| Summary: | A novel naturally-occurring bradykinin-related peptide (BRP) with an N-terminal extension, named RVA-Thr<sup>6</sup>-Bradykinin (RVA-Thr<sup>6</sup>-BK), was here isolated and identified from the cutaneous secretion of <i>Odorrana hejiangensis (O. hejiangensis)</i>. Thereafter, in order to evaluate the difference in myotropic actions, a leucine site-substitution variant from <i>Amolops wuyiensis</i> skin secretion, RVA-Leu<sup>1</sup>, Thr<sup>6</sup>-BK, was chemically synthesized. Myotropic studies indicated that single-site arginine (R) replacement by leucine (L) at position-4 from the N-terminus, altered the action of RVA-Thr<sup>6</sup>-BK from an agonist to an antagonist of BK actions on rat ileum smooth muscle. Additionally, both BK N-terminal extended derivatives (RVA-Thr<sup>6</sup>-BK and RVA-Leu<sup>1</sup>, Thr<sup>6</sup>-BK) exerted identical myotropic actions to BK, such as increasing the frequency of contraction, contracting and relaxing the rat uterus, bladder and artery preparations, respectively.
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