Nitric oxide and sodium nitroprusside-induced relaxation of the human umbilical artery

Abstract

In the human umbilical artery (HUA) pre-contracted with the thromboxane mimetic U46619 or with 5-hydroxytryptamine (5-HT), (and pretreated with indomethacin (3 microM) to suppress the synthesis of prostanoids), authentic nitric oxide (NO) evoked concentration-dependent relaxation (pEC(50) 7.05 and 5.99, respectively). In contrast, sodium nitroprusside (SNP) induced relaxation only in U46619 pre-contracted HUA (pEC(50) 6.52). At high (>300 mmHg) vs low (<55 mmHg) oxygen tension the dose-response curves to NO- and SNP-induced relaxations were biphasic and shifted leftward. Preincubation of the arterial rings with the soluble guanylyl cyclase (sGC) inhibitor 1H[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM) shifted the concentration-response curve to NO, reduced the maximal relaxation response to NO (E(max) 71%) and to SNP (E(max) 10%). Pre-exposure of HUA rings to high extracellular K(+) (50 mM) reduced E(max) relaxation responses to NO (36%) and SNP (1%). Pretreatment of the HUA with the K(+) channel inhibitors, tetraethylammonium (TEA, 1 mM), 4-aminopyridine (4-AP, 0.5 mM), charybdotoxin (0.1 microM) or iberiotoxin (0.1 microM) increased the pEC(30) for NO and SNP and changed the shape of the dose-response curves from biphasic to monophasic. Pre-incubation of HUA rings with TEA (1 mM), 4-AP (0.5 mM) and ODQ (10 microM) significantly reduced the NO-induced maximal relaxation (E(max) 26%) but not the pEC(50) (5.60). These data indicate that SNP-induced relaxation in the HUA is primarily mediated via sGC-cyclic GMP whereas NO-induced relaxation also involves the activation of K(V) and K(Ca) channels and a cyclic GMP/K(+) channel-independent mechanism(s).

Figure 1

Concentration-response curves for 5-HT and U46619 alone and after incubation with 0.3 μM nifedipine. Each point represents the arithmetic mean±s.e.mean of n=10 experimental determinations.

Figure 2

Concentration-response curves for NO- and SNP-induced relaxations in 5-HT and U46619 pre-contracted tissue. Each point represents the arithmetic mean±s.e.mean of n=5 and 6 experimental determinations.

Figure 3

Concentration-response curves for NO- and SNP-induced relaxation in U46619 pre-contracted tissue at low (<55 mmHg) versus high (>300 mmHg) oxygen tension. Each point represents the arithmetic mean±s.e.mean of n=5 experimental determinations.

Figure 4

NO-induced relaxation in U46619- or KCl (50 mM)- precontracted HUA. Effects of 0.5 mM 4-AP, 1 mM TEA or 4-AP and TEA together on NO-induced relaxation in U46619- precontracted vessels at high oxygen tension (300 mmHg). Each point represents the arithmetic mean±s.e.mean of n=4–7 experimental determinations.

Figure 5

The effect of selective K_Ca channel inhibitors on NO-induced relaxation in U46619 pre-contracted HUA: 0.1 μM charybdotoxin, and 0.1 μM iberiotoxin. Each point represents the arithmetic mean±s.e.mean of n=3–5 experimental determinations.

Figure 6

SNP-induced relaxation in U46619- and KCl (50 mM)-precontracted vessels. Effects of guanylyl cyclase inhibitors or potassium channel inhibitors on SNP-induced relaxation in U46619 pre-contracted tissue at high oxygen tension (>300 mmHg). Each point represents the arithmetic mean±s.e.mean of n=5–7 experimental determinations.

Figure 7

Concentration-response curves for NO-induced relaxation in U46619 pre-contracted tissue in the absence and presence of guanylyl cyclase inhibitors: 10 μM methylene blue and 10 μM ODQ. Effect of ODQ in the presence of 4-AP or 4-AP and TEA on NO-induced relaxation in U46619 pre-contracted HUA. Each point represents the arithmetic mean±s.e.mean of n=5–7 experimental determinations.

Figure 8

Effect of nifedipine on NO- and SNP-induced relaxation in U46619 pre-contracted HUA. Each point represents the arithmetic mean±s.e.mean of n=5–6 experimental determinations.