MT2 receptors mediate the inhibitory effects of melatonin on nitric oxide-induced relaxation of porcine isolated coronary arteries.

Previous studies from our laboratory demonstrated that melatonin inhibits nitric oxide (NO)-induced relaxation in porcine coronary arteries. The present study was designed to further characterize the mechanisms underlying this inhibitory effect of melatonin. Western immunoblot studies identified the presence of melatonin type 2 (MT(2)) receptors, but not MT(1) or MT(3) receptors, in porcine coronary arteries. Immunohistochemical analysis revealed that MT(2) receptors colocalized with α-actin in the smooth muscle cell layer. In coronary arterial rings suspended in organ chambers for isometric tension recording, melatonin (10(-7) M) inhibited relaxations induced by the exogenous NO donor sodium nitroprusside (SNP; 10(-9) to 10(-5) M) and by the α(2)-adrenoceptor agonist 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline (UK14,304; 10(-9) to 10(-5) M), an endothelium-dependent vasodilator. The inhibitory effect of melatonin on SNP- and UK14,304-induced relaxations was abolished in the presence of the selective MT(2) receptor antagonists 4-phenyl-2-propionamidotetralin (4P-PDOT; 10(-7) M) and luzindole (10(-7) M). In contrast to melatonin, the selective MT(3) receptor agonist 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT; 10(-7) M) had no effect on the concentration-response curves to either SNP or UK14,304. Melatonin (10(-7) M) had no effect on coronary artery relaxation induced by 8-bromoguanosine 3',5'-cyclic monophosphate, but it significantly attenuated the increase in intracellular cyclic GMP levels in response to SNP (10(-5) M). This effect of melatonin was abolished in the presence of 4P-PDOT (10(-7) M). Taken together, these data support the view that melatonin acts on MT(2) receptors in coronary vascular smooth muscle cells to inhibit NO-induced increases in cyclic GMP and coronary arterial relaxation, thus demonstrating a novel function for MT(2) receptors in the vasculature.

Bipolar multi-electrode balloon catheter radiofrequency renal denervation with the Vessix system: preclinical safety evaluation.

AIMS: A bipolar multi-electrode 7 Fr-compatible balloon-catheter radiofrequency (RF) renal denervation system (Vessix™ Renal Denervation System; Boston Scientific, Marlborough, MA, USA) was evaluated for safety in domestic swine. METHODS AND RESULTS: Renal arteries of 27 swine received overlapping treatments proximally/single treatments distally to mimic balloon overlap clinically. Each histopathology cohort (30, 90, 180 days) had four RF-treated and three sham-treated (no RF energy delivered) animals, with the response of artery/surrounding nerves to bilateral treatment examined (42 arteries). Scanning electron microscopy of the renal artery flow surface for endothelialisation was performed in six additional pigs (three at each of 30 and 90 days: 12 arteries) following unilateral whole artery treatment with proximal overlap: RF one side, sham the other side. Power was ~1 watt, treatment duration 30 seconds, target temperature 68°C. Renal histology and assessment for off-target injury was performed in all 27 swine. Renal artery thermal injury was transmural and segmental involving <10% to >90% of the circumference (typically 30-60%) with segmental neointimal hyperplasia exceeding shams but haemodynamically trivial (maximum stenosis 17.7%). Healing of necrotic arterial media was by replacement fibrosis. Overlying nerves also became fibrotic. Endothelialisation was focally incomplete at 30 days but confluent at 90 days. No off-target injury occurred outside the renal arteries. CONCLUSIONS: Safety was demonstrated.

Sirolimus causes relaxation of human vascular smooth muscle: a novel action of sirolimus mediated via ATP-sensitive potassium channels.

Little is known about the vasomotor effects of sirolimus, and preliminary studies using animal models have provided conflicting results. The present study was designed to determine the effects of sirolimus on vasomotor tone in human blood vessels. Human radial artery segments were cut into rings, denuded of endothelium, and placed into organ chambers for isometric tension recording. Sirolimus (10(-10) to 10(-6) M) caused concentration-dependent relaxation of human arteries contracted with U46619 (9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F(2alpha); 10(-8) M) [-log (M) EC(50) (pD(2)) = 7.28 +/- 0.1; E(max) = 57 +/- 6%] or phenylephrine (10(-6) M) (pD(2) = 7.16 +/- 0.4; E(max) = 45 +/- 9%). Sirolimus-induced relaxation was unaffected by treatment with indomethacin (10(-5) M) but was nearly abolished in tissues contracted by depolarization with elevated K(+) (60 mM). In U46619-contracted rings, the response to sirolimus was markedly inhibited in the presence of the specific ATP-sensitive potassium (K(ATP)) channel blocker, glyburide (10(-6) M), but was unaffected by treatment with blockers of large conductance, calcium-activated potassium channel (iberiotoxin, 10(-7) M), small conductance, calcium-activated potassium channel (apamin, 10(-6) M), or voltage-gated potassium channel (4-aminopyridine, 10(-3) M). The K(ATP) channel opener, aprikalim (10(-7) to 10(-5) M), caused concentration-dependent relaxations that were inhibited by glyburide (10(-6) M) and abolished in tissues contracted with elevated K(+) (60 mM), thus confirming that K(ATP) channel opening causes relaxation of these arteries. These data suggest that sirolimus, at concentrations attained in vivo, causes relaxation of human arteries, and this effect is mediated by opening of K(ATP) channels in vascular smooth muscle. Reduced vasomotor tone is a heretofore unrecognized action of sirolimus that could potentially contribute to its efficacy in drug-eluting stents.