Association of guinea pig lung bombesin receptors with pertussis toxin-sensitive guanine nucleotide binding proteins

Abstract

The possible interaction of bombesin receptors with guanine nucleotide binding protein in guinea pig lung was studied. The non-hydrolysable GTP analogue guanosine-5′-[γ-thio]triphosphate (GTPγS) was shown to decrease [ 125I-Tyr 4]bombesin binding in a concentration-dependent manner. The specificity of this effect was assessed by examining the effects of other guanine nucleotides on this binding at a concentration of 1mM. GMP and GDP weakly inhibited [ 125I-Tyr 4]bombesin binding (2 and 19%, respectively), whereas GTP, guanosine-5′-[β-thio]triphosphate (GDPβS), and 5-guanylylimidodiphosphate (GppNHp) exhibited similar potencies, inducing 52%, 46%, and 43% inhibition of [ 125I-Tyr 4]bombesin binding respectively. Saturation experiments performed in the absence and presence of 100 μM GTPγS indicated the presence of a single population of receptors in both cases. However, the addition of GTPγS induced a marked decrease in the number of receptors (from 1.76 fmol/mg protein to 0.78 fmol/mg protein) without significantly altering the dissociation constant ( K d). These results provide evidence that bombesin receptors are coupled to a G-protein signal transduction pathway in guinea pig lung. We have further characterised this G-protein on the basis of its toxin sensitivity. Pretreatment of the lung membranes with either pertussus (10 μm/ml) or cholera toxin (50 μg/ml) was performed. Cholera toxin treatment did not affect the ability of GTPγS to inhibit [ 125I-Tyr 4]bombesin binding to guinea pig lung membranes. However, pertussis toxin treatment induced a decrease in binding and resulted in the inability of GTPγS to inhibit [ 125I-Tyr 4]bombesin binding in a concentration-dependent manner. These results suggest that bombesin receptors of the guinea pig lung interact with pertussis toxin-sensitive G-proteins.