EFFECTS OF LITHIUM ON CAMP GENERATION IN CULTURED RAT INNER MEDULLARY COLLECTING TUBULE CELLS

The effects of lithium (Li) on the cAMP system in rat inner medullary collecting tubule cells were studied. While acute exposure to 5 mM Li was without effect, 10 mM, 25 mM and 50 mM Li significantly decreased AVP-stimulated cAMP formation. In contrast, cells grown in 5 mM Li for 72 hours which caused no morphologic changes enhanced cAMP formation (fmol/μg protein) in response to both 10 nM AVP (114.5 ± 9.2 vs. 71.6 ± 7.4, P < 0.005) and 100 nM AVP (182 ± 14 vs. 120 ± 8.3, P < 0.001), N = 16. A similar enhancement was observed when cAMP formation was stimulated by a post-receptor agonist, cholera toxin. The role of eicosanoids was examined with 5 μM meclofenamate which reversed Li-enhanced cAMP formation in response to both AVP and cholera toxin. To define the eicosanoid responsible, cyclooxygenase products were measured. Prostaglandin E2 and thromboxane B2 synthesis were unchanged by Li, but the production of prostacyclin was significantly (P < 0.02) increased. Prostacyclin (3 μM) mimicked the effect of Li to enhance the response to 10 nM AVP as cAMP levels increased from 100 ± 11 to 173 ± 13, P < 0.05). The experiments suggest that acute exposure of Li at concentrations of 10 mM or greater inhibit cAMP formation but prolonged Li exposure enhances cAMP formation by increasing the formation of prostacyclin.