Involvement of G(alpha q/11) in the contractile signal transduction pathway of muscarinic M(3) receptors in caecal smooth muscle

The nature of the pertussis toxin-insensitive G-protein involved in muscarinic-mediated phosphoinositides breakdown and contraction of isolated smooth muscle cells from the circular layer of the rabbit caecum was investigated. Immunoblotting of membrane proteins using affinity purified antibodies directed against different G-protein alpha-subunits revealed the expression of G(alpha q/11), G(alpha il) and G(alpha i2) in these cells. The carbachol-mediated [H-3]inositol phosphates accumulation in saponin-permeabilized cells was abolished by anti-G(alpha q/11)-antibodies whereas anti-G(alpha il,2)-antibodies were ineffective. Moreover, the carbachol-induced contraction of permeabilized cells, as determined by videomicrocopic measurements, was reversed by anti-G(alpha q/11)-antibodies bur not affected by anti-G(alpha il,2)-antibodies. From these data, we conclude that carbachol stimulates phosphoinositides hydrolysis and cell contraction through activation of specific muscai inic M(3) receptors coupled to the peaussis toxin-insensitive G(alpha q/11)-protein. This is the first demonstration of (alpha q/11) implication in the contractile signal transduction pathway of muscarinic M(3) receptors in smooth muscle cells.