Elevation of cAMP in cultured mesangial cells diminishes vasopressin-stimulated increases of phosphate uptake and ³²P-specific activity in ATP but has no effect on phosphoinositide metabolism

Agents known to elevate intracellular cyclic AMP (cAMP) is cultured mesangial cells (e.g., isoproterenol with and without isobutylmethylxanthine (MIX)) inhibit vasopressin-induced contraction. Since contraction of these cells in response to vasopressin is accompanied by release of inositol trisphosphate and increased intracellular ionized calcium, we wanted to determine whether cAMP is exerting its relaxing effect by altering phosphoinositide metabolism. Isoproterenol and MIX did not diminish the release of inositol trisphosphate in response to vasopressin. However, the stimulated ³a2P incorporation into phospholipids seen with vasopressin treatment was diminished by prior treatment with isoproterenol-MIX. Since incorporation of ³²P into phospholipids is not only dependent on phospholipid synthesis but also on the amount of label in the γ-phosphate of ATP, we determined the specific activity of ³²P in ATP. We found that suppression of ³²P incorporation into phospholipids in cells treated with isoproterenol-MIX was paralleled by a decline of specific acitity of ³²P in ATP. Furthermore, the changes in ATP specific activity were paralleled by similar changes in phosphate uptake into the cells. Thus, diminished phosphate uptake (transport) could account for the decline of ³²P content in phospholipids and ATP following treatment of mesangial cells with isoproterenol-MIX.