Presence of functional receptors for corticotropin releasing hormone in caecal circular smooth muscle cells of guinea pig

The presence of specific binding sites for corticotropin releasing hormone (CRH) in caecal circular smooth muscle cells of guinea pig was investigated by binding and pharmacological studies. The specific binding of I-125-CRH to these muscle cells reached an equilibrium after 90 minutes. Several peptides structurally unrelated to CRH did not affect the specific binding of I-125-CRH to these muscle cells. Unlabeled CRH completely inhibited the specific binding of I-125-CRH in a concentration-dependent manner, with an IC50 value of 13.5 nM. A CRH receptor antagonist, alpha-helical CRH (9-41), inhibited the specific binding of I-125-CRH in a concentration-dependent manner with a lower affinity than CRH. In pharmacological study, CRH inhibited the contractile response of these muscle cells to 1 nM cholecystokinin-octapeptide in a concentration-dependent manner, with an IC50 value of 14.1 nM. A CRH receptor antagonist, alpha-helical CRH (9-41), significantly antagonized this inhibitory effect produced by CRH. These results strongly suggest the presence of functional receptors for CRH that mediate relaxation of caecal circular smooth muscle cells of guinea pig.