This paper describes the localization of isomyosins in the pericytes of four rat microvascular beds: heart, diaphragm, pancreas, and the intestinal mucosa, by use of immunoperoxidase techniques and IgGs specific for either nonmuscle or smooth muscle isoforms. Based on the semiquantitative nature of the peroxidatic reaction, we concluded that the amount and distribution of these isoforms vary with the microvascular bed and also with vascular segments within the same bed. In the pericytes of small capillaries, nonmuscle isomyosin is the predominant form, whereas the smooth muscle isomyosin is present in very low concentration. A reversed relationship is found in the pericytes associated with larger capillaries and postcapillary venules. These results, taken together with previous findings on actin (Herman, I., and P. A. D'Amore, 1983, J. Cell Biol. 97:278a), tropomyosin (Joyce, N. C., M. F. Haire, and G. E. Palade, 1985, J. Cell Biol. 100:1379-1386), and cyclic GMP-dependent protein kinase (Joyce, N., P. DeCamilli, and J. Boyles, 1984, Microvasc. Res. 28:206-219), indicate that pericytes contain proteins essential for contraction in higher concentration than any other cells associated with the microvasculature, except smooth muscle cells. Pericytes appear to be, therefore, cells differentiated for a contractile function within the microvasculature.

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