Abstract

The effect of muscarinic-receptor stimulation on [3H]inositol mono-, bis-, tris- and tetrakisphosphate (InsP1, InsP2, InsP3 and InsP4) accumulation was examined in bovine tracheal smooth muscle slices prelabeled with myo-[3H]inositol. Carbachol (0.1 mM) caused a rapid increase in [3H]InsP3 and [3H]InsP2 followed by delayed increases in [3H]InsP1 and [3H]InsP4 accumulation. Analysis of the [3H]InsP3 isomers by HPLC showed an immediate although transient increase in [3H]Ins(1,4,5)P3 with a progressive and sustained accumulation of [3H]Ins(1,3,4)P3. [3H]Ins(1,3,4)P3 was confirmed as the predominant (greater than 80%) [3H]InsP3 isomer present at 1 and 30 min using an enzymatic method which causes selective hydrolysis of Ins(1,3,4)P3. Lithium enhanced markedly the carbachol-stimulated accumulation of [3H]InsP1 and [3H]InsP2 with a lower potency than that observed in other tissues but had no significant influence on [3H]InsP3 or [3H]InsP4 values. These data support a role for Ins(1,4,5)P3 in initiating airway smooth muscle contraction and indicate the importance of the Ins(1,3,4,5)P4/Ins(1,3,4)P4 pathway in this tissue.