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 su bsequent TxA(2)-induced increases in inositol phosphates and intracell ular Ca2+ levels by similar to 70%. This loss of receptor responsivene ss could be blocked by the TxA(2) receptor antagonist SQ-29548 and was reversible after removal of agonist from the incubation medium. Radio ligand binding studies using the TxA(2) agonist [I-125]BOP suggested t hat exposure of mesangial cells to U-46619 for 10 min reduced TxA(2) r eceptor responsiveness without a loss of receptor sites from plasma me mbrane fractions of the cell, although the density of mesangial cell T xA(2) receptors was decreased by similar to 60% after more prolonged e xposure of mesangial cells to thromboxane agonists. Both desensitizati on 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 c onclude that 1) agonist-specific decreases in TxA(2) receptor responsi veness may involve initial uncoupling of the receptor from its effecto r systems, followed by a loss of TxA(2) receptor sites from plasma mem brane fractions of the cell, and 2) PKC may be involved in these proce sses.