Sphingolipids regulate [Mg2+]o uptake and [Mg2+]i content in vascular smooth muscle cells: potential mechanisms and importance to membrane transport of Mg2+

Sphingolipids have a variety of important signaling roles in mammalian cells. We tested the hypothesis that certain sphingolipids and neutral sphingomyelinase (N-SMase) can regulate intracellular free magnesium ions ([Mg2+]i) in vascular smooth muscle (VSM) cells. Herein, we show that several sphingolipids, including C2-ceramide, C8-ceramide, C16-ceramide, and sphingosine, as well as N-SMase, have potent and direct effects on content and mobilization of [Mg2+]i in primary cultured rat aortic smooth muscle cells. All of these sphingolipid molecules increase, rapidly, [Mg2+]i in these vascular cells in a concentration-dependent manner. The increments of [Mg2+]i, induced by these agents, are derived from influx of extracellular Mg2+ and are extracellular Ca2+ concentration-dependent. Phospholipase C and Ca2+/calmodulin/Ca2+-ATPase activity appear to be important in the sphingolipid-induced rises of [Mg2+]i. Activation of certain PKC isozymes may also be required for sphingolipid-induced rises in [Mg2+]i. These novel results suggest that sphingolipids may be homeostatic regulators of extracellular Mg2+ concentration influx (and transport) and [Mg2+]i content in vascular muscle cells.

Progesterone regulation of vascular thromboxane A2receptors in rhesus monkeys

We hypothesized that progesterone regulates thromboxane A2 receptor (TxA2R) density in primate vascular muscle and that TxA2R density correlates with coronary reactivity in vivo and in vitro. Reactivity to serotonin + U-46619 was determined by angiography in surgically postmenopausal [ovariectomized (Ovx)] rhesus monkeys without progesterone replacement and after 2-wk progesterone treatment (1–2 ng/ml). In untreated Ovx animals, 100 μmol/l serotonin + 1 μmol/l U-46619 (syringe concentrations) provoked vasospasm-like constrictions in six of six monkeys; zero of six progesterone-treated monkeys developed vasospasms. Sustained Ca2+ responses in vascular muscle cells isolated from Ovx coronaries (208 ± 63% of basal 20 min after stimulation) treated with serotonin + U-46619 contrasted with transient Ca2+ responses (143 ± 18% of basal and decreasing 5 min after stimulation) in progesterone-treated monkeys. The maximum density of [1 S-(1 I,2 J(5 Z),3 I(1 E,3 R*),4 I)]-7-[3-(3-hydroxy-4-(4′-[125I]iodophenoxy)- 1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-heptenoic acid ([125I]-BOP) binding was greater ( P< 0.01) in carotid arteries and aortic membranes from Ovx (109 ± 11 fmol/mg) compared with progesterone-treated (43 ± 15 fmol/mg) monkeys. TxA2R immunolabeling revealed greater coronary TxA2R labeling in Ovx compared with progesterone-treated monkeys. The results suggest that progesterone can decrease arterial TxA2R in Ovx monkeys.

ANG II potentiates mitogenic effect of norepinephrine in vascular muscle cells: role of FGF-2

We examined the possible cooperation between norepinephrine (NE) and ANG II on proliferation of cultured vascular smooth muscle cells (VSMCs) and the involved cellular mechanisms. Nanomolar NE concentrations stimulated VSMC proliferation through a prazosin-sensitive effect. The pretreatment of cells with 100 nM ANG II for 24 h significantly potentiated the NE-induced VSMC proliferation; this potentiating effect of ANG II was blocked by losartan but was unaffected by the AT2 receptor antagonist PD-123177. ANG II pretreatment also potentiated the increase in inositol phosphate turnover and upregulated the cell expression of fibroblast growth factor (FGF-2) induced by NE. Anti-FGF-2 neutralizing antibodies prevented the potentiating effect of ANG II on NE-induced cell growth. Both ANG II and NE stimulated extracellular signal-related kinase (ERK1) activation, but an ANG II potentiation of the effect of NE on ERK1 activity was not detectable. Moreover, ANG II significantly increased protein synthesis but did not potentiate the hypertrophic effect of NE. These findings demonstrate that ANG II and NE cooperate in promoting VSMC growth and that FGF-2 upregulation is involved in this effect.

Effects of Ca2+ channel activity on renal hemodynamics during acute attenuation of NO synthesis in the rat

In cultured vascular muscle cells, nitric oxide (NO) has been shown to inhibit voltage-dependent Ca2+ channels, which are involved in renal blood flow (RBF) autoregulation. Therefore, our purpose was to specify in vivo the effects of this interaction on RBF autoregulation. To do so, hemodynamics were investigated in anesthetized rats during Ca2+ channel blockade before or after acute NO synthesis inhibition. Rats were treated intravenously with vehicle ( n = 10), 0.3 mg/kg body wt N G-nitro-l-arginine-methyl ester (l-NAME; n = 7), 4.5 μg ⋅ kg body wt−1 ⋅ min−1 nifedipine ( n = 8) alone, or with nifedipine infused before ( n = 8), after ( n = 8), or coadministered with l-NAME ( n = 10). Baseline renal vascular resistance (RVR) averaged 14.0 ± 1.2 resistance units and did not change after vehicle. RVR increased or decreased significantly by 27 and 29% afterl-NAME or nifedipine, respectively. Nifedipine reversed, but did not prevent, RVR increase after or coadministered withl-NAME. RBF autoregulation was maintained afterl-NAME, but the autoregulatory pressure limit (PA) was significantly lowered by 15 mmHg. Nifedipine pretreatment or coadministration with l-NAME limited PA resetting or suppressed autoregulation at higher doses. Results were similar with verapamil. Intrarenal blockade of Ca2+-activated K+ channels also prevented autoregulatory resetting by l-NAME ( n = 8). These findings suggest NO inhibits voltage-dependent Ca2+channels and thereby modulates RBF autoregulatory efficiency.