The receptor-binding properties of isolated rabbit colonic circular smooth muscle cells in primary culture have been investigated. In intact smooth muscle, acetylcholine, acting through M2 muscarinic receptors, and vasoactive intestinal polypeptide (VIP), acting through VIP receptors, are two of the principal neurotransmitters mediating contraction and relaxation, respectively. The muscarinic receptor was present in very high levels (600,000 receptors/cell) on freshly isolated colonic smooth muscle cells as shown by binding of the muscarinic receptor antagonist N-methylscopolamine (NMS). However, NMS binding sites decreased rapidly when the cells were placed in primary culture. After 21 h in culture, specific binding of [3H]NMS decreased to 20%, and after 48 h to less than 10% that of preculture values. This loss was not associated with a change in receptor affinity, since Kd was unchanged for the receptors still present. In contrast, high-affinity VIP receptors were expressed on cultured smooth muscle cells but could not be detected on freshly isolated cells. Cultured cells responded to VIP with an increase in intracellular adenosine 3',5'-cyclic monophosphate (cAMP), indicating that the VIP receptors were functionally coupled to adenylate cyclase. Cultured cells also responded to calcitonin gene-related peptide (CGRP) and forskolin with increased production of intracellular cAMP. In contrast, neither VIP nor CGRP elicited an increase in intracellular cAMP when added to freshly isolated cells. Furthermore, freshly isolated cells had a greatly diminished response to forskolin, suggesting that the isolation procedure not only destroyed cell surface receptors for VIP and CGRP, but also damaged the cells sufficiently to decrease cellular adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)
Interactive Mechanisms among Pituitary Adenylate Cyclase-Activating Peptide, Vasoactive Intestinal Peptide, and Parathyroid Hormone Receptors in Guinea Pig Cecal Circular Smooth Muscle Cells
