Bombesin-like peptides induce Ca2(+)-activated K+ conductance increases in mouse fibroblasts

Peptide receptor-activated membrane currents were studied in two mouse fibroblast cell lines, Swiss and Balb/c 3T3 cells, using a patch-electrode voltage-clamp technique. About 50% of the Swiss 3T3 cells examined responded to bombesin (Bn; 10(-9) to 10(-6) M), either by inducing outward current flow or inward current flow at the membrane holding potential (Vh) of -60 mV. The outward current type was more common (approximately 70%) than the inward current type (30%). The Bn-induced outward current (IBn) was reversed as the Vh was held to more negative than -90 mV (avg reversal potential, Erev = -82 mV). This Erev was closer to the equilibrium potential for K+ and shifted by altering the extracellular-to-intracellular K+ concentration ratio, in a Nernst-like relationship. The chance of recording this type of IBn was greatly reduced when K+ conductance blockers were present in the bathing solution (i.e., tetraethylammonium, Ba2+) or in the pipette solution (i.e., Cs). It was also reduced by recording with the pipette containing 5-10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Application of Ca2+ ionophore A23187 (5 microM) induced a similar membrane current with conductance increase. Thus the outward IBn in Swiss 3T3 cells appears to be induced by the intracellular Ca2(+)-dependent K+ conductance increase. Applications of bradykinin (Bk), arginine vasopressin (AVP), neuromedin B (NmB), and gastrin releasing peptide (GRP) to Swiss 3T3 cells also induced receptor-activated currents similar to IBn. Balb/c 3T3 cells rarely generated outward currents in response to Bn, GRP, and NmB but did not respond to both AVP and Bk with outward current flows.