Fractionation of rat ventricular nuclei

Myocardial cells were isolated after treatment with collagenase (0.05%) and hyaluronidase (0.1%) by discontinuous-gradient centrifugation on 3% Ficoll. Nuclei derived from these myocardial cells were then fractionated on a discontinuous sucrose density gradient with the following steps: (I) 2.0M/2.3M, (II) 2.3M/2.4M, (III) 2.4M/2.5M, (IV) 2.5M/2.6M, and (V) 2.6M/2.85M. The myocardial nuclei were sedimented in the interfaces of gradient fractions (II) and (III). Nuclei from whole ventricles that had been treated with the enzymes before isolation sedimented into five major subsets of nuclei. These findings suggest that nuclei sedimented in the isopycnic gradient at fractions (II) and (III) are most probably derived from myocardial cells. However, this procedure is laborious and lengthy, and the recovery of myocardial-cell nuclei is low. An alternative method was developed to isolate an enriched fraction of myocardial-cell nuclei from whole ventricular tissue without exposing the tissues to enzyme digestion. These ventricular nuclei could be fractionated into five nuclear subsets by using the same discontinuous sucrose density gradient as that described above. The content of DNA, RNA and protein per nucleus for each band was determined. Although the DNA content per nucleus was constant (10pg), that of RNA varied from 1.5 to 4.5pg and that of protein from 16 to 24pg. Nuclei from each band were examined by light-microscopy: large nuclei occurred in the ligher regions whereas smaller nuclei were found in the denser regions of the gradient. From the size distribution pattern of myocardial-cell nuclei compared with that of total ventricular nuclei, it was found that nuclear subsets (II), (III), and (IV) were similar to myocardial nuclei. Electrophoretic analyses of the proteins solubilized in sodium dodecyl sulphate/phenol or Tris/EDTA/2-mercaptoethanol/phenol obtained from each nuclear subset indicate that these fractions are similar, with limited qualitative differences. These findings indicate that isolation of an enriched fraction of myocardial-cell nuclei could be achieved by discontinuous-sucrose-density-gradient centrifugation.