Hypercalcemia stimulates expression of intrarenal phospholipase A2 and prostaglandin H synthase-2 in rats. Role of angiotensin II AT1 receptors.

In chronic hypercalcemia, inhibition of thick ascending limb sodium chloride reabsorption is mediated by elevated intrarenal PGE2. The mechanisms and source of elevated PGE2 in hypercalcemia are not known. We determined the effect of hypercalcemia on intrarenal expression of cytosolic phospholipase A2 (cPLA2), prostaglandin H synthase-1 (PGHS-1), and prostaglandin H synthase-2 (PGHS-2), enzymes important in prostaglandin production. In rats fed dihydrotachysterol to induce hypercalcemia, Western blot analysis revealed significant upregulation of both cPLA2 and PGHS-2 in the kidney cortex and the inner and outer medulla. Immunofluorescence localized intrarenal cPLA2 and PGHS-2 to interstitial cells of the inner and outer medulla, and to macula densa and cortical thick ascending limbs in both control and hypercalcemic rats. Hypercalcemia had no effect on intrarenal expression of PGHS-1. To determine if AT1 angiotensin II receptor activation was involved in the stimulation of cPLA2 and PGHS-2 in hypercalcemia, we treated rats with the AT1 receptor antagonist, losartan. Losartan abolished the polydipsia associated with hypercalcemia, prevented the increase in cPLA2 protein in all regions of the kidney, and diminished PGHS-2 expression in the inner medulla. In addition, losartan completely prevented the increase in urinary PGE2 excretion in hypercalcemic rats. Intrarenal levels of angiotensin II were unchanged in hypercalcemia. These data indicate that hypercalcemia stimulates intrarenal cPLA2 and PGHS-2 protein expression. Our results further support a role for angiotensin II, acting on AT1 receptors, in mediating this stimulation.