ß ₁-adrenoceptor stimulation evokes smooth muscle relaxation in rat small mesenteric arteries by releasing NO (Graves & Poston, 1993). ß-adrenoceptor agonists are also known to stimulate smooth muscle hyperpolarization (Holman et al., 1968), so the aim of this study was to investigate if hyperpolarization contributes to the relaxation evoked with these agonists in the mesenteric artery, and if so whether the endothelium provides any contribution to the response.

Male Wistar rats (200-250g) were killed by cervical dislocation and exsanguination. A 2mm segment of mesenteric artery was dissected and mounted in a Mulvany-Halpern myograph in oxygenated (95%O ₂/5%CO₂) Krebs’ buffer maintained at 37 °C. In some vessels, the endothelium was removed by gently rubbing with a human hair and endothelium-intact or denuded vessels identified by an ability to induce >90% or <10% relaxation with 3 µM ACh in phenylephrine (1-3 µM) constricted arteries, respectively. NO synthase was inhibited with 100 µM L-NAME in some experiments. Measurement of smooth muscle hyperpolarization was made with sharp glass microelectrodes in un-contracted arteries.

Each of the ß-adrenoceptor agonists, isoprenaline, dobutamine (ß ₁-adrenoceptor agonist) and salbutamol (ß ₂-adrenoceptor agonist) evoked 100% arterial relaxation, but with varying potency; EC₅₀=13.6nM, (n=4), 10.5nM, (n=8) and 161nM, (n=6), respectively. During incubation with L-NAME, the isoprenaline concentration-response curve was unaltered (n=14), that for salbutamol was slightly attenuated (n=10) and with dobutamine shifted significantly to the right (n=16, P<0.05). In endothelium-denuded arteries, the curves for all of the agonists were shifted to the right.

Resting membrane potential in endothelium intact and denuded arteries and arteries incubated with L-NAME was -54.5±0.6mV (n=30), -52.2±0.9mV (n=19) and -52.9±1.0mV (n=29), respectively. Each ß-adrenoceptor agonist evoked concentration dependent hyperpolarization, to a maximum of 23.8 ± 3.4 mV (n=7) with isoprenaline, but only ~10mV with dobutamine and salbutamol (n=14 and 9, respectively). Hyperpolarization with isoprenaline and dobutamine were not altered in the presence of L-NAME (n=7 and 16, respectively), but the response to salbutamol was depressed (n=6). In denuded arteries hyperpolarization with dobutamine and salbutamol was depressed by around 50%, while hyperpolarization to isoprenaline was maintained.

ß-adrenoceptor agonists can evoke smooth muscle hyperpolarization and relaxation by both a direct action on mesenteric artery smooth muscle and through the endothelium. While NO contributes to the latter, a component also persists when NO synthase is blocked.

Graves & Poston (1993) Br. J. Pharmacol. 108: 631-637
Holman et al. (1968) J Physiol (Lond). 196: 111-132

Supported by the Wellcome Trust