Effect of forskolin on bradykinin-induced calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on bradykinin (BK)-induced generation of inositol phosphates (IPs) and Ca²⁺ mobilization were investigated in canine cultured tracheal smooth muscle cells (TSMCs). Pretreatment of TSMCs with either forskolin or dibutyryl cyclic AMP attenuated BK-stimulated responses. The inhibitory effects of these agents produced both a depression of the maximal response and a shift to the right of the concentration-response curves of BK. The water-soluble forskolin analogue L-858051, 7-deacetyl-7β-(r-N-methylpiperazino)-butyryl forskolin, significantly attenuated BK-stimulated IPs accumulation, while 1,9-dideoxy forskolin, an inactive forskolin, had little effect on IPs response. Moreover, SQ-22536, 9-(tetrahydro-2-furanyl)-9-H-purin-6-amine, an inhibitor of adenylate cyclase, and both H-89, N-(2-aminoethyl)-5-isoquinolinesulfonamide and HA-1004, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide, inhibitors of cyclic AMP dependent protein kinase (PKA), reversed the ability of forskolin to attenuate BK stimulated IPs accumulation. The K(D) and B(max) values of the BK receptor for [³H]BK binding were not significantly changed by forskolin treatment for 30 min and 4 h. The AIF₄^--induced IPs accumulation was attenuated by forskolin, indicating that G protein(s) are directly activated by AIF₄^- and uncoupled to phospholipase C by forskolin treatment. These results suggest that activation of cyclic AMP/PKA might inhibit the BK-stimulated PI breakdown and consequently reduce the [Ca²⁺](i) increases or inhibit independently both responses, which is distal to the BK receptor in canine cultured TSMCs.