Evidence for involvement of both IKCa and SKCa channels in hyperpolarizing responses of the rat middle cerebral artery

Endothelium-derived hyperpolarizing factor responses in the rat middle cerebral artery are blocked by inhibiting IKCa channels alone, contrasting with peripheral vessels where block of both IKCa and SKCa is required. As the contribution of IKCa and SKCa to endothelium-dependent hyperpolarization differs in peripheral arteries, depending on the level of arterial constriction, we investigated the possibility that SKCa might contribute to equivalent hyperpolarization in cerebral arteries under certain conditions. METHODS: Rat middle cerebral arteries (approximately 175 microm) were mounted in a wire myograph. The effect of KCa channel blockers on endothelium-dependent responses to the protease-activated receptor 2 agonist, SLIGRL (20 micromol/L), were then assessed as simultaneous changes in tension and membrane potential. These data were correlated with the distribution of arterial KCa channels revealed with immunohistochemistry. RESULTS: SLIGRL hyperpolarized and relaxed cerebral arteries undergoing variable levels of stretch-induced tone. The relaxation was unaffected by specific inhibitors of IKCa (TRAM-34, 1 micromol/L) or SKCa (apamin, 50 nmol/L) alone or in combination. In contrast, the associated smooth-muscle hyperpolarization was inhibited, but only with these blockers in combination. Blocking nitric oxide synthase (NOS) or guanylyl cyclase evoked smooth-muscle depolarization and constriction, with both hyperpolarization and relaxation to SLIGRL being abolished by TRAM-34 alone, whereas apamin had no effect. Immunolabeling showed SKCa and IKCa within the endothelium. CONCLUSIONS: In the absence of NO, IKCa underpins endothelium-dependent hyperpolarization and relaxation in cerebral arteries. However, when NOS is active SKCa contributes to hyperpolarization, whatever the extent of background contraction. These changes may have relevance in vascular disease states where NO release is compromised and when the levels of SKCa expression may be altered

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses

Localization of relaxin receptors in arteries and veins, and region-specific increases in compliance and bradykinin-mediated relaxation after in vivo serelaxin treatment

Relaxin is a potent vasodilator of small resistance arteries and modifies arterial compliance in some systemic vascular beds, yet receptors for relaxin, such as RXFP1, have only been localized to vascular smooth muscle. This study first aimed to localize RXFP1 in rat arteries and veins from different organ beds and determine whether receptors are present in endothelial cells. We then tested the hypothesis that region-specific vascular effects of relaxin may be influenced by the cellular localization of RXFP1 within different blood vessels. The aorta, vena cava, mesenteric artery, and vein had significantly higher (P<0.05) RXFP1 immunostaining in endothelial cells compared with vascular smooth muscle, whereas the femoral artery and vein and small pulmonary arteries had higher (P<0.01) RXFP1 immunostaining in the vascular smooth muscle. Male rats were treated subcutaneously with recombinant human relaxin-2 (serelaxin; 4 μg/h) for 5 d; vasodilation and compliance in mesenteric and femoral arteries and veins were compared with placebo controls. Serelaxin significantly (P=0.04) reduced wall stiffness and increased volume compliance in mesenteric arteries but not in the other vessels examined. This was associated with changes in geometrical properties, and not compositional changes in the extracellular matrix. Serelaxin treatment had no effect on acetylcholine-mediated relaxation but significantly (P<0.001) enhanced bradykinin (BK)-mediated relaxation in mesenteric arteries, involving enhanced nitric oxide but not endothelium-derived hyperpolarization or vasodilatory prostanoids. In conclusion, there is differential distribution of RXFP1 on endothelial and smooth muscle across the vasculature. In rats, mesenteric arteries exhibit the greatest functional response to chronic serelaxin treatment.Maria Jelinic, Chen-Huei Leo, Emiel D. Post Uiterweer, Shaun L. Sandow, Jonathan H. Gooi, Mary E. Wlodek, Kirk P. Conrad, Helena Parkington, Marianne Tare, and Laura J. Parr