Differential effects of pinacidil and cromakalim on vascular relaxation and sympathetic neurotransmission

Potassium channel opener pinacidil induces relaxation of the isolated human radial artery

Taking into consideration that the search for drugs capable of modifying blood flow through human radial artery (RA) is warranted, the present study was designed to examine the vasodilatatory effects of the potassium channel opener, pinacidil on the RA and to define the contribution of different K+ -channel subtypes in the endothelium-independent pinacidil action on this blood vessel. Pinacidil relaxed the RA rings with endothelium and without endothelium with comparable potency. N-nitro-L-arginine methyl ester (L-NAME) and methylene blue did not affect the pinacidil-induced vasorelaxation in rings with endothelium. In the rings without endothelium, the K+ -channel blockers glibenclamide and tetraethylammonium (TEA) moderately antagonized the pinacidil-induced relaxation, while charybdotoxin and 4-aminopiridine did not. In endothelium-denuded rings, precontracted with 100 mM K+, the relaxant responses to pinacidil were highly significantly shifted to the right compared to those obtained in RA precontracted with phenylephrine, but pinacidil-induced maximal relaxation was not affected. Addition of nifedipine did not but addition of nifedipine and nickel (Na+ -Ca2+ exchanger inhibitor) did cause a statistically significant rightward shift of the pinacidil concentration-relaxation curve, although the effect 0.1 mM pinacidil was preserved. Thus, pinacidil induces relaxation of the human RA in endothelium-independent manner, and glibenclamide- and TEA-sensitive vascular smooth muscle K+ channels are probably involved. Its ability to completely relax the RA precontracted with K+ -rich solution suggests that pinacidil has additional K+ channel-independent mechanism(s) of action. It seems that stimulation of the forward mode of the Na+ -Ca2+ exchanger plays a part in this K+ channel-independent effect of pinacidil.

Ca2+-activated K+ channels in the endothelial cell layer involved in modulation of neurogenic contractions in rat penile arteries

The present study was designed to investigate the functional K+ channels involved in contractions induced by electrical field stimulation in isolated rat penile arteries. Blockers of Ca2+-activated K+ channels (KCa), tetraethylammonium, and of large-conductance KCa channels, charybdotoxin and iberiotoxin, as well as a blocker of voltage-dependent K+ channels (KV), 4-aminopyridine, increased resting tension in penile small arteries. In the presence of propranolol and NG-nitro-L-arginine (L-NOARG), electrical field stimulation evoked prazosin-sensitive contractions. In endothelium-intact preparations, these latter contractions were enhanced in the presence of tetraethylammonium and charybdotoxin. However, these blockers did not enhance contractions evoked by exogenously added noradrenaline. Endothelial cell removal increased the neurogenic contractions but tetraethylammonium had no further potentiating effect in these preparations. In the presence of an inhibitor of cyclooxygenase, indomethacin, and inhibitor of nitric oxide (NO) synthase, L-NOARG, acetylcholine evoked relaxations, which were abolished in the presence of either tetraethylammonium or charybdotoxin. In phenylephrine-contracted arteries treated with guanethidine and atropine, electrical field stimulation evoked relaxations, which were partially inhibited by L-NOARG and tetraethylammonium, without any additive effect of these drugs. These observations suggest that both large-conductance KCa channels and KV channels sensitive to iberiotoxin/tetraethylammonium and 4-aminopyridine, respectively, are directly involved in the modulation of myogenic tone of rat penile arteries. Furthermore, activation of endothelial intermediate-conductance KCa channels sensitive to tetraethylammonium and charybdotoxin leads to release of a non-NO nonprostanoid factor, which inhibits release of the neurotransmitter, noradrenaline, but these channels do not appear to be involved in inhibition of contraction evoked by exogenously applied noradrenaline in rat penile arteries.

Delayed rectifier potassium channels contribute to the depressed pulmonary artery contractility in pneumonia

We investigated the role of K(+) channels in the attenuated pulmonary artery (PA) contractility characteristic of acute Pseudomonas pneumonia. Contractility of PA rings from the lungs of control or pneumonia rats was assessed in vitro by obtaining cumulative concentration-response curves to the contractile agonists KCl, phenylephrine, or PGF(2 alpha) on PA rings before and after treatment with K(+) channel blockers. In rings from pneumonia rats, paxilline (10 microM), tetraethylammonium (2 mM) (blockers of large-conductance Ca(2+)-activated K(+) channels), and glybenclamide (ATP-sensitive K(+) channel blocker, 80 microM) had no significant effect on the attenuated contractile responses to KCl, phenylephrine, and PGF(2 alpha). However, 4-aminopyridine (2 mM), a blocker of voltage-gated K(+) channels (delayed rectifier K(+) channel) reversed this depressed contractility. Therefore, large-conductance Ca(2+)-activated K(+) and ATP-sensitive K(+) channels do not contribute to the attenuated PA contractility observed in this model of acute pneumonia. In contrast, 4-aminopyridine enhances contraction in PA rings from pneumonia lungs, consistent with involvement of a voltage-gated K(+) channel in the depressed PA contractility in acute pneumonia. Unraveling the precise mechanism of attenuated contractility in pneumonia could lead to innovative therapies for the pulmonary vascular abnormalities associated with this disease.

Sulphonylurea-sensitive channels and NO-cGMP pathway modulate the heart rate response to vagal nerve stimulation in vitro

Sulphonylurea-sensitive K(+)channels (K(ATP)) have been implicated in the release of acetylcholine (ACh) from the vagus nerve in the heart. Our aim was to establish the functional significance of this and to test whether this modulation could interact with stimulation of the NO-cGMP pathway that facilitates the decrease in heart rate (HR) in response to vagal nerve stimulation (VNS). We studied the effect of activation (diazoxide, 100 microM) and inhibition (glibenclamide 30 microM or tolbutamide 5 microM) of K(ATP)channels, and activation of the NO-cGMP pathway with the NO donor, sodium nitroprusside (SNP, 20 microM) or the cGMP analogue, 8-Br-cGMP (0.5 m M) on the HR response to VNS in the isolated guinea pig (Cavia porcellus) double atrial/right vagus preparation (n=40). Tolbutamide increased the bradycardia in response to vagal stimulation at 3 and 5 Hz (P<0.05); effects that were reversed by diazoxide. Glibenclamide also significantly increased the HR response to VNS at 1 and 3 Hz (P<0.05). Diazoxide alone significantly attenuated the HR response to VNS at 5 Hz (P<0.05). Neither glibenclamide nor diazoxide affected the HR response to carbamylcholine (CCh, 50-200 n M). In the presence of a maximal dose of tolbutamide, SNP or 8-Br-cGMP further increased the HR response to VNS at 5 Hz (P<0.05). These results are consistent with the hypothesis that inhibition of sulphonylurea-sensitive channels can increase the HR response to VNS by a pre-synaptic mechanism, and that this modulation may be independent of activation of the NO-cGMP pathway.

Channel activators regulate ATP-sensitive potassium channel (KIR6.1) expression in chick cardiomyocytes

ATP-sensitive potassium channels (K(ATP)) are widely expressed and yet little is known about the mechanisms regulating their expression. Here we report that expression of chick heart Kir6.1 is regulated by channel activators. Activation of K(ATP) with either ATP depletion or pinacidil, up-regulated Kir6.1 mRNA 1.8- to 2.4-fold in cultured ventricular myocytes as measured by competitive PCR. Pinacidil treatment also increased Kir6.1 protein as detected using an antibody to Kir6.1. Glibenclamide, a K(ATP) inhibitor, completely blocked the pinacidil-induced increase in Kir6.1 levels. It appears that Kir6.1 is up-regulated by an unknown signal transduction pathway initiated by K(ATP) opening.

Adrenotensin: an adrenomedullin gene product contracts pulmonary blood vessels

The purpose of the present study was to determine the effects of adrenotensin, a newly described product of the ADM gene, on cat pulmonary arterial (PA) rings. Under resting conditions, adrenotensin increased tension of PA rings in a concentration-dependent manner. Although addition of diphenhydramine, ONO-3708, phentolamine, methysergide, atropine, and meclofenamate did not alter the contractile response to adrenotensin, removal of the endothelial cell layer significantly reduced this response. Moreover, precontraction of PA rings with adrenotensin selectively attenuated the pulmonary vasorelaxant response to ADM but not to other vasodilator substances, including isoproterenol, pinacidil, nifedipine, and adenosine. The present data suggest that adrenotensin acts in an endothelium-dependent manner to contract PA rings. Moreover, the present data suggest that adrenotensin may act in a modulatory manner to influence vasorelaxation in response to ADM, a sister proADM product.

Effects of pinacidil on arterial and venous resistances and mean circulatory filling pressure in rats

1. The effects of the potassium channel opener, pinacidil, on mean arterial pressure (MAP), mean circulatory filling pressure (MCFP), total peripheral resistance (TPR), cardiac output (CO) and resistance to venous return (Rv) were studied in rats. 2. In pentobarbitone-anaesthetized rats given mecamylamine (ganglionic blocker, 3.7 micrograms kg-1) and noradrenaline (1.5 micrograms kg-1 min-1) to suppress autonomic reflexes, pinacidil (60 and 180 micrograms kg-1 min-1), relative to the vehicle, reduced MAP and TPR in a dose-dependent manner but did not significantly alter CO, MCFP or RV. 3. Pinacidil (10-300 micrograms kg-1 min-1) caused similar increases in MCFP, an inverse index of venous compliance, and similar dose-dependent reductions in mean arterial pressure (MAP) in conscious, intact rats and rats infused with the ganglionic blocker, hexamethonium (150 micrograms kg-1 min-1). In rats with vasomotor tone elevated by the infusion of noradrenaline (1.5 micrograms kg-1 min-1), pinacidil caused markedly greater depressor responses but did not significantly alter MCFP. 4. Our results show that pinacidil is an efficacious vasodilator of arterial resistance blood vessels but has little venodilator activity.

Femoral vasodilation to cromakalim is blocked by U37883A, a non-sulphonylurea that selectively inhibits KATP channels

The purpose of the present study was to determine the effects of U37883A, a non-sulphonylurea inhibitor of KATP channels, in the femoral vascular bed of the anaesthetized dog. Administration of U37883A, 4-morpholinecarboxamidine-N-1-adamentyl-N"-cyclohexyl hydrochloride (2.5 mg kg-1, i.v.), significantly inhibited the femoral vasodilator response to intra-femoral arterial injection of cromakalim, an activator of KATP channels. In contrast, U37883A had no effect on the femoral vasodilator responses to nitroglycerin, isoprenaline, 5-HT, or 5-carboxamidotryptamine, suggesting this agent is a novel and selective inhibitor of hindlimb vasodilation induced by KATP-channel activation. Since U37883A did not significantly alter baseline femoral blood flow and femoral vascular resistance, the present data suggest that KATP channels do not contribute, in large measure, to regulating the canine femoral vascular bed under resting conditions in-vivo.