DIVERSITY AND VARIABILITY OF SMOOTH-MUSCLE PHENOTYPES OF RENAL ARTERIOLES AS REVEALED BY MYOSIN ISOFORM EXPRESSION

The contractility and distensibility of renal arterioles are important in the regulation of glomerular filtration. However, little is known regarding the characteristics of contractile proteins in these arterio les. Recently it was demonstrated that vascular smooth muscles contain two types of myosin heavy chain (MHC) isoforms, SM1 and SM2, which ar e unique molecular markers of smooth muscle cell phenotypes. SM1 is co nstitutively expressed in all types of smooth muscles, whereas SM2 exi sts only in mature smooth muscles. We characterized the expression of MHC isoforms as well as the ultrastructural myofilament assembly of re nal arteriolar smooth muscles in human, rat and rabbit by immunohistoc hemical techniques. SM1 and alpha-smooth muscle actin were localized i n both the preglomerular vessels (including the afferent arterioles) a nd efferent arterioles, whereas SM2 was present only in the preglomeru lar vessels. Renin-producing cells in the afferent arterioles (juxtagl omerular granular cells, JG cells) were positive for alpha-smooth musc le actin but negative for SM2. When renin synthesis was stimulated, th e more proximal afferent arteriolar smooth muscles turned renin-positi ve and SM2 disappeared. Glomerular mesangial cells did not show immuno reactivities for SM1, SM2 or alpha-smooth muscle actin. The difference in MHC isoform expression in these arterioles was also reflected by u ltrastructures; the afferent arteriolar smooth muscles contained abund ant myofilaments including thick filaments, whereas the efferent arter iolar smooch muscles had a few myofilaments composed only of thin micr ofilaments. The JG cells displayed a myofilament assembly similar to t hat in the efferent arteriolar smooth muscles. We conclude from these observations that smooth muscles in pre- and postglomerular arterioles , the glomerular mesangial cells and JG cells differ in phenotypes, su ggesting that they may have different contractile properties which may be critically involved in the regulation of glomerular filtration.