capacity of sarcoplasmic reticulum membranes from skeletal muscle. Oral administration of 1,25(OH)2D3 to strontium-fed animals returned calcium transport values to normal. The changes observed could not be related to differences in the relative proportions of transport ATPase and calcium binding proteins. The results are consistent with a role of 1,25(OH)2D3 in muscle function.

Reversal of decreased phosphorylation of sarcoplasmic reticulum calcium transport ATPase by 1,25-dihydroxycholecalciferol in experimental uremia

When compared to that from sham-operated controls, sarcoplasmic reticulum isolated from skeletal muscle of uremic rabbits had a lower rate of calcium uptake and storing capacity. In vivo administration of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] restored the values in uremic animals toward normal. To obtain information about the mechanisms responsible for these differences, phosphorylation of the calcium transport ATPase was studied. The steady-state levels of phosphoprotein in uremic membranes were lower and returned to normal when the secosteroid was administered. Electrophoresis of the membranes phosphorylated with 32P-inosine triphosphate (32P-ITP) showed that the differences were related to a 100,000 dalton protein. The rate of phosphoprotein formation, determined with 32P-ITP and at 0 degrees C, was considerably lower in uremic than in control animals. Pretreatment with 1,25(OH)2D3 prevented this change. The hypothesis is advanced that the vitamin D metabolite affects the steady-state concentration and rate constant of formation of active sites in the Ca-ATPase. These results may partly explain the altered Ca transport function of the sarcoplasmic reticulum in experimental uremia.

Carbohydrate metabolism and uraemia-mechanisms for glycogenolysis and gluconeogenesis

Disturbances of carbohydrate metabolism during acute uraemia are characterized by the degradation of liver and muscle glycogen with a simultaneous activation of hepatic gluconeogenesis. After binephrectomy, the substitution of essential amino acids and keto analogues stimulate liver, but not skeletal muscle glycogen synthesis. Serine proves to be an optimal substrate for liver gluconeogenesis and muscle glycogen generation under acute uraemic conditions. Propranolol does not influence glycogenolysis of skeletal muscle in acutely uraemic rats. During starvation, acute uraemia leads to an increase of total carbohydrate content as well as of glycogen and glucose concentrations in heart muscle Alterations in carbohydrate contents are not observed in the kidney after ureter ligation. Enhanced glycogenolysis of skeletal muscle and liver during acute uraemia may be due to activation of phosphorylase kinase caused by the increased serum concentrations of various hormones (glucagon, catecholamines, parathormone) as well as free proteolytic activity, an increase of intracellular Ca2+-concentration and finally by alterations in the structure of contractile proteins.

Lack of involvement of sarcoplasmic reticulum in myopathy of acute phosphorous depletion

Acute and chronic hypophosphatemia are known to cause metabolic myopathy. It has been proposed that impaired Ca transport in subcellular membranes is involved in its genesis. In the present study, calcium transport in the sarcoplasmic reticulum (SR), concentrations of ions or nucleotides and transmembrane potential were investigated in muscles of acutely hypophosphatemic rats, i.e. animals with chronic dietary phosphorous deprivation (PD) and superimposed acute hypophosphatemia resulting from the administration of insulin and glucose. Despite hypophosphatemia and low muscle phosphorous concentration, no significant change of the initial rate of Ca uptake or Ca concentrating ability was observed in the SR of PD rats. Storing capacity was decreased; this may result from altered vesicle geometry. Water content, Na concentration, the concentration of several nucleotides and transmembrane potential of muscle were unchanged in PD rats. The findings document that no intrinsic abnormality of vectorial Ca transport is present in the SR of acutely hypophosphatemic PD animals.

Function of the sarcoplasmic reticulum (SR) in hypophosphatemic myopathy

Lipid composition and Ca2+ transport properties were examined in the isolated sarcoplasmic reticulum of non-exercised muscles of markedly phosphorus depleted rats. Phosphorous depletion with a fall of plasma Pi from 8.04 +/- 0.85 to 2.56 +/- 0.48 mg/dl was accompanied by a decrease in muscle Pi and ATP content and by a significant decrease in the phospholipid/protein ratio of sarcoplasmic reticulum. The phosphatidyl -choline/phosphatidyl-ethanolamine ration in sarcoplasmic membranes was increased. Storing capacity for Ca2+ was significantly diminished. In contrast, there was no significant change of kinetic parameters of Ca2+ transport, i.e. of the initial rate of uptake and concentrating ability. These findings do not necessarily exclude changes of cytosolic Ca2+ concentration in phosphate depletion, but they exclude alterations of intrinsic kinetic properties of the sarcoplasmic reticulum as a cause of any such potential changes.

Abnormal amino acid and protein metabolism in uremia

The many alterations in amino acid and protein metabolism in renal failure are often poorly defined, and the available data concerning them are usually descriptive. Nonetheless, certain factors play an important role in the altered amino acid and protein metabolism of uremia. These include malnutrition caused by poor nutrient intake, loss of nutrients during dialysis, and abnormal metabolism of nutrients. Other factors include uremic toxins, superimposed catabolic illnesses, endocrine disorders, and the reduced capacity of the failing kidney to synthesize or degrade certain hormones, amino acids, peptides, and small proteins. These aberrations have complex interrelationships which sometimes potentiate each other. It is possible that the administration of sufficient quantities of energy, vitamins, and minerals, as well as the dietary manipulation of protein, amino acid and ketoacid intake may improve the metabolism of amino acids and proteins. Vitamin B6 and zinc have special requirements that may affect protein or amino acid metabolism.

Changes in the kinetics of muscle contraction in vitamin D-depleted rats

Using an in situ rat soleus neuromuscular preparation, changes in the muscle contraction kinetics in response to vitamine D depletion were studied. For a single isometric contraction, the time-to-peak tension (Tp) and the time-for-recovery-half-way-to-resting tension (T1/2r) were recorded. For a 150 Hz, 300 msec tetanus, the T1/2r was determined. Animals raised on high-calcium, high-phosphate, vitamin D-depleted diets showed prolongation of all parameters. Repletion of vitamin D returned Tp and T1/2r values to normal. Neither dietary calcium deficiency nor thyroparathyroidectomy produced an prolongation of Tp or T1/2r values. Therefore, based upon the experimental data, it appears that vitamin D or one of its metabolites, independent of any effect on the serum calcium or serum phosphate concentration, is necessary for normal muscle relaxation.