Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: I. Erythrocytes

Regulatory differences between Ca(2+)-ATPase in plasma membranes from chicken (nucleated) and pig (anucleated) erythrocytes

Kinetic and regulatory properties of the plasma membrane Ca(2+)-ATPase activity from chicken (nucleated) erythrocytes were studied and compared to those from pig (anucleated) erythrocytes. In the absence of known activators: (1) Ca(2+) affinity for the Ca(2+)-ATPase activity from nucleated erythrocytes was 12-fold higher than that from pig erythrocytes, and thus the enzyme is sensitive to physiological Ca(2+) concentrations; (2) the enzyme from chicken erythrocytes showed two apparent Km values for ATP, as compared to one apparent Km value displayed by pig erythrocytes; (3) Ca(2+)-ATPase inserted in chicken erythrocyte membranes showed a low sensitivity to activation by phosphatidylinositol-4-phosphate; (4) when p-NPP was used as substrate, the activity of chicken erythrocytes was high, similar to that attained by pig erythrocytes, but barely sensitive to activation by dimethylsulfoxide and calmodulin. ATP hydrolysis was 10-fold lower than that displayed by pig erythrocytes and the maximal velocity was activated three-fold by calmodulin. The enzyme was insensitive to alkaline phosphatase treatment and showed a single phosphorylation band in electrophoresis, ruling out the possibility of previous modulation by endogenous kinases and/or by partial proteolysis. The differences may be attributed to some endogenous modulator, to distinct isoforms, or to a difference in the E(1)/E(2) states of the enzyme.

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.