Histamine decreases myogenic tone in rat cerebral arteries by H2-receptor-mediated KV channel activation, independent of endothelium and cyclic AMP

Long- and short-term effects of androgens in human umbilical artery smooth muscle

The aim of the present study was to determine the effects of androgens in the regulation of human umbilical artery (HUA) contractility. The short-term effects of testosterone on the tone of the HUA were investigated, as were the long-term effects of dihydrotestosterone (DHT) on the expression of some proteins involved in the contractile process. Endothelium-denuded HUA were treated for 24 h with DHT (2 μmol/L) or the vehicle control (ethanol) to analyse the genomic effects of androgens. Twenty-four hour treatment of HUA with DHT increased the mRNA expression of the β(1)-subunit of the large-conductance Ca(2+)-activated (BK(Ca)) channel and decreased expression of the α-subunit of L-type calcium channels. In organ bath studies, testosterone (1-100 μmol/L) produced similar relaxant responses in DHT- and vehicle-treated HUA rings precontracted with 5-HT, histamine and KCl. However, the relaxation response obtained by the combined application of testosterone (100 μmol/L) and nifedipine (10 μmol/L) was significantly greater in DHT- compared with vehicle-treated HUA. The results indicate that the rapid vasorelaxant effects of testosterone that are dependent on both BK(Ca) and voltage-sensitive potassium (K(V)) channel activity in control arteries become dependent solely on K(V) channel activity in DHT-treated HUA. Thus, the present study reveals the importance of the investigation of both the short- and long-term effects of androgens in human arteries.

Impaired mitochondria-dependent vasodilation in cerebral arteries of Zucker obese rats with insulin resistance

Mitochondria affect cerebrovascular tone by activation of mitochondrial ATP-sensitive K+ (K ATP) channels and generation of reactive oxygen species (ROS). Insulin resistance accompanying obesity causes mitochondrial dysfunction, but the consequences on the cerebral circulation have not been fully identified. We evaluated the mitochondrial effects of diazoxide, a putative mitochondrial K ATP channel activator, on cerebral arteries of Zucker obese (ZO) rats with insulin resistance and lean (ZL) controls. Diameter measurements showed diminished diazoxide-induced vasodilation in ZO compared with ZL rats. Maximal relaxation was 38 +/- 3% in ZL vs. 21 +/- 4% in ZO rats (P < 0.05). Iberiotoxin, a Ca2+-activated K+ channel inhibitor, or manganese(III) tetrakis(4-benzoic acid)porphyrin chloride, an SOD mimetic, or endothelial denudation diminished vasodilation to diazoxide, implicating Ca2+-activated K+ channels, ROS, and endothelial factors in vasodilation. Inhibition of nitric oxide synthase (NOS) in ZL rats diminished diazoxide-induced vasodilation in intact arteries, but vasodilation was unaffected in endothelium-denuded arteries. In contrast, NOS inhibition in ZO rats enhanced vasodilation in endothelium-denuded arteries, but intact arteries were unaffected, suggesting that activity of endothelial NOS was abolished, whereas factors derived from nonendothelial NOS promoted vasoconstriction. Fluorescence microscopy showed decreased mitochondrial depolarization, ROS production, and nitric oxide generation in response to diazoxide in ZO arteries. Protein and mRNA measurements revealed increased expression of endothelial NOS and SODs in ZO arteries. Thus, cerebrovascular dilation to mitochondria-derived factors involves integration of endothelial and smooth muscle mechanisms. Furthermore, mitochondria-mediated vasodilation was diminished in ZO rats due to impaired mitochondrial K(ATP) channel activation, diminished mitochondrial ROS generation, increased ROS scavenging, and abnormal NOS activity.

Role of K+ and Ca2+ fluxes in the cerebroarterial vasoactive effects of sildenafil

The aim of this study was to assess the role of K(+) and Ca(2+) fluxes in the cerebroarterial vasoactive effects of the phosphodiesterase-5 inhibitor sildenafil. We used isolated rabbit basilar arteries to assess the effects of extracellular K(+) raising on sildenafil-induced vasodilatation, and studied the pharmacological interaction of sildenafil with selective modulators of membrane K(+) and Ca(2+) channels. Expression of Kv1 subunits of K(+) channels was assessed at messenger and protein levels. Parallel experiments were carried out with zaprinast for comparison. Sildenafil (10 nM-0.1 mM) induced concentration-dependent relaxation of endothelin-1 (10 nM)-precontracted arteries, which was partially inhibited by depolarization with KCl (50 mM), 3 mM tetraethylammonium (non-selective K(+) channel blocker) or 1 mM aminopyridine (inhibitor of K(v) channels), but not by 1 microM glibenclamide (inhibitor of K(ATP) channels) or 50 nM iberiotoxin (inhibitor of K(Ca) channels). Arterial smooth muscle expressed messengers for Kv1.2, Kv1.3, Kv1.4, Kv1.5 and Kv1.6, and proteins of Kv1.1, Kv1.2 and Kv1.4. CaCl(2) (10 microM- 10 mM) induced concentration-dependent contraction in Ca(2+)-free, depolarizing (50 mM KCl) medium. Sildenafil (0.1-100 microM) produced reversible concentration-dependent inhibition of the response to CaCl(2), which was completely abolished by the highest sildenafil concentration. By contrast, only 100 microM zaprinast inhibited the response to CaCl(2). The L-type Ca(2+) channel activator Bay K 8644 (0.1 nM-1 microM) induced concentration-dependent potentiation of the response to CaCl(2) inhibited by 100 microM sildenafil. Moreover, Bay K 8644 (0.1 nM-1 microM) induced concentration-dependent contraction in slightly depolarizing (15 mM) medium, which was inhibited to the same extent and in a concentration-dependent way by sildenafil (0.1-100 microM) and zaprinast (1 or 100 microM). These results show that sildenafil relaxes the rabbit basilar artery by increasing K(+) efflux through K(v) channels, which in turn may affect Ca(2+) signalling. Expression of Kv1 subunits involved in this pharmacological effect occurs at the messenger and, in some cases, at the protein level. In addition to this phosphodiesterase-5-related effect, sildenafil and zaprinast inhibit cerebroarterial vasoconstriction at least in part by directly blocking L-type Ca(2+) channels, although a decrease in the sensitivity of the contractile apparatus to Ca(2+) can not be discarded.

Myogenic tone and reactivity of cerebral arteries in type II diabetic BBZDR/Wor rat

BBZDR/Wor rat is a new model of type II diabetes with spontaneous obesity and clinical characteristics close to human diabetes. In this study the time-course of cerebroarterial dysfunction was characterized. Posterior cerebral arteries from BBZDR/Wor rats and their age-matched lean controls were pressurized to 70 mm Hg in an arteriograph. Effects of intraluminal pressure and different pharmacological agents on myogenic tone were evaluated. Pressure-myogenic tone curves in diabetic arteries were similar to that in non-diabetic arteries at pre-diabetic age, showed leftward shift at 4 weeks and were significantly different with higher myogenic tone at 5 and 8 months of diabetes. Age-dependent decrease in myogenic tone was observed in non-diabetic arteries. Dilation to histamine was similar to that in non-diabetic arteries at pre-diabetic and at 4 weeks but significantly reduced at 5 and 8 months of diabetes. Bradykinin-mediated dilation was significantly reduced in early and chronic diabetes, whereas (+/-)-S-nitroso-N-acetylpenicillamine (SNAP)-mediated dilation was decreased modestly at 8 months of diabetes. Sensitivity and constriction to 5-hydroxytryptamine were increased in early and chronic diabetes. Responses to bradykinin and 5-hydroxytryptamine were decreased and increased, respectively. Myogenic tone was significantly less sensitive to (lower pIC(50)) U-73122 than normal arteries at 4 weeks and 8 months of diabetes suggesting an increased activation of phospholipase C (PLC). This study shows that pressure-mediated autoregulation of cerebral arteries in type II diabetes operates at higher resistance. Endothelium-dependent dilation was decreased with chronic diabetes with increased sensitivity to constrictor agonist. Endothelium-independent dilation was modestly affected. Arterial hyper-reactivity to pressure and constrictor agonist were likely due to increased PLC activation.

Functional serotonin and histamine receptor subtypes in porcine ciliary artery in comparison with middle cerebral artery

Functional serotonin (5-HT) and histamine receptor subtypes were investigated in porcine middle cerebral and ciliary arteries. An H(1) antagonist, mepyramine, antagonized histamine-induced responses with pK(B) values of 8.91-9.10. In the presence of 1 muM mepyramine, however, histamine caused dilation through H(2) receptors in the middle cerebral but not in the ciliary artery. A 5-HT(2A) antagonist, ketanserin, antagonized 5-HT-induced responses, causing rightward shifts in the concentration-response curves with pK(B) values of 8.52-8.71. A 5-HT(1B) antagonist, SB224289, produced rightward shifts of the concentration-response curves to sumatriptan with pK(B) values (6.66) only in the middle cerebral artery. In contrast, a 5-HT(1D) antagonist, BRL15572, had no effect in either artery. An RT-PCR study demonstrated the gene expression of the mRNAs of all three receptors (5HT(1B), 5HT(1D) and 5HT(2A)) in both arteries. These results suggest that histamine-induced contraction is mediated only through functional H(1) receptor in these arteries. Interestingly, there are functional 5-HT(2A) and 5-HT(1B) receptor subtypes in the middle cerebral artery, whereas the only functional receptor is 5-HT(2A) in the ciliary artery. The difference may be important for treatment with 5-HT(1B/1D) agonists (e.g. for migraine) without ocular side effect.