Are interatrial band myocytes maximally hypertrophied in normal canine hearts?

In canine right atrial hypertrophy, the cross-sectional area (A(xs)) of right atrial myocytes increases, whereas the A(xs) of the broader interatrial band myocytes does not. In the current study, myocyte reconstructions showed that right atrial myocyte length increased in proportion to A(xs) in right atrial hypertrophy. On the other hand, mean interatrial band myocyte length in both normal and right atrial hypertrophy dogs was roughly inversely proportional to mean A(xs), as expected if interatrial band myocyte volume was constant. Plotting mean A(xs) vs. myocyte length for individual interatrial band myocytes revealed a distribution whose border defined a maximal volume curve; many myocytes were well beneath that curve. Mononuclear myocytes (generally diploid) were limited by a 65,000-μm³ curve, which many binuclear myocytes (generally tetraploid) surpassed; myocyte ploidy thus constrained myocyte volume. However, because many mononuclear and binuclear myocytes had lower volumes, their failure to hypertrophy cannot be attributed to attainment of the maximal volume possible for their ploidy.