Involvement of Endothelial Nitric Oxide Synthase Activation in Midkine-Mediated Central Hypotensive Effects

The growth factor midkine (MK) has been implicated in various biologic and pathologic events. It has been shown that the peripheral influence of MK on cardiovascular regulation is due to an influence on the renin-angiotensin system (RAS). The nucleus tractus solitarii (NTS) is the primary integrative center for cardiovascular control and other autonomic functions in the central nervous system. However, the signaling mechanisms involved in MK-mediated cardiovascular effects in the NTS remain unclear. In this study, we investigated whether the RAS and/or N-methyl-D-aspartate (NMDA) receptor-calmodulin-endothelial nitric oxide synthase (eNOS) signaling pathways were both involved in MK-mediated blood pressure (BP) regulation in the NTS of Wistar-Kyoto (WKY) rats. Intra-NTS microinjection and immunoblot analysis were used to evaluate the signal pathway. WKY rats were anesthetized with urethane. Unilateral microinjection of MK (600 fmol) into the NTS produced a dose-dependent decrease in BP and heart rate (HR). The depressor effects were observed before and after microinjection of the angiotensin-converting enzyme (ACE) inhibitor lisinopril (2.4 fmol), or the angiotensin receptor blockers (ARB) inhibitor valsartan (7.5 pmol). However, lisinopril and valsartan did not diminish the MK-mediated cardiovascular effects in the NTS. Microinjection of the NMDA receptor antagonist MK801 (1 nmol) or the NOS inhibitor N-nitro l-arginine methyl ester (L-NAME), (33 nmol), into the NTS attenuated the MK-induced hypotensive effects. Pretreatment with an eNOS inhibitor N5-iminoethyl-l-ornithine (L-NIO) (6 nmol) attenuated the MK-induced hypotensive effects. In this study, the data showed that MK might play a role in central cardiovascular regulation in the NTS. These results suggest that MK decreased BP and HR in the NTS probably acting via the NMDA receptor-calmodulin-eNOS signaling pathway.