Role of efficacy in the assessment of the actions of alpha-adrenoceptor agonists in rat aorta with endothelium

Alpha lipoic acid treatment improved endothelium-dependent relaxation in diabetic rat aorta

The aim of this study was to ascertain the effects of α-lipoic acid (ALA) treatment on relaxant responses of acetylcholine (ACh) and isoprenaline (ISO) in aortic rings precontracted with serotonin (5-HT, 10(-6) M) obtained from streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in the rats by 50 mg/kg streptozotocin (STZ) via an intraperitoneal injection. Rat body and aorta weights were measured. The isometric tension to ACh (10(-9)-3×10(-6) M) and ISO (10(-9)-10(-4) M) of 5-HT-precontracted diabetic and non-diabetic rat (control), diabetic-ALA-treated, and ALA-treated aortas, in organ baths were recorded. Six weeks after STZ treatment blood glucose was elevated compared to control rats. In aortic rings from diabetic rats ACh and ISO-induced relaxations were impaired whereas endothelium-independent relaxation to sodium nitroprusside (SNP) was unaffected. ALA (100 mg/kg/day) treatment for 5 weeks enhanced ACh and ISO-induced relaxation in diabetic aortas. This recovering effect was via NO because prevented by incubating the vessels with N(G)-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor). It may be assumed that ALA treatment in vivo, can protect against impaired vascular responsiveness in STZ-induced diabetic rats.

Apocynin restores endothelial dysfunction in streptozotocin diabetic rats through regulation of nitric oxide synthase and NADPH oxidase expressions

AIM

Increased production of reactive oxygen species (ROS) in the diabetic vasculature results in the impairment of nitric oxide (NO)-mediated relaxations leading to impaired endothelium-dependent vasodilation. An important source of ROS is nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, and the inhibition of this enzyme is an active area of interest. This study aimed to investigate the effects of apocynin, an NADPH oxidase inhibitor, on endothelial dysfunction and on the expression of NO synthase (NOS) and NADPH oxidase in thoracic aorta of diabetic rats.

METHOD

Streptozotocin (STZ)-diabetic rats received apocynin (16 mg/kg per day) for 4 weeks. Endothelium-dependent and -independent relaxations were determined in thoracic aortic rings. Western blotting and RT-PCR analysis were performed for NOSs and NADPH oxidase in the aortic tissue.

RESULTS

Acetylcholine-induced relaxations and l-NAME-induced contractions were decreased in diabetic aorta. The decrease in acetylcholine and l-NAME responses were prevented by apocynin treatment without a significant change in plasma glucose levels. Endothelial NOS (eNOS) protein and mRNA expression exhibited significant decrease in diabetes, while protein and/or mRNA expressions of inducible NOS (iNOS) as well as p22(phox) and gp91(phox) subunits of NADPH oxidase were increased, and these alterations were markedly prevented by apocynin treatment.

CONCLUSION

NADPH oxidase expression is increased in diabetic rat aorta. NADPH oxidase-mediated oxidative stress is accompanied by the decreased eNOS and increased iNOS expressions, contributing to endothelial dysfunction. Apocynin effectively prevents the increased NADPH oxidase expression in diabetic aorta and restores the alterations in NOS expression, blocking the vicious cycle leading to diabetes-associated endothelial dysfunction.

Evidence against alpha2-adrenoceptors mediating relaxation in rat thoracic aortae: alpha2-agonists relaxation depends on interaction with alpha1-adrenoceptors

In rat aorta, the presence of functional alpha(2)-adrenoceptors (alpha(2)-AR) was investigated in ring preparations preconstricted with alpha(1)-adrenergic and non- alpha(1)-adrenergic agonists. Particularly, the hypothetical interference of alpha(2)-AR agonists with alpha(1)-AR-mediated vasoconstriction was evaluated. Relaxant and contractile responses to alpha(2)-AR agonists were obtained. In endothelium-intact and endothelium-denuded aortic rings preconstricted with phenylephrine (1 x 10(-6) m), the imidazoline derivatives, clonidine and UK14304, induced relaxations with similar order of potencies (-log EC(50)) and maxima relaxant effects respectively. Pretreatment with the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME) had no effect on the relaxant responses to clonidine and UK14304. In phenylephrine-constricted rings with endothelium, relaxations to clonidine and UK 14304 were not antagonized by the selective alpha(2)-AR antagonist, rauwolscine (< or =1 x 10(-6) m). Clonidine and UK 14304 induced only contractions on endothelium-intact and endothelium-denuded aortic rings contracted with prostaglandin F(2alpha) (3 x 10(-7) m). Moreover, clonidine and UK 14304-induced relaxation of endothelium-denuded arteries precontracted with methoxamine but not with serotonin. Finally, the concentration-contraction curves to clonidine and UK 14304 in endothelium-denuded aortic rings were significantly shifted to the right by the alpha(1D)-AR selective antagonist, BMY 7378, and rauwolscine. The pA(2) and pK(B) values for BMY 7378 and rauwolscine, respectively, against endothelium-independent actions of clonidine and UK 14304 were characteristic of an effect on the alpha(1D)-AR. The other selective alpha(2)-AR agonist tested BHT 933 (an azepine derivative), lacks considerable relaxant and contractile effects in rat aorta. The results provide no evidence for the presence of functional alpha(2)-AR in rat aorta. Respectively, the relaxant and contractile effects of the imidazoline derivatives, clonidine and UK 14304, may be due to an adjustable (in relation to the agonist-dependent active state of the alpha(1)-AR), inhibitory and excitatory, interaction with alpha(1)-ARs.

Cardiovascular pharmacology of aqueous extract of the leaves of Bridelia atroviridis Muell. Arg. (Euphorbiaceae) in the rat

Lyophilised decoction (10%) of the leaves of Bridelia atroviridis Muell. Arg. (Euphorbiaceae) was studied in the rat cardiovascular system. In vivo, the extract (15 and 30 mg/kg) caused a decrease of arterial pressure and a decrease of heart rate in an anaesthetized rat (ethylcarbamate 1.2 g/kg). If administrations of Bridelia were repeated (three times) a tachyphylaxie phenomena was observed. After administrations of adrenaline, noradrenaline, acethylcholine and isoprenaline the extract had no effect on the action of these neuromediators on blood pressure. In vitro the extract induced dose-dependent negative inotropic and chronotropic effects in isolated rat heart. It was ineffective in rat aorta preparations. Bridelia seemed to have a direct effect on rat heart. Hypotension is not due to an action on the vessels. The extract did not appear to interact with adrenergic nor cholinergic receptors. However, the extract was able to potentiate barium chloride induced contractions of rat aorta preparations. The extract might act through potential dependent calcium channels.

Role of nitric oxide and nitric oxide-independent relaxing factor in contraction and relaxation of rabbit blood vessels

It has been shown that spontaneous release of nitric oxide (NO) from the vascular endothelium attenuates contractile responses of vascular smooth muscles to norepinephrine, and that acetylcholine-induced relaxation is mediated by the evoked release of NO and endothelium-derived hyperpolarizing factor. Since the involvement of these substances (or factors) in mechanical responses is heterogeneous among blood vessels, we have investigated the role of these substances in agonist-induced contraction and relaxation in 6 rabbit blood vessels. Vascular reactivity for the contractile response to norepinephrine was potentiated after removal of endothelium and by 100 microM N(G)-nitro-L-arginine (L-NA) but not by 80 nM-0.4 microM clotrimazole. This potentiation was most marked in the mesenteric artery among the blood vessels tested, suggesting that the basal release of NO reduced the contractile response of the vascular smooth muscle to norepinephrine in this artery. Acetylcholine-induced relaxation was abolished by removal of the endothelium and was attenuated by L-NA (1-100 microM) in all blood vessels. The attenuation by 100 microM L-NA was most obvious in aorta and vein and least in mesenteric resistance artery in which the acetylcholine-induced, L-NA-resistant relaxation was inhibited by 80 nM-0.4 microM clotrimazole. These results suggested that there is a regional difference in the degree of involvement of NO in acetylcholine-induced relaxation. In mesenteric resistance artery, the NO-independent, clotrimazole-sensitive factor, possibly hyperpolarizing factor may also contribute to the response to acetylcholine at high concentrations.

Effects of long-term treatment with trandolapril on augmented vasoconstriction in rats with chronic heart failure

BACKGROUND

Despite the clinical relevance of angiotensin I-converting enzyme (ACE) inhibitors, their effects on impaired vascular function in patients and animals with chronic heart failure (CHF) have not been fully understood. This study was undertaken to determine whether long-term treatment with an ACE inhibitor improved the altered contractile properties of vessels from rats with CHF.

METHODS AND RESULTS

Twelve weeks after coronary artery ligation, the rats were sacrificed and the isometric tension development of thoracic aorta, pulmonary artery, and mesenteric artery with and without endothelium was examined. Contractile responses to norepinephrine and prostaglandin F2 alpha were augmented in endothelium-intact, but not in endothelium-denuded, thoracic aorta and pulmonary artery segments of the rat with CHF. The contractile response to angiotensin II was augmented in endothelium-denuded mesenteric artery segments of the rat with CHF, which was attenuated by indomethacin or diclofenac sodium but not by bunazosin. Trandolapril (3 mg/kg/d) was administered orally from the 2nd to 12th week after the operation. Treatment with trandolapril reversed the augmented contractile response of the rat with CHF to norepinephrine, prostaglandin F2 alpha, and angiotensin II almost to the levels in the sham-operated rat.

CONCLUSIONS

The results demonstrate that an ACE inhibitor is capable of reversing altered vascular function in the rat with CHF, suggesting that vascular beds are possible sites of action for ACE inhibitors in the therapy for CHF.

Endothelial modulation of alpha 1-adrenoceptor contractile responses in the tail artery of spontaneously hypertensive rats

1. Vascular contraction induced by phenylephrine was studied in tail artery rings from spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY) with particular focus on the role of endothelium. The influence of receptor reserve and the density of alpha 1-adrenoceptors on the possible differences observed were also analysed. 2. Phenylephrine (0.01-100 microM) induced concentration-dependent vasoconstrictions. The maximum response (alpha, P < 0.001) was greater but the pEC50 (P < 0.05) smaller in rings from SHR than from WKY rats irrespective of the presence or absence of endothelium. 3. Removal of endothelial cells resulted in a decrease of the maximum contraction with no modification in the pEC50 in arteries from both WKY and SHR. 4. The density of alpha 1-adrenoceptors (Bmax) and the dissociation constant (KD) were found to be the same for preparations from SHR and WKY rats in [3H]-prazosin binding experiments. 5. The apparent affinity (pKA) determined by the nested hyperbolic method and the operational model was similar in tail arteries from the two rat strains, irrespective of the presence or absence of endothelium. However, in endothelium-denuded rings, the pKA value was enhanced when compared with intact rings, in both SHR and WKY rats. 6. In rings from hypertensive rats, the operational parameter maximum possible effect (Em) was greater and the agonist efficacy (tau) was smaller than in rings from normotensive rats. When the endothelium was removed log tau and Em diminished in preparations from both rat strains. 7. In summary, the increased maximum responsiveness to phenylephrine in rings from SHR could be due to enhancement in Em. The log tau values indicate a deterioration in the transduction of the stimulus provided by the agonist in tail arteries from hypertensive animals. This study also suggests that the absence of endothelium modifies the alpha 1-adrenoceptor-mediated vasoconstriction probably by altering the transduction signalling mechanisms. The importance of analysing the degree of endothelium functionality when comparing results from different groups of rats is stated.

Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

1. The modulation by NG-nitro-L-arginine methylester (L-NAME) of alpha 1-adrenoceptor-mediated contraction was investigated on isolated segments of rat tail artery and aorta. The influence of L-NAME on inositol phosphates accumulation by alpha 1-adrenoceptor agonists was also investigated to elucidate the intracellular mechanism responsible for this modulation. 2. In aorta but not in tail artery L-NAME (30 microM) enhanced the sensitivity (3.3 times) and the maximum contraction (Emax) induced by the full agonist, phenylephrine. 3. St-587, a partial alpha 1-adrenoceptor agonist, behaved as a weak agonist in the aorta (22.2% of phenylephrine Emax). However, when the same agonist was studied in tail artery rings a maximum contraction that was 78.4% of the phenylephrine induced Emax was reached. 4. L-NAME increased (3.3 times) the Emax for St-587 contraction in the aorta but not in the tail artery. Sensitivity to St-587 was slightly but significantly (P < 0.001) enhanced (1.9 times) by L-NAME in tail artery segments. 5. Contractile responses to phenylephrine after partial alkylation with phenoxybenzamine were analyzed by the nested hyperbolic null method. To elicit 50% of Emax for contraction only 1.1% of the receptors in the tail artery and 21% of the receptors in the aorta need to be occupied. These results indicate a higher receptor reserve for the tail artery than the aorta. 6. In the tail artery but not in the aorta, St-587 activates phosphoinositide turnover. The presence of L-NAME was without effect on inositol phosphates accumulation induced by this partial alpha 1-adrenoceptor agonist. 7. The maximum contraction induced by phenylephrine, after partial alpha-adrenoceptor alkylation, was enhanced by L-NAME in tail artery rings. However, the NO synthase inhibitor was unable to modify the phenylephrine-induced accumulation of inositol phosphates in the presence of phenoxybenzamine. 8. These results indicate that the differences in St-587-induced contraction and the modulation by L-NAME of alpha 1-adrenoceptor-mediated contraction observed between the tail artery and aorta are associated with differences in receptor reserve. In addition, our biochemical studies indicate that the potentiating effect of L-NAME is independent of intracellular calcium release via phosphatidylinositol turnover.

Indigo carmine inhibits endothelium-dependent and -independent vasodilation

To investigate the potential mechanisms by which indigo carmine produces hypertension, we tested the hypothesis that indigo carmine inhibits endothelium-dependent vasodilation and determined the possible site of the inhibition (endothelium versus smooth muscle). Using isolated rat thoracic aortic rings that were precontracted with phenylephrine, we examined vasodilatory responses to acetylcholine, histamine, and Ca2+ ionophore A23187 (in endothelium-intact rings) and sodium nitroprusside and isoproterenol (in endothelium-denuded rings) in the presence and absence of indigo carmine. In addition, the effects of methylene blue on the acetylcholine- and sodium nitroprusside-induced vasodilation were compared with those of indigo carmine. Indigo carmine (10(-6), 10(-5), and 10(-4) mol/L) significantly inhibited receptor- and non-receptor-mediated endothelium-dependent vasorelaxation. Indigo carmine (10(-4) mol/L) also inhibited endothelium-independent vasorelaxation induced by sodium nitroprusside (an activator of vascular smooth muscle soluble guanylyl cyclase), although to a lesser extent than vasodilation from acetylcholine, histamine, and Ca2+ ionophore A23187. In contrast, indigo carmine (10(-4) mol/L) had no effect on the vasodilation induced by isoproterenol (an activator of adenylyl cyclase), indicating that indigo carmine selectively inhibits nitric oxide-mediated responses. Methylene blue, a known inhibitor of soluble guanylyl cyclase, inhibited both acetylcholine- and sodium nitroprusside-induced vasorelaxation. The inhibition was also greater in the acetylcholine- than the sodium nitroprusside-induced vasodilation. These results suggest that indigo carmine, like methylene blue, may inhibit endothelium-dependent relaxation by a mechanism that involves two levels. The major action of indigo carmine appears to be at the level of nitric oxide generation and/or release from the endothelial cell. In addition, indigo carmine appears to inhibit vascular smooth muscle guanylyl cyclase. Thus, indigo carmine may elevate blood pressure by interfering with these nitric oxide-mediated vasodilatory mechanisms.

Effect of the vascular endothelium on noradrenaline-induced contractions in non-pregnant and pregnant guinea-pig uterine arteries

1. The effect of pregnancy on noradrenaline-mediated contraction of guinea-pig uterine artery rings with both intact and denuded endothelium was investigated. 2. Noradrenaline (25 nM-100 microM) induced concentration-dependent contraction of non-pregnant and pregnant guinea-pig uterine arterial rings with intact endothelium with similar pD2 and maximal response values (non-pregnant: pD2 = 5.85 +/- 0.02, maximal response = 121 +/- 8.2%; pregnant: pD2 = 5.81 +/- 0.04, maximal response = 122 +/- 9.1%). Removal of endothelium did not affect noradrenaline-induced contractions in non-pregnant guinea-pig uterine artery (pD2 = 5.97 +/- 0.02, maximal response = 119 +/- 8.6%). In contrast, in arteries from pregnant guinea-pigs, removal of endothelium shifted concentration-response curve for noradrenaline to the left, without affecting maximal response value (pD2 = 6.36 +/- 0.03, maximal response = 120 +/- 9.0%). 3. The pKA values for noradrenaline were: 5.76 +/- 0.09 and 5.82 +/- 0.10 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively and 5.74 +/- 0.09 and 5.72 +/- 0.07 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. 4. The receptor occupancy-response relationship for noradrenaline was linear for all types of vessels, except for pregnant guinea-pig uterine artery with denuded endothelium, since half-maximal response to noradrenaline was obtained with 44.8 +/- 6.9% (non-pregnant guinea-pig uterine artery with intact endothelium), 43.3 +/- 6.1% (non-pregnant guinea-pig uterine artery with denuded endothelium) and 44.3 +/- 6.3% (pregnant guinea-pig uterine artery with intact endothelium) receptor occupancy. In pregnant guinea-pig uterine artery with denuded endothelium, occupancy-response relationship for noradrenaline was non-linear since half-maximal response to noradrenaline was obtained with 19.7 +/- 3.3% receptor occupancy. 5. NG-monomethyl-L-arginine (100 microM) and indomethacin (10 microM) did not affect concentration-response curve for noradrenaline in guinea-pig uterine arteries, regardless of pregnancy status or endothelial condition. 6. In quiescent preparations, the alpha-adrenoceptor antagonists, prazosin (5-50 nM) and yohimbine (1-10 microM) produced parallel rightward shifts of the curves for noradrenaline and the slopes of the Schild plots were not significantly different from unity. The plots constrained to a slope of unity gave the following - log Kb values: prazosin vs. yohimbine 8.78 +/- 0.03 vs. 6.41 +/- 0.02 for non-pregnant guinea pig uterine artery with intact endothelium, 8.95 +/- 0.03 vs. 6.34 +/- 0.02 for non-pregnant guinea-pig uterine artery with denuded endothelium, 8.91 +/- 0.01 vs. 6.44 +/- 0.03 for pregnant guinea-pig uterine artery with intact endothelium and 9.07 +/- 0.01 vs. 6.52 +/- 0.03 for pregnant guinea-pig uterine artery with denuded endothelium.7. It is concluded that initially there is no difference in noradrenaline action between uterine arteries from non-pregnant and pregnant guinea-pigs, but after removal of the endothelium the pregnant guinea-pig uterine artery is more sensitive to noradrenaline, which is related to increased receptor reserve for noradrenaline in this tissue. It is probable that relaxing factor derived from the endothelium mediates this effect, but it is unlikely to be nitric oxide or prostacyclin. Antagonist affinities and affinity of noradrenaline itself suggests that an identical subtype of alpha-adrenoceptor, probably the alpha 1 subtype, is involved in the noradrenaline-induced contraction of non-pregnant and pregnant guinea-pig uterine artery with or without endothelium.

Endothelium-dependent increase in vascular sensitivity to phenylephrine in long-term streptozotocin diabetic rat aorta

1. The effect of short- and long-term streptozotocin (STZ)-induced diabetes (12 and 52 weeks) on the vascular response to phenylephrine was examined in the isolated thoracic aorta with and without intact endothelium from diabetic, age matched control rats and diabetic rats treated with insulin. 2. Twelve weeks after induction of diabetes, aortae with intact endothelium demonstrated no changes either in sensitivity (defined as pD2) or contractility (defined as the maximal developed tension per aortic tissue wet weight) to phenylephrine. 3. In contrast, 52 weeks after induction of diabetes, aortae with intact endothelium demonstrated an increased sensitivity to phenylephrine while contractility to phenylephrine was not changed. Insulin treatment partially corrected the increased sensitivity to phenylephrine observed in diabetic rat aorta. 4. Removal of endothelium abolished the difference in phenylephrine sensitivity between diabetic and control aortae at 52 weeks. 5. Pretreatment of intact aortae with methylene blue, an inhibitor of endothelium-derived relaxing factor (EDRF), abolished the difference in phenylephrine sensitivity between control and diabetic rat aortae at 52 weeks, while pretreatment with indomethacin, an inhibitor of cyclo-oxygenase, had no effect. These results suggest that decreases in production or release of EDRF might be responsible for the increased vascular sensitivity to phenylephrine observed in long-term STZ diabetic rats. 6. Acetylcholine-induced relaxation, which is EDRF-dependent, was less in diabetic rat aortae with intact endothelium at 52 weeks, but not at 12 weeks. These results further support the theory that decreases in capacity of the endothelium to synthesize or release EDRF may occur in long-term STZ diabetic rats.

Effects of noradrenaline and prostaglandin F2 alpha on angiotensin-induced contraction and tachyphylaxis in rat aortic rings

The aim of this study was to examine the effects of noradrenaline (NA) and prostaglandin F2 alpha (PGF2 alpha) on angiotensin II (AII)-induced contraction and tachyphylaxis in aortic rings of the rat. Neither NA (10(-9) M) nor PGF2 alpha (10(-7) M) had significant effect on the response of the rings to the spasmogenic concentrations (10(-10) to 10(-7) M) of AII, but lowered significantly the threshold response of the aortic rings to AII (from 10(-9) to 10(-12) M). In rings that were tachyphylatic to AII, both NA and PGF2 alpha attenuated significantly the tachyphylaxis of the rings to AII at the concentrations of 10(-10) and 10(-7) of the octapeptide; and also lowered the threshold of the tachyphylatic rings to AII (from 10(-9) to 10(-11) M for NA, and from 10(-9) to 10(-10) M for PGF2 alpha). The specific properties of noradrenaline and PGF2 alpha were not shared by the non-specific potassium chloride. Because the lowering of threshold and attenuation of tachyphylaxis occurred at the physiological levels of AII and NA, it is possible that the in vivo actions of AII are under constant modulation by circulating and localised (higher than circulating) levels (e.g. of PGF2 alpha) of spasmogens. The results also call into question the physiological significance of angiotensin tachyphylaxis and may suggest that it is only an in vitro phenomenon occurring in the absence of endogeneous spasmogens.

Characterization of histamine-induced contraction in rat isolated aorta

High concentrations of histamine (greater than 10 microM) contract rat aortic rings and the effect is greatly enhanced when the endothelium is removed. The present study was aimed at characterizing the histamine-induced contractions of de-endothelialized rat aortic rings. These contractions were poorly inhibited by the histamine H1-receptor antagonist, mepyramine (1 and 10 microM) and insensitive to the histamine H2-receptor antagonist, cimetidine (10 microM), and to the cyclooxygenase inhibitor, indomethacin (5 microM). In contrast, the alpha-adrenoceptor antagonists, prasozin and pentholamine, antagonized these contractions in a concentration-dependent manner (respective apparent pKB values 9.7 and 7.9) and nifedipine (3 microM) reduced them by about 75%. Pretreatment of de-endothelialized rings with 8-bromo-cyclic GMP and of intact rings with methylene blue resulted in respective inhibition and enhancement of histamine-induced contractions, quite similarly to the effects in the presence and in the absence of endothelium, respectively. Histamine elicited endothelium-dependent relaxation of aortic rings precontracted by prostaglandin F2 alpha. This relaxation was abolished in the presence of mepyramine (1 microM). However, mepyramine failed to mimic the enhancing effect of endothelium removal on histamine-induced contractions of resting aortic rings. It is concluded that, in rat aorta, (1) contractions induced by high concentrations of histamine (greater than 10 microM) are probably mediated by alpha 1-adrenoceptors; and (2) spontaneous, but not histamine-stimulated, release of endothelium-derived relaxing factor is mainly involved in the modulation of histamine-induced contractions.

Technical considerations in evaluating the endothelial integrity of rat aortic preparations with silver staining

Assessment of endothelial integrity is an obligatory step in many pharmacological studies. Integrity of endothelium is affected by manipulations performed during the removal and cleaning of the vessel and by some of the silver-staining techniques utilized for demonstrating interendothelial junctions. When aortas were cleaned of periadventitial tissue in cold Tris-saline (once separated from the animal) by untrained personnel, only 45% of the endothelium was preserved. When cleaning was performed in situ by trained personnel while flushing with cold Krebs-Ringer-6% albumin, over 95% was left intact. AgNO3-staining performed before fixation produced a 50% loss of endothelium when using NH4Br and (NH4)2S as developers. AgNO3-staining performed after fixation produced over 95% recuperation of endothelium when 2% glutaraldehyde, 150 mM NaCl, 40 mM phosphate buffer, pH 7.4, were utilized as initial fixative, NH4Br and (NH4)2S being equally effective as developers. Chloride ions were necessary to intensify silver lines. Several patterns of deendothelization were produced by mechanical and chemical injury with saponin, NH4Br and (NH4)2S. In all cases, hematoxylin staining was employed as an auxiliary technique to interpret images of injured endothelium. Presence of albumin protected the endothelium from mechanical damage.

Modulation by endothelium of the responses induced by endothelin-1 and by some of its analogues in rat isolated aorta

1. The contractile effects of endothelin-1 and various analogues were studied in rat isolated rings. The potency of the peptides studied was endothelin-1 greater than sarafotoxin S6b greater than [Ala3,11]endothelin-1 greater than [Ala1,15]endothelin-1. [Ala1,3,11,15]endothelin-1 was neither agonist nor antagonist. 2. The concentration of endothelin-1 required to induced contractions equal to 50% of those induced by 1 microM noradrenaline was reduced from 5.8 nM when the vascular endothelium was present to 1.4 nM after it had been mechanically removed. 3. Contractions elicited by [Ala3,11]endothelin-1, [Ala1,15]endothelin-1 and sarafotoxin S6b were not modulated by the endothelium. 4. Endothelin-1 increased the tissue content of guanosine 3',5'-cyclic monophosphate (cyclic GMP) in rat aortic segments with endothelium by about 4 fold, suggesting that it increased the release of endothelium-derived relaxing factor. Sarafotoxin S6b did not reduce, or significantly increase, tissue cyclic GMP levels and therefore had little effect on EDRF release. 5. The concentration of sodium nitroprosside required to relax half-maximally aortae denuded of endothelium was 430 nM if the aortae had been precontracted with 10 nM endothelin-1 and 35 nM if 10 nM sarafotoxin S6b was used as the spasmogen. This indicates that differential sensitivities of the smooth muscle to cyclic GMP cannot explain differences between responses to endothelin-1 and sarafotoxin S6b in the presence of endothelium. 6. It is concluded that endothelin-1 contractions of rat aorta are modified by the endothelium, probably by enhancing the release of endothelium-derived relaxing factor (EDRF) and not by affecting the sensitivity of the smooth muscle to EDRF. This suggests that a stimulated release of an adequate amount of EDRF is necessary to modulate contractile responses to these peptides.

Calcium antagonists inhibit contractions to norepinephrine in the rat aorta, in the absence, but not in the presence of the endothelium

1. To compare the effect of diltiazem and verapamil on the responsiveness of vascular smooth muscle to norepinephrine in the presence and absence of the endothelium, rings of rat aorta were studied in organ chambers. 2. The removal of the endothelium decreased the ED50 to norepinephrine and augmented the maximal response to the catecholamine. 3. The contraction to norepinephrine consisted of a rapid initial (phasic) and a (tonic) part. The tonic part was reduced in the presence of the endothelium. 4. Diltiazem shifted the concentration-response curve to norepinephrine to the right only in rings without endothelium and reduced the difference in maximal response between rings with and without endothelium. 5. Verapamil abolished the difference in sensitivity (ED50) between rings with and without endothelium. 6. Oxyhemoglobin prevented the inhibitory effect of the endothelium on the response to norepinephrine, and unmasked a shift of the ED50 to the catecholamine to the right by diltiazem in rings with endothelium. 7. These experiments suggest that spontaneously released endothelium-derived relaxing factor(s) is a functional antagonist of norepinephrine-induced contractions, presumably by reducing the stimulated influx of extracellular Ca2+.

Effects of pregnancy and endothelial cell removal on alpha-adrenoceptor-mediated responses of rat thoracic aortae

1. The possibility was examined that changes in sensitivity of the aorta to alpha-adrenoceptor agonists during pregnancy in the rat are due to changes in smooth muscle receptor-mediated responses and/or endothelial cell-mediated responses. 2. Maximum constrictor responses to phenylephrine (PE) of both endothelium-intact and denuded thoracic aortic rings were greater when tissues were obtained from 20-day pregnant rats compared with non-pregnant ones. For endothelium-denuded thoracic aortic rings, the pA2 value for phentolamine as an antagonist of PE was not significantly different for rings from 20-day pregnant rats compared with rings from non-pregnant rats. 3. Prazosin (1-50 nM) markedly depressed maximum contractile responses to PE of endothelium-intact (but not denuded) thoracic aortic rings from both non-pregnant and 20-day pregnant rats. However, this marked depression of maximum responses by prazosin did not occur in the additional presence of yohimbine (10 nM), and yohimbine alone (10-500 mM) did not affect maximum responses to PE. There was no significant difference between the pA2 value for prazosin against PE-induced contractions of endothelium-denuded aortic rings obtained from either non-pregnant or 20-day pregnant rats (in the presence of yohimbine, 10 nM). 4. These results provide no evidence for a change in aortic smooth muscle alpha-adrenoceptor affinity during pregnancy, although maximum responsiveness of aortic smooth muscle to PE is increased on day 20 of pregnancy (c.f. non-pregnant controls).

Evidence for muscarinic receptors in endothelial cells from combined functional and binding studies

The aim of this study was to characterize muscarinic receptors of the bovine coronary artery by means of a combination of mechanical relaxation and contraction responses and radioligand binding data. Fresh helical strips of bovine coronary artery with intact endothelium relaxed in response to low concentrations (0.03-1 microM) of acetylcholine (ACh) and contracted at higher concentrations while endothelium-denuded strips only contracted. The ED50 for relaxation was 0.13 microM and that for contraction 1.8 microM (without endothelium); in the presence of endothelium, contraction dose-response curves were shifted to the right and the maximum contraction was reduced. In order to determine the location of the receptors mediating vasorelaxation, apparent affinity constants (KA) of ACh for relaxant and contractile effects were determined by irreversible blockade of a fraction of receptors with propyl benzilylcholine mustard (PBCM). The affinity constants (KA) were 0.22 microM for relaxation and 13 microM (with endothelium) and 20 microM (without endothelium) for contraction. In competition binding experiments against the muscarinic antagonist, [3H]N-methylscopolamine ([3H]NMS), the apparent affinity (KI) of ACh for binding sites in homogenates of endothelium-free coronary artery was 16 microM which was not different from the affinity constant determined in functional contraction experiments. Thus, the affinity constant of ACh determined for relaxation responses with endothelium-preserved vessels had no correlate in the binding affinity as determined with endothelium-free arteries. These findings indicate that bovine coronary arteries are relaxed by ACh through muscarinic receptors located on the endothelium whereas contractions are mediated by receptors on smooth muscle cells.

Endothelin: a review of its effects and possible mechanisms of action

The recently identified endothelium-derived peptide, endothelin, is a potent vasoconstrictor, but also binds specifically to many types of smooth muscle and to nerve tissue. Endothelin has been detected in plasma and may have physiological or pathological functions. Like other agonists, endothelin increases the turnover of phosphatidyl inositol and liberates intracellular stocks of Ca2+. It also increases plasmalemmal Ca2+ permeability, an effect that is antagonized by calcium entry blockers in some tissues. However, the characteristics of this antagonism are not always typical of that seen when other types of agonists are employed. It seems that in at least some cell types endothelin might activate specific L-type Ca2+ channels indirectly, perhaps secondarily to the activation of another type of cation channel. The endothelin originally described is one of a family of peptides that are closely related to the sarafotoxins. The comparative pharmacology of these peptides and of some analogues of the originally described endothelin have revealed some surprising differences and may indicate the existence of different endothelin receptors.

The modulatory role of vascular endothelium in the interaction of agonists and antagonists with alpha-adrenoceptors in the rat aorta

1. We have examined the effect of endothelium on the antagonistic action of prazosin, doxazosin, yohimbine and phentolamine against phenylephrine, clonidine and noradrenaline. 2. The action of prazosin against phenylephrine was similar to that earlier reported against noradrenaline, acting as a non-competitive antagonist in the presence of endothelium and as a competitive antagonist in the absence of endothelium. Prazosin also acted as a non-competitive antagonist against clonidine in the absence of endothelium. 3. Doxazosin behaved in a similar way to prazosin against noradrenaline, phenylephrine and clonidine acting as a non-competitive antagonist in the presence of endothelium and as competitive antagonist after removal of endothelium. In contrast, yohimbine and phentolamine acted as competitive antagonists both in the presence and in the absence of endothelium. 4. Analysis of the concentration-response curves for noradrenaline, phenylephrine and clonidine in the presence and in the absence of endothelium showed that the affinity for all three agonists was the same but not the efficacy and the receptor reserve, both of which were lower in the presence than in the absence of endothelium. 5. The rank order of agonist potency in the absence of endothelium was noradrenaline greater than phenylephrine greater than clonidine. The rank order of antagonist potency was prazosin greater than or equal to doxazosin greater than phentolamine greater than yohimbine. 6. The results show that vascular endothelium modulates the contractile response to alpha-adrenoceptor agonists and also modifies the action of the antagonists prazosin and doxazosin but not that of yohimbine and phentolamine. This effect of endothelium was related to a change in agonist efficacy and receptor reserve. These results also suggest that the alpha-adrenoceptors of the isolated aorta of the rat are predominantly, if not exclusively of the alpha 1-subtype.

Modulatory role of the vascular endothelium in the contractility of human isolated internal mammary artery

1. Endothelium-dependent relaxant responses and modulation of contractile responses were investigated in human isolated internal mammary artery (HIMA), a vessel widely used for coronary bypass surgery. 2. Acetylcholine and ionophore A23187 (both 10 nM-1 microM) elicited concentration-dependent relaxations of precontracted HIMA. These relaxations were abolished after rubbing of the endothelium, they were inhibited by methylene blue and were insensitive to indomethacin. 3. Histamine at concentrations lower than 10 microM elicited an endothelium-dependent, methylene blue-sensitive relaxation of precontracted HIMA. This effect of histamine was inhibited by the H1-receptor antagonist mepyramine. Bradykinin, noradrenaline and alpha 2-adrenoceptor agonists (in the presence of prazosin) did not relax unrubbed HIMA in which acetylcholine or A23187 were shown to be efficient. 4. Tissue levels of guanosine-3':5'-monophosphate (cyclic GMP) were found to be significantly higher in unrubbed HIMA rings than in matched rubbed rings. 5. Methylene blue evoked a slow contraction in resting HIMA, and this contraction was significantly greater in unrubbed than in rubbed preparations. Also, methylene blue enhanced the contractile response of HIMA to noradrenaline and this potentiating effect was significantly greater in unrubbed than in rubbed preparations. Indomethacin induced a slow contraction, of similar magnitude in unrubbed and rubbed HIMA rings. 6. In resting HIMA, the concentration-effect curve of noradrenaline-induced contraction was significantly shifted to the left after rubbing of the endothelium, without change in the maximal responses. In unrubbed rings the EC50 value of noradrenaline was about 2 fold that in rubbed rings. 7. Histamine also contracted resting HIMA in a concentration-dependent manner and in addition, it triggered rhythmic activity. This rhythmic activity was more prominent in unrubbed preparations and could be partially inhibited by indomethacin. The concentration-effect curve of histamine-induced contractions was displaced to the left after rubbing the endothelium, without changes in the maximal responses. The EC50 value of histamine in unrubbed rings was 4 to 9 fold that found in rubbed rings, depending on the level of tension taken into account for the concentration-effect curve during rhythmic contractions. 8. In the presence of nifedipine (3 microM), noradrenaline-induced contractions were not significantly altered, whereas histamine-induced contractions were found to be inhibited by about 70%. 9. It is concluded that in HIMA, both spontaneous and stimulated endothelium-dependent relaxing factor (EDRF) release may occur, and that basal EDRF can itself be responsible for the modulatory effect of endothelium on contractile responses.

Evidence for medetomidine as a selective and potent agonist at alpha 2-adrenoreceptors

The activity on alpha-adrenoreceptors of medetomidine ((+/-)-4-(alpha,2,3-trimethylbenzyl)imidazole), an alpha-methyl derivative of detomidine, has been characterized in vivo and in vitro using detomidine, MPV 207, MPV 295, azepexole, clonidine and xylazine for reference purposes. Medetomidine (1-100 micrograms/kg i.v.) was a hypotensive and bradycardic compound in anaesthetized rats. Furthermore, it induced vasopressor (PD50 1.7 microgram/kg) and sympatho-inhibitory (ID50 1.6 microgram/kg) actions in pithed rats, the effects being antagonized by idazoxan (0.3 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Medetomidine (30-300 micrograms/kg i.m.) had an alpha 2-adrenoreceptor mediated sedative effect on chicks. Medetomidine was, overall, more potent than detomidine, MPV 207, clonidine, xylazine, MPV 295 or azepexole in central (sedation in the chick) and peripheral (cardiac presynaptic in the pithed rat) actions on alpha 2-adrenoreceptors. Clonidine had, however, about an equal potency to medetomidine in the vascular smooth muscle of the pithed rat. Like detomidine and MPV 295, medetomidine had no agonistic activity in the rat aortic ring, but high concentrations antagonized methoxamine-induced contractions, giving a pA2 value of 5.68 for alpha 1-adrenoreceptor antagonism. The overall lipophilicity (log P') of medetomidine in the octanol/buffer (pH 7.4, 24-26 degrees C, HPLC technique) was 2.80. In summary, the experimental data suggest that medetomidine is a lipophilic compound with selective alpha 2-adrenoreceptor-stimulating properties and high potency. It may, therefore, prove to be a suitable pharmacologic tool for interventions in alpha 2-adrenoreceptor mediated effects in the autonomic nervous system.

Influence of the vascular endothelium on agonist-induced contractions and relaxations in rat aorta

The influence of the vascular endothelium on agonist-induced contractions and relaxations has been measured using intact segments of rat aorta. Contiguous rubbed segments were used as controls. Angiotensin II, histamine, noradrenaline, U46619 and UK14304 contracted both rubbed and intact tissues. The threshold spasmogenic concentrations of these agonists were lower in rubbed tissues than in intact preparations. The sensitivity and responsiveness of tissues to angiotensin II, histamine, noradrenaline and UK14304 were greater in rubbed than in intact tissues. Acetylcholine and histamine relaxed the established spasms of intact tissues but not those of rubbed preparations, These relaxant effects of acetylcholine were abolished by pre-incubation with haemoglobin. In the presence of prazosin, noradrenaline or UK14304 relaxed established contractions in intact tissues. These effects were antagonized by idazoxan or by pre-incubation with haemoglobin. In intact preparations, idazoxan had no effect on the spasmogenic sensitivity and responsiveness to UK14304. Pre-incubation with haemoglobin augmented the spasmogenic actions of noradrenaline, U46619 or UK14304 in intact tissues, but had no effect on these responses in rubbed preparations. Tissue concentrations of cyclic GMP were greater in intact than in rubbed tissues. A concentration of acetylcholine (10 microM) evoking just maximal mechanical inhibition produced a significant increase in cyclic GMP concentration in intact preparations. However, no detectable changes in cyclic GMP concentration were produced by UK14304 (10 microM) or by acetylcholine (30 nM), concentrations which were equi-effective in inhibiting mechanical activity. In the presence of threshold spasmogenic concentrations of noradrenaline, the contractile effects of angiotensin II were augmented and became comparable to those observed in rubbed preparations. In the presence of greater concentrations of noradrenaline, angiotensin II always produced an additional contraction. It is concluded that the presence of the vascular endothelium limits the spasmogenic action of a variety of agonists. Although spasmogens like noradrenaline and UK14304 can stimulate the release of endothelium-derived relaxing factor (EDRF) via alpha 2-adrenoceptors, the inhibitory effects of EDRF largely result from the spontaneous release of this substance.

Role of cyclic GMP in the modulation by endothelium of the adrenolytic action of prazosin in the rat isolated aorta

The effect of endothelium on the adrenolytic action of prazosin was studied in the rat isolated aorta. Prazosin showed a non-competitive type of antagonism in preparations with intact endothelium while in preparations where endothelium had been removed, prazosin at concentrations between 0.3 nM-10 nM acted as a competitive antagonist. Methylene blue, used to decrease tissue levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP), converted prazosin from a non-competitive antagonist into an apparently competitive antagonist in the presence of endothelium. Increasing tissue levels of cyclic GMP by incubation with 8-bromo-cyclic GMP converted prazosin from an apparently competitive antagonist into a non-competitive antagonist in the absence of endothelium. Analysis of concentration-response curves for noradrenaline in the presence and absence of endothelium showed that the affinity for noradrenaline was the same but the efficacy, measured by estimating the receptor reserve, was not; it was lower in the presence than in the absence of endothelium. It was concluded that the change in the mode of antagonism of prazosin after endothelium removal could be related to an alteration in the efficacy of the agonist, brought about by a change in the tissue levels of cyclic GMP.

Effect of endothelium on basal and alpha-adrenoceptor stimulated calcium fluxes in rat aorta

The rate of unstimulated influx of Ca2+ into rat aorta smooth muscle, measured as uptake of 45Ca, was inhibited in the presence of endothelium as compared to influx in the absence of endothelium. Efflux of 45Ca from unstimulated prelabelled tissues was also reduced in the presence of endothelium. In normal physiological solution the rate of influx and efflux of Ca2+ stimulated by B-HT 920 (1 and 10 microM), but not that stimulated by phenylephrine (30 nM and 1 microM), was also reduced in the presence of endothelium. In the presence of the calcium entry blocker flunarizine (3 microM), phenylephrine (1 microM) stimulated efflux of Ca2+ was inhibited by the presence of endothelium. A correlation between inhibition of Ca2+ influx and modulation of alpha-adrenoceptor agonist-induced contractions by endothelium could not be demonstrated, and methylene blue, an antagonist of endothelium mediated inhibition of B-HT 920 contractions, did not affect Ca2+ influx stimulated by the agonist. The effects of endothelium on Ca2+ influx and efflux are unlikely to be due to alterations by endothelium of diffusion of 45Ca or the agonists in the vessel. The results demonstrate that an endothelial derived factor or factors can reduce calcium influx into smooth muscle cells and also modulate the release of calcium from cells, perhaps by affecting intracellular calcium pumping mechanisms. A reduction of calcium influx cannot be the sole explanation for the modulatory effect of endothelium on alpha-adrenoceptor agonist-induced contractions but an effect on intracellular calcium metabolism may be important.