Mechanism of phorbol ester inhibition of histamine-induced IP3 formation in cultured airway smooth muscle

Abstract

Cytosolic calcium is a key determinant of the contractile state of airway smooth muscle (ASM). To investigate the mechanisms by which histamine affects cytosolic calcium, we measured changes in inositol 1,4,5-trisphosphate (IP3) following the addition of histamine to cultured canine ASM cells. The effect of phorbol 12-myristate 13-acetate (PMA) on IP3 formation was investigated under conditions previously shown to abolish histamine-induced calcium release. In both intact cells and ASM membranes, histamine produced a significant increase in IP3 formation, which was inhibited by PMA. The site of this blockade was investigated by examining the effect of PMA on guanine nucleotide-stimulated IP3 formation and on phosphoinositide-specific phospholipase C (PI-PLC) activity in ASM membranes. Guanine nucleotide-stimulated IP3 formation was inhibited by PMA pretreatment. Membrane-associated PI-PLC activity was also decreased, an effect that was not due simply to a shift in the calcium sensitivity of the enzyme. We conclude that in cultured canine ASM cells, PMA blocks histamine-induced IP3 formation and that this inhibition is caused, in part, by a postreceptor site of action of protein kinase C, possibly via a direct effect on PI-PLC.