Pre-constriction increases nerve-mediated responses in rat pressurised mesenteric arteries

Amjad Shatarat, William Dunn, Vera Ralevic. The University of Nottingham, Nottingham NG7 2UH, United Kingdom.

Under pressurised conditions (90 mmHg), activation of the sympathetic nerves supplying rat mesenteric arteries produced a vasoconstrictor response that was mediated in large part by ATP (Rummery et al, 2007). Since we have shown that partial vasoconstriction enhanced the functional role of ATP as a sympathetic neurotransmitter in the porcine mesenteric arteries (Shatarat et al. 2009), the present study investigated whether functional responses to ATP were enhanced in rat pressurised mesenteric arteries by pre-constriction with the thromboxane mimetic, U46619.

First and second order branch arteries from the mesenteric vascular bed of male Wistar rats (200–250 g) were dissected and set up in a pressure myograph system, as described previously (Rummery et al. 2007). Responses were obtained to electrical field stimulation (EFS) (0.5-10Hz, 20-30V, pulse width 1 ms) before and after partial constriction with U46619. The nature of the neurotransmitter involved in mediating responses to EFS was assessed using either 5-[2-[[2-(2-ethoxyphenoxy) ethyl aminojpropyl]-2-methoxy benzene-sulfonamide HC1 YM-12617 (α1-adrenoceptor antagonist), or 4,4’,4“4’‘’(Carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino)))tetrakis-1,3-benzenedisulfonic acidoctasodium salt (NF 449) (a P2X receptor antagonist). Additionally, responses to exogenous noradrenaline and α,β-methylene ATP were assessed in the absence and presence of U46619.

Under basal tone conditions, YM 12617 (0.1 μM) and NF 449 (0.1 μM) both reduced responses to EFS (at 10Hz by 83 ± 6 % and 70 ± 8 %, respectively (n = 5-7). Inducing vasoconstriction with U46619 (2-10 nM) (reduction in diameter by 21 ± 5 %), enhanced responses to EFS, at 10 Hz by 100 ± 19 % (n = 14). This enhanced response was inhibited by both YM 12617 and NF 449. In the presence of U46619, YM 12617 inhibited the contractile responses to EFS by 65± 9 % at 10 Hz, (n = 8), while NF 499 inhibited the electrically evoked contractile responses by 53 ± 10 %, (n = 5). Responses to exogenous NA (1 µM) were increased after inducing tone with U46619 from 50 ± 17 to 174 ± 25 µm (n = 5). Similarly, responses to α,β-methylene ATP (0.1 µM) were increased from 14 ± 4 to 54 ± 9 µm (n = 7).

In rat pressurised mesenteric arteries activation of the perivascular sympathetic nerves produced a vasoconstriction mediated by NA and ATP. Since YM12617 and NF 449 reduced the response by greater than 50% each, then this indicates a synergistic interaction between these neurotransmitters under basal tone conditions. Inducing vasoconstriction with U46619 greatly enhanced responses to activation of the sympathetic nerves. However, vasoconstriction did not produce a selective enhancement of ATP-mediated responses, since it increased responses to endogenous and exogenous stimulation of both α1-adrenoceptors and P2X receptors.

Rummery et al., (2007) J Physiol 582, 745-754.

Shatarat et al., (2009) pA2 online 7, 4.