Conformational changes in myocardial nuclei of rats

Myocardial nuclei have been found to undergo a dramatic change in nuclear conformation during the contraction-relaxation cycle. This was demonstrated with electron micrographs of longitudinally and transversely oriented nuclei in contracted and relaxed fibers of rat myocardium. The contracted fibers had a mean sarcomere length of l.7µ ± 0.1µ (±1 SD), and extended fibers had a mean sarcomere length of 2.1µ ± 0.3µ. The amount of membrane wrinkling was quantified by comparing a straight-line distance of 6.64µ between two points on the nuclear membrane to the distance along the membrane (NML) between the same points. The degree of wrinkling was expressed as "redundancy" which is [(NML - 6.64)/6.64] x 100. Longitudinal sections showed a marked difference in mean length and width in the two populations of nuclei, with an axial ratio of 2.2 in the contracted, and 4.8 in the extended fibers. The most obvious difference between the two populations, however, was the nuclear membrane redundancy, which was 2.1% and 27.0% for the extended and contracted fibers, respectively. Although a difference in nuclear membrane redundancy was clear in longitudinal sections, it was not apparent in transverse sections. It is concluded that during systole myocardial nuclei become longitudinally compressed with folding of the membrane perpendicular to the long axis. The correlation of the change in sarcomere length with the change in nuclear length and membrane redundancy suggests a coupling of the nuclear membrane to some sarcomere segment.