| The tachykinin SP, NKA, NKB and HK-1 are a family of peptides characterized by the presence of a common C-terminal motif FXGLM-NH2. There are several species-divergent HK-1 peptides, which include rat/mouse hemokinin-1 (r/m HK-1), human hemokinin-1 (h HK-1) and human hemokinin-1 (4-11) (h HK-1 (4-11)). HK-1 exerts their effects through the interaction with three types of tachykinin receptors, termed NK1, NK2 and NK3 receptors. In general, the rank order in potencies of SP and r/m HK-1 in binding to the tachykinin NK1 receptor is as follows:SP=r/m HK-1. H HK-1 and h HK-1 (4-11) bind to tachykinin NK1 receptors with 14-and 70-fold reduced affinities, respectively, relative to SP and r/m HK-1. There are extensive researches performed about the actions of SP, NKA and NKB in cardiovascular responses in vivo and in isolated heart. Meanwhile, since the discovery of r/m HK-1, h HK-1 and h HK-1(4-11), many studies have focused on its biological actions including modulation of pain, immunological regulation, female reproductive function. However, little if anything is known about the effects of r/m HK-1, h HK-1 and h HK-1(4-11) on cardiovascular responses in vivo and cardiac functions on isolated heart. Thus, the aim of the present study were therefore to 1) investigate the effect and mechanism of action of r/m HK-1, h HK-1 and h HK-1 (4-11) in the modulation of cardiovascular activity in anesthetized rats; 2) examine the pharmacologic actions of r/m HK-1, h HK-1 and h HK-1 (4-11) in the isolated guinea pig hearts without the influence of other in vivo organs, nerves and other peripheral factors. Moreover, TAC 4 which encodes r/m HK-1 is expressed in mouse gastrointestinal tract. It has been demonstrated that tachykinin NK1, NK2 and NK3 receptors were present in mouse colon. Based on the knowledge of tachykinin NK receptors distribution and the lack of colonic motility studies of r/m HK-1 in mouse, the present study in part 3 was designed to investigate the effect and mechanism of action of r/m HK-1 in the modulation of colonic motility in mouse by comparing it with that of SP.1. Our data showed that intravenously (i.v.) injection of r/m HK-1 (0.1,0.3,1,3, 10 and 30 nmol/kg) lowered mean arterial pressure (MAP) dose-dependently. This effect was significantly blocked by pretreatment with SR140333 (a selective tachykinin NK1 receptor antagonist) and the NO synthase inhibitor L-NAME, respectively, but was not affected by bilateral vagotomy or the muscarinic receptor blocker atropine, which indicated that the depressor response induced by r/m HK-1 is mediated by tachykinin NK1 receptors and endothelium-derived relaxing factor (NO). R/m HK-1 injected i.v. also produced a dose-dependent tachycardia response, which is mediated by the activation of tachykinin NK1 receptors to stimulate sympathetic ganglia and to release catecholamines from adrenal medulla. Lower doses of h HK-1 (0.1-3 nmol/kg) injected i.v. induced depressor response, whereas higher doses (10 and 30 nmol/kg) caused biphasic (depressor and pressor) responses. The depressor response is primarily due to the action on endothelial tachykinin NK1 receptor to release endothelium-derived relaxing factor (NO) and vagal reflex are absent in this modulation. The pressor response is mediated through the activation of tachykinin NK1 receptor to release catecholamines from sympathetic ganglia and adrenal medulla. Moreover, h HK-1 injected i.v. produced a dose-dependent tachycardia response along with blood pressure responses and the activation of sympathetic ganglia and adrenal medulla are involved in the tachycardia response. H HK-1(4-11) only lowered MAP dose-dependently (0.1-30 nmol/kg) and the mechanisms involved in the depressor response is similar to that of h HK-1. Additionally, h HK-1 (4-11) could also produce tachycardia response dose-dependently and the mechanisms involved in the tachycardia response are similar to that of h HK-1 except that bilateral adrenalectomy could not affect the tachycardia markedly, indicating that the tachycardia induced by h HK-1 (4-11) is primarily due to the stimulation of sympathetic ganglia. In a word, to a certain extent, h HK-1 (4-11) is the active fragment of h HK-1, however, the differences between h HK-1 and HK-1(4-11) involved in the effects of cardiovascular system suggest that the divergent amino acid residues at the N-terminus of h HK-1 produced different activation properties for tachykinin NK1 receptor.2. In the present study, the coronary vascular activities and cardiac functions of r/m HK-1, h HK-1 and h HK-1 (4-11) were investigated in isolated, spontaneously beating guinea pig hearts. Bolus injection of r/m HK-1 caused decrease in perfusion pressure indicative of coronary vasodilation, which was primarily due to the action on tachykinin NK1 receptors on vascular endothelial cells, causing the release of nitric oxide that relaxed the coronary vessels. H HK-1 caused biphasic perfusion pressure changes that were coronary vasodilation followed by vasoconstriction. The mechanism of the vasodilation was similar to that of r/m HK-1 while the coronary vasoconstriction was mediated through the activation of tachykinin NK2 receptors on coronary sympathetic neurons to release catecholamines. H HK-1 (4-11) only produced coronary vasoconstriction and the mechanism involved in this effect was similar to that of h HK-1 in vasoconstriction. Moreover, r/m HK-1 and h HK-1 produced similar decreases in heart rate indicative of negative chronotropic action, which were mainly mediated through the activation of tachykinin NK1 receptors to release ACh acting on muscarinic receptors. H HK-1(4-11) also produced negative chronotropic response, which was mainly mediated through tachykinin NK2 receptors and muscarinic receptors. Our present results provide evidence that all of these tachykinins could influence cardiac function and coronary vascular activity in the heart. Our novel findings should facilitate the analysis of the role of the tachykinins encoded by the newly identified TAC4 gene in pathophysiological conditions.3. R/m HK-1 induced substantial contractions on the circular muscle of mouse colon. The maximal contractile responses to r/m HK-1 varied significantly among proximal-, mid-and distal-colon, suggesting that the action of r/m HK-1 was region-specific in mouse colon. The contractile response induced by r/m HK-1 is primarily via activation of tachykinin NK1 receptors leading to activation of cholinergic excitatory pathways and with a minor contribution of NK2 receptors, which may be on the smooth muscle itself. A direct action on colonic smooth muscles may be also involved. In contrast, SP induced biphasic colonic responses (contractile and relaxant responses) on the circular muscle, in which the contractile action of SP was equieffective with r/m HK-1. SP exerted its contractile effect predominantly through neural and muscular tachykinin NK1 receptors, but unlike r/m HK-1 did not appear to act via NK2 receptors. The relaxation induced by SP was largely due to release of nitric oxide (NO) produced via an action on neural NK1 receptors. These results indicate that the receptors and the activation properties involved in r/m HK-1-induced mouse colonic contractile activity are different from those of SP. Therefore, our present results should facilitate the analysis of the role of r/m HK-1 and SP in gastrointestinal disease and may open novel possibilities for pharmacological interventions.
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