Microtubules in the heart muscle of the postnatal and adult rat.

In the postnatal rat heart, muscle cells continue to divide as well as increase in size. At the same time the cells in the soleus muscle (a slow skeletal muscle) do not divide, although they continue to grow in size. Since microtubules may have a role in orienting intracellular structures in muscle, we determined the numbers of microtubules/micron2 cross-sectional area in the rat heart papillary muscle during development. We have previously determined that in the soleus muscle, microtubule number/micron2 increases to a maximum at five to nine days of age, after which there is an abrupt decrease to a steady level characteristic of the adult [2]. The numbers of microtubules/micron2 in the heart were similar to those in the soleus muscle at the same age. The numbers of microtubules/micron2 increased from birth to a maximum at nine days, then decreased to a steady state. This decrease in microtubule number in heart muscle occurred at 9 to 11 days as in the soleus muscle. The distributions of microtubules are thus similar for cardiac and slow skeletal muscle, suggesting similar function(s) in these different muscle types.