Diabetic rat glomerular mesangial cells display normal inositol trisphosphate and calcium release

Since diabetes may cause cellular myo-inositol depletion, we investigated whether the observed in vitro hypocontractile response of streptozotocin (STZ)-treated rat glomeruli to angiotensin II (ANG II) is associated with an alteration in inositol trisphosphate (IP3) mobilization of intracellular Ca2+. Contraction of diabetic isolated glomeruli induced by ANG II (5 microM), measured in vitro by changes in the planar area, was reduced by 60%, compared with normal up to 60 min (P < 0.05). In cells of isolated glomeruli, preloaded with myo-[3H]inositol, production of [3H]inositol phosphates ([3H]IPs) and [3H]inositol trisphosphate ([3H]IP3) was analyzed by Dowex chromatography. ANG II (1 microM) evoked an immediate peak (5-10 s) in total [3H]IPs of 60.5 +/- 18.8% (mean +/- SE) above basal (nonstimulated state) in normal glomeruli, and 88.4 +/- 19.4% in diabetic condition [not significant (NS), n = 8]. At 60 s, the normal and diabetic total [3H]IPs responses were not significantly different from each other. The immediate (10 s) [3H]IP3 response from normal glomeruli, 8.1 +/- 7.9% above basal, was not significantly different from that of diabetic glomeruli, 15.7 +/- 7.4%. ANG II receptor-mediated rise in cytosolic Ca2+ in the cells of normal and diabetic isolated glomeruli was compared by measuring the efflux of 45Ca2+. Isolated glomeruli were preloaded with 45Ca2+. Following ANG II stimulation, peak 45Ca2+ efflux values at 1 min were 141.7 +/- 15.9% (normal) vs. 143.7 +/- 7.8% (diabetic) of baseline (100%), respectively (NS, n = 4). Thapsigargin, 2 microM, specifically prevented ANG II-stimulated and IP3-mediated 45Ca2+ efflux (73% inhibition, P < 0.001) from cells of whole glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)