Down‐regulation of bombesin binding to guinea‐pig pancreatic acinar cells during homologous desensitization

1 [125I]‐Tyr4‐bombesin exhibited saturable binding to pancreatic acinar cells. 2 Preincubation of cells at 37°C with 0.03 nM‐1 μm‐bombesin for 10 min followed by acid or neutral washes reduced subsequent binding of [125I]‐Tyr4‐bombesin in a concentration‐dependent manner by up to 90%. 3 In cell suspensions, bombesin raised the cytoplasmic Ca2+ concentration ([Ca2+]i) and the [Ca2+]i response was reduced by pre‐exposure to the agonist. 4 In individual superfused cells, bombesin at 1 nM normally caused a large [Ca2+]i transient followed by sustained [Ca2+]i oscillations. The cells exhibited a variable degree of desensitization when subsequently exposed to higher agonist concentrations. 5 Preincubation with bombesin for 10 min caused a concentration‐related reduction of subsequent amylase responses to bombesin. 6 Down‐regulation of receptor binding is a rapid phenomenon during bombesin exposure explaining, at least partially, tachyphylaxis of [Ca2+]i and amylase responses.