IgA immune complex blunts the contraction of cultured mesangial cells through the inhibition of protein kinase C and intracellular calcium

The effects of IgA immune complex (IgA-IC) on the contractile function of cultured mesangial cells were measured by the changes in planar surface area in response to treatment with agonists. Incubation of mesangial cells with IgA-IC for 24 hours significantly decreased the contractile responses to angiotensin II (10 ^-6 M) and phorbol 12-myristate 13-acetate (PMA, 10 ^-6 M). Pretreatment of mesangial cells with the protein kinase C (PKC) inhibitor, chelerythrine (10 ^-6 M), eliminated the difference in contractile responses to angiotensin II or PMA between the control and IgA-IC groups indicating IgA-IC may inhibit the activity of PKC. The contractile responses to ionomycin were not significantly different between IgA-IC treated and control mesangial cells, suggesting that the contractile machinery is not impaired by IgA-IC. Intracellular calcium, [Ca ²⁺ ] _i measured by changes in fura-2 level in response to ATP or bradykinin, was significantly inhibited in IgA-IC treated mesangial cells, compared to control cells. In contrast, treatment with thapsigargin did not result in significant differences in [Ca ²⁺ ] _i between IgA-IC and control mesangial cells, suggesting that a negligible role of endoplasmic reticulum in the effects of IgA-IC. Using PKC specific antibodies, IgA-IC significantly increased the particulate fraction of PKC-ι of mesangial cells to 141±13% of control, without significantly changing the protein content of PKC-α, -δ and -λ in the cytosolic and particulate fractions. In-summary, IgA-IC inhibits the contractile responses of cultured mesangial cells to agonists by inhibiting the activation of PKC and [Ca ²⁺ ] _i .