Two novel bombesin-like neuropeptides from the skin secretion of Pelophylax kl. esculentus: Ex vivo pharmacological characterization on rat smooth muscle types

Mammalian bombesin-like neuropeptides (BLPs) play an important role in regulation of physiological and pathophysiological processes. Frog skin-derived BLPs, of smaller size and diverse lengths and sequences at their N-terminus, have attracted the attention of many researchers. However, these N-terminal variants and the receptors modulating their pharmacological actions are poorly studied and less understood. In this study, two BLPs, namely, [Asn(3), Lys(6), Thr(10), Phe(13)]3–14-bombesin and [Asn(3), Lys(6), Phe(13)]3–14-bombesin with primary structures NLGKQWATGHFM and NLGKQWAVGHFM were isolated from the skin secretion of hybrid Pelophylax kl. esculentus. Both BLPs share a similar primary structure with only a single amino acid substitution at the eighth position (threonine to valine), while they have quite different myotropic potencies with EC(50) values in the range of 22.64 ± 9.7 nM (N = 8) to 83.93 ± 46.9 nM (N = 8). The potency of [Asn(3), Lys(6), Thr(10), Phe(13)]3–14-bombesin was approximately 3-fold higher than that of [Asn(3), Lys(6), Phe(13)]3–14-bombesin. Through the investigation of receptor selectivity using a canonical bombesin receptor antagonist, it was found that [Asn(3), Lys(6), Thr(10), Phe(13)]3–14-bombesin and [Asn(3), Lys(6), Phe(13)]3–14-bombesin had an affinity to both BB1 and BB2 receptors. Their contractile functions are mainly modulated by both BB1 and BB2 receptors on rat urinary bladder and BB2 alone on rat uterus smooth muscle preparations. These data may provide new insights into the design of potent and selective ligands for bombesin receptors. Moreover, [Asn(3), Lys(6), Thr(10), Phe(13)]3–14-bombesin and [Asn(3), Lys(6), Phe(13)]3–14-bombesin did not induce significant hemolysis and toxicity in normal human cells, suggesting that these two natural novel BLPs have great potential for development into new drug candidates.