Background and purpose: Bradykinin [BK-(1-9)] is an endogenous nonapeptide involved in multiple physiological and pathological processes. A long-held belief is that peptide fragments of BK-(1-9) are biologically inactive. Here, we have tested the biological activities of BK-(1-9) and two major peptide fragments in human and animal systems. Experimental Approach: Levels of BK peptides in male Wistar rat plasma were quantified by mass spectrometric methods. Nitric oxide was quantified in human, mouse and rat cells, and loaded with DAF-FM. We used aortic rings from adult male Wistar rats to test vascular reactivity. Changes in blood pressure and heart rate were measured in conscious adult male Wistar rats. Key results: Plasma levels of BK-(1-7) and BK-(1-5) in rats were increased following infusion of BK-(1-9). All tested peptides induced NO production in all cell types tested. However, unlike BK-(1-9), NO production elicited by BK-(1-7) or BK-(1-5) was not inhibited by B or B receptor antagonists. BK-(1-7) or BK-(1-5) also induced concentration-dependent vasorelaxation of aortic rings, without involving B or B receptors. In vivo, either intravenous or intra-arterial administration of BK-(1-7) or BK-(1-5) induced similar hypotension response. Conclusions and implications: BK-(1-7) and BK-(1-5) are endogenous peptides present in plasma. They are formed, at least partially, through the BK-(1-9) proteolysis. BK-related peptide fragments show biological activity, not mediated by B or B receptors. These BK-fragments could constitute new, active components of the kallikrein-kinin system.

Objective: Bradykinin [BK-(1–9)] is an endogenous peptide involved in many physiological and pathological processes, such as cardiovascular homeostasis and inflammation. The central dogma of the kallikrein-kinin system is that BK-(1–9) fragments are biologically inactive. In this work, we proposed to test whether these fragments were indeed inactive. Design and method: Nitric oxide (NO) was quantified in human, mouse and rat cells loaded with DAF-FM after stimulation with BK-(1–9), BK-(1–7), BK-(1–5) and BK-(1–3). We used adult male rat aortic ring preparation to test vascular reactivity mediated by BK-(1–9) fragments. Changes in blood pressure and heart rate was measured in conscious adult male rats by intraarterial catheter method. Results: BK-(1–9) induced NO production in all cell types tested by B2 receptor activation. BK-(1–7), BK-(1–5) and BK-(1–3) also induced NO production in all tested cell types but this response was independent of the activation of B1 receptor and/or B2 receptor. BK-(1–7), BK-(1–5) or BK-(1–3) induced only vasorelaxant effect and in a concentration-dependent fashion. Vasorelaxant effects for BK-(1–7), BK-(1–5) or BK-(1–3) were independent of the kinin receptors. BK-(1–9) induced in conscious adult male Wistar rats a dose-dependent hypotension while BK-(1–7), BK-(1–5) and BK-(1–3) induced a dose-independent hypotension. Importantly, these observations diverged from the BK-(1–9) results, highlighting that indeed the BK-(1–9) fragments do not seem to act via the classical kinin receptors. Conclusions: In conclusion, BK-(1–7), BK-(1–5) and BK-(1–3) are biologically active components of the kallikrein-kinin system. Importantly, observed pathophysiological outcomes of these peptides are independent of B1R and/or B2R activation.