Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: II. Ventricular myocytes

Influence of calcium on proliferation and phenotype alteration of cardiomyocyte in vitro

An accelerated weight gain is noted in the heart of Ca-deficient, hypertensive chick embryos maintained in a shell-less culture in vitro. We previously observed that the Ca handling property of cardiomyocytes isolated from the shell-less embryo is altered, i.e., faster Ca uptake, suggesting a requirement for adequate Ca supply and/or proper Ca handling in embryonic cardiac development. In this study, we have examined the function of Ca on cardiomyocytes by analyzing the effects of 1) various Ca concentration in the culture medium (NCa, 1.8 mmol/ L; HCa, 2.8 mmol/L; LCa, 0.9 mmol/L), and 2) various modulators of Ca handling on cell proliferation and phenotype regulation in chick embryonic cardiomyocytes. The analytical parameters included cell number, DNA content, expression of cell cycle-specific and cardiomyocyte-specific proteins, and creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) enzyme activities. Cell number and total DNA were significantly larger (P < 0.01) in LCa cultures compared with those in NCa. The level of LDH was elevated (P < 0.01), but that of CPK was lowered in LCa. Expression of the G1-S-specific protein PCNA was raised, but that of the contractile proteins myosin and tropomyosin was substantially suppressed in LCa; in HCa, the cells did not proliferate as well, whereas the level of contractile proteins was higher. Thapsigargin, a sarcoplasmic reticulum (SR)-specific, Ca-ATPase inhibitor, simulated the effects of LCa by enhancing cell proliferation and lowering the expression of tropomyosin. These results suggest that culturing in low Ca concentration and inhibition of SR Ca pumping enhance myocardial cell proliferation and suppress sarcomeric protein expression, perhaps by inducing cellular de-differentiation. The in vitro effects of medium Ca concentration and Ca handling modulators on cardiomyocytes also suggest that the in vivo cardiomegaly of the SL embryos is a direct result of Ca-deficiency, and that Ca is important in the phenotype regulation of cardiomyocytes.

Effects of cadmium on trophoblast calcium transport

Maternal exposure to cadmium (Cd) during pregnancy has been linked to low fetal birthweight, which may be attributed to placental damage and/or dysfunction in nutrient transport. Previous studies have suggested that Cd is accumulated in the placenta, and that placental transport of calcium (Ca) and zinc (Zn) is perturbed by Cd. To investigate the mechanism of Cd perturbation of Ca transport, we used JEG-3, a human choriocarcinoma cell line which exhibits trophoblastic properties, to analyse Cd effects in vitro. Treatment with Cd at low, physiologically relevant concentrations (e.g. 0.04 microM) did not result in obvious changes in cell morphology or integrity, whereas higher concentrations (> or = 0.16 microM) affected cell integrity. With lower concentrations of Cd treatment for 24 h, activities of cellular Ca uptake and transport, and Ca2+ binding were decreased, and intracellular [Ca2+] ([Ca2+]i) profile was also altered; however, membrane-associated Ca(2+)-activated ATPase activity remained relatively unchanged. Interestingly, cellular Ca uptake activity was unaffected by short-term (30 min) Cd pretreatment. The 24-h Cd treatment also resulted in elevated expression of the metal-binding protein, metallothionein, whereas the expression of a trophoblast-specific cytosolic Ca(2+)-binding protein (HCaBP) was drastically reduced. These results strongly suggest that Cd exposure significantly compromises the Ca handling ability of trophoblastic cells; this effect is probably not due to perturbations in Ca channel or membrane Ca pump activities, but rather a consequence of alterations in subcellular, cytosolic Ca2+ binding activities.

Transepithelial calcium transport in the chick chorioallantoic membrane. II. Compartmentalization of calcium during uptake

Calcium transport from the eggshell to the developing chick embryo is carried out by the ectoderm cells of the chick chorioallantoic membrane. Primary cells isolated from chick chorioallantoic membrane ectoderm were used to analyze the subcellular distribution of 45Ca2+ accumulated from the extracellular medium. We present evidence suggesting that calcium may be sequestered into endosome-like vesicles during the initial phase of uptake. A combination of techniques were utilized to monitor calcium fluxes and calcium compartmentalization in the cultured chorioallantoic membrane cells: (1) fura-2 fluorescence was used to indicate cytosolic free calcium concentrations, (2) 45Ca2+ tracer was used to follow calcium accumulation in all cellular compartments, and (3) digitonin was used to differentially permeabilize subcellular membranes in order to localize 45Ca2+ by following tracer release profiles. Differences between cytosolic calcium flux and whole cell calcium accumulation suggested that the pathway of calcium uptake from the medium involves sequestration into an internal compartment separate from the cytosol. Kinetic analysis of the digitonin-mediated release of specific subcellular markers (lactate dehydrogenase, NAD-dependent isocitrate dehydrogenase, [3H]inulin, and [3H]-2-deoxyglucose) and preloaded 45Ca2+ indicated that calcium was localized in a compartment similar to endosomal vesicles. Our results are consistent with a transcytotic mechanism for chorioallantoic membrane calcium transport.