Inhibition of KATP channels in the rat tail artery by neurally released noradrenaline acting on postjunctional alpha2-adrenoceptors

Sympathetic denervation facilitates L-type Ca2+ channel activation in renal but not in mesenteric resistance arteries

OBJECTIVES

Sympathetic denervation enhances agonist-induced vasoconstriction. This effect may involve altered function of signaling mechanisms such as Rho kinase (Rock) and L-type Ca channels downstream from vasoconstrictor receptors. We tested if enhanced Rock and L-type calcium channel activation contribute to exaggerated norepinephrine-induced vasoconstrictions in renal and mesenteric resistance arteries after sympathectomy.

METHODS

Rats underwent neonatal sympathectomy or sham sympathectomy. Resistance arteries were investigated by small vessel myography. Vascular Rock and L-type Ca channel expression as well as Rock activation were investigated by quantitative real-time PCR and Western blot. Vascular smooth muscle cell (VSMC) membrane potential was recorded with microelectrodes.

RESULTS

Sympathetic denervation enhanced norepinephrine sensitivity in renal and mesenteric arteries. Both, Rock inhibition or L-type Ca inhibition shifted the norepinephrine concentration-response curve to the right. This effect was more pronounced in renal than in mesenteric arteries from sympathectomized vs. sham-sympathectomized animals. The L-type Ca channel activator S-(-)-BayK8644 elicited strong vasoconstrictions only in renal arteries from sympathectomized rats. Rock activity and L-type Ca channel α-subunit expression were similar in renal arteries from sympathectomized and sham-sympathectomized animals. VSMC membrane potential was -57.5 ± 2.0 and -64.3 ± 0.3 mV (P < 0.01), respectively, in renal arteries from sympathectomized and from sham-sympathectomized rats. Depolarization enhanced and KATP channel activation abolished S-(-)-BayK8644-induced contractions in renal arteries from sympathectomized rats.

CONCLUSION

Sympathetic denervation enhances L-type Ca channel-dependent signaling in renal but not in mesenteric arteries. This effect may be partly explained by the decreased VSMC membrane potential in denervated renal arteries.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Removal of half the sympathetic innervation does not reduce vasoconstrictor responses in rat tail artery

Following reinnervation of denervated rat tail arteries, nerve-evoked contractions are at least as large as those evoked in normally innervated arteries despite a much lower nerve terminal density. Here nerve-evoked contractions have been investigated after transection of half the sympathetic innervation of normal tail arteries. After 1 week, the noradrenergic plexus 50-70 mm along the tail was about half as dense as control. Excitatory junction potentials recorded in smooth muscle cells of arterial segments isolated in vitro were half their normal amplitude. Surprisingly, nerve-evoked contractions of isometrically mounted segments were not reduced in amplitude, as was also the case after only 3 days. After 1 week, enhancement of nerve-evoked contractions by blocking either neuronal re-uptake of noradrenaline with desmethylimipramine or prejunctional α2-adrenoceptors with idazoxan was similar to control, suggesting that these mechanisms are matched to the number of innervating axons. The relative contribution of postjunctional α2-adrenoceptors to contractions evoked by long trains of stimuli was enhanced but that of α1-adrenoceptors was unchanged. Transiently, sensitivity to the α1-adrenoceptor agonist phenylephrine was slightly increased. After 7 weeks, amplitudes of nerve-evoked contractions remained similar to control, and sensitivity to phenylephrine had recovered but that to the α2-adrenoceptor agonist clonidine was slightly raised. The normal amplitude of nerve-evoked contractions after partial denervation is only partly explained by the greater contribution of α2-adrenoceptors. While the post-receptor mechanisms activated by nerve-released transmitter may be modified to amplify the contractions after partial denervation, our findings suggest that these mechanisms are normally saturated, at least in this artery.

K(ATP) channel action in vascular tone regulation: from genetics to diseases

ATP-sensitive potassium (K(ATP)) channels are widely distributed in vasculatures, and play an important role in the vascular tone regulation. The K(ATP) channels consist of 4 pore-forming inward rectifier K(+) channel (Kir) subunits and 4 regulatory sulfonylurea receptors (SUR). The major vascular isoform of K(ATP) channels is composed of Kir6.1/SUR2B, although low levels of other subunits are also present in vascular beds. The observation from transgenic mice and humans carrying Kir6.1/SUR2B channel mutations strongly supports that normal activity of the Kir6.1/SUR2B channel is critical for cardiovascular function. The Kir6.1/SUR2B channel is regulated by intracellular ATP and ADP. The channel is a common target of several vasodilators and vasoconstrictors. Endogenous vasopressors such as arginine vasopressin and α-adrenoceptor agonists stimulate protein kinase C (PKC) and inhibit the K(ATP) channels, while vasodilators such as β-adrenoceptor agonists and vasoactive intestinal polypeptide increase K(ATP) channel activity by activating the adenylate cyclase-cAMP-protein kinase A (PKA) pathway. PKC phosphorylates a cluster of 4 serine residues at C-terminus of Kir6.1, whereas PKA acts on Ser1387 in the nucleotide binding domain 2 of SUR2B. The Kir6.1/SUR2B channel is also inhibited by oxidants including reactive oxygen species allowing vascular regulation in oxidative stress. The molecular basis underlying such a channel inhibition is likely to be mediated by S-glutathionylation at a few cysteine residues, especially Cys176, in Kir6.1. Furthermore, the channel activity is augmented in endotoxemia or septic shock, as a result of the upregulation of Kir6.1/SUR2B expression. Activation of the nuclear factor-κB dependent transcriptional mechanism contributes to the Kir6.1/SUR2B channel upregulation by lipopolysaccharides and perhaps other toll-like receptor ligands as well. In this review, we summarize the vascular K(ATP) channel regulation under physiological and pathophysiological conditions, and discuss the importance of K(ATP) channel as a potentially useful target in the treatment and prevention of cardiovascular diseases.

Role of vascular K(ATP) channels in blood pressure variability after sinoaortic denervation in rats

AIM

To investigate the role of ATP-sensitive potassium (K(ATP)) channels on blood pressure variability (BPV) in sinoaortic denervated (SAD) rats.

METHODS

SAD was performed on male Sprague-Dawley rats 4 weeks before the study. mRNA expression of Kir6.1, Kir6.2 and SUR2 in aorta and mesenteric artery was determined using real-time quantitative polymerase chain reaction, and confirmed at the protein level using Western blotting and laser confocal immunofluorescence assays. Concentration-response curves of isolated aortic and mesenteric arterial rings to adenosine and pinacidil were established. Effects of K(ATP) channel openers and blocker on BPV were examined in conscious SAD rats.

RESULTS

Aortic SUR2 expression was significantly greater, while Kir6.1 was lower, in SAD rats than in sham-operated controls. In contrast, in the mesenteric artery both SUR2 and Kir6.1 expression were markedly lower in SAD rats than controls. For both arteries, Kir6.2 expression was indistinguishable between sham-operated and SAD rats. These findings were confirmed at the protein level. Responses of the aorta to both adenosine and pinacidil were enhanced after SAD, while the mesenteric response to adenosine was attenuated. Pinacidil, diazoxide, nicorandil, and glibenclamide significantly decreased BPV.

CONCLUSION

These findings indicate that expression of vascular K(ATP) channels is altered by chronic SAD. These alterations influence vascular reactivity, and may play a role in the increased BPV in chronic SAD rats.

Effects of nicorandil on sympathetic neurotransmission in rat caudal artery

1. We examined the effects of nicorandil, an ATP-sensitive potassium (K(ATP)) channel opener and nitric oxide donor, on the release of noradrenaline from vascular sympathetic nerves. This effect was compared to the effect on vascular smooth muscle. 2. Caudal artery preparations from Wistar rats were electrically stimulated (1 Hz, 0.5-ms duration) and noradrenaline release in the artery was detected using an high-pressure liquid chromatography-electrochemical detection technique. The pharmacological properties of the prejunctional effect of nicorandil were determined using the nonselective K(ATP) channel blocker glibenclamide, the pancreatic beta-cell and brain-type K(ATP) channel blocker tolbutamide, and the smooth muscle-type K(ATP) channel blocker PNU-37883A. 3. Nicorandil inhibited the electrical stimulation-evoked noradrenaline release in a concentration-dependent manner. This inhibitory effect was abolished by 1 micromol/L glibenclamide and 10 micromol/L tolbutamide, but was not affected by 10 micromol/L PNU-37883A or 0.3 micromol/L ODQ. Nicorandil did not affect the noradrenaline transporter uptake 1 in the adrenergic nerve terminals. 4. Nicorandil produced a relaxation response in a concentration-dependent manner in the caudal artery pre-contracted with 0.3 micromol/L noradrenaline. This relaxation response was significantly diminished in the presence of 1 micromol/L glibenclamide, 10 micromol/L PNU-37883A and 0.3 micromol/L ODQ but not by 10 micromol/L tolbutamide. 5. These findings suggest that nicorandil inhibits noradrenaline release via the K(ATP) channels of sympathetic nerves. These channels may be pharmacologically different from those of vascular smooth muscle.

ATP is the predominant sympathetic neurotransmitter in rat mesenteric arteries at high pressure

Most studies of neurovascular transmission in isolated small mesenteric arteries have used either isometric recording techniques or measured vasoconstriction in vessels with no distending pressure. Here we have used pressure myography to assess the contribution of noradrenaline and ATP to sympathetic neurotransmission in rat second-order mesenteric arteries. In arteries pressurized to 30 or 90 mmHg, activation of sympathetic axons with trains of electrical stimuli (50 pulses, 0.5-10 Hz) evoked frequency-dependent vasoconstrictions that increased in amplitude at higher pressure. In the presence of the P2-receptor antagonist suramin (0.1 mM), the amplitude of vasoconstrictions to trains at 2 and 10 Hz did not differ at 30 and 90 mmHg. In contrast, in the presence of the alpha(1)-adrenoceptor antagonist prazosin (0.1 microm) vasoconstrictions at 90 mmHg were larger than those at 30 mmHg. At both pressures, the combination of prazosin and suramin virtually abolished constrictions. The purinergic component of vasoconstriction (prazosin-resistant) was almost abolished by the L-type Ca(2+) channel antagonist nifedipine (1 microm). Increasing pressure from 30 to 90 mmHg decreased the resting membrane potential and increased the amplitude of purinergic excitatory junction potentials. These findings indicate that the contribution of ATP to neurovascular transmission increases when the pressure is raised from 30 to 90 mmHg, which is similar to the pressure second-order mesenteric arteries experience in vivo, and that Ca(2+) influx through L-type Ca(2+) channels is largely responsible for purinergic activation of the vascular smooth muscle.