Angiotensin II in human veins: development of rapid desensitization and effect of indomethacin

IMPORTANCE OF THE ANGIOTENSIN TYPE 1 RECEPTOR IN ANGIOTENSIN II–INDUCED BRONCHOCONSTRICTION AND BRONCHIAL HYPERRESPONSIVENESS IN THE GUINEA PIG

Although angiotensin II (Ang II) causes bronchoconstriction and bronchial hyperresponsiveness to methacholine in mildly asthmatic patients, the responsible mechanisms for these reactions are unclear. The authors examined the effect of intravenous infusion of Ang II on airway constriction in guinea pigs. Furthermore, the effects of subthreshold concentrations of Ang II on bronchial responsiveness to methacholine were investigated. Airway opening pressure (Pao), an index of bronchoconstriction, increased dose dependently after intravenous infusion of 3 and 10 nmol/kg Ang II (72.2 and 236.5 increase above the baseline value, respectively). In another set of experiments, animals received a methacholine inhalation challenge under a constant intravenous infusion of a subthreshold dose of Ang II (2 nmol/kg/min). The Ang II infusion elicited bronchial hyperresponsiveness to methacholine. The provocative concentration of methacholine, which produced a 200% increase above the baseline Pao (PC200), decreased from 306.9 to 156.1 micrograms/mL upon Ang II infusion. Pretreatment with TCV-116, a type 1 Ang II (AT1) receptor antagonist, but not PD123319, a type 2 Ang II (AT2) receptor antagonist, dose dependently prevented both the Ang II-induced bronchoconstriction and bronchial hyperresponsiveness to methacholine. The authors conclude that Ang II caused bronchoconstriction and induced bronchial hyperresponsiveness to methacholine via the AT1 receptors and that this effect did not involve the release of other bronchoactive mediators.

Marked synergism between vasopressin and angiotensin II in a human isolated artery

OBJECTIVE

To determine the direct contractile effects of angiotensin II (AII) and vasopressin (VP), and the effects of combinations of these agonists, in human isolated gastroepiploic arteries in vitro.

DESIGN

Laboratory and clinical investigation.

SETTING

University laboratory and hospital.

SUBJECTS

Ring segments were prepared from gastroepiploic arteries obtained from 57 patients undergoing gastrectomy. Blood samples were obtained from ten patients after severe hemorrhage and from five healthy volunteers.

INTERVENTIONS

Mechanical activity in the rings was assessed using a strain gauge. Plasma concentrations of AII and VP in the blood samples were measured using radioimmunoassay kits.

MEASUREMENTS AND MAIN RESULTS

Both AII (1 or 10 ng/mL) and VP (100 or 500 pg/mL) produced concentration-dependent contractions in the rings. However, whereas VP produced reproducible sustained contractions, the contractile responses induced by multiple applications of AII showed marked desensitization (i.e., tachyphylaxis). Indeed, by the sixth application of either 1 ng/mL AII or 10 ng/mL AII, the contractile responses were <20% of the initial (control) response. During applications of AII after the sixth, the co-application of a low concentration of VP (100 pg/mL) fully restored the contractile response to AII in a clearly more-than-additive fashion. Similarly, the tachyphylaxis seen on AII application alone did not occur with repeated applications of an AII + norepinephrine mixture. In patients who had experienced hemorrhage, there were marked elevations of both AII and VP plasma concentrations, with values as high as 2.2 ng/mL and 550 pg/mL, respectively.

CONCLUSION

These results indicate that there is a powerful synergism between the contractile effects of low-dose VP and AII in this human isolated artery. Moreover, the elevations of plasma AII and VP levels during hemorrhage suggest that this synergism may be both physiologically and clinically important in optimizing vasoconstriction and maintaining blood pressure in such critical states.

Vasoconstricting effect of angiotensin II in human hand veins: influence of aging, diabetes mellitus and hypertension

We examined human hand veins to determine whether venoconstricting response to angiotensin II (Ang II) and noradrenaline (NA) was influenced by aging or such diseases as diabetes mellitus (DM) and hypertension (HT). Twenty healthy male subjects (20-73 years), and 8 male patients with non-insulin-dependent DM and 8 male patients with essential HT were included in this study. A constant dose (50 ng/min) of Ang II or increasing dose (2-256 ng/min) of NA was infused into the dorsal hand vein and its diameter was measured using a linear variable differential transformer. The constant infusion of Ang II caused rapid desensitization or tachyphylaxis. The venoconstriction by Ang II in the 8 elderly subjects (58 to 73 years) was significantly (p<0.05) larger than that in the 8 young subjects (20 to 36 years) from 6 to 18 min after the start of the infusion (after 6 min: 63.6+/-11.6 (mean+/-SD)% vs. 39.9+/-20.8%, 12 min: 34.0+/-11.9% vs. 12.0+/-12.0%). However, the venoconstriction by Ang II in the patients with DM or HT was not significantly different from that in the 9 age-matched control subjects. No significant difference in venoconstrictor response to NA was observed between the young and elderly subjects, nor between the control subjects and the patients with DM or HT. These findings indicated that venoconstrictor response to Ang II might be greater in the elderly but might not be influenced by DM nor HT.

Comparison of angiotensin II (Ang II) effects in the internal anal sphincter (IAS) and lower esophageal sphincter smooth muscles

Studies were performed to compare the actions of Ang II in the internal anal sphincter (IAS) vs. lower esophageal sphincter (LES) smooth muscles in vitro, in opossum and rabbit. Studies also were carried out in isolated smooth muscle cells. In opossum, Ang II produced no discernible effects in the IAS, but did produce a concentration-dependent contraction in the LES. Conversely, in the rabbit, while Ang II caused a modest response in the LES, it caused a significant contraction in the IAS. The contractile responses of Ang II in the opossum LES were mostly resistant to different neurohumoral antagonists but were antagonized by AT1 antagonist losartan. AT2 antagonist PD 123,319, rather than inhibiting, prolonged the contractile action of Ang II. The contractile actions of Ang II in the opossum LES were not modified by the tyrosine kinase inhibitors (genistein and tyrphostin 1 x 10(-6) M) but were partially attenuated by the PKC inhibitor H-7 (1 x 10(-6) M), Ca2+ channel blocker nicardipine (1 x 10(-5) M), Rho kinase inhibitor HA-1077 (1 x 10(-7) M) or p(44/42) MAP kinase inhibitor PD 98059 (5 x 10(-5) M). The combination of HA-1077 and H-7 did not cause an additive attenuation of Ang II responses. Western blot analyses revealed the presence of both AT1 and AT2 receptors. We conclude that Ang lI-induced contraction of sphincteric smooth muscle occurs primarily by the activation of AT1 receptors at the smooth muscle cells and involves multiple pathways, influx of Ca2+, and PKC, Rho kinase and p(44/42) MAP kinase.

Local nitric oxide release does not affect tachyphylaxis to angiotensin II in dorsal hand veins in man in the presence of prostaglandin synthesis inhibition

AIMS

Local prostaglandin (PG) production contributes to tachyphylaxis to angiotensin II (ANGII) in veins. Our aim was to assess the hypothesis that local nitric oxide (NO) generation is also, in part, responsible for tachyphylaxis to ANGII in veins, using the Aellig dorsal hand vein technique.

METHODS

Eight healthy male volunteers received 600 mg of aspirin (orally) to inhibit PG production. The venoconstrictor effects of ANGII and noradrenaline (NA) were then compared in dorsal hand veins during co-infusion of the NO synthase inhibitor L-NMMA or saline, on separate occasions.

RESULTS

ANGII and NA produced a similar degree of initial venoconstriction. However, the response to ANGII was significantly attenuated by 12 min compared with NA (AUC 147 +/- 38 vs 196 +/- 40, respectively; [95% confidence interval for difference: 7, 92], P = 0.02). Infusion of L-NMMA did not influence the response to ANGII or NA (P = 0.2 and P = 0.3, respectively).

CONCLUSIONS

Tachyphylaxis to ANGII in dorsal hand veins is not dependent on local NO release.