Endothelium-dependent relaxation and hyperpolarization in aorta from control and renal hypertensive rats

Endothelium-dependent relaxation is not uniformly impaired in chronic heart failure

Abnormalities of vasomotor tone are characteristic of heart failure. This study was designed to assess the effects of chronic heart failure on endothelium-dependent relaxation in both large conduit arteries and small resistance vessels and to determine whether or not impaired nitric oxide (NO) production is involved. Segments of pulmonary artery (PA), abdominal aorta (AA), and small mesenteric artery (MA) were harvested from rats with heart failure resulting from coronary artery ligation and from sham-operated controls. Organ-bath experiments done in the presence of indomethacin to avoid the influence of vasodilatory prostanoids demonstrated that relaxation to acetylcholine (ACh) was impaired in the PA but not the AA or MA of the group with heart failure. Endothelium-independent relaxation to nitroglycerin was not significantly affected by the development of heart failure. Constriction to prostaglandin (PG) F(2alpha) was enhanced in PA but not in AA or MA segments. Preincubation with N(omega)-nitro-L-arginine (NNA) to inhibit the production of NO increased baseline force in vessels from all three beds, but the effect was greatest in the PA. Although relaxation to ACh was significantly diminished by NNA in the PA, it was not completely abolished. Furthermore, ACh-mediated relaxation in the presence of NAA was still impaired in the group with heart failure compared with the sham-operated control group. NNA had only mild effects on ACh-mediated relaxation in MA. These results demonstrate that (a) the mediators of endothelium-dependent relaxation may vary throughout the arterial circulation, (b) the contribution of NO to endothelium-dependent relaxation is substantial in PA and minimal in mesenteric resistance vessels, (c) endothelium-dependent relaxation is not uniformly impaired throughout the arterial bed by the development of heart failure, and (d) although a defect in NO production may account for enchanced vasoconstriction seen in response to PGF(2alpha), it does not account for the diminished vasodilatory response to ACh in this experimental model of heart failure.

Endothelium-derived hyperpolarizing factor

1. Not all endothelium-dependent relaxations can be fully explained by the release of either nitric oxide (NO) and/or prostacyclin. Another unidentified substance(s) that hyperpolarizes the underlying vascular smooth muscle cells (endothelium-derived hyperpolarizing factor; EDHF) contributes to endothelium-dependent relaxations. 2. In blood vessels from various species these hyperpolarizations are resistant to inhibitors of NO synthase (NOS) and cyclo-oxygenase. In canine, porcine and human blood vessels the hyperpolarization cannot be mimicked by nitrovasodilators or exogeneous NO. However, in other species (rat, guinea-pig, rabbit) endothelium-dependent hyperpolarizations resistant to inhibitors of NOS and cyclo-oxygenase and hyperpolarizations to endothelium-derived or exogeneous NO can be observed in the same vascular smooth muscle cells. 3. In blood vessels where NO causes hyperpolarization, the response is blocked by glibenclamide, suggesting the involvement of ATP-dependent potassium channels. Hyperpolarizations caused by EDHF are insensitive to glibenclamide but, depending on the tissue, are inhibited by relatively small concentrations of tetraethylammonium (TEA) or by apamin or the combination of charybdotoxin plus apamin, indicating that calcium-dependent potassium channels are likely to be involved. 4. Metabolites of arachidonic acid, through the cytochrome P450 mono-oxygenase pathway (epoxyeicosatrienoic acids), are produced by the endothelial cells, increase the open-state probability of calcium-activated potassium channels sensitive to TEA or charybdotoxin, and induce the hyperpolarization of arterial smooth muscle cells, indicating that epoxyeicosatrienoic acids could be EDHF. However, in blood vessels from various species, cytochrome P450 inhibitors do not affect endothelium-dependent hyperpolarizations, indicating that EDHF is not yet identified with certainty. 5. Endothelium-derived hyperpolarizing factor released from cultured endothelial cells reduces the intracellular calcium concentration in vascular smooth muscle cells and the EDHF component of the relaxation is proportionally more important in smaller than larger arteries. In aging animals and in various models of diseases, endothelium-dependent hyperpolarizations are diminished. 6. The identification of EDHF and/or the discovery of specific inhibitors of its synthesis and its action may allow a better understanding of its physiological and pathophysiological role(s).

Up-regulation of pressure-activated Ca(2+)-permeable cation channel in intact vascular endothelium of hypertensive rats

In endothelial cells, stretch-activated cation channels have been proposed to act as mechanosensors for changes in hemodynamic forces. We have identified a novel mechanosensitive pressure-activated channel in intact endothelium from rat aorta and mesenteric artery. The 18-pS cation channel responded with a multifold increase in channel activity when positive pressure was applied to the luminal cell surface with the patch pipette and inactivated at negative pipette pressure. Channel permeability ratio for K+, Na+, and Ca2+ ions was 1:0.98:0.23. Ca2+ influx through the channel was sufficient to activate a neighboring Ca2(+)-dependent K+ channel. Hemodynamic forces are chronically disturbed in arterial hypertension. Endothelial cell dysfunction has been implicated in the pathogenesis of arterial hypertension. In two comparative studies, density of the pressure-activated channel was found to be significantly higher in spontaneously hypertensive rats and renovascular hypertensive rats compared with their respective normotensive controls. Channel activity presumably leads to mechanosensitive Ca2+ influx and induces cell hyperpolarization by K+ channel activity. Both Ca2+ influx and hyperpolarization are known to induce a vasodilatory endothelial response by stimulating endothelial nitric oxide (NO) production. Up-regulation of channel density in hypertension could, therefore, represent a counterregulatory mechanism of vascular endothelium.

The effect of insulin treatment and of islet transplantation on the resistance artery function in the STZ-induced diabetic rat

1. This study was designed to investigate the influence of insulin treatment and islet transplantation on the smooth muscle contractility and endothelium-dependent and independent relaxation of resistance arteries in the chemically induced streptozotocin (STZ) diabetic rat after 6-8 weeks, and 12-14 weeks of diabetes, compared to non-diabetic age-matched controls. 2. The morphology, and contractile responses to high potassium physiological salt solution (KPSS), KPSS containing 10(-5) M noradrenaline (NAK), and concentration-response curves to noradrenaline (NA) of mesenteric resistance arteries were recorded, along with the endothelium-dependent relaxation responses to acetylcholine (ACh) and bradykinin (BK), and endothelium-independent relaxation to sodium nitroprusside (SNP). Concentration-response curves were then repeated in the presence of a nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG). 3. Insulin-treated diabetic rats in the 12 week study demonstrated enhanced vascular contractility to KPSS, NAK and NA, compared to age-matched non-diabetic controls. 4. Incubation with L-NOARG resulted in both a significant increase in maximum contractile response, and sensitivity (pD2) to NA in the untreated diabetic group (6 weeks). A significant shift in sensitivity was also seen in the insulin-treated diabetic group. In the 12 week study, incubation with L-NOARG resulted in an increased maximum contractile response and sensitivity to NA in the insulin-treated diabetics. An increase in sensitivity was also observed in the untreated diabetic group. 5. Endothelium-dependent relaxation to ACh was significantly augmented in the untreated diabetics (6-weeks), compared to the control group. In the 12-week study, relaxation to both ACh and BK was not significantly different in any of the experimental groups when compared to the sham-operated non-diabetic controls. 6. Incubation with L-NOARG resulted in a significant attenuation of the maximum relaxation response to ACh and BK in all of the experimental groups, in the 6- and the 12-week study. 7. There was no significant difference in the maximum relaxation response or sensitivity to sodium nitroprusside between the diabetic groups and their age-matched controls in either the 6-week or the 12-week study. 8. The results of this study suggest an enhanced release of nitric oxide in the early stages of diabetes, which is more evident in the untreated diabetic rats than the insulin treated, and appears to normalize as the duration of diabetes progresses. This study also shows that the alteration in vascular reactivity of the resistance arteries can be restored to within normal limits by the transplantation of islets of Langerhans, and that islet transplantation is an effective strategy in the correction of the metabolic abnormalities associated with insulin-dependent diabetes.

Impaired endothelium-dependent relaxation in isolated resistance arteries of spontaneously diabetic rats

1. Previous studies have shown that endothelium-dependent relaxation in the aorta of spontaneously diabetic bio bred rats (BB) is impaired. 2. We have investigated noradrenaline (NA) contractility, endothelium-dependent acetylcholine (ACh) and bradykinin (BK) relaxation, and endothelium-independent sodium nitroprusside (SNP) relaxation in mesenteric resistance arteries of recent onset BB rats and established insulin treated BB rats, compared to their age-matched non diabetic controls. 3. There was no significant difference in the maximum contractile response or sensitivity to noradrenaline in either of the diabetic groups compared to their age-matched controls. 4. Incubation with the nitric oxide synthetase inhibitor NG-nitro-L-arginine (L-NOARG) resulted in a significant increase in maximum contractile response to noradrenaline in the recent onset age-matched control group (P < 0.05). Analysis of the whole dose-response curve (using ANOVA for repeated measures with paired t test) showed a significant left-ward shift following the addition of L-NOARG (P < 0.001). A similar but less marked shift (P < 0.01) was evident in vessels from recent onset diabetics. An overall shift in both sensitivity and maximum response was also evident in the age-matched non diabetic controls of the insulin-treated group (P < 0.05). However, by contrast, there was no significant change in sensitivity in the insulin-treated diabetic rats. 5. ACh-induced endothelium-dependent relaxation was significantly impaired in the recent onset diabetic rats compared to their age-matched controls (47 +/- 11% versus 92 +/- 2%, P < 0.05, n = 6), and in the insulin treated diabetic rats (34 +/- 5% versus 75 +/- 6%, P < 0.05, n = 6). The relaxation responses to BK also were significantly impaired in the diabetic rats compared to their age-matched controls (recent onset: 20 +/- 3% versus 72 +/- 7%, P < 0.05, n = 6; insulin treated: 12 +/- 9% versus 68 +/- 7%, P < 0.05, n = 7). 6. Incubation with either the nitric oxide synthetase substrate, U-arginine, or the free radical scavenging enzyme superoxide dismutase (150 mu ml-1) failed to improve the attenuated response of acetylcholine-induced relaxation in the diabetic vessels. 7. Endothelium-dependent relaxation mediated by ACh and BK was significantly attenuated in both the diabetic and control vessels after incubation with L-NOARG. 8. Pretreatment with a cyclo-oxygenase inhibitor, indomethacin, significantly enhanced the relaxation to ACh in both the recent onset and insulin treated diabetic rats (42 +/- 10%, n = 7 versus 64 +/- 7%, n = 7, P < 0.05, and 40 +/- 5%, n = 7 versus 65 +/- 9%, n = 6, P < 0.05). 9. Following endothelium removal, there was a marked impairment in endothelium-dependent relaxation responses to ACh and BK in both the diabetic and control vessels. 10. Incubation with the thromboxane A2 receptor antagonist SQ29548, did not significantly improve the ACh endothelium-dependent relaxation response in the diabetic vessels. 11. Endothelium-independent relaxation to sodium nitroprusside was significantly impaired in the first group of diabetic vessels studied; however, subsequent studies showed no impairment of the sodium nitroprusside response in the diabetic vessels. 12. In conclusion, the ability of the endothelium to regulate vascular contractility is reduced in recent onset diabetic vessels, and significantly impaired in established insulin treated diabetics. Relaxation to the endothelium-dependent vasodilators ACh and BK was impaired in both the recent onset and the established insulin treated diabetics, and the ACh response was significantly improved following pretreatment with indomethacin, suggesting a role for a cyclo-oxygenase-derived vasoconstrictor. Preliminary studies with a thromboxane A2, receptor antagonist, SQ29548 did not significantly improve the impaired relaxation to ACh, indicating that the vasoconstrictor prostanoid is not thromboxane A2.

Prostanoid-induced contractions are blocked by sulfonylureas

The sulfonylureas glibenclamide and tolbutamide are blockers of ATP-regulated K+ channels. The present study shows that these drugs also block contractions induced by prostaglandin F2 alpha, prostaglandin E2 and the thromboxane A2 mimetic U-46619 on rat aorta. This effect of sulfonylureas is not related to the endothelium since it is also found in endothelium-denuded preparations. The blockade is specific for prostanoids since contractions with norepinephrine, phenylephrine, serotonin, endothelin-1 or K+ (120 mM) are not or much less affected. On the other hand, contraction induced by activation of G-proteins with aluminium tetrafluoride anion (AlF4-) is significantly blocked by the sulfonylureas. Also on rat carotid artery the contraction of prostaglandin F2 alpha is importantly blocked by glibenclamide. It is concluded that the sulfonylureas glibenclamide and tolbutamide exert a specific inhibitory influence on prostanoid-induced contractions. This inhibition might be due to interference at the level of regulatory G-proteins, since the contractions induced by agonists that, like the prostanoids, activate phospholipase C (serotonin, phenylephrine, norepinephrine, endothelin) are not blocked.

Endothelium-dependent rhythmic contractions induced by cyclopiazonic acid, a sarcoplasmic reticulum Ca(2+)-pump inhibitor, in the rabbit femoral artery

The vascular responses to cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase in the sarcoplasmic reticulum, were investigated in the rabbit femoral artery, suspended in an organ chamber for isometric tension recordings. CPA produced rhythmic contractions in the femoral artery which had been contracted with phenylephrine. CPA, however, did not induce the rhythmic responses in endothelium-denuded arteries. NG-nitro-L-arginine methyl ester and methylene blue, inhibitors of the formation and the action of nitric oxide, respectively, failed to antagonize the CPA-induced rhythmic contractions in the phenylephrine-contracted artery. In contrast, the CPA-induced rhythmic contractions were abolished by charybdotoxin, a Ca(2+)-activated K+ channel antagonist, but not by glibenclamide, a blocker of the ATP-sensitive K+ channel. Nifedipine also inhibited the CPA-induced rhythmic contractions in the endothelium-intact artery and relaxed the endothelium-denuded artery treated with CPA. These results indicate that the CPA-induced rhythmic contractions in the phenylephrine-contracted rabbit femoral artery may be attributed to the periodic inactivation of the voltage-dependent Ca2+ channel, presumably regulated by the Ca(2+)-activated K+ channel. The activation of the K+ channel by CPA might occur only when the endothelium is present.

Endothelium-derived relaxing factors in the kidney of spontaneously hypertensive rats

Acetylcholine (ACh)-induced vasodilation is mainly due to endothelium-derived nitric oxide (EDNO) and hyperpolarizing factor (EDHF). To explore the mechanisms underlying attenuated endothelium-dependent vasodilation in hypertensive arteries, we measured the EDNO released from isolated kidneys of spontaneously hypertensive rats (SHR) using a sensitive chemiluminescence assay system of NO. ACh-induced renal vasodilation was significantly smaller in SHR than in the normotensive control, Wistar-Kyoto rats (WKY). However, ACh-induced NO release did not differ between SHR and WKY (10(-7) M: SHR +37 +/- 2 [SE] vs. WKY +32 +/- 4 fmol/min/g kidney). Perfusion with a 20 mEq/L high-K+ buffer, which is reported to inhibit action of EDHF, significantly reduced ACh-induced vasorelaxation in WKY but not in SHR, resulting in identical renal perfusion pressure in SHR and wKY under these conditions. These results indicate that attenuated ACh-induced vasorelaxation in the SHR kidney may be attributed to a decrease in EDHF rather than that in EDNO.

Endothelium-derived relaxing factor released by 5-HT: distinct from nitric oxide in basilar arteries of normotensive and hypertensive rats

1. The role of the endothelium in cerebrovascular responses to 5-hydroxytryptamine (5-HT) was investigated in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in vitro. 2. Cumulative addition of 5-HT caused concentration-dependent contractions in ring preparations of SHR basilar arteries; the contractile response was smaller in WKY basilar arteries. 3. Removal of the endothelium enhanced markedly the contractile responses to 5-HT in WKY arteries but had only a slight effect in SHR arteries. The responsiveness to 5-HT in WKY arteries after removal of endothelium was comparable to that in SHR arteries. 4. The endothelium-dependent relaxation induced by acetylcholine in WKY basilar arteries was almost abolished by treatment with 10 microM methylene blue or 10 microM NG-nitro-L-arginine (L-NOARG). However, the response to 5-HT was not affected by treatment with methylene blue, L-NOARG or indomethacin. 5. Application of 10-20 mM K+ or 3.2 mM tetraethylammonium (TEA) did not change significantly, or only increased slightly, the resting tension, but markedly enhanced the contractile response to 5-HT in WKY arteries with endothelium. In contrast, the submaximal response to 5-HT in SHR arteries with endothelium was significantly enhanced by 0.3 mM TEA. 6. In the presence of 1 mM TEA, the application of 10 microM L-NOARG further enhanced the responses of 5-HT in WKY arteries with endothelium. In SHR arteries with endothelium, 10 microM L-NOARG per se enhanced slightly but significantly the responses to 5-HT. 7. These results indicate that 5-HT-induced contraction in basilar arteries is substantially attenuated by an endothelium-dependent mechanism in WKY, but to a much lesser extent in SHR. The major relaxing factor released by 5-HT from endothelium in WKY is distinct from NO and may exert its effect by activating K+ channels.

Is the senescent heart overloaded and already failing?

Heart failure mainly occurs during the last decades of life, and it is important to know if the senescent heart is not an already failing heart. During aging, both contraction and relaxation of papillary muscle are impaired. Such an impairment is compensated in vivo and the cardiac output remains normal. In spite of a loss in myocytes, the heart weight/body weight ratio is unchanged, but the myocytes are bigger. Arrhythmias are permanent and are accompanied by a loss of the normal heart rate variability. Changes in specific mRNAs include: a shift in myosin heavy chain (MHC) isogene expression leading to an increased beta MHC content; decreased densities of Ca2+ ATPase of the sarcoplasmic reticulum, beta 1-adrenergic receptor, and muscarinic receptors; and attenuation of the Na+/Ca2+ exchange activity. Most of these changes, but not all, resemble those observed during cardiac overload and are accompanied by an increased duration of both the action potential and the intracellular calcium transient. However, the senescent heart is still able to further modify its phenotype in response to mechanical overload. The senescent heart is a diseased heart, and the origin of the "disease" is multifactorial and includes the general process of senescence, hormonal changes, and the myocardial consequences of senescence of the vessels.

Interdependence of contractile responses of rat small mesenteric arteries on nitric oxide and cyclo-oxygenase and lipoxygenase products of arachidonic acid

1. We have examined the effects of nitric oxide inhibition, indomethacin and the dual lipoxygenase/cyclo-oxygenase inhibitor, 3-amino-1-[m-(trifluoromethyl)-phenyl]-2-pyrazoline (BW755C), on the responses of small mesenteric arteries of Wistar rats, with and without endothelium, to noradrenaline, potassium chloride, endothelin-1, acetylcholine and sodium nitroprusside. 2. Noradrenaline, potassium chloride and endothelin-1 caused concentration-dependent contraction of small mesenteric arteries. Indomethacin (14 microM) attenuated the contractile response to both noradrenaline and potassium chloride. The inhibitory action of indomethacin persisted in vessels treated with CHAPS. 3. Acetylcholine produced concentration-dependent relaxation in these vessels. Indomethacin (14 microM) had no significant effect on the acetylcholine concentration-response relationship. 4. NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) potentiated the contractile response to both noradrenaline and potassium chloride and inhibited acetylcholine-induced relaxation. Indomethacin attenuated the effects of L-NAME. 5. BW755C inhibited the contractile response to noradrenaline and potassium chloride but not to endothelin-1. The inhibitory effects of BW755C persisted in the presence of indomethacin and in vessels treated with CHAPS. 6. BW755C enhanced endothelium-dependent relaxation, as assessed by the response to acetylcholine. In the presence of indomethacin, BW755C produced a shift to the right of the concentration-response curve to acetylcholine. 7. Inhibition of nitric oxide synthase with L-NAME, reversed the inhibitory effect of BW755C on noradrenaline- and potassium-induced contraction. L-NAME and BW755C in combination resulted in a shift to the right of the concentration-response curve to acetylcholine. 8. Sodium nitroprusside produced concentration-dependent relaxation of the vessels. Endothelium removal reduced the maximum relaxation to nitroprusside. BW755C did not alter the response to sodium nitroprusside in vessels with or without endothelium.9 .These data support the existence of two vasoconstrictor products of arachidonic acid released during contraction of small mesenteric arteries with noradrenaline and potassium chloride: a cyclo-oxygenase product and a lipoxygenase product both of which appear to be largely endothelium-independent.

Regulation of baseline vascular resistance in the canine diaphragm by nitric oxide

1. The role played by nitric oxide (NO) in the regulation of blood flow to the canine isolated hemidiaphragm was evaluated by determining (a) the effects of the L-arginine analogues NG-nitro-L-arginine methyl ester (L-NAME), NG-nitro-L-arginine (L-NOARG), and argininosuccinic acid (ArgSA) on baseline vascular resistance and of the latter two agents on endothelium-dependent (acetylcholine, ACh) and endothelium independent (sodium nitroprusside, SNP) vasodilatation; (b) the effects of L- and D-arginine on baseline vascular resistance; and (c) the effects of L-glutamine, an inhibitor of intracellular recycling of L-citrulline to L-arginine, on baseline resistance and on the response to ACh and SNP. 2. L-NAME, L-NOARG and ArgSA (6 x 10(-4) M final concentration) increased baseline diaphragmatic vascular resistance to a similar extent (28.6 +/- 4.2%, 26.7 +/- 4.3% and 32.8 +/- 4.6% respectively). L-NOARG and ArgSA reversed the vasodilator effect of ACh but not of SNP. 3. L- and D-arginine had no effect on vascular resistance. 4. L-Glutamine (10(-3) M) increased baseline vascular resistance by 10 +/- 1.9% (P < 0.05) but did not alter responses to either ACh or SNP. 5. Basal NO release plays a role in the regulation of baseline diaphragmatic vascular resistance. L-Arginine analogues tested potently and specifically inhibited this process. Moreover, extracellular L-arginine appears to have no effect on baseline diaphragmatic vascular resistance.

Contribution of nitric oxide to the endothelium-dependent hyperpolarization in rat aorta

1. The effect of endogenous and exogenous nitric oxide on the membrane potential (Em) of smooth muscle cells of the thoracic aorta of rats was investigated. 2. In tissues with intact endothelium, application of ACh or carbachol generated a change of the membrane potential consisting of an initial hyperpolarization by 10-12 mV, followed by a partial recovery toward a level which was at 10 min still 6-8 mV more negative than in control conditions. 3. Application of NG-nitro-L-arginine methylester (L-NAME), an inhibitor of endogenous NO production, had no significant effect on the resting membrane potential. The initial peak endothelium-dependent hyperpolarization elicited by ACh or carbachol was not significantly diminished. However, the recovery was more accentuated. Similarly, NG-monomethyl-L-arginine (L-NMMA) significantly diminished the second component of the endothelium-dependent hyperpolarization without affecting the magnitude of the first transient peak Em change. 4. Nitroglycerin produced a small sustained hyperpolarization of 1-2 mV, and the NO donor SIN-1, the active metabolite of molsidomine, similarly increased Em by about 1 mV. Infusion of high doses of acidified NaNO2 solution caused a hyperpolarization smaller than that evoked by ACh or carbachol. 5. 8-Bromo-cyclic GMP caused little change of membrane potential. In the presence of 8-Br-cGMP, ACh evoked a membrane electrical response similar to that observed in the absence of the nucleotide. 6. It is concluded that, in the rat aorta, the initial peak endothelium-dependent hyperpolarization observed under the influence of ACh or carbachol is not directly related to the synthesis of NO.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cyclopiazonic acid on phenylephrine-induced contractions in the rabbit ear artery

1. Upon stimulation with phenylephrine, the rabbit ear artery displays endothelium-regulated rhythmic contractions, which may be attributed to the periodical activation of the dihydropyridine-sensitive Ca2+ channel, presumably regulated by the Ca(2+)-activated K+ channel. The effect of cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase of the sarcoplasmic reticulum (SR), on phenylephrine-induced contractions was examined in endothelium-denuded rabbit ear arteries suspended in an organ chamber for isometric tension recordings. 2. Phenylephrine-induced tonic contractions were converted to rhythmic ones by the addition of CPA. 3. The CPA-induced rhythmic contractions were abolished by the blockade of the dihydropyridine-sensitive Ca2+ channel and the Ca(2+)-activated K+ channel by nifedipine and charybdotoxin, respectively. In contrast, glibenclamide, an ATP-sensitive K+ channel antagonist, had no effect on the CPA-induced rhythmic responses. 4. CPA attenuated both Ca2+ repletion by the SR and Ca2+ influx across the plasmalemma without having a significant effect on Ca2+ release from the SR, as evaluated by phenylephrine-induced contractions. In contrast, these three parameters were not altered by the presence of the endothelium. 5. These findings indicate that the CPA-induced rhythmic contractions in the endothelium-denuded rabbit ear artery may be induced by the same ionic mechanism as endothelium-regulated rhythmic responses, by which the K+ channel could regulate the probability of the Ca2+ channel being opened. The CPA-induced rhythmic contractions may correlate with the inhibitory effects of CPA on the SR function, although this is not true for the endothelium-regulated rhythmic contractions.

Endothelium-derived hyperpolarizing factor and endothelium-dependent relaxations

The endothelial cells inhibit the tone of the underlying vascular smooth muscle by releasing endothelium-derived relaxing factors (EDRF). The existence of at least two such factors, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF), has been demonstrated. EDHF is an as yet unidentified substance that hyperpolarizes vascular smooth muscle cells and causes their relaxation. The contribution of endothelium-dependent hyperpolarization varies along the vascular tree. Particularly in smaller blood vessels, EDHF acts on vascular smooth muscle in cooperation with nitric oxide. Basal release of EDHF is not likely to occur, at least in vitro. The production and/or release of EDHF is regulated by the cytosolic concentration of Ca2+ ions, derived both from the extracellular space and intracellular stores. Calmodulin may be involved in its production and/or release. EDHF hyperpolarizes the vascular smooth muscle by opening K+ channels. The hyperpolarization closes voltage-dependent Ca2+ channels and, as a consequence, EDHF relaxes blood vessels. In the absence of chemical identification of EDHF, it is difficult to assess its contribution to endothelium-dependent relaxations in vivo.

Effect of NG-monomethyl-L-arginine on the beta-adrenoceptor-mediated relaxation of rat mesenteric resistance arteries

beta-Adrenoceptors are present on vascular smooth muscle and on endothelium. We investigated whether the endothelial beta-adrenoceptors induce relaxation of rat mesenteric resistance arteries by stimulation of endothelium-derived relaxing factor (EDRF) release. To this end, the relaxation was studied in the presence and absence of 100 microM NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of the production of EDRF. The maximal relaxation with isoprenaline, expressed as a percentage of the precontraction, was 44.0 +/- 4.0% (n = 12) in the L-NMMA treated group and 58.0 +/- 2.6% (n = 13) in the untreated group, a statistically significant difference (P = 0.008). However, the precontraction with 40 mM K+ tended to be higher in the presence of L-NMMA. The pD2-value for isoprenaline was not significantly changed by the L-NMMA treatment. We conclude that the isoprenaline-mediated relaxation of mesenteric resistance arteries is inhibited by L-NMMA, but that this effect can at least in part be ascribed to an inhibition of baseline EDRF-release.

Characterization of endothelium-dependent relaxations resistant to nitro-L-arginine in the porcine coronary artery

1. Previous studies, demonstrated that endothelium-dependent relaxations which are resistant to nitro-L-arginine (an inhibitor of nitric oxide synthase) are accompanied by membrane hyperpolarization in the porcine coronary artery. The present experiments were designed to characterize further this type of endothelium-dependent relaxation in response to bradykinin by measuring isometric force in isolated rings of that artery. The experiments were performed in the presence of indomethacin to rule out vasoactive prostanoids. 2. Bradykinin induced comparable endothelium-dependent relaxations of proximal and distal rings of porcine coronary arteries contracted with prostaglandin F2 alpha in the presence of nitro-L-arginine. 3. Bradykinin and SIN 1 (a donor of nitric oxide) reduced contractions induced by prostaglandin F2 alpha in an additive fashion in the presence of nitro-L-arginine. 4. Bradykinin (in the presence of nitro-L-arginine) relaxed the tissues contracted with tetraethylammonium, prostaglandin F2 alpha, phorbol 12, 13-diacetate or endothelin, with similar pD2 values. 5. The time course of the relaxations induced by bradykinin (in the presence of nitro-L-arginine) and UK14304 (an alpha 2-adrenoceptor agonist, in the absence of the inhibitor of nitric oxide synthase) were comparable. 6. These results suggest that, in the porcine coronary artery, nitro-L-arginine-resistant relaxations (a) are distributed similarly in the proximal and distal parts of the artery, (b) contribute to inhibition of vascular smooth muscle with nitric oxide in an additive fashion, (c) occur during contractions induced by various contractile agents and (d) do not precede those mediated by nitric oxide.

Calmodulin antagonists inhibit endothelium-dependent hyperpolarization in the canine coronary artery

1. The effects of the calmodulin antagonists, calmidazolium and fendiline were investigated on endothelium-dependent hyperpolarization in the canine coronary artery. The membrane potential of vascular smooth muscle cells was measured with the microelectrode technique. 2. Smooth muscle cells of the canine coronary artery had a resting membrane potential of -50 mV. Bradykinin and the Ca(2+)-ionophore, A23187, induced concentration- and endothelium-dependent hyperpolarization. The hyperpolarization induced by a supramaximal concentration of bradykinin (10(-6) M) reached approximately 20 mV. 3. Calmidazolium (10(-5) M) and fendiline (10(-4) M) inhibited hyperpolarization induced by bradykinin and A23187. By contrast, calmidazolium did not affect the hyperpolarization induced by lemakalim, an opener of ATP-sensitive K(+)-channels. 4. These observations suggest that calmodulin is involved in the generation of endothelium-dependent membrane hyperpolarization of vascular smooth muscle.

Endothelium-dependent modulation of resistance vessel contraction: studies with NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine

1. The effect of NG-nitro-L-arginine methyl ester (L-NAME) and NG-nitro-L-arginine (L-NOARG) on noradrenaline (NA)-induced contractility and acetylcholine (ACh)-induced endothelium-dependent relaxation was studied in rat mesenteric resistance arteries. 2. Third order branches of mesenteric arteries were dissected and mounted on two forty micron wires in a Mulvany myograph. 3. Incubation with L-NAME and L-NOARG (10 microM) caused a time-dependent shift in the 50% response to NA (ED50) (0.01 microM-10 microM) but was not associated with an increase in the maximum contractile response. 4. L-NAME and L-NOARG (10 microM) caused a time-dependent inhibition of ACh (1 microM)-induced relaxation with a maximum effect after 120 min. 5. Following endothelium removal, incubation with either L-NAME or L-NOARG caused no significant shift in the ED50, although the residual relaxation response to ACh (1 microM) was further attenuated. 6. Incubation with the cyclo-oxygenase inhibitor, indomethacin, enhanced the relaxation to ACh and reduced the inhibitory effects of L-NAME and L-NOARG. 7. In conclusion, L-NAME and L-NOARG are potent inhibitors of acetylcholine-induced endothelium-dependent relaxation in mesenteric resistance arteries. The shift in ED50 associated with these inhibitors suggests a probable role for the endothelium in modulating the contractility of the resistance vasculature.