Recent work carried out in our laboratory using glomeruli microperfused ex vivo has demonstrated that these are viscoelastic structures allowing volume expansion with increased intraglomerular pressure. since glomerular elasticity is evident ex vivo over the pathophysiological range of intraglomerular pressures, it is likely that this is an intrinsic structural property regulating volume in vivo. The axial location of the mesangial cell within the glomerular lobule and the attachment of its cytoplasmic projections to perimesangial areas indicate that glomerular expansion must be associated with marked tridimensional cell stretching. In recent work, we have also shown increased synthesis of extracellular matrix (ECM) components in mesangial cells subjected to mechanical strain during cyclic stretching. It is evident, therefore, that impaired control of glomerular volume, by allowing exaggerated expansion, will lead to stimulation of ECM formation by mesangial cells, even in circumstances of unaltered intraglomerular pressure, and will further aggravate the mesangial distensional effects of intracapillary hypertension. Preliminary results have shown that glomerular volume regulation may be evaluated according to: a) the level of glomerular dilation over a specific range of intraglomerular pressures (mean glomerular volume), and b) the prevalent glomerular compliance (degree of deformation resulting from an incremental intraglomerular pressure). Initial data suggests that the following factors are main contributors to the control of glomerular volume: a) intrinsic glomerular size, b) passive elastic component (supporting framework, content of glomerular collagen), and c) active viscoelastic component (mesangial cell tone). The work proposed will define separately the significance of these factors under normal conditions and in circumstances of glomerular hypertrophy and hyperfunction, i.e., in poorly controlled diabetes and after partial nephrectomy. In addition, the mechanical forces effectively stimulating mesangial cell ECM production will be defined. Based on the mechanical characteristics of glomerular expansion, specific forms of stretch, mimicking events in situ, have been selected for their application to cloned mesangial cells in tissue culture. The metabolic response of the mechanically stimulated mesangial cell will be evaluated as the rate of production of total collagen, collagen type I, collagen type IV, fibronectin, and laminin. In addition, the dual role of angiotensin II, as a constricting agent increasing mesangial cell tone (thus limiting glomerular expansion) and as a direct stimulator of extracellular matrix synthesis, will be investigated. Studies will be conducted in isolated microperfused glomeruli of the rat and rabbit under controlled conditions of perfusion pressure. Glomerular volume will be estimated using image analysis techniques and glomerular content of collagen by direct microtechniques for measurement of hydroxyproline. Formation of extracellular matrix components will be evaluated by ELISA, immunoprecipitation and radiolabeled proline incorporation. The study will explore a new pathophysiological concept pertaining to glomerular function and the modulation of extracellular matrix formation, i.e., glomerular volume regulation.
