E ndothelin (ET) is a vasoactive peptide produced by both endothelial epithelial cells with documented mitogenic action on mesangial cells. The present studies were designed to test the hypothesis that ET is also produced by human mesangial cells (HMC) and that other mitogens such as arginine vasopressin (AVP) and insulin stimulate cellular proliferation, in part, through modulation of endogenous production of this peptide. Studies were conducted on cultured normal HMC between the third and seventh passages. All mitogenesis experiments were carried out in 96-well plates and assessed by tritiated thymidine incorporation into DNA under various concentrations of AVP in the presence and absence of insulin, antiendothelin antisera (ETAS), a MAb against ET-1 (AbET), and a vasopressin-1 receptor antagonist. ET concentrations were measured daily from conditioned medium by a sensitive and specific RIA. ET was present in all concentrations of FCS as well as conditioned medium compared with medium alone. AVP (10(-6) M) in the presence of insulin increased ET production by quiescent HMC by 261% as well as cellular proliferation by 440% after 48 h incubation. In addition, cells cultured with ETAS or AbET demonstrated a blunted mitogenic response to AVP, a response not observed in cells cultured with ETAS where ET was added. Insulin significantly potentiated the mitogenic effects of AVP as well as media levels of ET, an effect significantly blunted by AbET. We conclude that ET is produced by HMC and its production is affected, in part, by both AVP and insulin. ET may thus serve to modulate the mitogenic effects of AVP on human mesangial cells.