An environment of high glucose concentration stimulates the synthesis of extracellular matrix (ECM) in mesangial cell (MC) cultures. This may result from a similar increase in intracellular glucose concentration. We theorized that increased uptake, rather than glucose concentration per se is the major determinant of exaggerated ECM formation. To test this, we compared the effects of 35 mM glucose on ECM synthesis in normal MCs with those of 8 mM glucose in the same cells overexpressing the glucose transporter GLUT1 (MCGT1). Increasing medium glucose from 8 to 35 mM caused normal MCs to increase total collagen synthesis and catabolism, with a net 81-90% increase in accumulation. MCs transduced with the human GLUT] gene (MCGT1) grown in 8 mM glucose had a 10-fold greater GLUT1 protein expression and a 1.9, 2.1, and 2.5-fold increase in cell myo-inositol, lactate production, and cell sorbitol content, respectively, as compared to control MCs transduced with bacterial f8-galactosidase (MCLacZ). MCGT1 also demonstrated increased glucose uptake (5-fold) and increased net utilization (43-fold), and greater synthesis of individual ECM components than MCLacZ. In addition, total collagen synthesis and catabolism were also enhanced with a net collagen accumulation 111-118% greater than controls. Thus, glucose transport activity is an important modulator of ECM formation by MCs; the presence of high extracellular glucose concentrations is not necessarily required for the stimulation of matrix synthesis. (J.

To define the interplay of glomerular hypertension and hypertrophy with mesangial extracellular matrix (ECM) deposition, we examined the effects of glomerular capillary distention and mesangial cell stretching on ECM synthesis. The volume of microdissected rat glomeruli (V ,), perfused ex vivo at increasing flows, was quantified and related to the proximal intraglomerular pressure (PIP). Glomerular compliance, expressed as the slope of the positive linear relationship between PIP and V was 7.68 x 103 gm3/mmHg. Total V, increment (PIP 0-150 mmHg) was 1.162 X 106 pm3 or 61% (n = 13). A 16% increase in V, was obtained over the PIP range equivalent to the pathophysiologial limits of mean transcapillary pressure difference. A similar effect of renal perfusion on V3 was also noted histologically in tissue from kidneys perfused/fixed in vivo. Cultured mesangial cells undergoing cyclic stretching increased their synthesis of protein, total collagen, and key components of ECM (collagen IV, collagen I, laminin, fibronectin). Synthetic rates were stimulated by cell growth and the degree of stretching. These results suggest that capillary expansion and stretching of mesangial cells by glomerular hypertension provokes increased ECM production which is accentuated by cell growth and glomerular hypertrophy. Mesangial expansion and glomerulosclerosis might result from this interplay of mechanical and metabolic forces. (J. Clin. Invest. 1992. 90:1932-1943

These findings support our hypothesis that CCN2 is up-regulated early in the evolution of glomerulosclerosis, including that of diabetes. We contend that urinary CCN2 may both stage nephropathy and predict those patients who are destined for progressive glomerulosclerosis and end-stage renal disease (ESRD). Cross-sectional and prospective studies of larger, well-defined diabetic patients groups will be required to prove this hypothesis, and are ongoing.

Glomerular extracellular matrix accumulation may derive from the stretching of mesangial cells caused by excessive glomerular dilatation. The relationship of glomerular volume (VG) to intraglomerular pressure, expressed as compliance or as mean VG in the isolated, perfused rat glomerulus, was used to analyze factors that regulate VG. Glomeruli were highly distensible over the normal and relevant abnormal range of pressure. Compliance increased directly with basal VG (P < 0.001), i.e., larger glomeruli dilated more than smaller ones at any given pressure. Perfusion with atrial natriuretic peptide did not alter compliance, and inhibitors of nitric oxide synthesis exerted only a trivial effect. VG expansion was consistently reduced by angiotensin II, but this effect was small (3.8%, P < 0.001). After subtotal nephrectomy, compliance increased by 59% in the remnant glomeruli (P < 0.001); 22% of this increase was attributable to structural changes, and the remainder was attributable to the large basal VG of the hypertrophied glomeruli. Thus the major determinants of VG expansion include capillary wall tension, basal VG, and intrinsic distensibility, which is markedly influenced by the character of the extracellular matrix and only slightly altered by an angiotensin II-modified mesangial cell tone.

Mesangial cells, but not podocytes, contain a cytoskeleton capable of contraction that is disorganized in long-term diabetes. Together with previous observations, the distribution of this cytoskeleton suggests that mesangial cell contraction may be involved in the redistribution of glomerular capillary blood flow, but not substantially in the modulation of glomerular distention. Disorganization of stress fibers may be a cause of hyperfiltration in diabetes.