MODULATION OF THROMBOXANE RECEPTOR ACTIVATION IN RAT GLOMERULAR MESANGIAL CELLS

Rat glomerular mesangial cells were used to investigate mechanisms of thromboxane A(2) (TxA(2)) receptor regulation in the kidney. Exposure of mesangial cells to the TxA(2) agonist U-46619 for 10 min reduced subsequent TxA(2)-induced increases in inositol phosphates and intracellular Ca2+ levels by similar to 70%. This loss of receptor responsiveness could be blocked by the TxA(2) receptor antagonist SQ-29548 and was reversible after removal of agonist from the incubation medium. Radioligand binding studies using the TxA(2) agonist [I-125]BOP suggested that exposure of mesangial cells to U-46619 for 10 min reduced TxA(2) receptor responsiveness without a loss of receptor sites from plasma membrane fractions of the cell, although the density of mesangial cell TxA(2) receptors was decreased by similar to 60% after more prolonged exposure of mesangial cells to thromboxane agonists. Both desensitization to U-46619 and loss of TxA(2) binding sites could be attenuated by the protein kinase C (PKC) inhibitors staurosporine, sphingosine, or H-7, and TxA(2) receptor responsiveness was reduced in cells incubated with phorbol esters before stimulation with thromboxane agonists. We conclude that 1) agonist-specific decreases in TxA(2) receptor responsiveness may involve initial uncoupling of the receptor from its effector systems, followed by a loss of TxA(2) receptor sites from plasma membrane fractions of the cell, and 2) PKC may be involved in these processes.