Development Of Calcium Pools In Early Embryonic Chicken
Jeffery D. Cromartie ('97), Sean M. Mercer ('96), Sallie R. Permar ('97) and A. Malcolm Campbell
Presented at the 10th American Society for Cell Biology, December 5 - 9, 1996, San Fransico, CA. (Mol. Biol. of the Cell , 7: 133a)
Calcium plays a critical role in cellular homeostasis, neurotransmitter release, and muscle contraction. Intracellular calcium pools are regulated by calcium ion pumps, channels, and buffers. Sarco/endoplasmic reticulum calcium ATPase (SERCA) is the transmembrane protein which actively transports calcium ions from the cytosol into the sarcoplasmic reticulum (SR) for storage. The ryanodine receptor and the inositol 1,4,5,-trisphosphate receptor (IP3R) are both calcium channels which allow calcium ions to be released from the SR. Calsequestrin (CS) is a lumenal SR protein that binds calcium with low affinity and high capacity and acts as a buffer for the mM levels of lumenal calcium. The initial expression of the cardiac SERCA isoform (SERCA2), IP3R, and CS was determined in cardiac myocytes of chicken embryos (embryonic day 1 through 4) using indirect immunofluorescence labeling. SERCA2 first appears as a punctate pattern along the right margin of the ventricular wall where myocytes exhibit the first visible contractions (Patten and Kramer Physiol. Rev. 29: 31-47, 1949). Our observations not only support an anterior to posterior gradient of SERCA2 (Jorgenson and BashirDev. Biol. 106: 156-165, 1984), but also suggest an additional lateral to medial gradiaent of SERCA2 expression in early chicken heart development. By embryonic day 3, the heart is beating regularly but IP3R was not detected until embryonic day 3.5 to 4. Calsequestrin expression begins during stage 14. Therefore, calcium pool development in cardiac myocytes is a sequential process that begins with SERCA2 expression and is not completed until embryonic day 4. Perhaps this sequential expression of proteins allows the SR to accumulate calcium via SERCA2 in order to fill the pool for later use when the SR becomes the major site of calcium regulation for cardiac contractions. Analysis for RYR expression is underway.
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