1,25-Dihydroxyvitamin D3 stimulates 45Ca2+ uptake by cultured adult rat ventricular cardiac muscle cells

Evaluation of the Relationship Between Vitamin D Deficiency and Subclinical Cardiac Dysfunction Using 2D/3D Strain Echocardiography in Healthy People

AIM

Vitamin D deficiency has a high prevalence in the population and is highly associated with cardiovascular diseases. The aim of this study was to evaluate subclinical left ventricular (LV) function using strain analysis in healthy individuals with vitamin D deficiency.

MATERIAL AND METHODS

113 healthy volunteers were enrolled in the study (age, 44.1±7 yrs, 34 male). All volunteers underwent two-dimensional (2D) and three-dimensional (3D) speckle tracking echocardiography after conventional echocardiographic evaluation. The subjects were divided into two groups according to their vitamin D concentrations. 61 subjects with vitamin D less than 20 ng / ml were included in the vitamin D deficiency group. The baseline clinical characteristics, laboratory measurements, echocardiographic data, including 2D and 3D global longitudinal strain (GLS) values, were compared between the groups.

RESULTS

The 2D GLS values of the subjects with vitamin D deficiency were lower (mathematically less negative) than subjects with normal vitamin D (-16.1±3.4 vs -19.3±4.2, p<0.001). Similarly, the 3D GLS results were lower in subjects with vitamin D deficiency (-18.3±5.2 vs -24.1±6.9, p<0.001). A significant correlation was detected between the vitamin D concentrations and the 2D and 3D GLS measurements. (r=0.765 and r=0.628, respectively, p<0.001). Vitamin D was found to be an independent predictor of impaired 2D and 3D LV GLS (p=0.031, p=0.023, respectively).

CONCLUSION

Subclinical LV dysfunction in healthy individuals with vitamin D deficiency was demonstrated by 3D and 2D strain analysis. Due to potential negative effects of vitamin D deficiency on cardiac function, more attention should be paid to healthy individuals with vitamin D deficiency.

Insulin Resistance and Vitamin D Deficiency: A Link Beyond the Appearances

Vitamin D is a steroid hormone that plays a key role in the regulation of body homeostasis, including cardiovascular function. Although the chronic deficiency of vitamin D is associated with cardiovascular risk factors, as well as with an adverse prognosis, randomized controlled trials have failed in demonstrating that dietary vitamin D supplementation could ameliorate the prognosis of patients with cardiovascular diseases, and suggested that vitamin D deficiency is the expression of the effects of other determinants of cardiovascular risk. Thus, the supplementation of vitamin D is not sufficient to improve the cardiovascular risk profile and prognosis. Insulin resistance is a complex phenomenon that plays a key role in the pathogenesis of conventional cardiovascular risk factors. Interestingly, defects of vitamin D and insulin resistance have a superimposable epidemiological distribution. According to the common view, Insulin resistance is considered the direct or indirect consequence of vitamin D deficiency. However, it is also reasonable to speculate that the deficit or the impaired action of vitamin D, in some circumstances, could be the result of the same pathogenic mechanisms responsible of insulin resistance development. In this case, vitamin D deficiency could be considered an epiphenomenon of insulin resistance. Insulin resistance is a reversible condition, being possibly ameliorated by physical activity and hypocaloric diets. Notably, both physical exercise and energy-restricted dietary regimens are associated with an increase of vitamin D levels. These findings indicate that improving insulin resistance condition is a necessary step to ameliorate vitamin D supplementation-based strategies in cardiovascular prevention.

Role of Vitamin D in Cardiovascular Diseases

Vitamin D represents a group of secosteroids involved in the calcium and phosphate metabolism. The active form of vitamin D, 1,25-dihydroxylcalciferol, exerts its biological mechanisms via the VDR (vitamin D receptor) which acts as a regulator of several target genes. Hypovitaminosis D is associated with many diseases, which are not only limited to the metabolism of the skeleton, but growing evidence links the deficit of vitamin D to cardiovascular, metabolic, immune, and neoplastic diseases. In regard to the cardiovascular system, current evidence shows the presence of VDR in endothelial cells. Moreover, both in vitro and animal experimental models demonstrated that the deficit of vitamin D can promote endothelial dysfunction and atherosclerosis development. Vitamin D can interfere with vascular functions also by affecting the production of vasodilator mediators. VDR is also expressed in left ventricle cardiomyocytes, and hypovitaminosis D can relate to cardiac hypertrophy and heart failure. Randomized clinical trials (RCT) designed to prove the therapeutic role of vitamin D supplementation have been inconclusive to date. The aim of this review is to highlight the main interactions between vitamin D metabolism and cardiovascular diseases; thus, focusing on pathogenic mechanisms and related clinical manifestations.

Vitamin D and cardiovascular disorders

Vitamin D is necessary for bone health but may also have many extra-skeletal effects. The vitamin D endocrine system has major effects on gene and protein expression in many cells and tissues related to the cardiovascular system. In addition, many preclinical studies in animals with vitamin D deficiency or genetically silenced expression of the vitamin D receptor or vitamin D metabolizing enzymes suggest that the absence of vitamin D action may result in cardiovascular events. This includes dysfunctions of endothelial cells, thereby accelerating the process of atherosclerosis, hypertension or abnormal coagulation, ultimately resulting in higher risks for all major cardiovascular or cerebrovascular events. A wealth of observational studies in different parts of the world have fairly consistently found a strong association between a poor vitamin D status and surrogate markers or hard cardiovascular events. A few Mendelian randomization studies did, however, not find a link between genetically lower serum 25OHD concentrations and cardiovascular events. Finally, many RCTs could not demonstrate a consistent effect on surrogate markers, and a limited number of RCTs did so far not find whatever effect on hard cardiovascular endpoints such as myocardial ischemia or infarction, stroke, or cardiovascular death. In conclusion, preclinical data generated a plausible hypothesis of a link between vitamin D status and extra-skeletal events, including cardiovascular endpoints. Whether the vitamin D endocrine system is redundant for the human vascular system or whether the RCTs have not been optimally designed to answer the research question is thus not yet settled.

Mechanical, Biochemical, and Structural Effects of Vitamin D Deficiency on the Chick Heart

The effects of vitamin D depriva tion on the chick heart were investi gated from three aspects: cardiac contractility (±dP/dT), intracellular high-energy phosphorus compounds, and structural differences. Four- week-old vitamin D-deficient chicks were divided into four groups: Group A served as the normal group and re ceived subcutaneous injections of cholecalciferol; Groups B and C were vitamin D-deficient hearts but per fused differently; Group D received daily subcutaneous injections of 5 μg of 1,25(OH) 2D3. When the isolated spontaneously beating hearts (modi fied Langendorff preparation) were perfused with Krebs-Henseleit (KH) solution containing a calcium concen tration of 2.5mM, the myocardial contractility of the vitamin D-defi cient hearts was significantly in creased when compared with group A. After the isolated heart had beaten for one hour, the myocardial contractility in the vitamin D-defi cient hearts was found to decline to significantly lower values. Presacri fice administration of 1,25(OH) 2D3 improved cardiac performance. Vita min D deficiency resulted in an en hanced rate of decline of the intracellular high-energy phosphorus compounds. No differences were found in the microscopic study. These observations suggest that vitamin D has a role in cardiac function.

Vitamin D and Heart Failure

PURPOSE

Vitamin D is principally known for its role in calcium homeostasis, but preclinical studies implicate multiple pathways through which vitamin D may affect cardiovascular function and influence risk for heart failure. Many adults with cardiovascular disease have low vitamin D status, making it a potential therapeutic target. We review the rationale and potential role of vitamin D supplementation in the prevention and treatment of chronic heart failure.

RECENT FINDINGS

Substantial observational evidence has associated low vitamin D status with the risk of heart failure, ventricular remodeling, and clinical outcomes in heart failure, including mortality. However, trials assessing the influence of vitamin D supplementation on surrogate markers and clinical outcomes in heart failure have generally been small and inconclusive. There are insufficient data to recommend routine assessment or supplementation of vitamin D for the prevention or treatment of chronic heart failure. Prospective trials powered for clinical outcomes are warranted.

Vitamin D and Cardiovascular Disease

Vitamin D is best known for its influence on skeletal health. There is growing recognition, however, that vitamin D has nonskeletal actions, which could have important implications for understanding the consequences of vitamin D deficiency. In epidemiologic studies, vitamin D deficiency has been consistently associated with an increased risk for cardiovascular disease and hypertension. Disruption of vitamin D signaling in animal models promotes hypertension, cardiac hypertrophy, and atherosclerosis. This evidence has led to the initiation of prospective randomized trials of vitamin D supplementation in individuals at risk for cardiovascular disease. The results of these trials should help to guide strategies for screening and management of vitamin D deficiency in the clinic and at the population level.

Risk of heart failure among postmenopausal women: a secondary analysis of the randomized trial of vitamin D plus calcium of the women's health initiative

BACKGROUND

Vitamin D supplementation may be an inexpensive intervention to reduce heart failure (HF) incidence. However, there are insufficient data to support this hypothesis. This study evaluates whether vitamin D plus calcium (CaD) supplementation is associated with lower rates of HF in postmenopausal women and whether the effects differ between those at high versus low risk for HF.

METHODS AND RESULTS

Analyses were restricted to 35 983 (of original 36 282) women aged 50 to 79 years old in the Women's Health Initiative randomized trial of CaD supplementation who were randomized 1:1 in a double-blinded fashion to receive 1000 mg/d of calcium plus 400 IU/d of vitamin D3 or placebo. Overall, 744 adjudicated incident HF cases (intervention, 363; control, 381) occurred during a median follow-up of 7.1 (interquartile range, 1.6) years. CaD supplementation, compared with placebo, was not associated with reduced HF risk in the overall population, hazard ratio, 0.95; P=0.46. However, CaD supplementation had differential effects (P interaction=0.005) in subgroups stratified by baseline risk status of HF defined by the presence (high risk=17 449) or absence (low risk=18 534) of pre-existing HF precursors including coronary heart diseases, diabetes mellitus, or hypertension: 37% (hazard ratio, 0.63 [95% confidence interval, 0.46-0.87]) lower risk of HF in the low-risk versus hazard ratio, 1.06; P=0.51, in the high-risk subgroups.

CONCLUSIONS

CaD supplementation did not significantly reduce HF incidence in the overall cohort, however, it was beneficial among postmenopausal women without major HF precursors while of little value in high-risk subgroups. Additional studies are warranted to confirm these findings and investigate the underlying mechanism.

CLINICAL TRIAL REGISTRATION URL

http://www.clinicaltrials.gov. Unique identifier: NCT00000611.

Could vitamin D supplements be a new therapy for heart failure? Possible pathogenic mechanisms from data of intervention studies

Vitamin D deficiency may play a role in the pathogenesis of chronic heart failure (HF), but whether giving patients supplements to raise vitamin D into the normal range improves their survival is not clear. It has been demonstrated that vitamin D deficiency is common in patients with HF, especially the elderly, in obese and in dark skinned people, and that low vitamin D levels are associated with adverse outcome. The epidemiological data have been confirmed by experimental data, which show that knockout mice for the vitamin D receptor developed myocardial hypertrophy and dysfunction. Data from interventional studies are scarce and discordant, and more research is urgently needed to confirm whether add-on supplementation therapy with vitamin D has a role in the management of patients with chronic HF.

Vitamin D as a potential therapy in amyotrophic lateral sclerosis

Vitamin D has been demonstrated to influence multiple aspects of amyotrophic lateral sclerosis (ALS) pathology. Both human and rodent central nervous systems express the vitamin D receptor (VDR) and/or its enzymatic machinery needed to fully activate the hormone. Clinical research suggests that vitamin D treatment can improve compromised human muscular ability and increase muscle size, supported by loss of motor function and muscle mass in animals following VDR knockout, as well as increased muscle protein synthesis and ATP production following vitamin D supplementation. Vitamin D has also been shown to reduce the expression of biomarkers associated with oxidative stress and inflammation in patients with multiple sclerosis, rheumatoid arthritis, congestive heart failure, Parkinson's disease and Alzheimer's disease; diseases that share common pathophysiologies with ALS. Furthermore, vitamin D treatment greatly attenuates hypoxic brain damage in vivo and reduces neuronal lethality of glutamate insult in vitro; a hallmark trait of ALS glutamate excitotoxicity. We have recently shown that high-dose vitamin D3 supplementation improved, whereas vitamin D3 restriction worsened, functional capacity in the G93A mouse model of ALS. In sum, evidence demonstrates that vitamin D, unlike the antiglutamatergic agent Riluzole, affects multiple aspects of ALS pathophysiology and could provide a greater cumulative effect.

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.

Protective role of the vitamin D receptor

Vitamin D receptor (VDR) is found in most tissues, not just those participating in the classic actions of vitamin D such as bone, gut, and kidney. The nonclassic actions are therefore potential targets for the active metabolite of vitamin D, 1,25(OH)₂D₃. This review is intended to highlight the actions of VDR in role of protection. Medline is searched for articles describing actions of VDR on secondary hyperparathyroidism, diabetic nephropathy, hypertension and atherosclerosis. VDR exerts its protective activities through the following mechanisms: inhibition of renin-angiotensin system (RAS); regulation of proliferation and differentiation; reduction of proteinuria; anti-inflammation and anti-fibrosis. The nonclassic actions of VDR provide a number of potential new clinical applications for 1,25(OH)₂D₃ and its analogs. We believe 1,25(OH)₂D₃/VDR can be of particular value in the prevention of kidney-related diseases.

Impact of vitamin D3 on cardiovascular responses to glucocorticoid excess

Although the cardiovascular system is not a classical target for 1,25-dihydroxyvitamin D3, both cardiac myocytes and vascular smooth muscle cells respond to this hormone. The present study aimed to elucidate the effect of active vitamin D3 on cardiovascular functions in rats exposed to glucocorticoid excess. Adult male Wistar rats were allocated into three groups: control group, dexamethasone (Dex)-treated group receiving Dex (200 μg/kg) subcutaneously for 12 days, and vitamin D3-Dex-treated group receiving 1,25-(OH)2D3 (100 ng/kg) and Dex (200 μg/kg) subcutaneously for 12 days. Rats were subjected to measurement of systolic (SBP), diastolic (DBP), and mean arterial (MAP) blood pressures and heart rate. Rate pressure product (RPP) was calculated. Rats' isolated hearts were perfused in Langendorff preparation and studied for basal activities (heart rate, peaked developed tension, time to peak tension, half relaxation time, and myocardial flow rate) and their responses to isoproterenol infusion. Blood samples were collected for determination of plasma level of nitrite, nitric oxide surrogate. Dex-treated group showed significant increase in SBP, DBP, MAP, and RPP, as well as cardiac hypertrophy and enhancement of basal cardiac performance evidenced by increased heart rate, rapid and increased contractility, and accelerated lusitropy, together with impaired contractile and myocardial flow rate responsiveness to beta-adrenergic activation and depressed inotropic and coronary vascular reserves. Such alterations were accompanied by low plasma nitrite. These changes were markedly improved by vitamin D3 treatment. In conclusion, vitamin D3 is an efficacious modulator of the deleterious cardiovascular responses induced by glucocorticoid excess, probably via accentuation of nitric oxide.

Hypocalcemia: a sometimes overlooked cause of heart failure in the elderly

Heart failure, a common condition affecting older patients, is associated with increased hospitalization and mortality rates among geriatric patients. We describe the case of an 86-year-old woman with moderate renal failure, who presented pulmonary edema and severe myocardial dysfunction due to hypocalcemia. Renal failure, but also the combination of additional factors, may have contributed to hypocalcemia, including vitamin D deficiency, loop diuretics and glucocorticoid therapy - which alone can give rise to sodium retention and calciuresis, and worsens hypocalcemia. Although in animal experiments hypocalcemia has been shown to lead to cardiac decompensation, heart failure from hypocalcemia is quite rare in clinical practice. Calcium plays a key role in cardiac muscle contraction and metabolism. It is recommended that physicians check serum calcium levels in the elderly, as hypocalcemia is a reversible cause of heart failure.

Vitamin D deficiency and myocardial diseases

Vitamin D deficiency is common among patients with myocardial diseases because sun-induced vitamin D production in the skin and dietary intake of vitamin D is often insufficient. Knockout mice for the vitamin D receptor develop myocardial hypertrophy and dysfunction. It has also been shown that children with rickets who suffered from severe heart failure could be successfully treated with supplementation of vitamin D plus calcium. In adults, almost all patients with heart failure exhibit reduced 25-hydroxyvitamin D levels, which are used to classify the vitamin D status. In prospective studies, vitamin D deficiency was an independent risk factor for mortality, deaths due to heart failure and sudden cardiac death. Several vitamin D effects on the electrophysiology, contractility, and structure of the heart suggest that vitamin D deficiency might be a causal factor for myocardial diseases. Data from interventional trials, however, are rare and urgently needed to elucidate whether vitamin D supplementation is useful for the treatment of myocardial diseases. In our opinion, the current knowledge of the beneficial effects of vitamin D on myocardial and overall health strongly argue for vitamin D supplementation in all vitamin D-deficient patients with or at high risk for myocardial diseases.

New therapies: calcimimetics, phosphate binders and vitamin D receptor activators

At present, new compounds are available to treat secondary hyperparathyroidism, namely calcimimetics, novel phosphorus binders and also novel vitamin D receptor activators. Calcimimetics increase the sensitivity of the parathyroid gland to calcium through spatial configurational changes of the calcium-sensing receptor. In addition, experimental studies have demonstrated that calcimimetics also upregulate both the calcium-sensing receptor and the vitamin D receptor. They are efficacious in children, though the experience in paediatric chronic kidney disease is still limited. Sevelamer, lanthanum carbonate and magnesium iron hydroxycarbonate are novel phosphorus binders available on the market. Several studies have demonstrated their efficacy and safety up to 6 years, though costs are the main limitation for a wider use. Since almost all the experience available on the new phosphorus binders comes from its use in adults, studies on children are needed in order to confirm the efficacy and safety of these products. Other new salts and polymers are also being developed. New vitamin D receptor activators, such as paricalcitol, are as effective at suppressing parathyroid hormone (PTH) as the traditional vitamin D receptor activators used for the past two decades, but they have a better and safer profile, showing fewer calcaemic and phosphoraemic effects while preserving the desirable effects of the vitamin D receptor activators on the cardiovascular system, hypertension, inflammation and fibrosis. Their use in children with chronic kidney disease has revealed similar responses to those of adults. The novel compounds discussed in this review should facilitate and improve the management of mineral and bone disorders in children with chronic kidney disease.

Vitamin D and its role in skeletal muscle

PURPOSE OF REVIEW

Vitamin D is best known for its role in regulating calcium homeostasis and in strengthening bone. However, it has become increasingly clear that it also has important beneficial effects beyond the skeleton, including muscle. This review summarizes current knowledge about the role of vitamin D in skeletal muscle tissue and physical performance.

RECENT FINDINGS

Molecular mechanisms of vitamin D action in muscle tissue include genomic and nongenomic effects via a receptor present in muscle cells. Knockout mouse models of the vitamin D receptor provide insight into understanding the direct effects of vitamin D on muscle tissue. Vitamin D status is positively associated with physical performance and inversely associated with risk of falling. Vitamin D supplementation has been shown to improve tests of muscle performance, reduce falls, and possibly impact on muscle fiber composition and morphology in vitamin D deficient older adults.

SUMMARY

Further studies are needed to fully characterize the underlying mechanisms of vitamin D action in human muscle tissue, to understand how these actions translate into changes in muscle cell morphology and improvements in physical performance, and to define the 25-hydroxyvitamin D level at which to achieve these beneficial effects in muscle.

A new role for vitamin D receptor activation in chronic kidney disease

Vitamin D has proven to be much more than a simple "calcium hormone." The fact that the vitamin D receptor has been found in cells not related to mineral metabolism supports that statement. The interest of nephrologists in vitamin D and its effects beyond mineral metabolism has increased over the last few years, evidencing the importance of this so-called "sunshine hormone." In the present review, we highlight the most recent developments in the traditional use of vitamin D in chronic kidney disease (CKD) patients, namely, the control of secondary hyperparathyroidism (sHPT). Furthermore, we also explore the data available regarding the new possible therapeutic uses of vitamin D for the treatment of other complications present in CKD patients, such as vascular calcification, left ventricular hypertrophy, or proteinuria. Finally, some still scarce but very promising data regarding a possible role of vitamin D in kidney transplant patients also are reviewed. The available data point to a potential beneficial effect of vitamin D in CKD patients beyond the control of mineral metabolism.

The use of vitamin D analogues in chronic kidney diseases: possible mechanisms beyond bone and mineral metabolism

Chronic kidney disease (CKD) is a common and costly medical condition, and currently available therapeutic options remain unsatisfactory. Vitamin D analogues are widely used for the bone and mineral disorder associated with CKD. However, accumulating evidence suggests that vitamin D analogues may have actions other than their effects on bone and mineral metabolism. In this article, we review the following aspects on the use of vitamin D analogues for the treatment of CKD: (1) epidemiological studies showing that patients with late-stage CKD have better survival than untreated patients; (2) animal studies showing that vitamin D analogues may retard the progression of CKD; (3) human studies on the anti-proteinuric and possibly renal protecting effects of vitamin D analogues in CKD and (4) the potential mechanisms of its therapeutic benefit. Nonetheless, definitive proof of the clinical benefits by randomized control trial would be necessary before one could advocate the routine use of vitamin D analogues for the treatment of CKD patients.

Vitamin D and skeletal muscle tissue and function

This review aims to summarize current knowledge on the role of vitamin D in skeletal muscle tissue and function. Vitamin D deficiency can cause a myopathy of varying severity. Clinical studies have indicated that vitamin D status is positively associated with muscle strength and physical performance and inversely associated with risk of falling. Vitamin D supplementation has shown to improve tests of muscle function, reduce falls, and possibly impact on muscle fiber composition and morphology in vitamin D deficient older adults. Molecular mechanisms of vitamin D action on muscle tissue include genomic and non-genomic effects via a receptor present in muscle cells. Genomic effects are initiated by binding of 1,25-dihydroxyvitamin D [1,25(OH)(2)D] to its nuclear receptor, which results in changes in gene transcription of mRNA and subsequent protein synthesis. Non-genomic effects of vitamin D are rapid and mediated through a cell surface receptor. Knockout mouse models of the vitamin D receptor provide insight into understanding the direct effects of vitamin D on muscle tissue. Recently, VDR polymorphisms have been described to affect muscle function. Parathyroid hormone which is strongly linked with vitamin D status also may play a role in muscle function; however, distinguishing its role from that of vitamin D has yet to be fully clarified. Despite the enormous advances in recent decades, further research is needed to fully characterize the exact underlying mechanisms of vitamin D action on muscle tissue and to understand how these cellular changes translate into clinical improvements in physical performance.

Calcitriol modulation of cardiac contractile performance via protein kinase C

Vitamin D(3) deficiency enhances cardiac contraction in experimental studies, yet paradoxically this deficiency is linked to congestive heart failure in humans. Activated vitamin D(3) (1alpha,25-dihydroxyvitamin D(3)) or calcitriol, decreases peak force and activates protein kinase C (PKC) in isolated perfused hearts. However, the direct influence of this hormone on adult cardiac myocyte contractile function is not well understood. Our aim is to investigate whether 1alpha,25-dihydroxyvitamin D(3) acutely modulates contractile function via PKC activation in adult rat cardiac myocytes. Sarcomere shortening and re-lengthening were measured in electrically stimulated myocytes isolated from adult rat hearts, and the vitamin D(3) response (10(-10) to 10(-7) M) was compared to shortening observed under basal conditions. Maximum changes in sarcomere shortening and relaxation were observed with 10(-9) M 1alpha,25-dihydroxyvitamin D(3). This dose decreased peak shortening, and accelerated contraction and relaxation rates within 5 min of administration, and changes in the Ca(2+) transient contributed to the peak shortening and relaxation effects. The PKC inhibitor, bis-indolylmaleimide (500 nM) largely blocked the acute influence of the most potent dose (10(-9) M) on contractile function. While peak shortening and shortening rate returned to baseline within 30 min, there was a sustained acceleration of relaxation that continued over 60 min. Phosphorylation of the Ca(2+) regulatory proteins, phospholamban, and cardiac troponin I correlated with the accelerated relaxation observed in response to acute application of 1alpha,25-dihydroxyvitamin D(3). Accelerated relaxation continued to be observed after chronic addition of 1alpha,25-dihydroxyvitamin D(3) (e.g. 2 days), yet this sustained increase in relaxation was not associated with increased phosphorylation of phospholamban or troponin I. These results provide evidence that 1alpha,25-dihydroxyvitamin D(3) directly modulates adult myocyte contractile function, and protein kinase C plays an important signaling role in the acute response. Phosphorylation of key Ca(2+) regulatory proteins by this kinase contributes to the enhanced relaxation observed in response to acute, but not chronic calcitriol.

The role of vitamin D in left ventricular hypertrophy and cardiac function

The role of vitamin D in left ventricular hypertrophy and cardiac function. Cardiovascular disease is the leading cause of death among patients with end-stage renal disease (ESRD). Traditional cardiac risk factors, as well as other factors specific to the ESRD population such as hyperphosphatemia, elevated calcium and phosphate product, abnormal lipid metabolism, hyperhomocysteinemia, and chronic inflammation play a role in the excessive risk of cardiovascular death in this population. Left ventricular disorders are proven risk factors for cardiac mortality in hemodialysis patients. These disorders are present in incident ESRD patients at rates far above the general population. There is an accumulating body of evidence that suggests that vitamin D plays a role in cardiovascular disease. Abnormal vitamin metabolism, through deficiency of the active form of 1,25-dihydroxyvitamin D(3), and acquired vitamin D resistance through the uremic state, have been shown to be important in ESRD. Vitamin D deficiency has long been known to affect cardiac contractility, vascular tone, cardiac collagen content, and cardiac tissue maturation. Recent studies using vitamin D receptor deficient mice as a model demonstrate a crucial role of vitamin D in regulation of the renin-angiotensin system. Additionally, there is emerging evidence linking treatment with vitamin D to improved survival on hemodialysis and improvement in cardiac function. The emergence of this data is focusing attention on the previously underappreciated nonmineral homeostatic effects of vitamin D that very likely play an important role in the pathogenesis of cardiac disease in ESRD.

Calcitriol Regresses Cardiac Hypertrophy and QT Dispersion in Secondary Hyperparathyroidism on Hemodialysis

BACKGROUND

Sudden cardiac death is common in patients on hemodialysis (HD), and its rate is as high as 25% of all cardiac deaths associated with left ventricular hypertrophy (LVH) and secondary hyperparathyroidism. A prolonged QT interval on standard electrocardiography is related to an increase in sudden death in various patient groups. It is also well known that LVH has been noted in uremic patients with high parathyroid hormone levels.

METHODS

To evaluate the response of intravenous calcitriol treatment on the QT interval and LVH in HD patients with secondary hyperparathyroidism (intact parathyroid hormone, iPTH, > 450 ng/ml), echocardiographic, electrocardiographic (ECG), and biochemical assessments were performed over a 15-week period in 25 HD patients before and after intravenous calcitriol treatment. We also evaluated 25 age-, sex-, HD duration-, and BMI-matched HD control patients with secondary hyperparathyroidism.

RESULTS

In patients receiving intravenous calcitriol, a significant reduction in iPTH levels (p < 0.05) and alkaline phosphatase levels (p < 0.01) was found without changes in values of serum calcium and ionized Ca2+, phosphorus, Na+, K+, Mg2+, hematocrit, blood pressure, or other hemodynamic changes. Echocardiograms showed significant decreases in the thickness of the interventricular septum (p < 0.05), left posterior wall thickness (p < 0.05), and left ventricle mass index (LVMi, p < 0.01). In addition, sequential ECG measurement in patients with calcitriol treatment showed significant reductions in QTcmax (QTmax interval corrected for heart rates, p < 0.01) and QTc dispersion (QT dispersion corrected for heart rates, p < 0.01). However, in the control patients, biochemical, hemodynamic, and ECG changes, as well as myocardial structural and functional changes were not seen. Multiple regression analysis in all patients indicated that iPTH and LVMi levels were independent predictors of QTcmax while the LVMi level was the only independent predictor of QTc dispersion (p < 0.05).

CONCLUSIONS

Our study showed a significant correlation between LVMi and QT dispersion in HD patients with secondary hyperparathyroidism. Intravenous calcitriol treatment, to be used for the control of secondary hyperparathyroidism, was found to cause regression of myocardial hypertrophy and a reduction in the QTc interval and dispersion, without biochemical and hemodynamic changes. These findings suggest that an active vitamin D metabolite has a cardioprotective action in HD patients.

1,25-dihydroxyvitamin D inhibits human ANP gene promoter activity

1,25-dihydroxyvitamin D, through association with its cognate nuclear receptor, has been shown to have important effects in the cardiovascular and renal systems. We have shown previously that the liganded vitamin D receptor (VDR) inhibits hypertrophy and expression of hypertrophy-sensitive genes (i.e. those encoding atrial natriuretic peptide [ANP], brain natriuretic peptide and alpha skeletal actin) in neonatal cardiac myocytes. In the present study we confirm a time-, ligand- and retinoid X receptor-dependent, VDR-mediated suppression of human ANP gene promoter activity. Conventional deletion analysis demonstrated that the promoter region positioned between -217 and -104 is required for the VDR-dependent suppression of the hANP promoter. Mutation of two functional CArG elements, including one located within this critical region, failed to reverse the suppression. We found no evidence that the liganded VDR is capable of associating directly with regulatory elements positioned between -217 and -104. We conclude that the inhibition may arise from protein-protein interactions between the liganded VDR and stimulatory transcription factors that bind in this region.

1,25-dihydroxyvitamin D3 receptor is upregulated in aortic smooth muscle cells during hypervitaminosis D

Several studies have demonstrated that excess of vitamin D3 is toxic particularly to vascular tissues. A notable pathological feature is arterial calcification. The nature of the toxic metabolite in hypervitaminosis D and the pathogenesis of arterial calcification are not clearly understood. The present study was undertaken to explore whether arterial calcification is a sequel of increased calcium uptake by arterial smooth muscle mediated by up regulation of vitamin D receptor in the cells in response to elevated circulating levels of vitamin D3 in serum. The experimental study was performed in 20 New Zealand white female rabbits aged 6 months. Animals in the test group were injected 10,000 IU of cholecalciferol intramuscularly twice a week for one month. Six control animals were given intra-muscular injections of plain cottonseed oil. Animals were sacrificed and aortas were examined for pathological lesions, 1,25-dihyroxyvitamin D3 (1,25(OH)2 D3) receptor levels and 45Ca uptake in smooth muscle cells. Serum samples collected at intervals were assayed for levels of 25-OH-D3 and calcium. The results showed that in animals given injections of cholecalciferol, serum levels of 25-OH-D3 were elevated. In four of these animals calcification and aneurysmal changes were seen in the aorta. Histological lesions comprised of fragmentation of elastic fibers as well as extensive loss of elastic layers. 1,25(OH)2 D3 receptor levels were up regulated and 45Ca uptake enhanced in aortas of animals which were given excessive vitamin D3. The evidences gathered suggest that excess vitamin D is arteriotoxic and that the vitamin induces arterial calcification through up regulation of 1,25(OH)2D3 receptor and increased calcium uptake in smooth muscle cells of the arteries.

Suppression of ANP gene transcription by liganded vitamin D receptor: involvement of specific receptor domains

We showed previously that liganded vitamin D receptor (VDR) effects a suppression of human atrial natriuretic peptide (hANP) gene-promoter activity in cultured neonatal rat atrial myocytes. In the present study, we have attempted to identify the structural domains of the VDR that are involved in mediating this suppression. We examined the effects of a series of VDR mutants on a cotransfected hANP promoter-driven chloramphenicol acetyltransferase (CAT) reporter. Neither the native VDR nor any of the mutants tested displayed inhibitory activity in the absence of the 1,25-dihydroxyvitamin D3 (VD3) ligand. Delta134, a deletant harboring solely the DNA binding region of the VDR, and L254G, a mutant shown to be defective in retinoid X receptor (RXR) heterodimer formation in other systems, were as effective as the native VDR in reducing promoter activity. HBD, a deletant containing only the hormone-binding domain of the VDR, and K246G, a point mutant that is defective in the activation function of the receptor, did not attenuate reporter activity. A similar activity profile was displayed when a positively regulated promoter containing a direct-repeat vitamin D responsive element (DR3-CAT) was examined in these cells. Liganded VDR, the delta134 mutant, and liganded L254G effected increases in DR3-CAT activity of 2.5-, 2-, and 4-fold, respectively. Two nonhypercalcemic analogues of VD3 (RO 23-7553 and RO 25-6760) displayed the same inhibitory activity as VD3. These studies suggest that the inhibition of hANP promoter activity requires both the DNA binding and activation functions of the receptor but does not appear to require formation of a classic RXR alpha-VDR heterodimer.

Homologous up-regulation of vitamin D receptors is tissue specific in the rat

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptors (VDR) are expressed in multiple tissues within the body. VDR levels are increased by 1,25(OH)2D3 in intestine and kidney and in numerous cell models. The ability of 1,25(OH)2D3 to affect VDR levels in other target tissues in vivo was studied by assessing VDR levels by the 3H-1,25(OH)2D3 binding assay under varied physiological conditions in the rat. When compared with vitamin D-deficient (-D) controls, rats raised on a normal vitamin D-sufficient (+D) diet showed elevated VDR levels in kidney (391 +/- 53 vs. 913 +/- 76 fmol/g of tissue;p < 0.05), but not in testis, heart, or lung. Up-regulation of the VDR also occurred in kidney of +D rats 1 day after a single 100-ng dose of 1,25(OH)2D3 (454 +/- 43 vs. 746 +/- 113 fmol/mg of DNA; p < 0.05), but no changes were seen in intestine, testis, or lung. Because 1,25(OH)2D3-induced hypercalcemia may independently affect VDR regulation, 1,25(OH)2D3 was infused into -D rats, and normocalcemia was maintained by reduced dietary calcium intake. In this model, the renal VDR was again up-regulated (446 +/- 115 vs. 778 +/- 58 fmol/mg of DNA; p < 0.05), but VDR levels in testis and lung were unaffected. Scatchard analysis and tests of 1,25(OH)2D3 dose (1-100 ng/day for 7 days) and temporal (100 ng/day for 1-7 days) responsiveness further supported the tissue-specific nature of the homologous VDR regulation. Assay of VDR levels by L-1-tosylamido-2-phenylethyl chloromethyl ketone-3H-1,25(OH)2D3 exchange assay ruled out differences in endogenous 1,25(OH)2D3 occupancy as the basis for the observed differences in VDR regulation. Finally, coidentity of the VDR-like sites in kidney versus testis was confirmed by competitive binding analysis comparing their relative affinities for 25(OH)D3 versus 1,25(OH)2D3 (30.5 +/- 6.4 vs. 35.6 +/- 3.6 in kidney and testis, respectively) and by immunoblot analysis using a highly specific monoclonal anti-rat VDR antibody. Thus, under a wide variety of experimental conditions, homologous up-regulation of the VDR occurs in the rat kidney in vivo, but not in several other target tissues which do not regulate plasma calcium homeostasis. Moreover, this differential VDR regulation did not result from secondary changes in plasma calcium, from differential 1,25(OH)2D3 responsiveness in the various tissues, nor from differences in endogenous 1,25(OH)2D3 occupancy of the VDR. These studies thus establish that, in contrast to observations in vitro, the widely described phenomenon of homologous VDR up-regulation in kidney and intestine is not a universal property of 1,25(OH)2D3 target tissues in vivo in the rat.

Noncalcemic actions of 1,25-dihydroxyvitamin D3 and clinical applications

Vitamin D is absolutely essential for the maintenance of a healthy skeleton. Without vitamin D, children develop rickets and adults exacerbate their osteoporosis and develop osteomalacia. Casual exposure to sunlight is the major source of vitamin D for most people. During exposure to sunlight, ultraviolet B photons photolyze cutaneous stores of 7-dehydrocholesterol to previtamin D3. Previtamin D3 undergoes a thermal isomerization to form vitamin D3. Increased skin pigmentation, changes in latitude, time of day, sunscreen use, and aging can have a marked influence on the cutaneous production of vitamin D3. Once vitamin D3 is formed in the skin or ingested in the diet, it must be hydroxylated in the liver and kidney to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. It is now recognized that a wide variety of tissues and cells, both related to calcium metabolism and unrelated to calcium metabolism, are target sites for 1,25(OH)2D3. 1,25(OH)2D3 stimulates intestinal calcium absorption and mobilizes stem cells to mobilize calcium stores from bone. Noncalcemic tissues that possess receptors for 1,25(OH)2D3 respond to the hormone in a variety of ways. Of great interest is that 1,25(OH)2D3 is a potent antiproliferative and prodifferentiation mediator. As a result, 1,25(OH)2D3 and its analogs have wide clinical application in such diverse clinical disorders as rheumatoid and psoriatic arthritis; diabetes mellitus type I; hypertension; cardiac arrhythmias; seizure disorders; cancers of the breast, prostate, and colon; some leukemias and myeloproliferative disorders; chemotherapy-induced hair loss; and skin rejuvenation as well as skin diseases like psoriasis and ichthyosis.

Vitamin-D3 derivatives and breast-tumor cell growth: effect on intracellular calcium and apoptosis

Vitamin-D3 derivatives are now well-recognized growth inhibitors of numerous tumoral cells and in particular breast-cancer cells. However, the mechanisms by which they operate are not well established. Among the wide range of physiological and biological functions of vitamin-D3 derivatives, the best described include their action on calcium homeostasis. In this study, we sought to establish whether the effects of vitamin-D3 derivatives on breast-cancer cell growth may be in part related to intracellular calcium modulation and induction of apoptosis. To address these questions, we used, in addition to 1,25(OH)2D3, the active metabolite of vitamin D3, a non-calcemic 1,25(OH)2D3 derivative: Ro 23-7553 [16-ene-23-yne-1,25(OH)2D3], which in our hands was more potent than the parent compound in inhibiting breast-cancer cell growth. We showed that the efficiency of both compounds in growth inhibition was higher in the estradiol-receptor-positive-breast-tumor MCF-7 cells than in the estradiol-receptor-negative MDA-MB 231 cells. In MCF-7 cells in particular, important modifications of intracellular calcium related to the emptying of intracellular pools were observed. The depletion of Ca++ from intracellular stores was followed by the induction of apoptosis. Such a phenomenon was never observed in MDA-MB 231 cells. Our results suggest that the action of vitamin-D3 derivatives on the depletion of calcium stores, which was more significant in MCF-7 than in MDA-MB 231 cells, may induce apoptosis in the former cells and account for the high efficiency of vitamin-D3 derivatives on growth inhibition of MCF-7 breast-tumor cells.

1,25(OH)2D3 suppresses expression and secretion of atrial natriuretic peptide from cardiac myocytes

We have examined the effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on the expression of the rat atrial natriuretic peptide (ANP) gene and the secretion of the encoded protein product in neonatal rat cardiac myocyte cultures. 1,25(OH)2D3 effected a dose- and time-dependent inhibition of agonist-stimulated ANP secretion, which was accompanied by a reduction in the levels of the ANP mRNA transcript. The latter effect appeared to derive, at least in part, from suppression of ANP gene transcription. Of interest, both the reduction in mRNA levels and the inhibition of transcriptional activity were amplified by simultaneous treatment with retinoic acid, suggesting that heterodimerization of liganded 1,25(OH)2D3 receptor and retinoic acid receptor (likely retinoid X receptor) may underlie the inhibitory mechanism in the cardiac myocyte. Neither the secretory effect nor the effect on transcription proved to be calcium dependent. 22-Oxacalcitriol, a nonhypercalcemic analogue of 1,25(OH)2D3, was equally effective in suppressing ANP mRNA levels and transcription of the gene. These findings add to a growing body of data that imply an important role for 1,25(OH)2D3 in the regulation of cardiovascular function.

Congestive heart failure caused by vitamin D deficiency?

We describe a child, 3.5 months old, with severe vitamin D deficiency, profound hypocalcaemia, hyperphosphataemia, dilated left ventricle, severely reduced myocardial contractility and congestive heart failure. She also had depressed thyroid function with subnormal thyroxine and non-detectable serum thyrotropin (TSH) levels. The child promptly responded to calcium infusions, conventional anticongestive therapy and calcitriol. She is now 3 years old and received no medication. Myocardial function is normal but she has motor delay. We believe that her transitory congestive heart failure was caused by severe vitamin D deficiency with profound hypocalcaemia.

Lack of stimulatory effect of 1,25(OH)2 vitamin D3 on beta-endorphin and cortisol secretion

The aim of the work was to investigate the effect of 1,25 (OH)2 vitamin D3 on beta-endorphin and cortisol secretion in vivo. beta-Endorphin, cortisol, calcium and phosphate in blood were measured in ten healthy unstressed volunteers before and after 4-day oral administration of 1,25 (OH)2 vitamin D3 in doses of 3 micrograms/d. The biological effectiveness of the treatment was proved by a significant increase of calcemia as compared with control values (2.71 +/- 0.03 vs 2.57 +/- 0.02 mmol/l, mean +/- SE, p < 0.01) and phosphatemia (1.18 +/- 0.06 vs 0.89 +/- 0.01 mmol/l, p < 0.01). beta-Endorphin after treatment was 39.1 +/- 9.2 (vs 42.7 +/- 7.6) ng/l and cortisol 369 +/- 35 (vs 377 +/- 31) nmol/l. In conclusion, in spite of the good compliance of the treatment, 1,25 (OH)2-vitamin D3 stimulates neither beta-endorphin nor cortisol secretion in healthy subjects.

Non-genomic signal transduction pathway of vitamin D in muscle

The secosteroid hormone 1,25(OH)2-vitamin D3 rapidly activates voltage-dependent Ca2+ channels of the L-type in skeletal and cardiac muscle cells by a non-genomic mechanism which involves guanine nucleotide binding (G) protein-medicated stimulation of the adenylate cyclase/cAMP/protein kinase A messenger system. Modifications in calmodulin intracellular distribution induced by PKA-dependent membrane protein phosphorylation may participate in the fast regulation of muscle Ca2+ influx by 1,25(OH)2D3. The protein kinase C pathway also plays a role modulating 1,25(OH)2D3 signal transduction in muscle by cross-talk with the PKA system. The hormone sequentially activates phospholipases C and D providing diacylglycerol for PKC activation and inositol triphosphate for intracellular Ca2+ mobilization. In addition, 1,25(OH)2D3 rapidly stimulates phospholipase A2 generating arachidonic acid for the eicosanoid pathway. Specificity of hormone effects suggests that binding to a muscle membrane-bound receptor mediates these events.

Involvement of protein kinase C in 1,25(OH)2-vitamin D3 regulation of calcium uptake by cultured myocytes

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] produces an acute stimulation of calcium influx in cultured chick embryo myocytes through activation of voltage-gated Ca2+ channels and involvement of cyclic AMP-dependent protein kinase A (PKA). To investigate the participation of protein kinase C (PKC) in this hormone-induced response, calcium uptake was measured in myocytes treated with PKC activators 12-O-tetradecanoyl phorbol 13-acetate (TPA, 50 nM) or 1,2-dioctanoyl-rac-glycerol (DOG, 50 microM). TPA and DOG decreased 45Ca2+ uptake 37% below control cultures. Contrarily, the PKC inhibitors H7 and staurosporine increase myocyte Ca2+ uptake 51% and 54%, respectively. In addition, PKC activity was augmented in cytosol (39%) and membranes (31%) of myocytes after 5 min of treatment with 0.1 nM 1,25(OH)2D3. Likewise, the hormone induced a fast biphasic formation of diacylglycerol, the natural PKC activator, peaking at 30 s (26%) and 3 min (39%). On the other hand, the stimulation of Ca2+ uptake induced by compound H7 as well as 1,25(OH)2D3 was completely abolished with a specific PKA inhibitor. H7 also produced an increase in cAMP levels (172%) and PKA activity (204%). These results suggest the participation of PKC in 1,25(OH)2D3 regulated calcium influx in heart cells and the operation of a cross-talk mechanism between the PKC and PKA pathways.

Absence of calbindin-D28 expression in nonclassical 1,25-dihydroxyvitamin D targets: analysis by polymerase chain reaction

CaBP-D28 mRNA expression in rat heart, testis, and lung was assessed by polymerase chain reaction (PCR). The animal model used was the hyperinduced vitamin D-treated rat (100 ng 1,25-dihydroxyvitamin D subcutaneously, daily for 7 days). For the PCR studies, two pairs of 20 mer oligonucleotide primers (designated 1-4 according to their position on the coding strand, but with primers 3 and 4 in reverse orientation) derived from the rat CaBP-D28 cDNA sequence were tested in various combinations. Optimal conditions were established using a 1:100 dilution of cDNA from normal rat kidney. Bands of the predicted sizes of 869 (1, 3), 994 (1, 4), 725 (2, 3), and 850 (2, 4) nucleotide base pairs resulted, but with varying intensities: 2,4 approximately 1,3 > 1,4 > 2,3. Repeat PCR (recycling after 1:100 dilution and readdition of reagents and primers with at least one different primer) provided strong additional amplification, particularly with the 1,4/2,4 combination. Under these conditions, mixing experiments showed that CaBP-D28 transcripts were detectable at 10(-7)- to 10(-9)-fold lower levels of expression than in D+ kidney. When RNA was isolated and cDNA generated from test tissues from 4 individual vitamin D-stimulated (D+) and vitamin D-deficient (D-) rats, repeat PCR (1,4/2,4 primer combination) provided no evidence of significant CaBP-D28 mRNA expression in the nonclassic target tissues, in contrast to strong bands in both the D- kidney (undiluted) and D+ kidney (1:100 dilution) preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological effects of 1,25-dihydroxyvitamin D3 in TM4 Sertoli cell line

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] receptors have been previously described in Sertoli cells. This study was performed to assess biological activity of the receptor in the mouse Sertoli cell line TM4. A 2-h preincubation with 0.01-25 nM 1,25(OH)2D3 resulted in a dose-dependent rapid uptake of 45Ca2+ within 5 min of addition of the isotope to the cells (27 +/- 8%, n = 4 experiments; P less than 0.05). This response was specific for 1,25(OH)2D3, in that it was not induced by 25-hydroxyvitamin D3, estradiol, cortisol, R 5020 (promegestone), or testosterone. However, a combination of testosterone and 1,25(OH)2D3 inhibited uptake by 23 +/- 8% (n = 3 experiments, P less than 0.01). That the mechanism responsible for 1,25(OH)2D3-stimulated uptake may involve 1,25(OH)2D3 receptor interaction is supported by the observation that cycloheximide inhibited the response. Conversely, there was no detectable change in uptake by 1,25(OH)2D3-treated cells after 24-h incubation with 0.1-5 nM 1,25(OH)2D3. Increased levels of DNA and protein content also resulted from a 2-h incubation with the steroid and were sustained up to 24 h without a concomitant increase in cell number or a detectable change in cell morphology. The presence of specific 1,25(OH)2D3 receptor-like binding sites was demonstrated by sucrose gradient analysis and hydroxylapatite assay. These data demonstrate that 1,25(OH)2D3 may play an important role in testicular function through regulation of receptor-mediated events.

Evidence on the participation of the 3',5'-cyclic AMP pathway in the non-genomic action of 1,25-dihydroxy-vitamin D3 in cardiac muscle

Several studies have suggested that vitamin D plays a role in cardiovascular function. It has been recently shown that in vitro treatment of vitamin D-deficient chick cardiac muscle with physiological concentrations of 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) induces a rapid (1-10 min) increase of tissue 45Ca uptake which can be suppressed by Ca channel blockers. The hormone simultaneously stimulated heart microsomal membrane protein phosphorylation. Experiments were performed to investigate the existence of a relationship between these changes and to obtain information about the mechanism involved in 1,25(OH)2D3-induced modifications in cardiac protein phosphorylation. Dibutyryl cyclic AMP (10 microM) and forskolin (10 microM), known activators of the cAMP pathway, produced time courses of changes in 45Ca uptake by chick heart tissue similar to 1,25(OH)2D3 (10(-10) M). Analogously to the hormone, the effects of both compounds were abolished by nifedipine (30 microM) and verapamil (10 microM). In agreement with these observations, 1,25(OH)2D3 significantly increased (34-70%) heart muscle cAMP levels within 1-10 min of treatment. In addition, 1,25(OH)2D3 and forskolin caused similar changes in cardiac microsomal membrane protein phosphorylation (e.g. stimulation in 43 kDa and 55 kDa proteins). These changes were also evidenced by direct exposure of isolated heart microsomes to 1,25(OH)2D3, suggesting a direct membrane action of the hormone. The fast effects of 1,25(OH)2D3 on dihydropyridine-sensitive cardiac muscle Ca uptake could be reproduced in primary-cultured myocytes isolated from chick embryonic heart. Furthermore, the effects of the hormone could be suppressed by a specific protein kinase A inhibitor. These results suggest that 1,25(OH)2D3 affects heart cell calcium metabolism through regulation of Ca channel activity mediated by the cAMP pathway.

Evidence for two classes of 1,25-dihydroxyvitamin D3 binding sites in classical vs. nonclassical target tissues

Possible differences in 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] binding sites in classical and nonclassical target tissues were tested by Scatchard analysis of [3H]1,25(OH)2D3 binding in parallel chromatin preparations of rat kidney vs. testis. Two distinct binding components were resolved in kidney (p less than 0.005). Moreover, the single binding site in testis exhibited a 10-fold lower Kd (p less than 0.05) than did the principal binding site in kidney (50 +/- 4 vs. 405 +/- 142 pM). Secondly, regulation of [3H]1,25(OH)2D3 binding sites also differed. 1,25(OH)2D3 injection resulted in increased 1,25(OH)2D3 binding (p less than 0.05) in kidney (92%) and intestine (415%), but not in testis, lung or heart. These results suggest that the principal 1,25(OH)2D3 binding sites in classical targets kidney and intestine may be intrinsically different from those in at least some nonclassical targets.

Rapid stimulation of calcium uptake and protein phosphorylation in isolated cardiac muscle by 1,25-dihydroxyvitamin D3

1,25-Dihydroxyvitamin D3 has been shown to induce rapid changes in calcium fluxes in skeletal muscle and other target tissues independently of gene activation. The possibility that the hormone would produce similar effects in heart where 1,25-dihydroxyvitamin D3 receptors and activities have been shown, was studied. A significant increase of 45Ca uptake by left ventricular slices from vitamin D-deficient chicks was observed upon incubation for 1-10 min with physiological doses of 1,25-dihydroxyvitamin D3. This stimulation was dose-dependent and specific for the hormone when compared with vitamin D3, 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 and could not be associated to changes in lipid synthesis as assessed by measurements of [3H]glycerol incorporation into cardiac tissue lipids. The Ca channel blockers nifedipine (30 microM) and verapamil (10 microM) abolished the increase in Ca uptake produced by 1,25-dihydroxyvitamin D3. The rapid effects of the hormone on heart Ca influx were accompanied by a stimulation of the phosphorylation of two microsomal proteins of 43 kDa and 55 kDa. These results further support a direct action of 1,25-dihydroxyvitamin D3 in the regulation of cardiac muscle Ca metabolism which may involve activation of Ca channels.

1,25 (OH)2 vitamin D3-induced 45CA uptake in vascular myocytes cultured from spontaneously hypertensive and normotensive rats

The effect of 1,25 (OH)2 vitamin D3 on basal 45Ca uptake was examined in vascular smooth muscle cells cultured from mesenteric arteries of spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) normotensive rats. Basal uptake of 45Ca was significantly greater in myocytes of WKY than SHR at 5, 10, 30 and 60 min incubation with the isotope. Incubation with 1 ng/ml 1,25 (OH)2 vitamin D3 for 48 hr increased basal 45Ca uptake between 1-10 min in SHR and between 5-10 min in WKY. The dose-response relationship indicated that cells from both strains are equally sensitive to the calciotropic effects of 1,25 (OH)2 vitamin D3 with half-maximal stimulation occurring at approximately 0.3-0.4 ng/ml. In cells of both strains maximal stimulation of 45Ca uptake was achieved only after a 12-24 hr period of incubation with hormone and pretreatment with cycloheximide inhibited 1,25 (OH)2 vitamin D3-enhanced 45Ca uptake. Although 45Ca binding by extracellular matrix material was significantly greater in WKY than SHR, 1,25 (OH)2 vitamin D3 had no effect on the amount of matrix 45Ca binding in either strain. These results suggest that 1,25 (OH)2 vitamin D3 induces an increase in intracellular protein synthesis that results in enhanced 45Ca uptake. The similar responses of the two strains indicate that hypertensive smooth muscle is not more sensitive to 1,25 (OH)2 vitamin D3 and the Ca2+ response is a general property of vascular muscle.

Calcitropic hormones, platelet calcium, and blood pressure in essential hypertension

Plasma ionized calcium, platelet cytosolic calcium (using the fura-2 method in gel-filtered platelets), parathyroid hormone (both the intact hormone and a midmolecule portion), calcitriol, and calcidiol were measured in 19 untreated male patients with essential hypertension and 19 age-matched normotensive male research subjects. Mean levels of platelet cytosolic calcium, parathyroid hormone, calcitriol, and calcidiol were all significantly higher, whereas plasma ionized calcium was significantly lower, in the hypertensive group compared with the normotensive group. Both platelet cytosolic calcium and intact parathyroid hormone were positively correlated with mean arterial pressure (r = 0.58, p less than 0.001; r = 0.54, p less than 0.001, respectively), whereas plasma ionized calcium was inversely correlated with mean arterial pressure (r = -0.60, p less than 0.001) in the combined group of all study subjects. All three of these correlations were significant in the hypertensive group alone but not in the normotensive group alone. When analyzed with plasma ionized calcium, body mass index, serum calcitriol, and calcidiol in a multivariable regression model, the significance of the partial regressions of platelet cytosolic calcium and parathyroid hormone with mean arterial pressure persisted. Intact parathyroid hormone was positively correlated to platelet cytosolic calcium (r = 0.43, p less than 0.01) and plasma ionized calcium was inversely correlated to platelet cytosolic calcium (r = -0.44, p less than 0.01). These results confirm previous reports of disturbances of calcium metabolism in essential hypertension and suggest that the elevated platelet cytosolic calcium observed in essential hypertension may be linked to one or more of these alterations of calcium metabolism.

Effects of vitamin D3 metabolites on thyrotropin secretion from rat pituitary cells in culture

The effects of several vitamin D3 metabolites on TSH secretion were investigated using rat pituitary cells in culture. After a 24-h incubation in Dulbecco's medium, only 25,26-dihydroxyvitamin D3 [25,26(OH)2D3] at 10(-6) M increased the basal TSH secretion (36% above control). The TRH-induced TSH secretion increased dose-dependently with increasing concentrations of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] showing a significant increase at 10(-8) M (41% above control). At 10(-5) M 25,26(OH)2D3 the TRH-stimulated TSH secretion was also increased (63% above control). In medium containing charcoal-treated serum, 1, 24,25-trihydroxyvitamin D3 [1,24,25(OH)3D3] decreased the TRH-induced TSH secretion dose dependently (34% below control at 10(-8) M), while the effects of both 1,25(OH)2D3 and 25,26(OH)2D3 were blunted. Our results indicate that several vitamin D3 metabolites may affect pituitary TSH secretion.