Programme Contents

Do K⁺ channels play a role in noradrenergic signalling in vascular smooth muscle?

The mechanisms underlying depolarization of arterial smooth muscle by nerve-released noradrenaline (NA) remain largely unknown. In isolated vascular smooth muscle cells, applied NA produces an inward current by activating Ca²⁺-activated Cl^- channels (Hogg et al., 1994) and transient receptor potential (TRP)-like cation ion channels (Albert & Large, 2006). In rat iridial arterioles (Gould and Hill, 1996) and guinea-pig mesenteric veins (Van Helden, 1988), nerve-released NA produces a transient depolarization that is mediated by Ca²⁺-activated Cl^- channels. However, there is no evidence that TRP-like cation channels contribute to nerve-evoked depolarization. In rat tail artery and guinea-pig mesenteric vein, nerve-released NA produces a slow phase of depolarization that is associated with a decrease in membrane conductance, indicating closure of K⁺ channels (Cassell et al., 1988; Van Helden, 1988). During ongoing nerve activity, this slow synaptic potential produces 15-20 mV of depolarization and contributes to constriction of the tail artery (Brock et al., 1997). We have been investigating the mechanisms that underlie this depolarization.

Tail arteries were isolated from rats that had exsanguinated under deep anaesthesia (80 mg/kg pentobarbitone, i.p.). Artery segments were mounted in a recording chamber and the perivascular axons were electrically stimulated. Intracellular recordings were made from the smooth muscle cells. In rat tail artery, short trains of stimuli evoke both ATP-mediated excitatory junction potentials (EJPs) and a slow NA-mediated depolarization (NAD). Application of the α₁-antagonist prazosin (0.1 μM) slowed the rising phase of the NAD but did not change its amplitude. In contrast, the α₂-antagonist rauwolscine (1 μM) did not change the onset of the NAD but it did reduce its amplitude. In the presence of prazosin, the NAD was completely blocked by the K_ATP channel blockers, glybenclamide (10 μM, n = 6) and PNU 37883A (5 μM, n = 6). These agents also produced membrane depolarization. The α₂-adrenoceptor-mediated component of the NAD is produced by closure of K_ATP channels.

The NAD remaining when α₂-adrenoceptors were blocked with rauwolscine (1 μM) was increased in amplitude by glybenclamide (10 μM, n = 5). In rat tail artery, the time constant of decay of the EJP (τEJP) is determined by the membrane time constant (Cassell et al., 1988). The τEJP of EJPs evoked at the peak of the rauwolscine-resistant NAD was prolonged (relative change 1.16, p < 0.01, n = 6). Similarly, the τEJP was prolonged during depolarization induced by the α₁-agonist, phenylephrine (0.5-1 μM, n = 5). These findings indicate a decrease in membrane conductance, suggesting that α₁-adrenoceptor-mediated depolarization is also produced by closure of K⁺ channels. The rauwolscine-resistant NAD was unaffected by the Cl^- channel blockers, 9-anthracene carboxylic acid (100 μM, n = 5) and niflumic acid (10 μM, n = 5) or by the non-selective cation channel blocker, SKF 96365 (10 μM, n = 4).

Broad-spectrum K⁺ channel blockers (tetraethylammonium, 4-aminopyridine, Ba²⁺) did not inhibit the rauwolscine-resistant NAD. In CNS neurones, NA produces depolarization by closing the two-pore domain K⁺ channel, TASK-1, but the selective blocker of these channels, anandamide (10 μM, n = 5), did not change the NAD. In heart, NA closes a Na⁺-dependent K⁺ channel that is blocked by quinidine. Quinidine (10 μM, n = 5) produced depolarization, slowed the τEJP and reduced the NAD. However, quinidine is reported to be an α₁-adrenoceptor antagonist.

These findings indicate that the NAD has two components: one of which is due to activation of α₁-adrenoceptors and the other to activation of α₂-adrenoceptors. The α₂-adrenoceptor-mediated component is produced by closure of K_ATP channels whereas α₁-adrenoceptor-mediated component is most likely mediated by closure of another type of K⁺ channel.

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