Sharon H. Nelson

Comparison of Nitroprusside and Hydralazine in Isolated Uterine Arteries from Pregnant and Nonpregnant Patients

The purpose of the present study was to determine the relative potency of nitroprusside and hydralazine with respect to inhibition of norepinephrinc-induced contraction of isolated, uterine arteries from pregnant and nonpregnant patients. The arteries, obtained after hysterectomy, were dissected free from surrounding tissue, and arterial rings were prepared and mounted in tissue chambers filled with Krebs-bicarbonate solution. Isometric tension was recorded. At concentrations of 10-9 M to 10-5 M, both nitroprusside and hydralazine produced concentration-dependent inhibition of the contractile response to norepinephrine. Nitroprusside and hydralazine were more potent in relaxing arteries contracted by a lower concentration (3 X 10-6 M) of norepinephrine than by a higher concentration (10-5 M) of norepinephrine. Regardless of the concentration of norepinephrine, nitroprusside was considerably more potent than hydralazine. The concentrations of nitroprusside that produced 50% inhibition (IC50) of the contractile response to norepinephrine (3 X 10-6 M) in uterine arteries from pregnant and nonpregnant patients were 3.2 ± 0.5 X 10-9 M (n = 5) and 1.2 ± 0.1 X 10-9 M (n = 6), respectively. The IC50 values for hydralazine acting against norepinephrine (3 X 10-6 M) in the uterine arteries from pregnant and nonpregnant patients were 5.1 ± 0.5 X 10-7 M (n = 5) and 4.0 ± 0.5 X 10-7 M (n = 6), respectively. Nitroprusside (10-6 M), compared to hydralazine (10-5 M), produced the greater maximal inhibition of norepinephrine-induced contraction. The results demonstrate that, although both nitroprusside and hydralazine inhibit norepinephrine-induced uterine artery contraction, nitroprusside has a greater potency compared to hydralazine in producing direct vasodilation of the uterine arteries from pregnant and nonpregnant humans.

Relaxation by calcitonin gene-related peptide may involve activation of K+ channels in the human uterine artery

The vasodilatory role of calcitonin gene-related peptide in activating K+ channels was examined in isolated, suffused human uterine arteries. Calcitonin gene-related peptide produced a concentration-dependent relaxation of norepinephrine (1 microM)-induced contractions. Calcitonin gene-related peptide was antagonized by glybenclamide (1-100 microM), an inhibitor of ATP-sensitive K+ channels, but not by tetraethylammonium (1 mM), an inhibitor of calcium(2+)-activated K+ channels. Glybenclamide (10 microM) produced a 6.7 fold and an 11-fold shift to the right of calcitonin gene-related peptide (0.1 to 100 nM) in uterine arteries from pregnant patients (n = 3) and nonpregnant patients (n = 6), respectively. Calcitonin gene-related peptide (10 nM) less effectively (P < 0.05) relaxed contractions produced by KCl (50 mM) (29.4 +/- 1.6%) than by norepinephrine and glybenclamide (10 microM) did not reverse this relaxation (22.2 +/- 6.8%, n = 4 nonpregnant patients). Pinacidil (1 microM), an ATP-sensitive K+ channel opener, relaxed norepinephrine-induced contractions of uterine arteries. Glybenclamide (10 microM) also antagonized pinacidil. These results suggest that calcitonin gene-related peptide relaxes norepinephrine-contracted human uterine arteries, at least in part, by activation of a K+ channel, perhaps of the ATP-sensitive type.

A role for nitric oxide in endotoxin-induced depletion of the peripheral catecholamine stores.

Endotoxemia is associated with increased sympathetic nerve activity and depletion of norepinephrine (NE) and epinephrine (EPI) contents in the adrenal gland and in sympathetically innervated tissues. Endotoxin (bacterial lipopolysacchride [LPS]) causes an increased production of nitric oxide (NO) by inducing nitric oxide synthase (iNOS) expression in various tissues. This increased production of NO contributes significantly to the hypotension associated with endotoxemia. At high concentrations, NO also can act as a neurotoxin. In this study we tested the hypothesis that increased production of NO is involved in depletion of catecholamine content in various tissues from rats treated with a nonlethal dose of LPS. Catecholamine content was measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC) and NOS activity was measured by the 3H-I-arginine to 3H-I-citrulline conversion method. The NE content was decreased in rat adrenal gland, lung, spleen, tail artery, and aorta after LPS. The maximal decrease was at 24 h and returned to control levels at 6 days (144 h). There was no depletion of the NE content in the heart. The EPI content in the adrenal gland was greatly depleted (91%) from 12 to 72 h after LPS. LPS increased the NOS activity in all tissues examined. The time course for NOS activity showed an increase at 3 h, a further increase at 6 h, and a return to control level at 48 h after LPS. The increase in NOS activity occurred before maximal catecholamine depletion. Aminoguanidine, a relatively selective iNOS inhibitor, completely prevented NE depletion in all tissues and partially prevented adrenal EPI depletion induced by LPS. These results are consistent with the hypothesis that LPS-induced production of NO plays a role in depletion of tissue NE and EPI.

DIFFERENTIAL EFFECTS OF PROLONGED SEPTICEMIA ON ISOLATED PULMONARY ARTERIES AND VEINS FROM SHEEP

Isolated third-order pulmonary arteries and veins from sheep were examined for the effects of septicemia on norepinephrine-induced contractions, nitric oxide (NO)-mediated dilation, and basal cyclic GMP levels. The groups studied were as follows: control sheep (n = 7); sheep given live Pseudomonas aeruginosa (Ps, n = 6) for 48 h; and sheep given NG-mono-methyl-L-arginine during the last 24 h of Ps infusion (Ps-L-NMMA, n = 4). The norepinephrine-induced contractions were significantly greater (p < .05) in arteries from septic (Ps and Ps-L-NMMA) sheep. Basal cyclic GMP levels were similar in all of the arteries. The norepinephrine-induced contractions were significantly depressed (p < .05) in veins from septic (Ps and Ps-L-NMMA) sheep. Basal cyclic GMP levels in veins from Ps sheep were markedly elevated (p < .01). N omega-nitro-L-arginine methyl ester (L-NAME) ex vivo decreased cyclic GMP in both arteries and veins. Removal of endothelium enhanced contractions and decreased cyclic GMP in arteries and veins only from control sheep. The results show that septicemia differently affects the pulmonary artery and vein. The enhanced vasoconstriction of the artery is due to decreased endothelium-dependent NO release; the attenuated vasoconstriction of the vein is associated with NO-mediated increased cyclic GMP levels.

Isolated, perfused rabbit ear artery: a model for studying segmental vasoconstriction and dilatation.

Summary Severe increases in blood pressure (BP) are associated with a segmental pattern of constriction and dilatation in small arteries and arterioles, but the pathogenesis is poorly understood. We showed that the isolated, perfused rabbit ear artery typically develops segmental constriction and dilatation when intraluminal pressure is > 160–180 mm Hg during field stimulation of perivascular nerves (>6 Hz) or extra- or intraluminal infusions of norepinephrine (NE > 10-7 M) or phenylephrine (PE) (>5 x 10-7 M). Light, transmission, and scanning electron microscopy showed that the dilated vessel segments initially show endothelial injury with no smooth muscle lesions. After repeated or prolonged exposure to high intraluminal pressure, dilated segments manifest extensive and severe endothelial and smooth muscle damage. Dilated regions also became abnormally permeable to tracer particles (ferritin). Constricted segments did not show evidence of endothelial or smooth muscle injury or hyperpermeability. These changes, i.e., segmental vaso-constriction/dilatation, hyperpermeability, and vessel wall damage localized to dilated segments, are comparable to those that occur in small arteries and arterioles during severe hypertension. We discuss the potential usefulness of the isolated ear artery as a model for studying the pathogenesis and morphology of segmental vasocon-striction/dilatation.

d -Tubocurarine as a Dopaminergic Antagonist in the Rabbit Ear Artery

d-Tubocurarine has been reported to inhibit the action of dopamine on mollusc neurons. The purpose of the present study was to determine whether or not d-tubocurarine acts as an antagonist on mammalian dopamine receptors. The isolated, perfused rabbit ear artery was the experimental preparation used. In this preparation dopamine and apomorphine produced concentration-dependent inhibition of the response to electrical field stimulation of the periarterial sympathetic nerves. d-Tubocurarine antagonized the inhibitory effect of dopamine and apomorphine in a competitive manner. The calculated dissociation constants for d-tubocurarine acting against dopamine and apomorphine were 1.9 ± 0.6 μM and 1.7 ± 0.5 μM, respectively. d-Tubocurarine (1–100 μM), by itself, did not affect the response of the artery to sympathetic nerve stimulation. Other nicotinic antagonists hexamethonium, pancuronium and mecamylamine did not affect the dopamine action. These results suggest that d-tubocurarine could attenuate the therapeutic benefit of dopamine if both of these drugs are administered concomitantly.