Binding of 125I-physalaemin to rat parotid acinar cells.

1. The binding of [125I]physalaemin to rat parotid acinar cells was investigated. 2. The [125I]physalaemin exhibited a small degree of specific binding that was rapid, reversible and saturable. 3. The EC50 values for inhibition of binding by four peptides were well correlated with their ability to activate Ca-dependent K release from the rat parotid gland. 4. The number of binding sites, which may represent substance P receptors, was estimated to be in the range of 200/cell, a value quite different from those reported previously for muscarinin (840/cell) or alpha-adrenergic (15,000/cell) receptors. 5. It is concluded that if, as previously suggested, these receptors regulate the same population of Ca channels, then the mechanism or perhaps efficiency by which this is achieved may differ for the three receptors.