Effects of 1,25(OH)2D3 on enterocyte basolateral membrane Ca transport in rats

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Vitamin D and adaptation to dietary calcium and phosphate deficiencies increase intestinal plasma membrane calcium pump gene expression

The effect of vitamin D and other variables on the synthesis of the chicken intestinal plasma membrane calcium pump (PMCA) mRNA was assessed. The DNA probe for Northern analysis was obtained by reverse transcription and PCR with intestinal poly(A)+ RNA, using two 20-mer oligonucleotide primers homologous to the 3' coding region of the human teratoma PMCA. An EcoRI restriction fragment of the PCR product was cloned into the pBluescript II KS(-) phagemid vector, and the chimeric plasmid was used to transform Escherichia coli. The amino acid sequence deduced from the nucleotide DNA sequence of the PCR product and the cloned DNA were 96% homologous with the teratoma sequence. Northern blots of intestinal poly(A)+ RNA with 32P-labeled DNA showed the presence of three major species of chicken PMCA mRNAs at about 6.6, 5.4, and 4.5 kb. Northern analysis with the chicken PMCA DNA indicated that repletion of vitamin D-deficient chickens with vitamin D increased PMCA mRNAs in the duodenum, jejunum, ileum, and colon. After injection of 1,25-dihydroxyvitamin D3 intravenously into vitamin D-deficient chickens, duodenal PMCA mRNA tended to increase by 2 hr, reached a maximum at about 16 hr, and returned to baseline levels at 48 hr. Adaptation of chickens to either a calcium- or phosphorus-deficient diet resulted in a 2- to 3-fold increase in duodenal PMCA mRNA. These results indicate that vitamin D and specific variables that affect calcium absorption through the vitamin D-endocrine system increase intestinal PMCA gene expression.