ATP and UTP stimulate spreading dilatation responses in rat isolated small mesenteric arteries

Polly Winter & Kim A Dora, Department of Pharmacy & Pharmacology, University of Bath, Bath BA1 7AY.

Levels of ATP achieved in the circulation are consistent with a key autacoid role. Luminal infusion of ATP and UTP each stimulate robust endothelium-derived hyperpolarizing factor (EDHF)-mediated dilatation in rat small mesenteric arteries (Liu et al., 2004) involving stimulation of Ca2+-activated K+ channels (Winter et al., 2005). Therefore we explored whether the dilatation to these agonists could propagate through the wall of these resistance arteries and evoke spreading dilatation responses, as we have previously observed with ACh (Takano et al., 2004).

Male Wistar rats (200-250g) were killed by schedule 1 methods of the Animals (Scientific Procedures) Act 1986 (UK). For luminal application of agonists, the three ends of 2nd and 3rd order mesenteric arteries with a bifurcation at the downstream end were cannulated, mounted in a heated chamber and pressurized to 50mmHg. In the continued presence of luminal flow through the feed artery, solutions containing agonists and carboxyfluorescein (0.1μM) were infused though a side branch using a multichannel syringe pump. For abluminal delivery, agonists were applied focally from micropipettes at the downstream end of cannulated arteries as previously described (Takano et al., 2004). All arteries were maintained at 37oC and continuously superfused at 2mL.min-1 with MOPS buffer containing phenylephrine (at a concentration sufficient to evoke a 30-50% decrease in artery diameter) and the NO synthase inhibitor L-NAME (100μM). Arteries were visualized with conventional brightfield and confocal fluorescence microscopy (4x objective) and outer diameter and fluorescence from the infused dye simultaneously measured at multiple positions along the artery.

Spreading responses to infused ATP (1-3μM, n=6) and UTP (3-10μM, n=6) consisted of a near maximal local (Branch 1) EDHF-type dilatation that spread rapidly from the branch into the adjacent feed artery and upstream against the direction of luminal flow (ATP, Branch 90.0±3.7%, 0μm 80.8±3.5%, 2000μm 8.7±2.5%; UTP, Branch 81.5±6.7%, 0μm 75.8±10.8, 2000μm 9.8±1.1%). The extent of decay of dilatation was similar to acetylcholine (n=7). In contrast, focal, abluminal application of either ATP (1mM, 30-300ms, n=4) or UTP (1mM 30-1000ms, n=4) evoked a local constriction (ATP, 50.2±6.6%; UTP, 66.2±13.9%) that did not spread and only in the case of ATP, was accompanied by a spreading dilatation response (0μm 17.5±7.5%, 2000μm 21.1±11.5%). The non-hydrolyzable ADP analogue ADPβS caused robust local and spreading vasodilatation responses, whether applied luminally (Branch 1, 85.8±5.0%, 0μm 80.0±6.6%, 2000μm 9.3±4.2%) or abluminally (0μm 90.3±5.9%, 2000μm 76.1±4.7%). These data demonstrate that direct stimulation of the endothelium of rat mesenteric arteries with ATP or UTP can facilitate a robust local and spreading dilatation response that may play an important physiological role in controlling blood flow.

Liu, C. et al. 2004. Am J Physiol Heart Circ Physiol 286: H1688-1695
Takano, H. et al. 2004. J Physiol 556: 887-903
Winter, P. et al. 2005. From the Institute of Education, London Winter 2005http://www.pa2online.org/abstracts/Vol3Issue4abst039P.pdf