Adrenomedullin is a potent vasodilatory peptide that has a variety of effects in a number of different systems including kidney. In cultured rat glomerular mesangial cells adrenomedullin increases cAMP, decreases proliferation and increases apoptosis. Associated with the anti-proliferative and apoptotic effects, adrenomedullin also causes a decrease in extracellular signal-regulated kinase2 (ERK2) and an increase in cJun N-terminal kinase 1 (JNK1) and P38 mitogen-activated protein kinase (P38 MAPK) activities. The purpose of the present study was to examine the role of P38 MAPK on adrenomedullin-mediated inhibition of [3H]thymidine incorporation (an index of proliferation) and on adrenomedullin-stimulated nucleosome-associated cytoplasmic DNA fragmentation (an index of apoptosis) in mesangial cells, using a selective inhibitor of P38 MAPK, SB203580 [[4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-im idazole], and also to characterize the proximal signal transduction pathways of the three MAPKs in relation to [3H]thymidine incorporation and cytoplasmic DNA fragmentation using a phosphotidyl inositol-3-kinase inhibitor, wortmannin [[1S-(1alpha,6b alpha,9alphabeta,11alpha,11b beta)]-11-(acetyloxy)-1,6b,7,8,9a,10,11,11b-octahydro-1-(methoxyme thyl)-9a,11b-dimethyl-3H-furo[4,3,2-de]indeno[4,5-h]-2-benzopyran-3,6,9- trione]. SB203580 significantly reversed the effects of adrenomedullin on [3H]thymidine incorporation and cytoplasmic DNA fragmentation, and inhibited only P38 MAPK activity. It had no effect on ERK2 and JNK1 activities. Wortmannin, on the other hand, inhibited only adrenomedullin-stimulated cytoplasmic DNA fragmentation and did not affect adrenomedullin-mediated inhibition of [3H]thymidine incorporation. Wortmannin also inhibited adrenomedullin-stimulated P38 MAPK activity without affecting ERK2 and JNK1 activities. These results indicate that: (a) In rat mesangial cells adrenomedullin-mediated inhibition of [3H]thymidine incorporation and stimulation of nucleosome-associated cytoplasmic DNA fragmentation are sensitive to SB203580, and (b) adrenomedullin activates a P38 MAPK through a wortmannin-sensitive kinase. The data using SB203580 suggest an important physiological role for P38 MAPK in rat mesangial cell proliferation and apoptosis.