Effects of pinacidil on the sympatho-adrenal system in dogs

Roles of ATP sensitive potassium channel in modulating gastric tone and accommodation in dogs

OBJECTIVE

The ATP sensitive potassium (K_ATP) channel plays an important role in the regulation of resting membrane potential and membrane excitability. The role of the K_ATP channel in modulating gastric motility is unclear. The aim of this study was to investigate the role and mechanism of the K_ATP channel in modulating gastric tone and accommodation in dogs.

MATERIALS AND METHODS

Gastric volume under a constant pressure reflecting gastric tone was measured using a barostat device in dogs equipped with a gastric cannula. Gastric accommodation was evaluated by the difference in gastric volume before and after a liquid meal. The roles of cholinergic and nitrergic pathways in the inhibitory effect of pinacidil (a K_ATP opener) were assessed.

RESULTS

1) Pinacidil dose-dependently decreased gastric tone at a dosage of 30 (p = 0.628), 100 (p = 0.013) and 300 μg kg^-1 (p < 0.001). 2) Glibenclamide, a K_ATP blocker, completely blocked the inhibitory effect of pinacidil on gastric tone. 3) Atropine did not block the inhibitory effect of pinacidil on gastric tone but N_ω-Nitro-L-arginine methyl ester markedly attenuated the inhibitory effect of pinacidil (p = 0.004). 4) Glibenclamide significantly reduced gastric accommodation (p < 0.001) while pinacidil had no effects on gastric accommodation. 5) Glibenclamide significantly reduced nitric oxide donor sodium nitroprusside-induced gastric relaxation.

CONCLUSIONS

These findings indicate that the K_ATP channel plays an important role in modulating gastric tone and accommodation in dogs. The inhibitory effect of pinacidil on gastric tone was through the nitrergic pathway as well as acting directly on smooth muscle cells.

Adenosine production and its interaction with protection of ischemic and reperfusion injury of the myocardium

Adenosine exerts cardioprotective effects on the ischemic myocardium. A flexibly mounted microdialysis probe was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5'-nucleotidase (a key enzyme responsible for adenosine production) in in vivo rat hearts. The level of adenosine during perfusion of adenosine 5'-adenosine monophosphate (AMP) was given as an index of the activity of ecto-5'-nucleotidase in the tissue. Endogenous norepinephrine (NE) activates both alpha(1)-adrenoceptors and protein kinase C (PKC), which, in turn, activates ecto-5'-nucleotidase via phosphorylation thereby enhancing the production of interstitial adenosine. Histamine-release NE activates PKC, which increased ecto-5'-nucleotidase activity and augmented release of adenosine. Opening of cardiac ATP sensitive K(+) (K(ATP)) channels may cause hydroxyl radical (.OH) generation through NE release. Lysophosphatidylcholine (LPC), an endogenous amphiphiphilic lipid metabolite, also increases the concentration of interstitial adenosine in rat hearts, through the PKC-mediated activation of endogenous ecto-5'-nucleotidase. Nitric oxide (NO) facilitates the production of interstitial adenosine, via guanosine 3',5'-cyclic monophosphate (cGMP)-mediated activation of ecto-5'-nucleotidase as another pathway. These mechanisms play an important role in high sensitivity of the cardiac adenosine system. Adenosine plays an important role as a modulator of ischemic reperfusion injury, and that the production and mechanism of action of adenosine are linked with NE release.

In vivo effects of fenoldopam on autonomic nervous system after inhibition or activation of ganglionic transmission

The study investigated the effects of dopamine D1-like receptor stimulation on the autonomic nervous system. Fenoldopam (20 microg/kg) was injected i.v. in conscious sinoaortic denervated dogs, that is, surgically deprived of baroreflex pathways. In barodenervated dogs, fenoldopam (20 microg/kg) induced arterial hypotension as well as bradycardia and reduced noradrenaline plasma levels. Pentolinium (0.1 mg/kg i.v.), used to induce partial blockade of nicotinic ganglionic receptors, suppressed the fenoldopam-induced decrease in sympathetic tone, suggesting a ganglionic location for the dopamine D1-like receptor. Moreover, the inability of fenoldopam to reduce the nicotine-induced increase in sympathetic tone suggests that a postsynaptic ganglionic location can be excluded for the dopamine D1-like receptor. The results of these "in vivo" experiments strongly suggest a presynaptic location for the ganglionic dopamine D1-like receptor, stimulation of which results in a reduction of sympathetic tone.

[Adenosine production and its role in protection against ischemic and reperfusion injury of the myocardium]

Adenosine exerts cardioprotective effects on the ischemic myocardium. A flexibly mounted microdialysis apparatus was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5'-nucleotidase (a key enzyme responsible for adenosine production) in in vivo rat hearts. The level of adenosine during adenosine 5'-adenosine monophosphate (AMP) perfusion serves as an index of the activity of ecto-5'-nucleotidase in the tissue. Endogenous norepinephrine (NE) activates alpha 1-adrenoceptors to lead to the activation of protein kinase C (PKC), which, in turn, activates ecto-5'-nucleotidase via phosphorylation, thereby enhancing the production of interstitial adenosine. Histamine-induced release of NE activates PKC, which increases ecto-5'-nucleotidase activity and augments release of adenosine. Nicorandil, a hybrid of an ATP sensitive K+ (KATP) channel opener and nitrate, increases the level of interstitial adenosine via cGMP-mediated activation of ecto-5'-nucleotidase. Opening of cardiac KATP channels may cause hydroxyl radical (.OH) generation. Nitric oxide (NO) facilitates the production of interstitial adenosine, via activation of ecto-5'-nucleotidase. However, singlet oxygen (1O2) is a very powerful oxidant that causes inactivation of ecto-5'-nucleotidase to result in a decrease in the concentration of adenosine in rat heart. Adenosine plays an important role as a modulator of ischemic reperfusion injury. The production and action of adenosine are intimately linked to the release of NE.

Block of cardiac ATP-sensitive K(+) channels reduces hydroxyl radicals in the rat myocardium

The present study examined whether opening of an ATP-sensitive K(+) (K(ATP)) channel can induce hydroxyl free radical ((*)OH) generation in the rat myocardium. Sodium salicylate in Ringer's solution (0.5 nmol/microl/min) was infused directly through a microdialysis probe to detect the generation of (*)OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Induction of cromakalim (100 microM), a K(ATP) channel opener, through the microdialysis probe significantly increased the level of 2,3-DHBA. Another K(ATP) channel opener, nicorandil, also increased the level of 2,3-DHBA. When iron(II) was administered to cromakalim-pretreated animals, a marked elevation of DHBA was observed, compared with iron(II) only-treated animals. A positive linear correlation between iron(II) and formation of (*)OH, trapped as DHBA in the dialysate, was shown (r(2) = 0.988). When corresponding experiments were performed with nicorandil-treated animals, a positive linear correlation between iron(II) and DHBA in the dialysate was shown (r(2) = 0.988). However, the presence of glibenclamide (1-50 microM) decreased the cromakalim-induced 2,3-DHBA formation in a concentration-dependent manner (IC(50) = 9.1 microM). 5-Hydroxydecanoate (5-HD; 100 microM), another K(ATP) channel antagonist, also decreased cromakalim-induced (*)OH formation. The IC(50) value for 5-HD against cromakalim-evoked increase in 2,3-DHBA was 107.2 microM. In the presence of glibenclamide (10 microM), the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, the normal elevation of 2,3-DHBA in the heart dialysate was not observed in animals pretreated with glibenclamide (10 microM). When corresponding experiments were performed with 5-HD (100 microM) pretreated animals, the same results were obtained. These results suggest that opening of cardiac K(ATP) channels may cause (*)OH generation.

Renal morphological changes after sinoaortic denervation in dogs

The present study investigates morphological renal lesions in sinoaortic-denervated dogs 1 (n = 6) and 18 (n = 5) months after sinoaortic denervation compared with sham-operated controls (n = 8). After 1 month, a marked hyalinization and moderate thickening of the media of arterioles and small interlobular arteries were observed. These changes associated with edema and intimal thickening led to a narrowing of the lumen. In glomeruli, increase of mesangial matrix was focally present in all cases and associated with mesangial proliferation. In four of six cases, some glomeruli appeared retracted, with a large urinary space. A focal area of interstitial fibrosis occurred in just one case. After 18 months, similar but more pronounced vascular lesions were present, with marked hyperplasia of the media. Glomerular changes were characterized by mesangial lesions associated with focal glomerular sclerosis and thickening of Bowman's capsule. Tubulointerstitial lesions were more prominent in this group, with the presence of tubular epithelial changes and casts. Focal interstitial fibrosis, infiltrates, or both were demonstrated in all cases. These morphological lesions were associated with an increase in arterial blood pressure, proteinuria, and natriuresis and a decrease in urinary kallikrein. These results show that chronic sinoaortic denervation in dogs is associated with renal lesions similar to those observed in other well-established experimental and clinical hypertensive states.

Effects of cicletanine on vasoactive systems in conscious sinoaortic-denervated dogs

The present study investigates the antihypertensive action of cicletanine, a new antihypertensive compound with diuretic properties (or placebo), on vasopressor (catecholamines, renin-aldosterone) as well as vasodepressor (prostaglandins, kallikrein-kinin) systems in conscious chronic sinoaortic denervated (SAD) dogs. Cicletanine (10 mg/kg twice a day, per os, for one month) lowered blood pressure and heart rate. The antihypertensive action does not involve an effect on sympathetic tone (since plasma catecholamine levels were unmodified) or on plasma aldosterone levels. By contrast, urinary 6 keto PGF1 or PGE2 levels and kallikrein activity were enhanced. This result indicates that the antihypertensive effect of cicletanine is associated with a stimulation of potential vasodepressor systems (such as prostaglandins or kinins).

Clonidine but not propranolol decreases plasma neuropeptide Y (NPY) levels

The effects of acute administration of two antihypertensive drugs, clonidine and propranolol, on plasma NPY and catecholamine levels were compared in sinoaortic denervated (a model associated with a marked increase in sympathetic tone and a rise in blood pressure) and normal conscious dogs. Clonidine decreased plasma noradrenaline and NPY concentrations in both groups of animals. Propranolol failed to change plasma noradrenaline and NPY levels in sinoaortic denervated dogs but elicited a decrease in plasma noradrenaline with no change in NPY levels in normotensive animals. The present experiments show that changes in plasma noradrenaline and NPY concentrations are not always simultaneous. The decrease in plasma NPY concentrations could contribute to the sympatholytic effect of clonidine.

Effects of inhibition of nitric oxide formation on basal vasomotion and endothelium-dependent responses of the coronary arteries in awake dogs

The role of nitric oxide in basal vasomotor tone and stimulated endothelium-dependent dilations in the coronary arteries in chronically instrumented awake dogs was studied by examining the consequences of inhibiting endogenous nitric oxide formation with the specific inhibitor of nitric oxide formation, NG-monomethyl-L-arginine (L-NMMA). In four awake dogs, coronary dimension crystals were chronically implanted on the circumflex artery for the measurement of epicardial coronary diameter, and Doppler flow probes were implanted for quantitation of phasic coronary blood flow (vasomotion of distal regulatory resistance vessels). Basal epicardial coronary diameter, acetylcholine-stimulated endothelium-dependent dilation, and flow-induced endothelium-dependent dilation of the epicardial arteries and phasic blood flow were recorded before, and after 5, 15, 50, and 120 mg/kg of L-NMMA. L-NMMA induced a dose-related increase in basal epicardial coronary vasomotor tone. There was an accompanying increase in aortic pressure and a decrease in heart rate. At doses greater than or equal to 50 mg/kg, rest phasic coronary blood flow was also decreased. Left ventricular end-diastolic pressure and contractility were not significantly changed. In contrast, the flow-induced or acetylcholine-stimulated endothelium-dependent responses were attenuated only after infusion of the highest does of L-NMMA (120 mg/kg). The changes in the basal vasomotor tone and acetylcholine-stimulated endothelium-dependent responses returned towards the control states in the presence of L-arginine (660 mg/kg). These data support the view that nitric oxide plays a significant role in modulating basal vasomotion and endothelial-dependent dilation stimulated by acetylcholine or increase in blood flow in epicardial coronary arteries and also influence the regulation of coronary blood flow during physiologic conditions.

Pinacidil. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in the treatment of hypertension

Pinacidil is an orally administered antihypertensive drug that acts via direct relaxation of vascular smooth muscle to produce peripheral vasodilatation and a reduction in blood pressure without significant direct effects on cardiac electrophysiology. Pinacidil is unrelated to other antihypertensive drugs in clinical use, either in structure or mechanism of action. It belongs to a new class of agents called 'potassium channel openers' which act via potassium efflux to hyperpolarize cell membranes, indirectly causing a net reduction in intracellular calcium that leads to relaxation of vascular smooth muscle. Pinacidil is indicated in the management of essential hypertension. In clinical trials of up to 1 year duration, pinacidil administered twice daily in a controlled release capsule formulation has been shown to achieve adequate blood pressure control both in previously untreated patients and in those with blood pressure inadequately controlled by beta-adrenoceptor blocking drugs or thiazide diuretics. In long term (up to 1 year) comparative studies pinacidil was at least as effective as hydralazine, prazosin or nifedipine in maintaining blood pressure control. Pinacidil may also have a potential use in the treatment of patients with secondary renal hypertension. Clinical trials to date have usually allowed the addition of a thiazide diuretic and/or beta-adrenoceptor blocking drug to enhance the efficacy of pinacidil and/or to reduce the incidence of adverse effects. The main adverse effects of pinacidil treatment, which result from its peripheral vasodilator activity, are headache, oedema, palpitations and tachycardia. Although the overall incidence of adverse effects is quite high, they are usually mild, transient in nature and respond to a reduction in dose. Nevertheless, these effects may occasionally be severe, necessitating withdrawal from therapy. Thus, pinacidil is an effective antihypertensive drug for the treatment of mild to moderate essential hypertension. Despite its novel mechanism of action pinacidil causes adverse effects typical of peripheral vasodilators; during long term use with twice daily administration of the controlled release capsule formulation, the addition of a diuretic is often necessary to attenuate peripheral oedema and maintain adequate control of blood pressure. Further long term controlled trials are needed to determine the precise role of pinacidil relative to that of the angiotensin converting enzyme (ACE) inhibitors and calcium channel blocking drugs.