RECEPTOR-ACTIVATED CURRENTS IN MOUSE FIBROBLASTS EXPRESSING TRANSFECTED BOMBESIN RECEPTOR SUBTYPE CDNAS

BALB/c 3T3 cells do not normally express receptors for bombesin-like p eptides [bombesin (Bn), gastrin-releasing peptide (GRP), and neuromedi n B (NmB)]. Trnasfection of BALB/c 3T3 cells with complementary DNA-en coding GRP receptors or NmB receptors leads to stable expression of fu nctional GRP receptors (GRP R(t)) or NmB receptors (NmB R(t)), respect ively, which are coupled to cell membrane ion channels. Whole cell cur rent analysis using patch electrodes shows that the activation of thes e newly expressed receptors induces cation conductance increases, most frequently a Ca2+-activated plasma membrane K-conductance. The dose-r esponse (peak-current) relations of both transfected receptor subtypes were sigmoidal and exhibited threshold activation concentration in th e picomole range and the saturation of responses to higher concentrati ons than 10(-8) M. The GRP R(t) responded about equally to GRP, NmB, a nd Bn when compared at equimolar levels, despite their known differenc e in binding affinity for the three peptides (GRP, Bn > NmB). In contr ast, for the NmB R(t), the NmB was more potent than GRP or Bn. Among f our GRP/Bn-receptor antagonists tested, the [D-Phe6]Bn(6-13) ethyl est er suppressed GRP R(t) responses at low concentrations (10(-7) M). N-a cetyl-GRP-(20-26) amide, [Leu13-PSI(CH2NH)-Leu14]Bn, and [D-Arg1,D-Phe 5,D-Trp7,9,Leu11]substance P also blocked GRP R, responses but at high er concentrations (10(-5) M). However, at these concentrations, these four antagonists had little effect on NmB R(t) responses, thereby show ing a specificity of these antagonists for the GRP receptors.