Phospholipase A2 and phospholipase C are activated by distinct GTP-binding proteins in response to alpha 1-adrenergic stimulation in FRTL5 thyroid cells

Abstract

In FRTL5 rat thyroid cells, norepinephrine, by interacting with alpha 1-adrenergic receptors, stimulates inositol phosphate formation, through activation of phospholipase C, and arachidonic acid release. Recent studies have shown that GTP-binding proteins couple several types of receptors to phospholipase C activation. The present study was undertaken to determine whether GTP-binding proteins couple alpha 1-adrenergic receptors to stimulation of phospholipase C activity and arachidonic acid release. When introduced into permeabilized FRTL5 cells, guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S]), which activates many GTP-binding proteins, stimulated inositol phosphate formation and arachidonic acid release. Neomycin inhibited GTP[gamma-S]-stimulated inositol phosphate formation but was without effect on GTP[gamma-S]-stimulated arachidonic acid release, suggesting that separate GTP-binding proteins mediate each process. In addition, pertussis toxin inhibited norepinephrine-stimulated arachidonic acid release but not norepinephrine-stimulated inositol phosphate formation. Norepinephrine-stimulated arachidonic acid release but not inositol phosphate formation was also inhibited by decreased extracellular calcium and by TMB-8, suggesting a role for a phospholipase A2. To confirm that arachidonic acid was released by a phospholipase A2, FRTL5 membranes were incubated with 1-acyl-2-[3H]arachidonoyl-sn-glycero-3-phosphocholine. GTP[gamma-S] slightly stimulated arachidonic acid release, whereas norepinephrine acted synergistically with GTP[gamma-S] to stimulate arachidonic acid release. The results show that phospholipase C and phospholipase A2 are activated by alpha 1-adrenergic agonists. Both phospholipases are coupled to the receptor by GTP-binding proteins. That coupled to phospholipase A2 is pertussis toxin-sensitive, whereas that coupled to phospholipase C is pertussis toxin-insensitive.