We have documented and characterized the down-regulation of the125I-secretin binding sites and the associated desensitization of the secretin receptor-cAMP system in rat gastric glands. Secretin induced a rapid decrease of the high-affinity125I-secretin binding sites with t1/2=30 min at 37°C. Half-maximal down-regulation and desensitization occurred at 10−9 M secretin, a physiological concentration corresponding to the half-maximal activation of the secretin receptor. The Scatchard parameters of the low-affinity125I-secretin binding sites were unaffected by the pretreatment. This desensitization is heterologous in view of the loss of responsiveness to the truncated glucagon-like peptide 1 (TGLP-1), and pharmacologically selective since the sectetin-related analogue VIP (10−7 M) does not alter the secretin-induced cAMP generation in rat gastric glands. The glycoprotein nature of the secretin receptor has also been demonstrated using WGA-agarose affinity chromatography of the solubilized125I-secretin receptor complex.

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