Characterization of opioid receptors on isolated canine gallbladder smooth muscle cells.

Abstract Smooth muscle cells were isolated from the fundus of the canine gallbladder and examined for the presence of opioid receptors. The cells contracted in a concentration-dependent manner in response to three opioid peptides (Met-enkephalin, dynorphinl-13 and Leu-enkephalin), which are known derivatives of opioid precursors present in myenteric neurons of the gut. The order of potency was Met-enkephalin > dynorphinl-13 > Leu-enkephalin. The contractile response to opioid agonists was selectively inhibited by opioid antagonists (naloxone and Mr2266) but not by muscaarinic, CCK/gastrin or tachykinin antagonists. Equivalent responses to the three opioid peptides exhibited differential sensitivity to preferential antagonists of μ (naloxone) and κ (Mr2266) opioid receptors consistent with the presence of the three main types of opioid receptors (μ, δ and κ) on canine gallbladder muscle cells.