Endothelium‐dependent relaxation to acetylcholine in bovine oviductal arteries: mediation by nitric oxide and changes in apamin‐sensitive K⁺ conductance

Abstract

1 Mechamisms underlying the relaxant response to acetylcholine (ACh) were examined in bovine oviductal arteries (o.d. 300-500 pm and i.d. 150-300 pm) in vitro. Vascular rings were treated with indomethacin (10 pM) to prevent the effects of prostaglandins. 2 ACh elicited a concentration-related relaxation in ring segments precontracted with noradrenaline (NA), which was abolished by endothelium denudation. 3 The ACh-induced relaxation was attenuated but not abolished by NG-nitro-L-arginine (L-NOARG, 1 gM-1 mM), an inhibitor of nitric oxide (NO) formation. The inhibition caused by L-NOARG (10 pM) was reversed by addition of excess of L-arginine but not D-arginine (1 mM). 4 In high K+ (40-60 mM)-contracted rings, ACh was a much less effective vasodilator and its relaxant response was completely abolished by L-NOARG (100 pM).5 In NA (10 gM)-contracted rings, ACh induced sustained and concentration-dependent increases in cyclic GMP, which were reduced below basal values by L-NOARG (100 pM), while potent relaxation persisted. Similar increases in cyclic GMP were evoked by ACh in high K+ (50 mM)-treated arteries and under these conditions, both cyclic GMP accumulation and relaxation were L-NOARG-sensitive. 6 S-nitroso-L-cysteine (NC), a proposed endogenous precursor of endothelial NO, also induced cyclic GMP accumulation in NA-contracted oviductal arteries. 7 Methylene blue (MB, 10 pM), a proposed inhibitor of soluble guanylate cyclase, inhibited both endothelium-dependent relaxation to ACh and endothelium-independent response to exogenous NO, whereas relaxation to NC remained unaffected. 8 The L-NOARG-resistant response to ACh was not affected by either ouabain (0.5 mM), glibenclamide (3 pM), tetraethylammonium (TEA, 1 mM) or charybdotoxin (50 nM), but was selectively blocked by apamin (0.1-1 pM). However, apamin did not inhibit either relaxation to ACh in high K+-contracted rings or endothelium-independent relaxation to either NO or NC. 9 Apamin and MB inhibited ACh-induced relaxation in an additive fashion, suggesting the involvement of two separate modulating mechanisms. 10 These results suggest that ACh relaxes bovine oviductal arteries by the release of two distinct endothelial factors: a NO-like substance derived from L-arginine, which induces cyclic GMP accumulation in smooth muscle, and another non-prostanoid factor acting by hyperpolarization mechanisms through alterations in apamin-sensitive K+ conductance.

Cardiovascular Effects Elicited by Milonine, a New 8,14‐Dihydromorphinandienone Alkaloid

“…TEA was used for blocking BK Ca channels [37], apamin for SK Ca channels [41] and charybdotoxin for IK Ca as well as BK Ca channels [42]. In our experiments, treatment with TEA (1 mM) produced a shift to the right of the concentrationresponse curve for milonine, suggesting a role for the BK Ca .…”

Role of potassium channels in the nitrergic nerve stimulation‐induced vasodilatation in the guinea‐pig isolated basilar artery

“…Our results also differ from those obtained in various regions of gastrointestinal tract, where the evidence strongly supports the involvement of apamin‐sensitive K Ca channels in nitrergic transmission (Christinck et al , 1991; Ward et al , 1992; Martins et al , 1995; Serio et al , 1995). The concentration of apamin used in the present study was shown to be effective in blocking vasodilator responses mediated by an acetylcholine‐induced endothelium‐derived hyperpolarizing factor (EDHF) in the rat and rabbit mesenteric and bovine oviductal arteries (Adeagbo & Triggle, 1993; Murphy & Brayden, 1995; Garcia‐Pascual et al , 1995). However, our result is consistent with recent findings in the horse deep penile arteries in which it has been shown that nitrergic stimulation‐induced vasodilatations were partly mediated by large‐conductance K Ca channels (Simonsen et al , 1995).…”

Urothelium‐derived inhibitory factor(s) influences on detrusor muscle contractility in vitro

“…Not all potassium channels however are blocked by TEA and one of the major TEA‐insensitive channels is the small calcium activated potassium channel which can be blocked by apamin. The inhibition induced by the urothelium was not affected by apamin, which has also been shown to inhibit the effects of endothelium‐derived hyperpolarizing factor (EDHF) in some vascular tissues (Garcia‐Pascual et al ., 1995). In rat hepatic arteries, potassium has been identified as the EDRF responsible for mediating relaxation responses to acetylcholine, local elevation of myoendothelial potassium levels activating K + channels and Na + K + ATP'ase (Edwards et al ., 1998).…”