1. The effects of increase in intracellular adenosine 3':5'-cyclic monophosphate (cAMP) on endothelin-1 (ET-1)-induced generation of inositol phosphates (IPs) and increase in intracellular Ca2+ ([Ca2+](i)) were investigated in canine cultured tracheal smooth muscle cells (TSMCs). 2. Pretreatment of TSMCs with either cholera toxin (CTX; 10 μg ml-1, 4 h), forskolin (10 μM, 30 min), or dibutyryl cAMP (1 mM, 30 min) inhibited ET-1-stimulated Ca2+ mobilization (by 23 ± 5%, n = 8) and IPs accumulation (by 32 ± 6%, n = 4). While after treatment with forskolin for 24 h, the cells retained the ability to respond to ET-1-induced Ca2+ mobilization to the same extent as the control group. 3. Forskolin (1-100 μM) inhibited the ET-1-induced increase in [Ca2+](i), but the lower concentrations had little effect on this response. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration-response curve of ET-1 without changing the -logEC50 values. 4. The water-soluble forskolin analogue L-858051, 7-deacetyl-7β-(γ-N-methylpiperazino)-butyryl forskolin, significantly inhibited ET-1-stimulated IPs accumulation. In contrast, the addition of 1,9-dideoxy forskolin, an inactive analogue of forskolin, had little effect on stimulated responses. Moreover, SQ-22536, 9-(tetrahydro-2-furanyl)-9H-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 cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit ET-1-induced IPs accumulation. These results suggest that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. 5. The locus of this inhibition of forskolin treatment on A1F4--stimulated IPs accumulation was investigated in canine TSMCs. The A1F4-induced IPs accumulation was inhibited by forskolin, supporting that G protein(s) are directly activated by A1F4- and uncoupled to phospholipase C by forskolin treatment. 6. We conclude that cAMP elevating agents inhibit ET-1-stimulated generation of IPs and Ca2+ mobilization in canine cultured TSMCs. Since generation of IPs and increases in [Ca2+](i) are very early events in the activation of ET-1 receptors, attenuation of these events by cAMP elevating agents might well contribute to the inhibitory effect of cAMP on tracheal smooth muscle function.
ASJC Scopus subject areas
Yang, C. M., Pan, S. L., Chiu, C. T., Lin, C. C., & Hsu, Y. M. (1998). Effect of forskolin on endothelin-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells. Journal of Autonomic Pharmacology, 18(4), 213-221. https://doi.org/10.1046/j.1365-2680.1998.18485.x