Pascual Medina

Potentiation by vasopressin of adrenergic vasoconstriction in the rat isolated mesenteric artery

1 The aim of the present study was to investigate in rat mesenteric artery rings whether low concentrations of vasopressin could modify the contractile responses to noradrenaline and electrical stimulation of perivascular nerves. 2 Vasopressin (10−10–10−7 M) caused concentration‐dependent contractions (pD2=8.36±0.09). The V1‐receptor antagonist d(CH2)5Tyr(Me)AVP (10−9–10−8 M) produced parallel rightward shifts of the control curve for vasopressin. Schild analysis yielded a pA2 value of 9.83 with a slope of 1.10±0.14. 3 Vasopressin (3×10 −10 and 10−9 M) caused concentration‐dependent potentiation of the contractions elicited by electrical stimulation (2–8 Hz; 0.2 ms duration for 30 s) and produced leftward shifts of the concentration‐response curve for noradrenaline. The V1‐receptor antagonist induced concentration‐dependent inhibitions of potentiation induced by vasopressin. The selective V1‐receptor agonist [Phe*, Orn8]‐vasotocin (3×10 −10 and 10−9 M) induced potentiation of electrical stimulation‐evoked responses which was also inhibited in the presence of the V1 antagonist (10−8 M). In contrast, the V2‐receptor agonist deamino‐8‐D‐arginine vasopressin (desmopressin 10−8–10−7 M) did not modify the electrical stimulation‐induced responses and the V2‐receptor antagonist [d(CH2)5, D‐Ile*, Ile4, Arg8]‐vasopressin (10−8–10−7 M) did not affect the potentiation evoked by vasopressin. 4 In artery rings contracted by 10−6 M noradrenaline in the presence of 10−6 M guanethidine and 10−6 M atropine, electrical stimulation (2, 4 and 8 Hz) produced frequency‐dependent relaxations which were unaffected by 10−9 M vasopressin but abolished by 10−6 M tetrodotoxin. 5 Vasopressin also potentiated contractions elicited by KCl and contractions induced by addition of CaCl2 to KCl depolarized vessels. The augmenting effects were inhibited by the V1 antagonist. 6 In the presence of the calcium antagonist nifedipine (10−6 M), vasopressin failed to enhance the contractile responses to electrical stimulation, noradrenaline and KCl. 7 The results demonstrate that low concentrations of vasopressin strongly potentiate the contractions to adrenergic stimulation and KCl depolarization. This effect appears to be mediated by V1 receptor stimulation which brings about an increase in calcium entry through dihydropyridine‐sensitive calcium channels.

Effects of Some Guanidino Compounds on Human Cerebral Arteries

BACKGROUND AND PURPOSE Accumulation of endogenous guanidino-substituted analogues of L-arginine in chronic renal failure might contribute to some of the vascular and neurological disorders of this pathology. We tested the hypothesis that in human cerebral arteries, some guanidino compounds may increase vascular tone, through nitric oxide (NO) synthase inhibition, and impair endothelium-dependent relaxation. METHODS Rings of human middle cerebral artery were obtained during autopsy of 26 patients who had died 3 to 12 hours before. The rings were suspended in organ baths for isometric recording of tension. We then studied the responses to N(G)-monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (asymmetrical dimethylarginine; ADMA), aminoguanidine (AG), and methylguanidine (MG). RESULTS L-NMMA (10(-6) to 3x10(-4) mol/L) and ADMA (10(-6) to 3x10(-4) mol/L) caused concentration- and endothelium-dependent contractions (median effective concentrations [EC(50)]=1.1x10(-5) and 1.6x10(-5) mol/L, respectively; E(max)=35. 5+/-7.9% and 43.9+/-5.9% of the response to 100 mmol/L KCl). AG (10(-5) to 3x10(-3) mol/L) and MG (10(-5) to 3x10(-3) mol/L) produced endothelium-independent contractions (E(max)=44.3+/-8.8% and 45.7+/-5.8% of the response to 100 mmol/L KCl, respectively). L-Arginine (10(-3) mol/L) prevented the contractions by L-NMMA and ADMA but did not change contractions induced by AG and MG. L-NMMA and ADMA inhibited endothelium-dependent relaxation induced by acetylcholine in a concentration-dependent manner; AG and MG were without effect. CONCLUSIONS The results suggest that the contractions induced by L-NMMA and ADMA are due to inhibition of endothelial NO synthase activity, whereas AG and MG do not affect the synthesis of NO. An increase in the plasma concentration of L-NMMA and ADMA associated with uremia is likely to represent a diminished release or effect of NO, and consequently, an increased cerebrovascular tone in uremic patients is highly conceivable.

Relaxation of human isolated mesenteric arteries by vasopressin and desmopressin

1 The effects of vasopressin and deamino‐8‐D‐arginine vasopressin (DDAVP, desmopressin) were studied in artery rings (0.8‐1 mm in external diameter) obtained from portions of human omentum during the course of abdominal operations (27 patients). 2 In arterial rings under resting tension, vasopressin produced concentration‐dependent, endothelium‐independent contractions with an EC50 of 0.59 ± 0.12 nm. The V1 antagonist d(CH2)5Tyr(Me)AVP (1 μ) and the mixed V1‐V2 antagonist desGly‐d(CH2)5D‐Tyr(Et)ValAVP (0.01 μm) displaced the control curve to vasopressin to the right in a parallel manner without differences in the maximal responses. In the presence of indomethacin (1 μm) the contractile response to vasopressin was significantly increased (P < 0.01). 3 In precontracted arterial rings, previously treated with the V1 antagonist, d(CH2)5Tyr(Me)AVP (1 μm), vasopressin produced endothelium‐dependent relaxation. This relaxation was reduced significantly (P < 0.05) by indomethacin (1 μm) and unaffected by the V1‐V2 receptor antagonist desGly‐d(CH2)5D‐Tyr(Et)ValAVP (1 μm) or by NG‐nitro‐L‐arginine methyl ester (L‐NAME, O.1 μm). 4 The selective V2 receptor agonist, DDAVP, caused endothelium‐independent, concentration‐dependent relaxations in precontracted arterial rings that were inhibited by the mixed V1‐V2 receptor antagonist, but not by the V1 receptor antagonist or by pretreatment with indomethacin or L‐NAME. 5 Results from this study suggest that vasopressin is primarily a constrictor of human mesenteric arteries by V1 receptor stimulation; vasopressin causes dilatation only during V, receptor blockade. The relaxation appears to be mediated by the release of vasodilator prostaglandins from the endothelial cell layer and is independent of V2 receptor stimulation or release of nitric oxide. In contrast, the relaxation induced by DDAVP is largely dependent on stimulation of V2 receptors.

Inhibition of neuroeffector transmission in human vas deferens by sildenafil.

Sildenafil (0.1–30 μM), a cyclic GMP phosphodiesterase 5 (PDE 5) inhibitor, induced inhibition of electrically evoked contractions of ring segments of human vas deferens from 34 vasectomies. Zaprinast (0.1–100 μM), another PDE 5 inhibitor, and the nitric oxide (NO) donor sodium nitroprusside (SNP) (0.1–100 μM) had no effect on neurogenic contractions. The inhibition induced by sildenafil was not modified by the inhibitor of guanylate cyclase 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxaline‐1‐one (ODQ) (1–30 μM) but it was abolished by the K+ channel blockers tetraethylammonium (TEA, 1 mM), iberiotoxin (0.1 μM) and charybdotoxin (0.1 μM). Sildenafil, zaprinast and SNP did not affect the contractions induced by noradrenaline. SNP (10 μM) caused elevation of cyclic GMP levels that was potentiated by sildenafil (10 μM) and zaprinast (100 μM). ODQ (10 μM) inhibited the increase in cyclic GMP. Sildenafil inhibits adrenergic neurotransmission in human vas deferens. The inhibition is not related to accumulation of cyclic GMP but is probably due to activation of prejunctional large‐conductance Ca2+‐activated K+ channels.

Vascular aging in women: is estrogen the fountain of youth?

Aging is associated with structural and functional changes in the vasculature, including endothelial dysfunction, arterial stiffening and remodeling, impaired angiogenesis, and defective vascular repair, and with increased prevalence of atherosclerosis. Cardiovascular risk is similar for older men and women, but lower in women during their fertile years. This age- and sex-related difference points to estrogen as a protective factor because menopause is marked by the loss of endogenous estrogen production. Experimental and some clinical studies have attributed most of the protective effects of estrogen to its modulatory action on vascular endothelium. Estrogen promotes endothelial-derived NO production through increased expression and activity of endothelial nitric oxide synthase, and modulates prostacyclin and thromboxane A2 release. The thromboxane A2 pathway is key to regulating vascular tone in females. Despite all the experimental evidence, some clinical trials have reported no cardiovascular benefit from estrogen replacement therapy in older postmenopausal women. The “Timing Hypothesis,” which states that estrogen-mediated vascular benefits occur only before the detrimental effects of aging are established in the vasculature, offers a possible explanation for these discrepancies. Nevertheless, a gap remains in current knowledge of cardiovascular aging mechanisms in women. This review comprises clinical and experimental data on the effects of aging, estrogens, and hormone replacement therapy on vascular function of females. We aim to clarify how menopause and aging contribute jointly to vascular aging and how estrogen modulates vascular response at different ages.

Accumulation of Symmetric Dimethylarginine in Hepatorenal Syndrome

In patients with cirrhosis, nitric oxide (NO), asymmetric dimethylarginine (ADMA), and possibly symmetric dimethylarginine (SDMA) have been linked to the severity of the disease. We investigated whether plasma levels of dimethylarginines and NO are elevated in patients with hepatorenal syndrome (HRS), compared with patients with cirrhosis without renal failure (no-HRS). Plasma levels of NO, ADMA, SDMA, and l-arginine were measured in 11 patients with HRS, seven patients with no-HRS, and six healthy volunteers. SDMA concentration in HRS was higher than in no-HRS and healthy subjects (1.47 ± 0.25 vs. 0.38 ± 0.06 and 0.29 ± 0.04 μM, respectively; P < 0.05). ADMA and NOx concentrations were higher in HRS and no-HRS patients than in healthy subjects (ADMA, 1.20 ± 0.26, 1.11 ± 0.1, and 0.53 ± 0.06 μM, respectively; P < 0.05; NOx, 94 ± 9.1, 95.5 ± 9.54, and 37.67 ± 4.62 μM, respectively; P < 0.05). In patients with HRS there was a positive correlation between serum creatinine and plasma SDMA (r2 = 0.765, P < 0.001) but not between serum creatinine and ADMA or NOx. The results suggest that renal dysfunction is a main determinant of elevated SDMA concentration in HRS. Accumulation of ADMA as a result of impaired hepatic removal may be the causative factor initiating renal vasoconstriction and SDMA retention in the kidney.

Effects of sildenafil on human penile blood vessels.

OBJECTIVES To investigate the effects of sildenafil on human penile blood vessels and evaluate the interaction of sildenafil with neurogenic-mediated responses. Sildenafil is currently used in the treatment of erectile dysfunction. METHODS Penile dorsal arteries and deep dorsal veins were obtained from 14 multiorgan donors. Vascular rings were suspended in organ bath chambers, and the isometric tension was recorded. We then studied the effects of sildenafil on precontracted vessels and the neurogenic (noradrenergic and nitrergic) responses. RESULTS Sildenafil (10(-9) to 3 x 10(-6) M) caused concentration-dependent relaxation and amplified the relaxation induced by sodium nitroprusside. Relaxation was unaffected by the inhibitor of nitric oxide synthase N(G)-monomethyl-L-arginine (10(-4) M). Compared with zaprinast, sildenafil was 8 to 10 times more potent in terms of the median effective concentration (EC(50)) values. Electrical field stimulation of the vessels under resting tension caused frequency-dependent contractions that were attenuated in the presence of sildenafil. When penile vessels were contracted after blockade of norepinephrine release with guanethidine (10(-6) M), electrical stimulation induced frequency-dependent, nitric oxide-dependent relaxations that were enhanced by sildenafil. CONCLUSIONS These results indicate that the relaxation of human penile arteries and veins induced by sildenafil involves inhibition of noradrenergic contraction, enhancement of neurogenic nitric oxide-mediated relaxation, and inhibition of smooth muscle contraction.

Arginine Vasopressin Enhances Sympathetic Constriction Through the V1 Vasopressin Receptor in Human Saphenous Vein

BACKGROUND Arginine vasopressin (AVP) not only acts directly on blood vessels through V1 receptor stimulation but also may modulate adrenergic-mediated responses in animal experiments in vivo and in vitro. The aim of the present study was to investigate whether AVP can contribute to an abnormal adrenergic constrictor response of human saphenous veins. METHODS AND RESULTS Saphenous vein rings were obtained from 32 patients undergoing coronary artery bypass surgery. The vein rings were suspended in organ bath chambers for isometric recording of tension. AVP (3x10[-9] mol/L) enhanced the contractions elicited by electrical field stimulation at 1, 2, and 4 Hz (by 80%, 70%, and 60%, respectively) and produced a leftward shift of the concentration-response curve to norepinephrine (half-maximal effective concentration decreased from 6.87x10[-7] to 1.04x10[-7] mol/L; P<.05). The V1 vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP (10[-6] mol/L) prevented the potentiation evoked by AVP. The selective V1 receptor agonist [Phe,2 Orn8]-vasotocin (3x[-10]-9 mol/L) induced potentiation of electrical stimulation-evoked responses, which was also inhibited in the presence of the V1 receptor antagonist (10[-6] mol/L). In contrast, the V2 receptor agonist desmopressin (10[-9] to 10[-7] mol/L) did not modify neurogenic responses, and the V2 receptor antagonist [d(CH2)5, D-Ile,2 Ile,4 Arg8]-vasopressin (10[-8] to 10[-6] mol/L) did not prevent the potentiation induced by AVP. The dihydropyridine calcium antagonist nifedipine (10[-6] mol/L) did not affect the potentiating effect of AVP. CONCLUSIONS The results suggest that low concentrations of AVP facilitate sympathetic neurotransmission and potentiate constrictor effects of norepinephrine in human saphenous veins. These effects appear to be mediated by V1 receptor stimulation and are independent of calcium entry through dihydropyridine calcium channels. Thus, AVP may contribute to vascular mechanisms involved in acute ischemic syndromes associated with venous grafts, particularly if the sympathetic nervous system is activated.

Relaxation induced by cGMP phosphodiesterase inhibitors sildenafil and zaprinast in human vessels.

BACKGROUND Sildenafil is currently used in the treatment of erectile dysfunction. However, assessment of direct effects of sildenafil on coronary arteries and on arteries used as coronary grafts is unknown. This study was designed to investigate the effects of sildenafil on contracted human coronary, internal mammary, and radial arteries obtained from multiorgan donors. The observations were extended to forearm veins. Zaprinast was included in this study for comparison. METHODS Segments of left coronary, internal mammary, and radial arteries, and forearm veins were obtained from 16 multiorgan donors. Vascular rings were suspended in organ bath chambers and isometric tension was recorded. Then the effects of sildenafil, zaprinast, and sodium nitroprusside on precontracted vessels were studied. RESULTS Sildenafil (10(-8) - 3 x 10(-5) mol/L) caused concentration-dependent relaxation in the internal mammary arteries, radial arteries, and forearm veins. In the coronary arteries, sildenafil had a modest relaxant effect. In addition, sildenafil amplified the relaxation induced by sodium nitroprusside in all four vessels. Relaxation was unaffected by the inhibitor of nitric oxide synthase NG-monomethyl-L-arginine (10(-4) mol/L). Compared with zaprinast, sildenafil was eight to ten times more potent in terms of EC50 values. CONCLUSIONS The direct relaxant effects of sildenafil together with its synergistic interaction with nitric oxide donors should be considered in patients undergoing coronary bypass surgery, patients with low blood pressure, and patients receiving antihypertensive regimes.

Inhibition of nitric oxide activity by arginine analogs in human renal arteries

BACKGROUND Plasma levels of endogenous guanidine compounds are increased in various pathologic conditions, including chronic renal failure. In the present study we tested the effects of some of these compounds on basal and stimulated nitric oxide activity in human renal arteries. METHODS Rings from human renal arteries were obtained from 22 patients undergoing nephrectomy. The rings were suspended in organ baths for isometric recording of tension. We then studied the effects of N(G)-monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (asymmetrical dimethylarginine [ADMA]), aminoguanidine (AG), and methylguanidine (MG) on artery rings under basal and stimulated conditions. RESULTS In precontracted arteries, L-NMMA (1 micromol/L to 1 mmol/L) and ADMA (1 micromol/L to 3 mmol/L) caused concentration- and endothelium-dependent contractions (median effective concentrations [EC50] = 13.3 micromol/L and 17.5 micromol/L, respectively; Emax = 15+/-4% and 17+/-4% of the response to 100 mmol/L KCl, respectively). Aminoguanidine (0.01 to 3 mmol/L) and MG (0.01 to 3 mmol/L) produced endothelium-independent contractions (Emax = 9+/-3% and 16+/-2% of the response to 100 mmol/L KCl, respectively). L-arginine (1 mmol/L) but not D-arginine (1 mmol/L) prevented the contractions by L-NMMA and ADMA, but did not change contractions induced by AG and MG. In precontracted arteries, the relaxation to acetylcholine was decreased but not abolished by L-NMMA and ADMA. The remaining relaxation was reduced by charybdotoxin (0.1 mol/L) and tetraethylammonium (1 mmol/L). CONCLUSIONS The results demonstrate that L-NMMA and ADMA reduce basal and stimulated nitric oxide activity in human renal arteries. An increase in the plasma concentrations of methylarginines associated with renal disease should be considered as a risk factor for endothelial dysfunction and abnormal vasomotor tone in human renal arteries.