A role for inositol 1,4,5‐trisphosphate in the initiation of agonist‐induced contractions of dog tracheal smooth muscle

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Abstract
To elucidate the role of inositol 1,4,5‐trisphosphate (Ins‐P3) in the initiation of agonist‐induced contraction of the smooth muscle cells of the dog trachea, we investigated the effects of acetylcholine (ACh) on the concentrations of Ins‐P3, phosphatidylinositol‐4,5‐bisphosphate (PI‐P2) or phosphatidic acid (PA). The effects of Ins‐P3 on the Ca2+ stored in the smooth muscle cells were also studied in saponin‐permeabilized smooth muscle cells. A half maximal or maximal Ca2+ accumulation into the cells was observed in the dispersed single, smooth muscle cells treated by saponin, in free Ca2+ concentrations of 4.6 × 10−7 or 5 × 10−5 M, respectively. The ATP‐dependent Ca2+ accumulation was maximal at 0.63 nmol/105 cells. Effects of Ins‐P3 on stored Ca2+ were observed at a free Ca2+ concentration of 3.7 × 10−7 M, which induces about half maximal ATP‐dependent Ca2+‐accumulation. Ins‐P3 released the Ca2+ accumulated by ATP, in a dose‐dependent manner. About 40% of the total Ca2+ was released following application of 3 μM Ins‐P3. The release of stored Ca2+ induced by application of Ins‐P3 was followed by its re‐uptake into the smooth muscle cells. Thus, the stored Ca2+ was repeatedly released with repetitive applications of Ins‐P3. Application of ACh (10−5 M) to the dog trachea stimulated the production of Ins‐P3 in the soluble fraction and 10 s after this application, the relative amount of Ins‐P3 was 290% of the control value. Concomitantly, ACh (10−5 M) either reduced or increased the contents of phosphatidyl inositol 4,5‐biphosphate (PI‐P2) or phosphatidic acid (PA) in the lipid fraction of the smooth muscle cells to 60% or to 350% of the control value, respectively, thereby indicating that ACh stimulates the phosphodiesteric hydrolysis of PI‐P2. 5‐Hydroxytryptamine (5‐HT; 10−5 M) also reduced or increased the contents of PI‐P2 or PA to 80 or to 200% of the control values, respectively. However, neither histamine (10−5 M), in the presence or absence of cimetidine (10−5 M), nor prostaglandin F2α (PGF2α 10−7 M) showed any effect on the contents of PI‐P2 or PA in the lipid fraction of the smooth muscle cells. These results indicate that in muscle cells of the dog trachea, Ins‐P3 may play the role of intracellular second messenger in the initiation of ACh or 5‐HT‐induced contraction, but not in the case of histamine or PGF2α‐induced contraction.