Effects of an ethanolic extract of mulberry fruit on blood pressure and vascular remodeling in spontaneous hypertensive rats.

Mulberry (Morus alba) has been used in traditional oriental medicine since ages. Recently, it has been reported that mulberry produces hypotensive effects through the eNOS signaling pathway. However, the mechanism underlying the hypotensive effects of mulberry is not entirely clear. Moreover, the effects of mulberry on vascular remodeling events such as hyperplasia, an important etiology in the pathogenesis of hypertension and arteriosclerosis, are also ambiguous. Here, we hypothesized that an ethanolic extract of mulberry fruit (EMF) has beneficial effects on vascular remodeling and produces hypotensive effects. The effects of a 6-week oral administration of EMF were examined in spontaneously hypertensive rats (SHRs). The animals were divided into four groups: normotensive control (Wistar Kyoto rats), non-treated SHR, low-dose (100 mg/kg) EMF-treated SHR, and high-dose (300 mg/kg) EMF-treated SHR. Our results showed that the EMF-diet normalizes hypertension in SHRs in a dose-dependent manner, by preventing smooth muscle proliferation, thickening of the tunica media, and vascular hyper-reactivity. The endothelial functions were not substantially affected by the EMF diet in our experimental setting. In conclusion, we suggest that the mulberry fruit could act as a food supplement for reducing blood pressure in hypertensive subjects through its effects on smooth muscle proliferation and vascular contractility.

Hydrogen peroxide constricts rat arteries by activating Na+-permeable and Ca2+-permeable cation channels.

Oxidative stress is associated with many cardiovascular diseases, such as hypertension and arteriosclerosis. Oxidative stress reportedly activates the L-type voltage-gated calcium channel (VDCCL) and elevates [Ca2+]i in many cells. However, how oxidative stress activates VDCCL under clinical setting and the consequence for arteries are unclear. Here, we examined the hypothesis that hydrogen peroxide (H2O2) regulates membrane potential (Em) by altering Na+ influx through cation channels, which consequently activates VDCCL to induce vasoconstriction in rat mesenteric arteries. To measure the tone of the endothelium-denuded arteries, a conventional isometric organ chamber was used. Membrane currents and Em were recorded by the patch-clamp technique. [Ca2+]i and [Na+]i were measured with microfluorometry using Fura2-AM and SBFI-AM, respectively. We found that H2O2 (10 and 100 µM) increased arterial contraction, and nifedipine blocked the effects of H2O2 on isometric contraction. H2O2 increased [Ca2+]i as well as [Na+]i, and depolarised Em. Gd3+ (1 µM) blocked all these H2O2-induced effects including Em depolarisation and increases in [Ca2+]i and [Na+]i. Although both nifedipine (30 nM) and low Na+ bath solution completely prevented the H2O2-induced increase in [Na+], they only partly inhibited the H2O2-induced effects on [Ca2+]i and Em. Taken together, the results suggested that H2O2 constricts rat arteries by causing Em depolarisation and VDCCL activation through activating Gd3+-and nifedipine-sensitive, Na+-permeable channels as well as Gd3+-sensitive Ca2+-permeable cation channels. We suggest that unidentified Na+-permeable cation channels as well as Ca2+-permeable cation channels may function as important mediators for oxidative stress-induced vascular dysfunction.

Caveolar remodeling is a critical mechanotransduction mechanism of the stretch-induced L-type Ca2+ channel activation in vascular myocytes.

Activation of L-type voltage-dependent Ca2+ channels (VDCCL) by membrane stretch contributes to many biological responses such as myogenic contraction of arteries. However, mechanism for the stretch-induced VDCCL activation is unclear. In this study, we examined the hypothesis that caveolar remodeling and its related signaling cascade contribute to the stretch-induced activation of VDCCL in rat mesenteric arterial smooth muscle cells. The VDCCL currents were recorded with nystatin-perforated or with conventional whole-cell patch-clamp technique. Hypotonic (~230 mOsm) swelling-induced membrane stretch reversibly increased the VDCCL currents. Electron microscope and confocal imaging analysis revealed that both hypotonic swelling and cholesterol depletion by methyl-ß-cychlodextrin (MßCD) similarly disrupted the caveolae structure and translocated caveolin-1 (Cav-1) from membrane to cytosolic space. Accordingly, MßCD also increased VDCCL currents. Moreover, subsequent hypotonic swelling after MßCD treatment failed to increase the VDCCL currents further. Western blotting experiments revealed that hypotonic swelling phosphorylated Cav-1 and JNK. Inhibitors of tyrosine kinases (genistein) and JNK (SP00125) prevented the swelling-induced facilitation of VDCCL currents. Knockdown of Cav-1 by small interfering RNA blocked both the VDCCL current facilitation by stretch and the related phosphorylation of JNK. Taken together, the results suggest that membrane stretch is transduced to the facilitation of VDCCL currents via caveolar structure-dependent tyrosine phosphorylation of Cav-1 and subsequent activation of JNK in rat mesenteric arterial myocytes.