Glomerular dysfunction in the aging Fischer 344 rat is associated with excessive growth and normal mesangial cell function.

BACKGROUND: Fischer 344 (F344) rats display focal and diffuse glomerulosclerosis with aging postulated to result from loss of normal mesangial cell intrinsic function, e.g., vasoactive hormone signaling, or preservation of normal responsiveness to extrinsic growth factors. METHODS: In 3-, 17-, and 24-month-old F344 male rats, glomerular structure, measured by PC-based morphometry, and function were compared. Immunoperoxidase staining of glomerular proliferating cell nuclear antigen (PCNA) detected cellular proliferation. Primary cultured mesangial cells from the 3 age groups were studied in parallel. Calcium (Ca2+) signaling, measured by Fura-2 fluorescence, contraction to vasopressin (AVP) 1 microM, measured by videomicroscopy, and proliferative response to platelet-derived growth factor-beta beta (PDGF) were compared. RESULTS: Proteinuria was 13 +/- 4, 38 +/- 17, and 110 +/- 35 mg/24 hours at 3, 17, and 24 months, respectively (n = 5, mean +/- SE, p < .01, 3 vs 24 months), with no change in 24-hour creatinine clearances. Glomerular volumes (n = 200/group) for 3, 17, and 24 months, respectively, were .30 +/- .01, .60 +/- .02, .74 +/- 0.2 x 10(6) micron3 (p < .001, 3 months vs 17 months, and 17 vs 24 months). Glomerular basement membrane (GBM) widths and fractional mesangial volumes increased significantly with aging. Glomerular cell PCNA staining remained positive at 24 months. Cultured mesangial cell Ca2+ signaling and contraction to AVP were unchanged with aging. Proliferation to PDGF, which was partially inhibited with verapamil, was similar at 3 and 24 months. CONCLUSIONS: In the Fischer 344 rat, mesangial cell Ca2+ signaling, contraction, and proliferation responsiveness are unchanged with aging. Continued growth is associated with the glomerulosclerosis of aging.

Isolated rat glomerular cells demonstrate L-type Ca(2+)-channel activity.

The presence of L-type calcium (Ca2+)-channels and the effects of Ca(2+)-channel antagonists on cells of rat glomeruli were investigated. Glomeruli were isolated by graded sieving and after preincubation (10 min) in zero Ca2+, the uptake of 45Ca2+ by glomerular cells was measured. Depolarization with KCl (50 mM) or the dihydropyridine agonist Bay K 8644 (10 microM) stimulated 45Ca2+ uptake by 13% and 24%, respectively, above control (100%), which was inhibited by nifedipine (Nif, 10 microM), P < 0.05, and by both S and R isomers of verapamil (Ver, 10 microM), P < 0.001. In a separate experimental preparation, isolated glomeruli were preloaded (45 min) with 45Ca2+. Following a 45 min perifusion (37 degrees C, CaCl2 1.26 mM, in the absence of 45Ca2+), both KCl (50 mM) and Bay K 8644 (10 microM) induced cellular 45Ca2+ efflux with peak values above control of 11% and 15%, respectively, (P < 0.05). Exposure to Bay K 8644 preceded by depolarization with KCl resulted in enhanced 45Ca2+ efflux identifying the presence of voltage-dependent Ca(2+)-channel activity. Cultured rat mesangial cells grown to confluence on coverslips were preloaded with Fura-2 and cytosolic Ca2+ was measured by microfluorometry. KCl (50 mM), gramicidin (2 microM) and/or Bay K 8644 (6 microM) stimulated Ca2+ influx which was inhibited by Ver (10 microM). Ver did not alter endothelin-stimulated Ca2+ signalling. We conclude that L-type Ca2+ channels are present on both rat glomerular (endothelial and/or mesangial) cells in vivo and on cultured mesangial cells, and their activation may be hormone specific.