Evidence Showing Bombesin-Like Peptides Contract Guinea Pig Vas Deferens Smooth Muscle

In this study, we investigated (1) the functional role of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in the regulation of guinea pig vas deferens smooth muscle (VDSM) contractions and (2) the potential contractile effects of 33 physiologically active substances and related chemicals that have not been previously reported to contract VDSM. Iberiotoxin (an inhibitor of BK_Ca channels, 10^-7 M) was the most potent enhancer of both noradrenaline (10^-5 M)- and ATP (10^-6 M)-induced contractions among the 6 types of K⁺ channel inhibitors. In addition, BK_Ca channel mRNA expression was the highest among the 32 types of K⁺ channel mRNAs. Iberiotoxin also enhanced the contractions induced by acetylcholine (10^-6 M), histamine (5 × 10^-5 M), bradykinin (10^-6 M), neurokinin A (10^-6 M), neurokinin B (10^-6 M), and substance P (10^-6 M). In the 33 tested physiologically active substances and related chemicals (15 peptides, 5 amino acids and their derivatives, 11 prostanoid- and isoprostane-related drugs, 1 endocannabinoid, and 1 phospholipid), we found that 3 bombesin-like peptides, neuromedin B (NMB) (10^-6 M), gastrin-releasing peptide (GRP, 10^-8 M), and NMC (10^-6 M), contracted VDSM in the absence of iberiotoxin, and these contractions were strongly enhanced in the presence of iberiotoxin. Among the 3 bombesin receptor subtypes, the mRNA expression level of Grpr (BB₂ receptor) was the highest. These findings suggest that (1) BK_Ca channels are the most powerful negative regulator of VDSM contractility and (2) NMB, GRP, and NMC are physiologically active substances that contract VDSM.