Identification of both NK1 and NK2 receptors in guinea‐pig airways
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abstract
NK1 and NK2 receptors have been characterized in guinea‐pig lung membrane preparations by use of [125I‐Tyr8]‐substance P and [125I]‐neurokinin A binding assays in conjunction with tachykinin‐receptor selective agonists ([Sar9Met(O2)11]substance P for NK1 and [βAla8]neurokinin A (4–10) for NK2) and antagonists (CP‐99,994 for NK1 and SR48968 for NK2).The presence of high affinity, G‐protein‐coupled NK1 receptors in guinea‐pig lung parenchymal membranes has been confirmed. The rank order of affinity for competing tachykinins was as predicted for an NK1 receptor: substance P = [Sar9Met(O2)11]substance P > substance P‐methyl ester = physalaemin > neurokinin A = neurokinin B >> [βAla8]neurokinin A (4–10). The novel NK1 antagonist CP‐99,994 has a Ki of 0.4 nm at this NK1 site.In order to characterize [125I]‐neurokinin A binding to guinea‐pig lung, the number of [125I]‐neurokinin A specific binding sites was increased 3–4 fold by purification of the parenchymal membranes over discontinuous sucrose gradients. The rank order of affinity determined for NK1‐ and NK2‐receptor agonists and antagonists in competition for these sites showed that the majority (80%) of [125I]‐neurokinin A specific binding was also to the NK1 receptor.Under conditions where the guinea‐pig lung parenchymal NK1 receptor was fully occupied by a saturating concentration of either [Sar9Met(O2)11]substance P (1 μm) or CP‐99,994 (2.7 μm), residual [125I]‐neurokinin A specific binding was inhibited in a concentration‐dependent manner by both [βAla8]neurokinin A and SR48968. This result shows that the NK2 receptor is also present in these preparations.Similar studies using guinea‐pig tracheal membranes demonstrated that [125I]‐neurokinin A specific binding was composed of a NK1‐receptor component (60%), inhibited by both [Sar9Met(O2)11]substance P and CP‐99,994, and a significant NK2‐receptor component, inhibited by both [βAla8]neurokinin A and SR48968.In summary, these data demonstrate that guinea‐pig lung parenchyma and guinea‐pig trachea express both NK1 and NK2 receptors.
