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During development fast-contracting atrial and ventricular chambers develop from a peristaltic-contracting heart tube. This study addresses the question of whether chamber formation is paralleled by a matching expression of the sarcoplasmic reticulum (SR) [Ca.sup.2+] pump. We studied indo-1 [Ca.sup.2+] transients elicited by field stimulation of linear heart tube stages and of explants from atria and outflow tracts of the prototypical preseptational E13 rat heart. [Ca.sup.2+] transients of H/H 11+ chicken hearts, which constitute the prototypic linear heart tube stage, were sensitive to verapamil only, indicating a minor contribution of [Ca.sup.2+]-triggered SR [Ca.sup.2+] release. Outflow tract transients displayed sensitivity to the inhibitors similar to that of the linear heart tube stages. Atrial [Ca.sup.2+] transients disappeared upon addition of ryanodine, tetracaine, or verapamil, indicating the presence of [Ca.sup.2+]-triggered SR [Ca.sup.2+] release. Quantitative radioactive in situ hybridization on sections of E13 rat hearts showed ~10-fold higher SERCA2a mRNA levels in the atria compared to nonmyocardial tissue and ~5-fold higher expression in compact ventricular myocardium. The myocardium of atrioventricular canal, outflow tract, inner curvature, and ventricular trabecules displayed weak expression. Immunohistochemistry on sections of rat and human embryos showed a similar pattern. The significance of these findings is threefold. (i) A functional SR is present long before birth. (ii) SR development is concomitant with cardiac chamber development, explaining regional differences in cardiac function. (iii) The pattern of SERCA2a expression underscores a manner of chamber development by differentiation at the outer curvature, rather than by segmentation of the linear heart tube. [C] 2000 Academic Press Key Words: cardiac development; sarco(endo)plasmic reticulum [Ca.sup.2+] pump; chicken; rat; human.