MESANGIAL CELL HYPERTROPHY INDUCED BY NH4CL - ROLE OF DEPRESSED ACTIVITIES OF CATHEPSINS DUE TO ELEVATED LYSOSOMAL PH

Enhanced ammoniagenesis is currently thought to play an important role in renal hypertrophy and subsequent tubulointerstitial fibrosis. Unde r certain conditions glomeruli also may be affected by ammonia toxicit y. Exposure of glomeruli to augmented ammonia levels may occur: (i) in advanced liver diseases due to elevated blood ammonia concentrations; (ii) in conditions of enhanced tubular ammoniagenesis following corti cal ''trapping;'' and (iii) due to increased ammonia formation in the glomeruli in the presence of impaired renal function. To elucidate the potential role of ammonia in glomerular injury, we investigated the e ffect of NH4Cl on protein turnover as well as on activities of various cathepsins in cultured rat mesangial cells. The results show that NH4 Cl (20 mM) induced cell hypertrophy as defined by an increase in both cell protein content and cell volume (+38% and +10.1%, respectively, a fter 48 hr). This hypertrophy was associated with suppression of the a ctivities of cathepsins B and L+B (-56.8% and -51.3% after 48 hr) and reduction of protein degradation rate (-61% after 48 hr), but without enhanced protein synthesis. Inhibiton of Na+/H+ antiport by amiloride (1 mM) neither prevented the reduction of cathepsin activities nor the hypertrophy of the mesangial cells. Upon NH4Cl application lysosomal pH was elevated. This alkalinization may be causatively involved in th e impairment of cathepsin B and L+B due to shifting the lysosomal pH a bove the optimum of their activities. In conclusion, NH4Cl induces hyp ertrophy but not hyperplasia in mesangial cells. This hypertrophy is c aused by the reduction of protein degradation, mainly due to depressed activities of cathepsin B and L+B in the absence of enhanced protein synthesis. A shift of lysosomal pH above the optimum of the acidic cat hepsins seems to be a key factor in their impaired activities in mesan gial cells.