Multiple tachykinin binding sites in hamster, rat and guinea-pig urinary bladder

Abstract

Binding of the 125I-Bolton-Hunter labelled tachykinins substance P, substance K, eledoisin and neuromedin K (BHSP, BHSK, BHE, BHNK) was examined in urinary bladders of hamster, rat and guinea-pig using crude membrane suspensions and by autoradiography. High-affinity binding of BHSK was observed in hamster and rat bladder and high-affinity binding of BHSP was seen in rat and guinea-pig bladder. Characterization of this binding showed that the hamster bladder contains very large numbers of K-type binding sites, where BHSK is displaced by substance K > kassinin > eledoisin > neuromedin K > substance P > physalaemin, and has very few P-type binding sites, where BHSP is displaced by substance P > substance K⪢ neuromedin K. In contrast, the rat bladder contains moderate and approximately equal numbers of both K (K D, 0.74 nM; B max 2.9 fmol/mg wet weight tissue) and P (K D, 0.12 ± 0.01 nM; B max 2.6 ± 0.2 fmol/mg wet weight tissue) sites. The guinea-pig bladder possesses predominantly P sites. Most tachykinin binding sites are localized over smooth muscle and probably represent functional receptors mediating the direct contractile effects of tachykinins in these tissues. Few E-type binding sites, as previously described in rat brain, were found, although some BHNK binding sites were seen in the mucosa of guinea-pig bladder.