Increase in calcium content and Ca2+-ATPase activity in the brain of fasted rats: Comparison with different ages

The effect of fasting on calcium content and Ca²⁺-ATPase activity in the brain tissues of 5 weeks and 50 weeks old rats was investigated. Brain calcium content and Ca²⁺-ATPase activity in the microsomal and mitochondrial fractions of the brain homogenate from young and elderly rats were significantly increased by overnight–fasting. These increases were appreciably restored by a single oral administration of glucose solution (400 mg/100 g body weight) to fasted rats. In comparison with young and elderly rats, brain calcium content and microsomal Ca²⁺-ATPase activity were significantly elevated by increasing ages. The effect of ageing was not seen in the brain mitochondrial Ca²⁺-ATPase activity. When calcium (50 mg/100 g) was orally administered to young and elderly rats, brain calcium content was significantly elevated. The calcium administration–induced increase in brain calcium content was greater in elderly r crease in Ca²⁺-ATPase activity in the microsomal and mitochondrial fractions of brain homogenates from young rats. In aged rats, the microsomal Ca²⁺-ATPase activity was not further enhanced by calcium administration, although the mitochondrial enzyme activity was significantly raised. The present study demonstrates that the fasting–induced increase in brain calcium content is involved in Ca²⁺-ATPase activity raised in the brain microsomes and mitochondria of rats with different ages, supporting a energy–dependent mechanism in brain calcium accumulation.     

Streptozotocin-induced diabetes increases (Ca2+-Mg2+)-ATPase activity in hepatic plasma membranes of rats: Involvement of protein kinase C

The alteration in calcium transport in the liver of rats with streptozocin(STZ)-diabetic state was investigated. STZ (6 mg/100 g body weight) was subcutaneously administered in rats, and 1 or 2 weeks later they were sacrificed by bleeding. STZ administration caused a remarkable elevation of serum glucose concentration. Liver calcium content was significantly increased by STZ administration. Hepatic plasma membrane (Ca²⁺-Mg²⁺)-ATPase activity was markedly elevated by STZ administration. This increase was completely abolished by the presence of staurosporine (10^-7-10^-5 M), an inhibitor of protein kinase C, in the enzyme reaction mixture, suggesting an involvement of protein kinase C signalling. Moreover, the STZ-induced increase in liver plasma membrane (Ca²⁺-Mg²⁺)-ATPase activity was significantly raised by the presence of okadaic acid (10^-5 and 10^-4 M). Meanwhile, the STZ-increased (Ca²⁺-Mg²⁺)-ATPase activity was not appreciably altered by the presence of anti-regucalcin IgG in the reaction mixture, indicating that the activatory protein regucalcin does not participate in the elevation of the enzyme activity. The present study demonstrates that STZ-induced diabetes causes the increase in hepatic plasma membrane (Ca²⁺-Mg²⁺)-ATPase activity of rats.     

Characterization of calcium accumulation in the brain of rats administered orally calcium: The significance of energy-dependent mechanism

The characterization of calcium accumulation in the brain of rats administered orally calcium chloride solution was investigated. Rats received a single oral administration of calcium (15–50 mg/100 g body weight), and they were sacrificed by bleeding-between 15 and 120 min after the administration. The administration of calcium (50 mg/100 g) produced a significant increase in serum calcium concentration and a corresponding elevation of brain calcium content, indicating that the transport of calcium into the brain is associated with the elevation of serum calcium levels. The increase in brain calcium content by calcium administration was not appreciably altered by the pretreatment with Ca²⁺ channel blockers (verapamil or diltiazem with the doses of 1.5 and 3.0 mg/100 g). In thyroparathyroidectomized rats, the administration of calcium (50 mg/100 g) caused a significant increase in brain calcium content, indicating that calcium-regulating hormones do not participate in the brain calcium transport. Now, brain calcium content was clearly elevated by fasting (overnight), although serum calcium level was not significantly altered. Calcium administration to fasted rats induced a further elevation of brain calcium content as compared with that of control (fasted) rats. The fasting-induced increase in brain calcium content was appreciably restored by refeeding. This restoration was also seen by the oral administration of glucose (0.4 g/100 g) to fasted rats. The present study demonstrates that serum calcium is transported to brain, and that the increased brain calcium is released promptly. The release of calcium from brain may be involved in energy metabolism, and this release may be weakened by the reduction of glucose supply into brain. The finding suggests a physiological significance of energy-dependent mechanism in the regulation of brain calcium.     

Characterization of calcium accumulation in the brain of rats administered orally calcium: The significance of energy-dependent mechanism

Primary cultures of rat hepatocytes maintained on different matrix proteins such as collagen (Co IV) fibronectin (Fn), Laminin (Ln) or different tissue biomatrices were metabolically labelled with ³⁵[S]-SO₄ and the synthesis of sulphated proteoglycans was studied. The incorporation of the label into total glycosaminoglycan (GAG) was significantly higher in cells maintained on Co IV compared to those maintained on Fn or Ln. Similarly the incorporation of label was maximum in those cells maintained on the aortic biomatrix compared to liver or mammary gland biomatrix. About 80–95% of the GAG synthesised and secreted by cells maintained on individual matrix proteins and liver biomatrix was heparan sulphate (HS). But in the case of cells maintained on collagen IV aortic or mammary biomatrix in addition to HS, significant amount of chondroitin sulphate (CS) was also found. Nearly 50% of the total ³⁵[S]-GAG was associated with the cell layer after 24 h in culture in the case of cells maintained on individual matrix protein while those maintained on tissue biomatrix, retained about 70% of the ³⁵[S]-labelled proteoglycans (PG) with the cell layer. Analysis of the cell surface ³⁵[S]-labelled proteoglycans isolated from cells maintained on different biomatrix showed that it is a hybrid proteoglycan consisting of CS and HS. While the PG isolated from cells maintained on liver biomatrix consists of HS and CS in the ratio of 3:2 that from cells maintained on aorta or mammary gland matrix was about 2:3 indicating an alteration in the nature of the cell surface PGs produced by cells maintained on different tissue biomatrix. These results indicate that depending on the nature of the matrix substratum with which the cells are in contact, the nature and quantity of sulphated proteoglycans produced by hepatocytes vary.