Mediation of excitatory neurotransmission by the release of ATP and noradrenaline in sheep mesenteric lymphatic vessels.

Abstract

1. Spontaneous isometric contractions were measured in rings of sheep mesenteric lymphatics. Field stimulation at short pulse widths increased the frequency of spontaneous contractions and this response was blocked by 3 x 1O-7 M w-conotoxin and by 10-6 M guanethidine. 2. Rings that had been incubated with [3H]noradrenaline released 3H in response to field stimulation in a frequency-dependent manner. 3. Exogenous ATP mimicked the response to field stimulation and this was blocked by 10-4M suramin but not by prior desensitization with 10-6 M a,fl-methylene ATP. Exogenous noradrenaline was not blocked by 10-4 M suramin. 4. The excitatory response to field stimulation was not blocked by to-4 M suramin but a combination of 1o-4 M suramin and 3 x 10-6 M phentolamine did block the response.5. In rings taken from sheep that had been pretreated with reserpine, 10-4 M suramin alone blocked the response to field stimulation. 6. The results of this study suggest that the excitatory response to stimulation of intramural nerves in sheep mesenteric lymphatics is mediated by the release of both ATP and noradrenaline.Stimulation of the splanchnic nerve in anaesthetized sheep increases the pumping activity of the main intestinal lymph duct (Harty, McGeown, McHale & Thornbury, 1988) but this response cannot be blocked by adrenergic antagonists (Harty, 1990). This is a surprising finding in view of the considerable body of evidence, both histochemical and functional, that lymphatics of other species have a noradrenergic innervation (Todd & Bernard, 1973;Alessandrini, Gerli, Sacchi, Pucci & Fruschelli, 1981; McHale, 1985 McHale, , 1991McHale, , 1993 VanHelden, 1993 METHODSSegments of main lymphatic duct 5 cm in length and 2 mm in diameter were dissected from the mesenteries of sheep approximately o min after slaughter. The vessels were transported in warmed oxygenated Krebs solution to the laboratory where the surrounding fat and connective tissue was removed from the lymphatic by sharp dissection. Rings of lymphatic 2 mm in diameter and 8 mm in length were then dissected from the main duct, suspended between stainlesssteel hooks and placed into a water-jacketed organ bath (volume 5 ml) maintained at 37 'C. The rings were perfused with Krebs solution of composition (mM): NaCl, 120; NaHCO3, 25-0; KCl, 5.9; Na2HP04, 1-2; CaCl2, 2-5; MgCl2, 12; glucose, 11-0; gassed with 95% 02, 5% CO2. The rings were then adjusted to a tension of [2][3][4]

ATP inhibits pump activity of lymph vessels via adenosine A1 receptor-mediated involvement of NO- and ATP-sensitive K+ channels

“…ATP-induced constrictor responses of bovine and guinea pig mesenteric lymph vessels were significantly antagonized by 10 Ϫ4 M suramine, a blocker of P2X and P2Y receptors (7,13,50). The same concentration of suramine used in the present study did not antagonize the ATPinduced inhibitory responses of intrinsic pump activity in these lymph vessels of rats.…”

“…ATP is also an important physiological substance, in that it regulates contraction and relaxation of smooth muscles in the vascular system (3,38). ATP increased the frequency of spontaneous contractions of lymphatic smooth muscles in sheep (13) and guinea pigs (7,50). In contrast, isolated and precontracted ring preparations of bovine (33), porcine (10), and canine (41) lymph vessels caused relaxation in response to ATP.…”

“…ATP increases the frequency of intrinsic pump activity of mesenteric lymph vessels in sheep (13) and guinea pigs (7,50). The ATP-mediated constrictor response is independent of the function of the endothelium in mesenteric lymph vessels of guinea pigs (50).…”

Tetrodotoxin‐Sensitive Sodium Current in Sheep Lymphatic Smooth Muscle

“…The first of these was that when we used TTX to block the effects of field stimulation of intramural nerves in ring preparations the force of spontaneous contractions was significantly reduced or contractions were completely abolished. This meant that we had to use w-conotoxin to block the effects of intramural nerve stimulation (Hollywood & McHale, 1994) whereas other investigators have used TTX for this purpose in many other smooth muscle preparations (Brock & Cunnane, 1992). The second observation, which fits very well with the existence of a fast sodium current in these vessels, is that cannulated preparations set up to study pumping show remarkably good co-ordination of contraction, with lengths of 6 cm or more contracting almost synchronously, suggesting an efficient propagation of the action potential without significant decrement (Convery, Hollywood, Cotton, Thornbury & McHale, 1997).…”

Kit-like immunopositive cells in sheep mesenteric lymphatic vessels

“…He argued that distension of a segment due to filling with lymph initiated a contraction which propelled lymph into the adjacent segment which dilated it and the process was repeated. This simple explanation, however intuitively appealing, cannot adequately account for the fact that isolated rings of sheep mesenteric lymphatic can contract regularly for many hours under conditions where there is no intraluminal distending force (Hollywood and McHale 1994). This would suggest that there are pacemaker cells capable of generating a regular rhythm even when vessels are empty.…”