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 Ca 2+ levels by ~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 [ 125 I]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 ~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.
Spurney, RF; Middleton, JP; Raymond, JR; Coffman, TM
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