Vascular actions of 17β-oestradiol in rat aorta and mesenteric artery

1 It has been proposed that the cardiovascular protective actions of 17β-oestradiol may involve calcium antagonistic actions. We have examined the effects of 17β-oestradiol on contractions to noradrenaline and KCl in male rat small mesenteric artery and aorta. 2 In rat mesenteric artery, 17β-oestradiol (10 μm ) significantly reduced the maximum contraction to noradrenaline (67.7 ± 5.8% of control) and KCl (38.8 ± 3.1% of control) without affecting potency. 3 In rat aorta, 17β-oestradiol (10 μm ) also significantly reduced contractions to noradrenaline (77.5 ± 4.8% of control), and the effects were mimicked by droloxifene (10 μm ). The effect of oestrogen was not prevented by the protein synthesis inhibitor cycloheximide (10 μm ). In experiments carried out in calcium-free solution in which calcium stores were depleted, 17β-oestradiol (10 μm ) significantly reduced the contraction to calcium restoration in rat aorta. 4 In aorta from female rats, 17β-oestradiol (10 μm ) significantly reduced contractions to noradrenaline (73.6 ± 10.8% of control), but this effect of oestrogen was not prevented by cycloheximide (10 μm ). 5 In summary, 17β-oestradiol diminishes the maximum contractile response to noradrenaline in both rat small mesenteric artery and aorta, an effect which at least in the aorta is mimicked by the oestrogen receptor antagonist/partial agonist droloxifene, and may be due to restriction of calcium entry by a nongenomic action.     

Vasodilator actions of abnormal-cannabidiol in rat isolated small mesenteric artery

1. The nonpsychoactive cannabinoid abnormal-cannabidiol (trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) (abn-cbd) produced concentration-dependent relaxation of methoxamine-precontracted rat small mesenteric artery. Endothelial removal reduced abn-cbd potency six-fold without affecting the maximum relaxation. 2. In endothelium-intact vessels, abn-cbd was less potent under 60 mM KCl-induced tone and inhibited by combination of L-N(G)-nitroarginine methyl ester (L-NAME) (nitric oxide synthase inhibitor; 300 micro M), apamin (small conductance Ca(2+)-activated K(+) channels inhibitor; 50 nM) and charybdotoxin (inhibitor of intermediate conductance Ca(2+)-activated K(+) channels and large conductance Ca(2+)-activated K(+) channels BK(Ca); 50 nM). L-NAME alone or in combination with either toxin alone had little effect. 3. In intact vessels, relaxations to abn-cbd were inhibited by SR 141716A (cannabinoid receptor antagonist; 1 or 3 micro M). Concomitant addition of L-NAME, apamin and charybdotoxin had no further effect. Other cannabinoid receptor antagonists either had little (SR 144528; 1 micro M and AM 251; 1 micro M) or no effect (AM 630; 10 micro M and AM 281; 1 micro M). Inhibition of gap junctions, G(i/o) protein coupling and protein kinase A also had no effect. 4. Endothelium-independent relaxation to abn-cbd was unaffected by L-NAME, apamin plus charybdotoxin or capsaicin (10 micro M). Abn-cbd inhibited CaCl(2)-induced contractions in vessels with depleted intracellular Ca(2+) stores and stimulated with methoxamine or KCl. This was insensitive to SR 141716A (3 micro M) but greatly reduced in vessels stimulated with ionomycin (Ca(2+) ionophore; 1 micro M). 5. We conclude that abn-cbd relaxes the rat small mesenteric artery by endothelium-dependent activation of K(+) channels via SR 141716A-sensitive pathways, which do not involve CB(1) and CB(2) receptors. It also causes endothelium-independent, SR 141716A-insensitive, relaxation by inhibiting Ca(2+) entry through voltage-gated Ca(2+) channels.     

Vascular relaxation by ifenprodil in the isolated perfused rat mesenteric artery

The possible mechanism of vascular relaxation by ifenprodil has been examined in the isolated perfused rat mesenteric artery preparation. Ifenprodil (5.0 X 10(-9)-5.0 X 10(-8) M) noncompetitively blocked noradrenaline- and cirazoline-induced vasoconstrictor responses. This concentration range did not affect responses to 5-hydroxytryptamine and potassium chloride. However, in concentrations above 10(-6) M, ifenprodil caused (a) reversal of constrictor responses to excess potassium given either by bolus injections (3.4 X 10(-2)-5.4 X 10(-1) M) or by continuous perfusion (55 mM) and (b) inhibition of Ca2+-induced constrictor effects in arteries perfused with Ca2+-free depolarizing Krebs solution. It is concluded that ifenprodil blocks Ca2+ mobilization through both the ion channels coupled to alpha-adrenoceptors and the potential-sensitive channels.     

Mechanisms of vasorelaxation to 17beta-oestradiol in rat arteries

We have investigated the involvement of the endothelium, K+ channels, oestradiol receptors, and Ca2+ influx in 17beta-oestradiol-induced vasorelaxation in rat mesenteric arterial beds and aortae. 17beta-Oestradiol (10 pM-1 mM) caused acute vasorelaxations in mesenteric arterial beds and aortae from male and female rats. In male rat mesenteric vessels and aortae, the vasorelaxations were mostly independent of the endothelium and nitric oxide (NO). However, indomethacin (10 microM) enhanced the relaxant responses to 17beta-oestradiol. In male rat mesenteric beds, 60 mM KCl, tetrabutylammonium chloride (300 microM), 4-aminopyridine (1 mM), and barium chloride (30 microM), charybdotoxin (100 nM), but not glibenclamide (10 microM) and tamoxifen (10 microM), inhibited vasorelaxation to 17beta-oestradiol. In male rat aortae, 60 mM KCl did not affect vasorelaxation to 17beta-oestradiol. However, in the presence of indomethacin, vasorelaxation to 17beta-oestradiol was enhanced but this was sensitive to 60 mM KCl. Pre-treatment with 17beta-oestradiol (100 microM) inhibited CaCl2-induced contraction. The present findings indicate that, in rat mesenteric beds and aortae, 17beta-oestradiol causes acute and potent vasorelaxation which may be enhanced in the presence of a cyclooxygenase inhibitor. In mesenteric arterial bed, 17beta-oestradiol-induced vasorelaxation occurs primarily via activation of K+ channels. In the aorta, vasorelaxations involved activation of K+ efflux when the cyclooxygenase pathway was inhibited, and also inhibition of Ca2+ influx.     

Effects of portal hypertension on responsiveness of rat mesenteric artery and aorta.

1. We have examined the effects of pre-hepatic portal hypertension on the responsiveness of rat small mesenteric arteries and aorta. Rats were made portal hypertensive by creating a calibrated portal vein stenosis, or sham-operated. 2. In rat mesenteric arteries, there was no significant difference between portal hypertensive and sham-operated animals in the contractile potency of noradrenaline (NA), but the maximum contractile responses to NA, U46619 and KCl were significantly increased in vessels from portal hypertensive animals. This altered maximum contractile response was not due to alterations in smooth muscle mass. 3. In rat mesenteric arteries, there were no significant differences between portal hypertensive and sham-operated animals in endothelium-dependent relaxations to acetylcholine (ACh). The difference between portal hypertensive and sham-operated rats in the maximum response to U46619 was maintained following a combination of methylene blue (1 microM) and NG-monomethyl-L-arginine (100 microM), suggesting that any differences in endothelial function do not explain differences in the response to vasoconstrictors. 4. In rat aorta, there were no significant differences between portal hypertensive and sham-operated animals in the contractile response to NA or KCl or in the endothelium-dependent relaxations to ACh. 5. In pithed rats, there was no difference between portal hypertensive and sham-operated animals in the pressor potency of NA. 6. It is concluded that portal hypertension produces an increase in the contractile response to the vasoconstrictors NA, U46619 and KCl in rat mesenteric arteries but not in the aorta. This suggests that the diminished responsiveness to vasoconstrictors reported in portal hypertensive rats in vivo is not due to a diminished responsiveness at the level of the vascular smooth muscle.     

The actions of some cannabinoid receptor ligands in the rat isolated mesenteric artery

1. The actions of a number of cannabinoid receptor ligands were investigated using the myograph-mounted rat isolated mesenteric artery. Anandamide, CP 55,940, HU-210, palmitoylethanolamide and WIN 55,212-2 all caused concentration-dependent relaxations of methoxamine-precontracted vessels which were not affected by removal of the endothelium.
2. Precontracting vessels with 60 mM KCl instead of methoxamine greatly reduced the vasorelaxant effects of anandamide and palmitoylethanolamide. High K⁺ solution caused a modest decrease in the relaxant potency of CP 55,940 and HU-210, and had no effect on relaxations induced by WIN 55,212-2.
3. Relaxations of methoxamine-induced tone by anandamide, CP 55,940 and HU-210, but not palmitoylethanolamide and WIN 55,212-2, were attenuated by the cannabinoid receptor antagonist, SR 141716A. Relaxation of vessels contracted with 60 mM KCl by CP 55,940 was also sensitive to SR 141716A.
4. Anandamide and CP 55,940 caused small but concentration-dependent contractions in resting vessels in the absence of extracellular calcium. These were not sensitive to SR 141716A. Palmitoylethanolamide and WIN 55,212-2 produced smaller contractions only at higher concentrations.
5. Anandamide and CP 55,940, but not palmitoylethanolamide and WIN 55,212-2, caused concentration-dependent inhibition of the phasic contractions induced by methoxamine in calcium-free conditions, but only anandamide caused inhibition of contractions to caffeine under such conditions. These inhibitory effects were not antagonised by SR 141716A.
6. The present study provides the first detailed investigation of the actions of cannabinoid agonists on vascular smooth muscle. Our results show that these compounds exert both receptor-dependent and -independent effects on agonist-induced calcium mobilization in the rat isolated mesenteric artery.

     

Effects of the oestrous cycle and gender on acute vasodilatory responses of isolated pressurized rat mesenteric arteries to 17 beta-oestradiol

The influence of the oestrous cycle and gender on responses of isolated pressurized mesenteric arteries to acute 17 beta-oestradiol was investigated. All vessels, pre-contracted with 60 mM KCl or 10 microM U46619 (9,11 dideoxy-11alpha, 9alpha-epoxy methano-prostaglandin), exhibited concentration-dependent vasodilatory responses to 17 beta-oestradiol (3 - 30 microM). The largest responses were seen in vessels from female rats in pro-oestrous (38.9+/-5.4% U46619 max and 63.1+/-4.0% KCl max for 30 microM oestradiol), the smallest from animals in di-oestrous (20.1+/-3.7% U46619 and 50.1+/-4.5% KCL - both P:<0.05 cf pro-oestrous (all n=8)). Responses of vessels from male rats were similar to those from pro-oestrous rats (41.5+/-9.1% U46619 (n=10) and 54.9+/-2.9% KCl (n =8)). All responsees were unaffected by inhibition of nitric oxide synthase (NOS). Female rats in pro-oestrous had the highest plasma concentrations of 17 beta-oestradiol and testosterone (40.76+/-4.73 pg ml(-1) and 0.29+/-0.05 ng ml(-1) respectively (n=8)) while those in di-oestrous had the lowest (15.24+/-3.94 pg ml(-1) for oestradiol and 0.08+/-0.03 ng ml(-1) for testosterone (n=8)). In male rats the concentration of oestrogen was 10.29+/-1.21 pg ml(-1) (n=7) while that of testosterone was 3.15+/-0.36 ng ml(-1) (n=7). Incubation of arteries isolated from male rats and from female rats in pro-oestrous and di-oestrous with testosterone (1 microM, 3 h) significantly enhanced the subsequent vasodilatory responses to acute 17 beta-oestradiol. Following incubation, the responses to 17 beta-oestradiol were similar in all groups. These observations suggest that gender and the oestrous cycle may influence the vascular responses to acute 17 beta-oestradiol administration.     

Endothelium-independent relaxation of rabbit coronary artery by 17 beta-oestradiol in vitro.

1. We assessed the relaxant effect of 17 beta-oestradiol (10(-7), 10(-6) and 10(-5) M) on rabbit isolated coronary arteries precontracted with prostaglandin F2 alpha (3 x 10(-6) M), high extracellular potassium (30 mM) and Bay K 8644 (10(-6) M) plus high extracellular potassium (15 mM) by measuring isometric tension. 17 beta-Oestradiol (10(-6) and 10(-5) M) induced significant relaxation in coronary arteries from male and female rabbits. No differences were seen between arteries with or without endothelium. There were also no differences between coronary arteries isolated from male and female rabbits. 2. Inhibitors of endothelium-derived relaxing factor and vasodilator prostanoids, namely, reduced haemoglobin, N omega-nitro-L-arginine methyl ester and indomethacin, did not affect the relaxation induced by 17 beta-oestradiol in endothelium-intact coronary arteries. 3. Methylene blue, an inhibitor of guanylate cyclase, did not affect the coronary artery relaxation induced by 17 beta-oestradiol. 4. The calcium concentration-dependent contraction curve in potassium-depolarization medium was shifted to the right by 17 beta-oestradiol (10(-6) and 10(-5) M) in the rabbit coronary artery and rat aorta. The -log EC50s of calcium in control and after incubation with 17 beta-oestradiol (10(-6) and 10(-5) M) were 3.7 +/- 0.09, 3.1 +/- 0.10 and 2.8 +/- 0.08 respectively in rabbit coronary arteries and 3.8 +/- 0.11, 3.3 +/- 0.14 and 2.9 +/- 0.15 in rat aorta. 5. The results indicate that 17 beta-oestradiol induces rabbit coronary artery relaxation by an endothelium-independent mechanism in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of 17 beta-oestradiol induced vasodilatation in isolated pressurized rat small arteries

1. The influence of 17 beta-oestradiol on pressurized isolated rat mesenteric and coronary small arteries was investigated. 2. 17 beta-oestradiol caused rapid (t1.0 < 5 mins) concentration-dependent relaxations of pre-contracted pressurized (50 mmHg) isolated rat mesenteric and coronary arteries. Similar responses were observed in both vessel types. Significant relaxations were only observed at concentrations exceeding 3 microM. 3. The vasodilatory responses in both types of artery were unaffected by 10 microM L-nitro arginine (L-NNA) alone or in the presence of 10 microM indomethacin, inhibitors of nitric oxide and prostaglandin synthesis respectively. They were also unaffected by the pre-contracting agent used i.e. high K+ or U46619 (a thromboxane analogue). 4. Neither the oestrogen receptor antagonist ICI 182,780 (10 microM) nor the protein synthesis inhibitor cycloheximide (100 microM) had any effect on the responses of mesenteric arteries to 17 beta-oestradiol. 5. 17 alpha-oestradiol had only a minor effect on mesenteric arterial diameter over a concentration range similar to the effective vasodilatory range for 17 beta-oestradiol. 6. Membrane impermeant 17 beta-oestradiol conjugated to bovine serum albumin (beta-oestradiol-17-hemisuccinate-BSA) (E-H-BSA) resulted in a vasodilatation of pressurized arteries. 7. Wortmannin, an inhibitor of myosin light chain kinase, near maximally relaxed pressurized mesenteric arteries although the time course for the response was significantly slower than that for 17 beta-oestradiol. 8. These results taken together suggest that the acute effects of 17 beta-oestradiol on isolated pressurized arterial tone may be due to effects directly on the vascular smooth muscle via non-genomic mechanisms that involve a stereospecific interaction at the plasma membrane.     

Vascular actions of thrombin receptor-derived polypeptides: structure-activity profiles for contractile and relaxant effects in rat aorta.

1. Using endothelium-denuded and intact rat aortic rings, we have determined the contractile and relaxant structure-activity profile for a series of thrombin receptor-derived polypeptides (TRPs) based on the human and rat receptor sequences: SFLLR (P5), SFLLR-NH2 (P5-NH2) SFFLR (Rat P5), SFFLR-NH2 (Rat P5-NH2), SFLLRNP (P7), SFLLRNP-NH2 (P7-NH2), SFFLRNP (Rat P7), SFFLRNP-NH2 (Rat P7-NH2), and SFLLRNPNDKYEPF (P14). 2. A contractile response to thrombin and the TRPs in the endothelium-denuded aortic tissue was minimal or absent in preparations obtained from animal weighing less than 180 g (< 6 weeks of age), but increased with animal size, plateauing in tissues derived from animals weighing between 320 and 420 g (about 9 to 14 weeks of age). In contrast, the contractile responses to KCl and noradrenaline did not differ in the tissues and relaxant responses to the TRPs in endothelium-intact aortic preparations were comparable for tissues obtained from either young (< or = 180 g) or older (> or = 320 g) animals. 3. The contractile response of the endothelium-denuded preparation to thrombin and the TRPs showed marked cross-desensitization: the relaxation response of the intact rings did not desensitize to the TRPs. 4. The relative potencies for the TRPs in the aortic contraction assay were comparable to those for the relaxation assay, but were distinct from the relative potencies we measured previously in a rat gastric longitudinal muscle contraction assay. Further, P5 behaved as a partial agonist in the aortic contraction assay, whereas it had been observed to be a full agonist in the gastric contraction assay.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of endothelial factors involved in the vasodilatory effect of simvastatin in aorta and small mesenteric artery of the rat

1. Vascular effects of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, simvastatin, were studied in conductance (aorta) and resistance vessels (branch II or III of superior mesenteric artery, SMA) of the rat (12 - 14 weeks old). 2. Simvastatin produced relaxation of both aorta and SMA, with and without functional endothelium. These responses were inhibited by the product of HMG-CoA reductase, mevalonate (1 mmol l(-1)). 3. In vessels with functional endothelium, the NO-synthase inhibitor, L-N(G)-nitroarginine (L-NOARG, 30 micromol l(-1)), inhibited simvastatin-induced relaxation. In the presence of L-NOARG, relaxation to simvastatin was lower in vessels with endothelium than in endothelium-denuded arteries without L-NOARG. 4. The cyclo-oxygenase inhibitor, indomethacin (10 micromol l(-1)), abolished endothelium-dependent component of the response to simvastatin in both arteries. The combination of L-NOARG plus indomethacin did not produce further inhibition. The T(p) receptor antagonist, GR 32191B (3 micromol l(-1)), did not affect relaxation in aorta but it reduced response to low concentrations of simvastatin in SMA. However, the inhibitory effect of L-NOARG was less marked in the presence of GR 32191B in aorta but not in SMA. 5. The endothelium-dependent relaxation to simvastatin was inhibited by the superoxide dismutase (SOD, 100 u ml(-1)) or by the tyrosine kinase inhibitor, genistein (30 micromol l(-1)) in the two arteries. 6. The present study shows that simvastatin produces relaxation of conductance and small arteries through mevalonate-sensitive pathway. The endothelium-dependent relaxation to simvastatin involves both NO and vasodilator eicosanoids by a mechanism sensitive to SOD, and to genistein. Also, the results highlighted participation in the aorta of endothelial vasoconstrictor eicosanoids acting on the T(p) receptor after blockage of NO synthase only.     

Vascular actions of octreotide in the portal hypertensive rat

1. We have investigated the actions of the somatostatin analogue octreotide in the portal hypertensive Wistar rat in vivo and in rat small mesenteric artery and aorta in vitro.
2. In small mesenteric artery, octreotide (0.1–0.3 μM) failed to produce any direct contraction, nor did it affect contractions to noradrenaline (NA, 10 μM) or endothelium-dependent relaxations to acetylcholine.
3. In rat aorta, octreotide (0.3 μM) and somatostatin (1 μM) failed to affect contractions to NA (1 μM), or concentration-contractile response curves to NA.
4. In rat vas deferens, octreotide and somatostatin significantly reduced contractile responses to electrical stimulation with pD₂ values (−log IC₅₀) of 8.19±0.10 (n=4) and 8.16±0.26 (n=4), respectively. Hence, the lack of effect of these agents in aorta or mesenteric artery was not due to lack of efficacy or inappropriate choice of concentration.
5. In the anaesthetized portal hypertensive rat, intravenous injection of octreotide (1–100 μg kg⁻¹) did not significantly affect systemic blood pressure, nor did it affect mesenteric vascular conductance as measured by laser doppler flow probes. However, octreotide (100 μg kg⁻¹) significantly reduced vascular conductance to 74.2±7.7% of control (n=6) in porto-systemic shunt vessels as measured by laser doppler flow probes.
6. Phenylephrine (1 μg kg⁻¹) significantly raised blood pressure and significantly decreased vascular conductance in both mesenteric (66.6±3.7% of control) and porto-systemic shunt vessels (58.7±10.0% of control).
7. It was concluded that octreotide has selective effects on porto-systemic shunt vessles in vivo in the portal hypertensive rat.

     

The actions of the cannabinoid receptor antagonist,SR 141716A,in the rat isolated mesenteric artery

1. The actions of the cannabinoid receptor antagonist, SR 141716A, were examined in rat isolated mesenteric arteries. At concentrations greater than 3 μM, it caused concentration-dependent, but endothelium-independent, relaxations of both methoxamine- and 60 mM KCl-precontracted vessels.
2. SR 141716A (at 10 μM, but not at 1 μM) inhibited contractions to Ca²⁺ in methoxamine-stimulated mesenteric arteries previously depleted of intracellular Ca²⁺ stores. Neither concentration affected the phasic contractions induced by methoxamine in the absence of extracellular Ca²⁺.
3. SR 141716A (10 μM) caused a 130 fold rightward shift in the concentration-response curve to levcromakalim, a K⁺ channel activator, but had no effect at 1 μM.
4. SR 141716A (10 μM) attenuated relaxations to NS 1619 (which activates large conductance, Ca²⁺-activated K⁺ channels; BK_Ca). The inhibitory effect of SR 141716A on NS 1619 was not significantly different from, and was not additive with, that caused by a selective BK_Ca inhibitor, iberiotoxin (100 nM). SR 141716A (1 μM) did not effect NS 1619 relaxation.
5. SR 141716A (10 μM) had no effect on relaxations to the nitric oxide donor S-nitroso-N-acetylpenicillamine, or relaxations to carbachol in the presence of 25 mM KCl.
6. The results show that, at concentrations of 10 μM and above, SR 141716A causes endothelium-independent vasorelaxation by inhibition of Ca²⁺ entry. It also inhibits relaxations mediated by K⁺ channel activation. This suggests that such concentrations of SR 141716A are not appropriate for investigation of cannabinoid receptor-dependent processes.

     

Investigation of the actions of chloroethylclonidine in rat aorta.

1. The interaction between chloroethylclonidine (CEC) and noradrenaline (NA) has been examined at alpha-adrenoceptors mediating contractions of rat aorta. 2. In rat aorta, the competitive antagonist prazosin, over the concentration-range 0.01-10 microM, produced concentration-dependent shifts in the contractile potency of NA, so that there was no component of the NA contraction resistant to prazosin. 3. The irreversible alpha 1-adrenoceptor antagonists, phenoxybenzamine (PBZ) (1-10 microM) and benextramine (10 microM) produced shifts in potency of NA and reduced the maximum response in a concentration-dependent manner. 4. The irreversible alpha 1-adrenoceptor antagonist, CEC (100 microM), produced a non-parallel shift in the NA concentration-response curve so that low concentrations of NA produced relatively small contractions but relatively high concentrations produced further contractions, so that the maximum response was not significantly reduced. 5. The combination of CEC pretreatment and subsequent prazosin (0.1 microM) produced a parallel shift in the potency of NA. However, prazosin (10 microM) failed to produce any further effect on the response to high concentrations of NA following CEC pretreatment. Hence, a component of the contraction to NA in the presence of CEC was resistant to subsequent prazosin. Likewise, this component was resistant to a combination of prazosin (10 microM) and yohimbine (10 microM). 6. Receptor protection experiments were carried out in which tissues were exposed to NA (100 microM), yohimbine (10 microM) or prazosin (0.1 microM) prior to and during exposure to CEC. Receptor protection with NA, yohimbine or prazosin (0.1 microM), followed by washout prevented the shift in potency of NA produced by CEC. 7. Further experiments examined the effects of prazosin (10 microM) on responses to NA following receptor protection with NA (100 microM), yohimbine (10 microM), prazosin (10 microM), or xylazine (100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasorelaxing effects of propranolol in rat aorta and mesenteric artery: a role for nitric oxide and calcium entry blockade

1. Propranolol has been prescribed successfully to patients with cardiovascular diseases, but the exact mechanisms by which it reduces peripheral vascular resistance have been poorly investigated. 2. The present study was designed to investigate the relaxing effects of propranolol in the rat isolated aorta and mesenteric artery, focusing on the contribution of the nitric oxide (NO)-cGMP pathway and calcium entry blockade. Relaxation responses to propranolol were obtained in precontracted rat aortic and mesenteric artery rings. 3. DL-Propranolol (10-100 micromol/L) produced concentration-dependent relaxations in the aorta and mesenteric artery rings with intact endothelium. The isomers D- and L-propranolol produced relaxation responses that were equipotent to the racemic mixture. 4. Metoprolol (10-100 micromol/L) produced slight relaxations, whereas atenolol (10-100 micromol/L) had no relaxant activity. 5. The NO inhibitor N(G)-nitro-L-arginine methyl ester (100 micromol/L) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (1 micromol/L), as well as removal of the endothelium, significantly reduced the relaxation responses induced by the lower concentrations of propranolol without affecting maximal responses. In addition, DL-propranolol markedly increased cGMP levels in endothelium-intact preparations. 6. In Ca(2+)-free Krebs' solution, DL-propranolol (10-100 micromol/L) caused marked rightward shift in the concentration-response curves to CaCl(2), with a decrease of maximal responses in tissues with either intact or denuded endothelium. Nifedipine (1 micromol/L) in combination with DL-propranolol virtually abolished the CaCl(2)-induced contractile responses. 7. The relaxation responses induced by DL-propranolol were significantly reduced in aortic and mesenteric rings precontracted with phorbol-12,13-dibutyrate (1 micromol/L). 8. In conclusion, DL-propranolol relaxes arterial smooth muscle by mechanisms involving activation of the NO-cGMP pathway and calcium influx blockade, independent of beta-adrenoceptor blockade.     

Characterization of receptors mediating contraction of the rat isolated small mesenteric artery and aorta to arginine vasopressin and oxytocin

1. The exact nature of the receptor subtype(s) involved in the action of arg-vasopressin (AVP) on the rat aorta and small mesenteric artery (SMA) is controversial. Therefore, we have studied the effects of the selective V_1A receptor antagonists, OPC 21268 and SR 49059, and the oxytocin (OT) receptor antagonist, atosiban, on the AVP- and OT-induced contractions of the two vessels.
2. AVP and OT displayed similar intrinsic activities in the rat aorta and SMA, but AVP was ∼130 fold and ∼500 fold more potent than OT, respectively. In the rat aorta, Hill slopes (n_H) were similar for OT and AVP. However, in rat SMA, the OT concentration-effect (E/[A]) curve was significantly steeper than the AVP E/[A] curve (n_H,=3.3±0.20, 2.3±0.15; P<0.001).
3. In the aorta OPC 21268, SR 49059 and atosiban competitively antagonized the AVP and OT E/[A] curves. Except for atosiban and SR 49059 against AVP, competitive antagonism was also observed in the SMA. Atosiban caused concentration-dependent steepening of the AVP E/[A] curve, whereas SR 49059 decreased the upper asymptote.
4. Schild analysis yielded affinities indicative of V_1A receptor involvement in both vessels: pK_B/pA₂=9.20–9.48, 7.56–7.71 and 6.19–6.48 for SR 49059, OPC 21268 and atosiban, respectively.
5. Neither AVP nor OT relaxed U46619 pre-contracted aorta or SMA in the presence of SR 49059, suggesting no interference of a vasodilatory component.
6. Despite predominant involvement of V_1A receptors in both vessels, the different Hill slopes of AVP and OT E/[A] curves as well as the steepening of the AVP E/[A] curves by atosiban are indicative of receptor heterogeneity in the rat SMA.

     

Vascular effects of capsaicin in isolated perfused rat mesenteric bed

The effects of intra- and extraluminal capsaicin administration were evaluated in isolated perfused rat mesenteric bed. Capsaicin (10 nM-1 microM) produced a potent concentration-dependent relaxation of the tonic vasoconstriction induced by norepinephrine (1 microM) but not by high-K+ (60 mM). The capsaicin-induced relaxation was nearly abolished in preparations pretreated in vitro with a high concentration of capsaicin (1 microM, for 10 min, 1 h before). Capsaicin-induced relaxation was reduced but not abolished in preparations obtained from rats pretreated neonatally with capsaicin. The capsaicin effects were unaffected by atropine, guanethidine, propranolol, hexamethonium or tetrodotoxin. The observation that capsaicin (0.1 microM)-induced relaxation was virtually abolished in presence of the proteolytic enzyme alpha-chymotrypsin (1 U/ml) supports the involvement of neuropeptide(s) in this response. Bolus injections of calcitonin gene-related peptide (CGRP) elicited a potent and rapidly ensuing relaxation which underwent tachyphylaxis. However, no cross-desensitization with capsaicin was observed. It is concluded that activation of capsaicin-sensitive sensory fibers could release neuropeptides locally with a potent effect on intestinal blood flow.     

Vascular inhibitory actions of 17β‐oestradiol in rat portal vein

1 We have examined the effects of 17β‐oestradiol on spontaneous spikes and contractions to noradrenaline in rat portal vein. 2 The 17β‐oestradiol (10 μM ) significantly reduced the maximum height of spontaneous spike contractions and significantly reduced the maximum contraction to noradrenaline (10 μM ) in portal vein from both male and female rats. 3 The protein synthesis inhibitor cycloheximide (10 μM ) did not significantly affect the inhibitory actions of 17β‐oestradiol (10 μM ) against spontaneous spike contractions and contractions to noradrenaline. 4 In summary, 17β‐oestradiol diminishes spontaneous spike contractions and the contractile response to noradrenaline in rat portal vein, by a non‐genomic action.