Modulation of fluoroaluminate-induced inositol phosphate formation by increases in tissue cyclic AMP content in bovine tracheal smooth muscle.

1. The effect of fluoroaluminate complexes (AlCl3 plus NaF) upon smooth muscle tone, [3H]-inositol phosphate accumulation and [3H]-cyclic AMP accumulation has been investigated in slices of bovine tracheal smooth muscle. 2. Fluoroaluminate (10 microM AlCl3 + various concentrations of NaF) elicited concentration-dependent contractions of bovine tracheal smooth muscle strips at concentrations of NaF in the range 1-10 mM. The resultant contractile response was reversed by isoprenaline (50 nM) and was preserved in calcium-free medium. 3. Fluoroaluminate stimulated [3H]-inositol phosphate formation at concentrations of NaF over 1 mM. The response to 20 mM NaF + 10 microM AlCl3 was 164 +/- 29% of the response to 1 mM histamine. Fluoroaluminate also increased the incorporation of [3H]-myo-inositol into membrane phospholipids. 4. Fluoroaluminate produced a small rise in [3H]-cyclic AMP levels (2.1 fold increase over basal with 20 mM NaF). The response to forskolin (1 microM, 8.6 fold over basal) was reduced by fluoroaluminate in a concentration-dependent manner, but still remained significantly (P less than 0.05) elevated over the response to fluoroaluminate alone. 5. The [3H]-inositol phosphate response to fluoroaluminate was inhibited by salbutamol (maximum inhibition 60%, IC50 = 0.08 microM), forskolin (1 microM, 46% inhibition) and isobutylmethylxanthine (1 mM, 73% inhibition). 6. These data suggest that inhibition of agonist-induced inositol phospholipid turnover by cyclic AMP in this tissue can occur at the post-receptor level.