Am J Physiol Regul Integr Comp Physiol 2004 Sep 01;2873:R575-85. doi: 10.1152/ajpregu.00051.2004.

Molecular identification and characterization of three isoforms of tachykinin NK(1)-like receptors in the cane toad Bufo marinus.

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The tachykinin peptide bufokinin, isolated from the cane toad intestine, is important in intestinal and cardiovascular regulation in the toad. In this study, three tachykinin NK(1)-like receptor isoforms, bNK(1)-A, bNK(1)-B, and bNK(1)-C, encoding proteins of 309, 390, and 371 amino acids, respectively, were cloned from the toad brain and intestine. These isoforms differ only at the intracellular COOH terminus. The bNK(1)-A and bNK(1)-B isoforms are similar to the truncated and full-length forms of the mammalian NK(1) receptor, whereas bNK(1)-C is unique and does not correspond to any previously described receptor. RT-PCR studies demonstrated that three isoform transcripts are widely distributed in the toad with high expression in gut, spinal cord, brain, lung, and skeletal muscle. When expressed in COS-7 cells, bufokinin showed similar high affinity (IC(50) 0.6-0.8 nM) in competing for (125)I-labeled Bolton-Hunter bufokinin binding at all receptors, but the binding affinities of substance P (SP) and neurokinin A (NKA) were very different at each isoform. When expressed in Xenopus oocytes, the truncated isoform, bNK(1)-A, was inactive, whereas bNK(1)-B and bNK(1)-C produced changes in chloride current when stimulated by tachykinins (minimum concentrations: bufokinin, 0.1 nM; SP, 1 nM; and NKA, 10 nM). A marked desensitization of the response was seen to subsequent applications of tachykinins, as experienced by the mammalian NK(1) receptor. In summary, our study describing three isoforms of NK(1)-like receptor from the toad suggests that the alternative splicing of NK(1) receptor is a physiologically conserved mechanism and raises a fundamental question as to the physiological role of each isoform.

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Species referenced: Xenopus laevis
Genes referenced: tac1