N''-cyano-N-4-pyridyl-N'-1,2,2-trimethylpropylguanidine, monohydrate (P 1134): a new, potent vasodilator

Pharmacological Profiling of KATP Channel Modulators: An Outlook for New Treatment Opportunities for Migraine

Migraine is a highly disabling pain disorder with huge socioeconomic and personal costs. It is genetically heterogenous leading to variability in response to current treatments and frequent lack of response. Thus, new treatment strategies are needed. A combination of preclinical and clinical data indicate that ATP-sensitive potassium (KATP) channel inhibitors could be novel and highly effective drugs in the treatment of migraine. The subtype Kir6.1/SUR2B is of particular interest and inhibitors specific for this cranio-vascular KATP channel subtype may qualify as future migraine drugs. Historically, different technologies and methods have been undertaken to characterize KATP channel modulators and, therefore, a head-to-head comparison of potency and selectivity between the different KATP subtypes is difficult to assess. Here, we characterize available KATP channel activators and inhibitors in fluorescence-based thallium-flux assays using HEK293 cells stably expressing human Kir6.1/SUR2B, Kir6.2/SUR1, and Kir6.2/SUR2A KATP channels. Among the openers tested, levcromakalim, Y-26763, pinacidil, P-1075, ZM226600, ZD0947, and A-278637 showed preference for the KATP channel subtype Kir6.1/SUR2B, whereas BMS-191095, NN414, and VU0071306 demonstrated preferred activation of the Kir6.2/SUR1 subtype. In the group of KATP channel blockers, only Rosiglitazone and PNU-37783A showed selective inhibition of the Kir6.1/SUR2B subtype. PNU-37783A was stopped in clinical development and Rosiglitazone has a low potency for the vascular KATP channel subtype. Therefore, development of novel selective KATP channel blockers, having a benign side effect profile, are needed to clinically prove inhibition of Kir6.1/SUR2B as an effective migraine treatment.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Pinacidil enhances survival of cryopreserved human embryonic stem cells

Human embryonic stem cells (hESCs) can be maintained as undifferentiated cells in vitro and induced to differentiate into a variety of somatic cell types. Thus, hESCs provide a source of differentiated cell types that could be used to replace diseased cells of a tissue. The efficient cryopreservation of hESCs is important for establishing effective stem cell banks, however, conventional slow freezing methods usually lead to low rates of recovery after thawing cells and their replating in culture. We have established a method for recovering cryopreserved hESCs using pinacidil and compared it to a method that employs the ROCK inhibitor Y-27632. We show that pinacidil is similar to Y-27632 in promoting survival of hESCs after cryopreservation. The cells exhibited normal hESC morphology, retained a normal karyotype, and expressed characteristic hESC markers (OCT4, SSEA3, SSEA4 and TRA-1-60). Moreover, the cells retained the capacity to differentiate into derivatives of all three embryonic germ layers as demonstrated by differentiation through embryoid body formation. Pinacidil has been used for many years as a vasodilator drug to treat hypertension and its manufacture and traceability are well defined. It is also considerably cheaper than Y-27632. Thus, the use of pinacidil offers an efficient method for recovery of cryopreserved dissociated human ES cells.

Novel regulators of stem cell fates identified by a multivariate phenotype screen of small compounds on human embryonic stem cell colonies

Understanding the complex mechanisms that govern the fate decisions of human embryonic stem cells (hESCs) is fundamental to their use in cell replacement therapies. The progress of dissecting these mechanisms will be facilitated by the availability of robust high-throughput screening assays on hESCs. In this study, we report an image-based high-content assay for detecting compounds that affect hESC survival or pluripotency. Our assay was designed to detect changes in the phenotype of hESC colonies by quantifying multiple parameters, including the number of cells in a colony, colony area and shape, intensity of nuclear staining, and the percentage of cells in the colony that express a marker of pluripotency (TRA-1-60), as well as the number of colonies per well. We used this assay to screen 1040 compounds from two commercial compound libraries, and identified 17 that promoted differentiation, as well as 5 that promoted survival of hESCs. Among the novel small compounds we identified with activity on hESC are several steroids that promote hESC differentiation and the antihypertensive drug, pinacidil, which affects hESC survival. The analysis of overlapping targets of pinacidil and the other survival compounds revealed that activity of PRK2, ROCK, MNK1, RSK1, and MSK1 kinases may contribute to the survival of hESCs.

The antihypertensive effect of pinacidil versus prazosin in mild to moderate hypertensive patients seen in general practice

The antihypertensive effect of a new vasodilating drug, pinacidil, was compared with prazosin in a randomized, open study in general practice including 131 patients with a sitting diastolic blood pressure (DBP) between 100-115 mmHg. At inclusion 108 patients were untreated and the remaining patients were on treatment with thiazide diuretics and/or beta-blockers. The aim was to reduce the sitting DBP to less than or equal to 95 mmHg, which was achieved in 85% of the patients treated with pinacidil and in 77% of the patients treated with prazosin (NS). In the responding patients the reductions were (mean +/- SD) 16 +/- 7 mmHg (p less than 0.001) and 13 +/- 6 mmHg (p less than 0.001) in the pinacidil group (n = 60) and the prazosin group (n = 46), respectively (p less than 0.10). During 5 months of maintenance therapy no statistically significant differences in blood pressures between the two treatment groups were present. Side-effects were typical of vasodilator therapy, i.e. headache, dizziness, tachycardia and edema, leading to discontinuation of therapy in 10 patients in each treatment group. Heart rate (HR) was increased with pinacidil and unchanged with prazosin. Edema was frequently seen with pinacidil and dizziness with prazosin. Because of edema a thiazide diuretic was given to nine patients in the pinacidil group and two patients in the prazosin group. No clinically significant changes in ECG and biochemical variables were observed. In conclusion, the study has demonstrated that pinacidil is as effective an antihypertensive agent as prazosin. Pinacidil may be used as monotherapy. However, the study suggests that pinacidil should be used as add-on therapy to thiazide diuretics.

ATP-sensitive K+ channels in the kidney

ATP-sensitive K+ channels (KATP channels) form a link between the metabolic state of the cell and the permeability of the cell membrane for K+ which, in turn, is a major determinant of cell membrane potential. KATP channels are found in many different cell types. Their regulation by ATP and other nucleotides and their modulation by other cellular factors such as pH and kinase activity varies widely and is fine-tuned for the function that these channels have to fulfill. In most excitable tissues they are closed and open when cell metabolism is impaired; thereby the cell is clamped in the resting state which saves ATP and helps to preserve the structural integrity of the cell. There are, however, notable exceptions from this rule; in pancreatic beta-cells, certain neurons and some vascular beds, these channels are open during the normal functioning of the cell. In the renal tubular system, KATP channels are found in the proximal tubule, the thick ascending limb of Henle's loop and the cortical collecting duct. Under physiological conditions, these channels have a high open probability and play an important role in the reabsorption of electrolytes and solutes as well as in K+ homeostasis. The physiological role of their nucleotide sensitivity is not entirely clear; one consequence is the coupling of channel activity to the activity of the Na-K-ATPase (pump-leak coupling), resulting in coordinated vectorial transport. In ischemia, however, the reduced ATP/ADP ratio would increase the open probability of the KATP channels independently from pump activity; this is particularly dangerous in the proximal tubule, where 60 to 70% of the glomerular ultrafiltrate is reabsorbed. The pharmacology of KATP channels is well developed including the sulphonylureas as standard blockers and the structurally heterogeneous family of channel openers. Blockers and openers, exemplified by glibenclamide and levcromakalim, show a wide spectrum of affinities towards the different types of KATP channels. Recent cloning efforts have solved the mystery about the structure of the channel: the KATP channels in the pancreatic beta-cell and in the principal cell of the renal cortical collecting duct are heteromultimers, composed of an inwardly rectifying K+ channel and sulphonylurea binding subunit(s) with unknown stoichiometry. The proteins making up the KATP channel in these two cell types are different (though homologous), explaining the physiological and pharmacological differences between these channel subtypes.

Comparison of the effects of several potassium-channel openers on rat bladder and rat portal vein in vitro

1. The ability of several K-channel openers to inhibit KCl-induced contractions of rat bladder detrusor and spontaneous mechanical activity in rat portal vein was examined. 2. Lemakalim, pinacidil, Ro 31-6930, RP 49356, P1060 and S 0121 dose-dependently relaxed rat detrusor, precontracted with 20 mM KCl. With the exception of pinacidil, concentrations of these agents below 30 microM did not inhibit 80 mM KCl-included contractions. Pinacidil (10 microM) produced a small, but significant (P < 0.05) relaxation of 80 mM KCl-induced mechanical activity. Minoxidil sulphate and BRL 38226 produced some relaxation of 20 mM but not 80 mM KCl-induced contractions. 3. Glibenclamide (0.3-3 microM) antagonized the relaxant effects of lemakalim, pinacidil, Ro 31-6930, RP 49356, P1060 and S 0121 in a competitive manner (pA2 values 6.3-6.6). The effects of minoxidil sulphate and BRL 38226 were fully antagonized by 3 microM glibenclamide. 4. Lemakalim, pinacidil, S 0121, BRL 38226 and minoxidil sulphate were each approximately 8 times more potent as inhibitors of the spontaneous contractions of rat portal vein than KCl-induced contractions of the rat detrusor. Minoxidil sulphate was approximately 30 times more potent in the rat portal vein than in the bladder. This may indicate that either minoxidil sulphate is acting at different recognition sites in these two tissues, or that this compound has an additional mechanism of action in the portal vein. 5. With the exception of minoxidil sulphate, all the compounds tested stimulated 86Rb efflux and 42K efflux from preloaded rat detrusor strips. The stimulated 86Rb efflux was qualitatively but not quantitatively similar to the stimulated 42K efflux. Minoxidil sulphate stimulated 42K efflux from rat portal vein but not from rat bladder. 6. It is concluded that all the compounds tested cause relaxation of rat detrusor predominantly by Kchannel opening. Selectivity for bladder rather than vascular smooth muscle was not shown by any compound.

Glibenclamide enhances but pinacidil reduces attenuation in sympathetic responsiveness after acute coronary artery occlusion

To investigate the role of ATP-sensitive K+ channels in modulating the efferent autonomic response following acute myocardial ischemia/infarction, we examined the effects of a blocker (glibenclamide) and an opener (pinacidil) of ATP-sensitive K+ channels on the time course and extent of the attenuation in efferent cardiac sympathetic responsiveness in anesthetized dogs. We measured the effective refractory periods (ERPs) at nonischemic sites basal and apical to the area of myocardial ischemia/infarction in the baseline state and during bilateral stimulation of the ansae subclaviae before and after each drug administration and 5, 30, 60, 120, and 180 minutes after latex injection of a diagonal branch of the left anterior descending coronary artery. Animals received either vehicle (n = 12), glibenclamide (0.3 mg.kg-1, n = 10), pinacidil (0.15 mg.kg-1 + 0.2 mg.kg-1 infusion, n = 10), or a combination of these two drugs (n = 9) intravenously. In another group of dogs receiving just pinacidil (n = 10), an intra-aortic balloon was inflated distal to the renal arteries to prevent pinacidil-induced hypotension. Another group of dogs received either high-dose glibenclamide (0.3 mg.kg-1 + 0.15 mg.kg-1, n = 4), low-dose glibenclamide (0.06 mg.kg-1, n = 4), medium-dose pinacidil (0.03 mg.kg-1 + 0.04 mg.kg-1 infusion, n = 4), or low-dose pinacidil (0.0075 mg.kg-1 + 0.01 mg.kg-1 infusion, n = 4). In all dogs, basal sites exhibited no attenuation of sympathetically induced shortening of the ERP throughout the period of acute myocardial ischemia/infarction. Cumulative attenuation in sympathetic responsiveness (shortening of ERP < or = 2 milliseconds induced by bilateral stimulation of the ansae subclaviae) at nonischemic test sites apical to the area of ischemia/infarction during a 3-hour period was greater in the glibenclamide group (26 of 44 sites, P = .008) and less in the pinacidil (2 of 44 sites, P = .002) and pinacidil-balloon (1 of 48 sites, P < .001) groups compared with the vehicle group (14 of 46 sites). Glibenclamide abolished the protective effect of pinacidil so that 10 of 45 sites had < 2-millisecond shortening during a 3-hour period in the glibenclamide + pinacidil group (P = .018 versus pinacidil group, P = .286 versus vehicle group). Such effects of glibenclamide and pinacidil on sympathetic attenuation were dose dependent. Maintaining the blood glucose level during glibenclamide administration did not affect the sympathetic attenuation after acute coronary artery occlusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Stereoselective disposition and metabolism of pinacidil in rat

1. An hplc method has been established for the determination of the enantiomer ratios of pinacidil and pinacidil-pyridine-N-oxide (M-1), using a beta-cyclodextrin-containing mobile phase. 2. Shortly after the administration of racemic pinacidil (2 mg/kg, i.v. or oral) to rat, the enantiomer ((+)/(-)) ratios of pinacidil in plasma were about 1 (i.v.) or > 1 (oral), whereas those of M-1 were < 1; both ratios increased with time. The (+)/(-) ratios of M-1 and pinacidil in 0.24 h urine samples were 0.65 (i.v.), 0.63 (oral), and 2.09 (i.v.), and 1.56 (oral), respectively, in male rat, and 0.76 (i.v.), 0.77 (oral) and 1.79 (i.v.), and 1.48 (oral), respectively, in female rat. 3. No isomerization of (+)- or (-)-pinacidil was observed during incubation with liver slices, and there was no stereoselectivity in the protein binding of the drug. An (-)-enantiomer-rich M-1, however, was produced after incubation of (+/-)-pinacidil with liver slices. The N-oxidation of (-)-pinacidil exhibited a higher stereoselectivity in males than females (the (+)/(-) ratio of M-1 in male, 0.55; female, 0.68). 4. These results indicated that the stereoselective disposition of pinacidil in rat was due, in large part, to the stereoselective N-oxidation of this drug in the liver.

Urinary metabolites of pinacidil. II. Species difference in the metabolism of pinacidil

1. Pinacidil was given orally to rabbit (10 mg/kg), dog (10 mg/kg), monkey (10 mg/kg) and mouse (150 mg/kg), the urinary metabolites were separated by h.p.l.c. and their structures determined by mass spectrometry. 2. Three new metabolites, namely, omega-hydroxy-pinacidil-O-glucuronide (M-8), pinacidil-pyridine-N-oxide-O-glucuronide (M-9) and pinacidil-pyridine-N-glucuronide (M-10) were isolated from rabbit urine, and one new metabolite, namely, pinacidil-pyridine-phenolic-glucuronide (M-11) was isolated from mouse urine. 3. M-9 is a unique glucuronide because the glucuronic acid is linked to the oxygen of pyridine-N-oxide. 4. In man, similar to rat, dog and mouse, pinacidil-pyridine-N-oxide (M-1) was the main urinary metabolite, with pyridine-N-oxidation being the major metabolic pathway. On the other hand, M-8 and M-9 were the most abundant metabolites in monkey and rabbit urine, respectively. Therefore, rat, dog and mouse have similar metabolism of pinacidil to man, but monkey and rabbit are significantly different in their metabolism of the drug.

Differential vasorelaxant effects of K(+)-channel openers and Ca(2+)-channel blockers on canine isolated arteries

The vasorelaxant effects of the K(+)-channel openers, pinacidil and cromakalim, were compared with those of the Ca(2+)-channel blockers, verapamil and KB-2796 (1-[bis(4-fluorophenyl)methyl]-4-(2,3,4-trimethoxybenzyl)piperazine dihydrochloride), in canine isolated coronary, renal, basilar and mesenteric arteries precontracted with U46619, a thromboxane A2 mimetic. The relaxation induced by pinacidil and cromakalim was greater in coronary than in other arteries, the magnitude of relaxation being in the order of coronary > renal > basilar > mesenteric arteries. The relaxant responses to both drugs were inhibited by glibenclamide, a blocker of ATP-sensitive K+ channels. The relaxation induced by verapamil and KB-2796, in contrast, was greater in basilar than in other arteries, the magnitude of relaxation being in the order of basilar > coronary > renal and mesenteric arteries. In fura-2-loaded, U46619-stimulated arteries, pinacidil and cromakalim produced a greater reduction in intracellular Ca2+ concentration and muscle tension in coronary than in mesenteric arteries, while verapamil and KB-2796 reduced these values more potently in basilar than in mesenteric arteries. These results suggest that K(+)-channel openers exhibit a vasorelaxant selectivity for coronary arteries, whereas Ca(2+)-channel blockers exhibit such selectivity for cerebral arteries. The selective vasorelaxant action induced by these drugs appears to correspond, in part, to their effects on the concentration of intracellular Ca2+.

Urinary metabolites of pinacidil: I. Isolation and identification of the metabolites in rat urine

1. The urinary metabolites of pinacidil administered orally to rats, were analysed by h.p.l.c. Seven metabolites were isolated from rat urine following fractionation on an HP-20 column and purified by t.l.c. 2. The major metabolite was pinacidil-pyridine-N-oxide (M-1). omega-Hydroxypinacidil (M-2) and dealkylpinacidil (M-3) were less abundant metabolites, and minor metabolites were the M-2-aldehyde, in which an intramolecular ring had formed (M-4), M-2-pyridine-N-oxide (M-5), carbamoyl-pinacidil (M-7) and M-7-pyridine-N-oxide (M-6). 3. Metabolic pathways of pinacidil in rats were postulated from the isolated metabolites.

Cardiotoxicity of vasodilators and positive inotropic/vasodilating drugs in dogs: an overview

Standard toxicological studies in dogs using high doses of vasodilators and positive inotropic/vasodilating agents give rise to a species-specific cardiotoxicity. The reason may be the extreme sensitivity of the dog to the pharmacological effects of these drugs; exaggerated pharmacodynamic effects and prolonged disturbance of homeostasis mechanisms often are responsible for the observed organ lesions. An assessment of the toxicological relevance and the risk for patients taking the drugs at therapeutic doses cannot be made without taking into account their pathomechanisms and the pathophysiological basis of the exceptional reaction patterns occurring in dogs. A large series of vasodilating and positive inotropic agents are presented, their pharmacological properties are described, and toxicological effects in dogs are compared. In view of the poor correlation between the distinct cardiac lesions induced in dogs and a lack of comparable toxicity in humans, it appears desirable to reassess the adequacy of the standard toxicological approaches for these substances.

Lipid and apolipoprotein levels during therapy with pinacidil combined with hydrochlorothiazide

This study determined the effect of pinacidil on the concentration of plasma lipids and apolipoproteins in male patients previously equilibrated with 25 mg hydrochlorothiazide twice daily. Pinacidil therapy given to 52 hypertensives at 25 to 100 mg daily for 8 weeks resulted in a reduction of systolic and diastolic blood pressure concurrently to reductions in plasma cholesterol and triglycerides with no change in low density lipoprotein-cholesterol (LDL-C) and high density lipoprotein-cholesterol (HDL-C). There was an associated decrease in apolipoproteins (Apo)B, C-III and E and elevation in ApoA-I. A parallel placebo group of 44 patients experienced reduction in diastolic blood pressure and an elevation in ApoA-I. These changes indicate that pinacidil will be a useful antihypertensive agent having properties on lipoprotein metabolism which would favor decreased risks of atherosclerosis.

Acute hemodynamic effects of pinacidil in hypertensive patients with and without propranolol pretreatment

To study the systemic and regional hemodynamic effects of the new antihypertensive agent pinacidil, the authors administered intravenously two doses of pinacidil (0.1 mg/kg) to patients with hypertension after 3 days of randomized, double-blind pretreatment with either propranolol or placebo. Pinacidil administration decreased systemic arterial pressure and total peripheral vascular resistance in both groups of patients. It also decreased pulmonary artery wedge pressure, and increased cardiac output, heart rate, and plasma norepinephrine levels; the changes in cardiac output and heart rate were attenuated by propranolol pretreatment. In addition, propranolol-pretreated patients responded to pinacidil with a decrease in forearm blood flow. In contrast, pinacidil administration exerted no significant effects on right atrial pressure, stroke volume, or mean pulmonary arterial pressure alone or in combination with propranolol. The results show that pinacidil is a potent arterial dilator but has little effect on the venomotor tone in patients with hypertension.

Arterial vasodilating profile and biological effects of pinacidil in healthy volunteers

1. The effects of pinacidil (25 mg, sustained release formulation) a) on systemic (arterial pressure, cardiac output) and regional (brachial and carotid arteries' diameters and flows) haemodynamics (pulsed Doppler techniques), b) on sympathetic (plasma noradrenaline) and renin-angiotensin (plasma renin activity) systems, and c) on atrial natriuretic factor have been investigated and compared with those of a placebo during the 12 h period following oral administration in a randomized, double-blind and cross-over study performed in six healthy volunteers. Simultaneously, the plasma levels of pinacidil and of its active metabolite, pinacidil N-oxide, were determined. 2. As compared with placebo, pinacidil decreased systemic vascular resistance and arterial blood pressure but cardiac output was not modified. 3. Pinacidil significantly increased brachial and carotid arteries' diameters (by 7 and 8% respectively) and flows (by 60 and 17% respectively) and decreased forearm vascular resistance (by 43%). Thus, pinacidil dilates both large and small arteries, increases large vessels' compliance and redistributes blood flow towards the muscular vascular bed. These effects peaked at 4 h and their duration at the brachial level was 8 h. 4. Pinacidil administration resulted in a stimulation of both sympathetic (increases in heart rate and plasma noradrenaline) and renin-angiotensin systems, and induced a transient increase in atrial natriuretic factor. 5. The duration of pinacidil haemodynamic effects at the brachial level is consistent with the pharmacokinetic data which show that pinacidil and pinacidil N-oxide plasma levels almost plateaued between 3 and 8, and 2 and 8 h respectively after oral administration of the sustained release formulation used.

Pinacidil inhibits neuromuscular transmission indirectly in the guinea-pig and rabbit mesenteric arteries

1. Effects of pinacidil were investigated on neuromuscular transmission in smooth muscle tissues of the rabbit and guinea-pig mesenteric arteries by both electrophysiological procedures and a bioassay of noradrenaline (NA) outflows. 2. Pinacidil (over 1 microM) hyperpolarized smooth muscle cell membranes in both tissues, in a concentration dependent manner. Pinacidil hyperpolarized and increased the ionic conductance of smooth muscle membrane more markedly in the rabbit mesenteric artery than in the guinea-pig. The hyperpolarization induced by pinacidil occurred in the presence or absence of endothelial cells and was blocked by glibenclamide. 3. Perivascular adrenergic nerve stimulation produced excitatory junction potentials (e.j.ps) and repetitive stimulation produced a facilitation of e.j.ps in both tissues. Pinacidil (over 1 microM) reduced the amplitude and the decay time of e.j.ps to a consistently greater extent in the rabbit mesenteric artery than in the guinea-pig. However, the facilitation process of e.j.ps was not modified following application of pinacidil (1 microM). The pinacidil-induced inhibition of e.j.ps was prevented by pretreatment with glibenclamide. 4. Pinacidil (30 microM) marginally increased the overflows of NA and its metabolite, 3,4-dihydroxyphenylglycol (DOPEG) released following repetitive perivascular nerve stimulations. 5. Pinacidil (10 microM) partly inhibited the voltage-dependent Ca channel, as estimated from the recovery process following removal of pinacidil, of action potentials evoked on e.j.ps. 6. It is concluded that pinacidil increases ionic conductance and hyperpolarizes smooth muscle cell membranes of the guinea-pig and rabbit mesenteric arteries and as a consequence, inhibits the neuromuscular transmission process occurring on adrenergic nerve stimulation with no reduction in the amount of released transmitter.

Haemodynamic profile of the potassium channel activator EMD 52692 in anaesthetized pigs

1. The systemic and regional haemodynamic effects of the potassium channel activator EMD 52692 or its solvent were investigated after intravenous and after intracoronary administration in anaesthetized pigs. 2. Consecutive intravenous 10 min infusions of EMD 52692 (0.15, 0.30, 0.60, 1.20 micrograms kg-1 min-1; n = 7) dose-dependently decreased mean arterial blood pressure by up to 50%. This was entirely due to peripheral vasodilatation, since cardiac output did not change. Heart rate increased by up to 50%, while left ventricular end diastolic pressure decreased dose-dependently from 6 +/- 1 mmHg to 3 +/- 1 mmHg (P less than 0.05), and stroke volume decreased from 30 +/- 2 ml to 21 +/- 2 ml (P less than 0.05). Left ventricular dP/dtmax was not affected. 3. Although cardiac output did not change, EMD 52692 caused a redistribution of blood flow from the arteriovenous anastomoses to the capillary channels. Blood flow to the adrenals, small intestine, stomach, bladder, spleen and brain increased, while renal blood flow decreased and blood flow to several muscle groups and skin were not altered. Vascular conductance was increased dose-dependently in all organs, except for the kidneys, where after the initial increase, vascular conductance returned to baseline with the highest dose. Particularly striking were the effects on the vasculature of the brain. With the highest dose of EMD 52692 blood flow more than doubled, while vascular conductance increased four fold. 4. Transmural myocardial blood flow increased slightly, which was entirely due to an increase in subepicardial blood flow. Myocardial O2-consumption and segment length shortening were not significantly affected. 5. After consecutive 10 min intracoronary infusions (0.0095, 0.019, 0.0375 and 0.075 microgram kg-1 min-1; n = 7) into the left anterior descending coronary artery (LADCA), mean arterial blood pressure was maintained with the lowest two doses, but decreased by up to 15% with the higher doses, whereas heart rate increased by up to 24%. Blood flow to the LADCA-perfused myocardium doubled with the highest dose, the subepicardium benefitting the most. Coronary venous O2-saturation increased dose-dependently from 23 +/- 2% to 60 +/- 4%, while myocardial O2-consumption of the LADCA-perfused myocardium was not affected by the drug. 6. It is concluded that EMD 52692 is a potent vasodilator, with particularly pronounced effects on vasculature of the brain. Its selectivity for vascular smooth muscle cells exceeds that for the myocytes, since with doses that are much higher than those of potential clinical interest no negative inotropic effects were observed. The compound primarily dilates arteries but some venodilatation may also occur.

Inhibition by the putative potassium channel opener pinacidil of the electrically-evoked release of endogenous dopamine and noradrenaline in the rat vas deferens

The effect of pinacidil on the release of endogenous noradrenaline and dopamine from the sympathetic innervation of the rat vas deferens was examined. Amine release was evoked by electrical stimulation (1, 2, 5 and 10 Hz) or by depolarization with high potassium (75 mmol/l) in the medium. Dopamine and noradrenaline were measured by means of high pressure liquid chromatography with electrochemical detection. Pinacidil (1, 5, 10 and 50 mumol/l) produced a concentration-dependent inhibition of the electrically stimulated (2 Hz) overflow of noradrenaline and dopamine. Only pinacidil 50 mumol/l increased the spontaneous loss of dopamine and noradrenaline. The inhibitory effects of pinacidil (5 mumol/l) on amine overflow were also observed at other frequencies of stimulation (1, 5 and 10 Hz). The magnitude of the inhibitory effect on noradrenaline release was approximately the same at all frequencies (63% to 56% reduction); for dopamine, the higher the frequency of stimulation, the greater the inhibitory effect of pinacidil (up to 73% reduction). When the preparations were continuously stimulated for 70 min at 2 Hz, pinacidil (5 mumol/l) reduced the overflow of dopamine and noradrenaline during the first 40 or 30 min of stimulation only. The addition of phentolamine (1 mumol/l) to the perifusion medium slightly reduced the inhibitory effect of pinacidil on amine overflow, but the inhibition by pinacidil remained statistically significant. Tetraethylammonium (10 mmol/l) completely abolished the inhibitory effect of pinacidil (10 mumol/l). Pinacidil (5 mumol/l) did not reduce the potassium-evoked release of the amines. The results demonstrate that pinacidil impairs transmitter release from the sympathetic innervation of the rat vas deferens, probably as a consequence of the opening of potassium channels.

Cytoplasmic calcium and the relaxation of canine coronary arterial smooth muscle produced by cromakalim, pinacidil and nicorandil

1. In order to investigate the vasodilator mechanisms of the K+ channel openers, cromakalim, pinacidil and nicorandil, we measured changes in cytoplasmic Ca2+ concentration [( Ca2+]i) simultaneously with force by a microfluorimetric method using fura-2, a calcium indicator, in canine coronary arterial smooth muscle cells. 2. The three K+ channel openers all produced a concentration-dependent reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS) but failed to affect [Ca2+]i in 45 and 90 mM KCl-PSS. 3. Cromakalim only partly inhibited (-45%) the 30 mM KCl-induced contractures, whereas pinacidil and nicorandil nearly abolished contractions produced by 45 mM, 90 mM and 30 mM KCl-PSS. 4. Tetrabutylammonium (TBA), a nonselective K+ channel blocker, or glibenclamide, a supposed adenosine 5'-triphosphate (ATP)-sensitive K+ channel blocker, abolished the reduction of [Ca2+]i caused by the three K+ channel openers and the relaxant effect of cromakalim, whereas they only slightly attenuated the relaxant effects of pinacidil and nicorandil. 5. The increase in [Ca2+]i produced by 45 or 90 mM KCl-PSS in the presence of pinacidil or nicorandil was abolished by 10(-5) M verapamil, indicating that the increase in [Ca2+]i was caused by the influx of extracellular Ca2+ and that pinacidil and nicorandil did not affect the voltage-dependent Ca2+ channel directly. 6. The [Ca2+]i-force relationship in the presence of cromakalim was not distinguishable from that of control. 7. The [Ca2+]i-force curve was shifted to the right by pinacidil and nicorandil. 8. These results show that cromakalim is a more specific K+ channel opener than pinacidil and nicorandil, and that vasodilatation produced by cromakalim in this study is predominantly a result of a reduction of [Ca2+]i due to the closure of voltage-dependent Ca21 channels by hyperpolarization. In contrast, additional mechanisms are involved in the vasodilator actions of pinacidil and nicorandil. One of these is related to a reduction in the sensitivity of contractile proteins to Ca2 . The latter mechanism of nicorandil is akin to that of nitroglycerin. K+ channels opened by these K+ channel openers may be ATP-sensitive ones which are blocked by glibenclamide.

Effect of pinacidil on the electrophysiological properties in guinea-pig papillary muscle and rabbit sino-atrial node

The electrophysiological effect of the antihypertensive drug pinacidil has been examined in preparations of guinea-pig papillary muscle and rabbit sino-atrial node using a standard microelectrode method. In papillary muscle preparations, pinacidil (greater than 30 microM) shortened the action potential duration (APD), whereas it did not affect the maximum rate of rise (Vmax). Pinacidil (greater than 1 microM) also decreased APD of slow action potentials evoked by high K+ (27 mM) solution containing 0.2 mM Ba2+. At 30 microM, the drug reduced the Vmax of slow action potentials. In the spontaneously beating sino-atrial node, pinacidil (greater than 30 microM) shortened APD. At 100 microM, it also decreased the heart rate, Vmax, action potential amplitude and the rate of diastolic depolarization. It is concluded that pinacidil modifies the electrical activity of myocardial cells probably due to an increase in the potassium conductance although in high concentrations the compound might also reduce Ca2+ influx through the cell membrane, which would contribute to an obvious negative chronotropic action.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.

Pinacidil activates the ATP-sensitive K+ channel in inside-out and cell-attached patch membranes of guinea-pig ventricular myocytes

Patch-clamp techniques were used to study the effects of pinacidil on the adenosine-5'-triphosphate (ATP)-sensitive K+ channel current in guinea-pig ventricular myocytes. In inside-out patches, the ATP-sensitive K+ channel current could be recorded at an internal ATP concentration of 0.5 mM or less and almost complete inhibition was achieved by raising the concentration to 2 mM. Application of pinacidil (10-30 microM) in the presence of 2 mM ATP restored the current, whereas 5 mM ATP antagonized the effect of pinacidil. The conductance of the channel at symmetrical K+ concentrations of 140 mM was 75 pS with a slight inward rectification at voltages positive to + 40 mV. There was no significant change in the conductance after application of pinacidil. In 0.5 mM ATP, at -80 mV, both the distributions of the open time and the life-time of bursts could be fitted by a single exponential. An increase in ATP concentration decreased the mean life-time of bursts, whereas pinacidil increased it with little increase in the mean open time. Closed time distributions of the channel were fitted by at least two exponentials, with a fast and a slow time constant. An increase in ATP concentration markedly increased the slow time constant associated with a decrease in the number of bursts, whereas the effect of pinacidil was opposite to that of increased ATP. These results indicate that pinacidil increases the open-state probability of the ATP-sensitive K+ channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of pinacidil on sympathetic neuroeffector transmission in rabbit blood vessels

The effects of pinacidil on vascular sympathetic neuroeffector transmission were studied. Pinacidil (3 x 10(-7)-3 x 10(-4) M) inhibited the contractions of rabbit isolated pulmonary artery evoked by nerve stimulation. This was the case both in the absence and presence of cocaine plus corticosterone. The inhibition reached a steady state and was reversible. Pinacidil (10(-5) -3 x 10(-4) M), in the absence or presence of cocaine plus corticosterone, markedly enhanced the 3H-overflow evoked by electrical-field stimulation of the artery preloaded with 3H-noradrenaline (3H-NA). The enhancement was dependent on frequency (1-30 Hz) in a complex manner. Phentolamine (3 x 10(-6) M), but not rauwolscine (10(-6) M) prevented the enhancement. Pinacidil (10(-4) M) did not antagonize the alpha-methylnoradrenaline-induced inhibition of 3H-overflow evoked by stimulation. Pinacidil (10(-5)-3 x 10(-5) M) in a non-competitive manner antagonized the contractions of isolated aorta elicited by noradrenaline (3 x 10(-9)-3 x 10(-5) M), phenylephrine (2 x 10(-8)-3 x 10(-4) M), adrenaline (10(-9)-3 x 10(-5) M), histamine (10(-6) 6 x 10(-4) M), serotonin (10(-8)-10(-4)M), and potassium (10(-3) M). Pinacidil (10(-6)-3 x 10(-4) M) and methacholine (3 x 10(-8)-10(-6) M) relaxed aorta preconstricted with noradrenaline (10(-7) M). The relaxation caused by pinacidil was not dependent on the presence of endothelial cells, while that seen with methacholine was.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiohemodynamic effects of cromakalim and pinacidil, potassium-channel openers, in the dog, special reference to venous return

The cardiohemodynamic effects of the potassium-channel openers, cromakalim and pinacidil, were studied by the use of the closed-loop method (for both drugs) and the cardiopulmonary bypass technique (for cromakalim only) in anesthetized, open-chest dogs. In closed-loop preparations, both drugs (cromakalim, 3 to 100 micrograms/kg; pinacidil, 10 to 300 micrograms/kg) administered intravenously decreased systemic blood pressure and increased venous return (sum of the flow through the inferior and the superior vena cava) and cardiac output (CO). With the highest doses of both drugs, venous return and CO decreased in some preparations in which right atrial pressure rose, and the maximum rate of rise of left ventricular pressure (LV dP/dt max) diminished. Except such preparations, right atrial pressure, heart rate, and atrioventricular conduction time remained virtually unchanged. The fall in systemic blood pressure produced by intermediate doses of pinacidil was greater in the preparations in which baroceptor reflexes were eliminated (denervated preparations) than in those with the reflexes left intact (nerve-intact preparations). The increases in venous return and CO, however, were not different when comparing the nerve-intact and the denervated preparations. In cardiopulmonary-bypass preparations, higher doses of cromakalim increased venous return while producing a fall in systemic blood pressure, suggesting that the decreased venous return and CO seen with higher doses of the potassium-channel openers in the closed-loop preparations were secondary to the decreased cardiac contractility. The potassium-channel openers should be characterized as vasodilators, which preferentially reduce afterload and increase venous return.

Effects of pinacidil on the sympatho-adrenal system in dogs

1. The aim of the present work was to study the antihypertensive effect of pinacidil, a potassium channel opener, in sinoaortic denervated (SAD) conscious dogs and to investigate whether the involvement of the sympathetic nervous system induced by this vasodilator compound is only of baroceptor reflex origin. 2. Pinacidil (0.1, 0.2, 0.4 mg kg-1 i.v.) induced a dose-dependent decrease in blood pressure in normal as well as in SAD dogs. In contrast, the induced-tachycardia observed in normal dogs was not found in SAD animals. 3. Since pinacidil induced an increase in plasma catecholamines, free fatty acids, glucose, plasma renin activity and aldosterone in SAD dogs it is suggested that this sympathetic activation is independent of the baroreceptor reflex pathways. 4. The sympathetic activation is mainly of peripheral origin, since pinacidil (0.7 mg kg-1 i.v.) induced an increase in adrenaline release from the adrenal gland after section of the great splanchnic nerve in anaesthetized dogs. This increase is probably due to an effect that does not involve K+ channel opening. However, this effect of pinacidil was not observed during splanchnic nerve stimulation (in this case, the involvement of K+ channel opening is suggested).

Interaction of potassium channel openers and blockers in canine atrial muscle

1. The possibility that the interaction between potassium channel openers, e.g. cromakalim, pinacidil and nicorandil, and some potassium channel blockers involves a common site was investigated in canine atrial muscle. 2. Cromakalim, pinacidil and nicorandil produced a negative inotropic effect, their pD2 (-log EC50) values being 6.11 +/- 0.07, 5.37 +/- 0.09 and 4.55 +/- 0.07, respectively. 3. The potassium channel blockers, tetraethylammonium (TEA), tetrabutylammonium (TBA), 3,4-diaminopyridine (DAP), CsCl and BaCl2 all produced a positive inotropic effect. 4. The concentration-effect curves for the negative inotropic actions of pinacidil were shifted in a parallel way to the right by low concentrations of TEA, TBA or BaCl2. Maximum responses to pinacidil were depressed by higher concentrations of the blockers. An analysis of the non-competitive antagonism by TEA yielded pKA (-log KA) values of 4.00-4.05 for pinacidil. 5. The concentration-effect curves for cromakalim and nicorandil were shifted by TEA similarly to those for pinacidil, and a similar analysis yielded pKA values of 4.47-4.68 for cromakalim and 3.47-3.74 for nicorandil. 6. The KA values of cromakalim, pinacidil and nicorandil were about 10-30 times greater than their EC50 values, indicating that there are non-linear stimulus-effect relationships between the binding of the three potassium channel openers to their binding sites at potassium channels and their negative inotropic effects. 7. The dissociation constants for TEA could also be estimated from pA2 and pKB values for antagonizing competitively and non-competitively the negative inotropic effects of the three potassium channel openers; they were 3.47-3.89, and did not differ between the potassium channel openers. 8. The concentration-effect curves for the three potassium channel openers were not affected by DAP or CsCl. 9. These results suggest the following: (i) quaternary ammonium compounds like TEA and TBA antagonize the negative inotropic effect of cromakalim, pinacidil and nicorandil by binding to potassium channels, thus preventing binding of the channel openers to the same sites or closely related sites in canine right atrial muscles.

Glibenclamide is a competitive antagonist of cromakalim, pinacidil and RP 49356 in guinea-pig pulmonary artery

The relaxant effect of cromakalim (BRL 34915), pinacidil and RP 49356 (N-methyl-2-(3-pyridyl)-tetrahydro-thiopyran-2-carbothioamide-1-ox ide) on the sustained contractions induced by 20 mM KCl were compared with the effects of nicorandil. The preparation used was vascular smooth muscle of phenoxybenzamine-treated pulmonary artery rings from reserpinized guinea-pigs. Cromakalim, pinacidil, RP 49356 and nicorandil relaxed the tissues with -log EC50 values of 6.78, 6.12, 6.02 and 5.46, respectively. The inhibitory effect of cromakalim, pinacidil and RP 49356, but not of nicorandil, was competitively antagonized by glibenclamide (10(-7)-3 X 10(-6) M), yielding uniform pA2 values of 7.17-7.22 against all three relaxant drugs. The order of potency of other K+ channel blocking agents for the inhibition of vasorelaxation by cromakalim, pinacidil and RP 49356 was procaine greater than 4-aminopyridine greater than tetraethylammonium. The mainly competitive type of inhibition of the RP 49356-induced response was more comparable to that with pinacidil than with cromakalim. The relaxation caused by nicorandil was only attenuated by a high concentration of 4-aminopyridine or tetraethylammonium but was markedly antagonized by methylene blue (3 X 10(-6)-10(-5) M) and potentiated by M & B 22948 (3 X 10(-6)-10(-5) M). These results suggest that the vascular relaxation caused in guinea-pig pulmonary artery by cromakalim, pinacidil and RP 49356 is mediated through the same glibenclamide-sensitive K+ channel whereas the major mechanism for the effect of nicorandil seems to involve stimulation of guanylate cyclase.

Nicorandil as a hybrid between nitrates and potassium channel activators

Nicorandil, although structurally a nitrate, differs from classic nitrates in several respects. It preferentially dilates resistive vessels. Its effect on venous return in dogs is not unanimously a decrease but rather an increase. In high doses or concentrations it suppresses myocardial contraction and ventricular automaticity, nearly sparing sinoatrial nodal automaticity and atrioventricular nodal conduction. It shortens the effective refractory period of myocardium. These cardiac effects of nicorandil have been explained by its mechanism of action as a potassium (K) channel activator. However, what part of the vascular effects of nicorandil this mechanism is responsible for has not been determined. BRL 34915 and pinacidil, nonnitrate vasodilators with a K-channel activator action, have essentially the same cardiovascular profile as nicorandil in isolated, blood-perfused canine heart preparations. In anesthetized, open-chest dogs the 2 K-channel activators decreased systemic blood pressure and increased venous return and cardiac output without elevating heart rate, unless the cardiodepressant effect emerged. The increase in venous return or cardiac output survived elimination of baroceptor functions. These results taken together with previous results on nicorandil suggest the following: (1) The property of nicorandil as a resistive vessel dilator highly selective for vasculature originates in its mechanism of action as a K-channel activator. The nonunanimous effect of nicorandil on venous return is a result of the opposing actions as a capacitive (action as a nitrate) and a resistive vessel dilator. Nicorandil, with its hybrid nature, is advantageous over specific K-channel activators and classic nitrates in therapeutic implications.

Pinacidil uptake and effects in the isolated rabbit heart

The myocardial accumulation of pinacidil showed one-compartment characteristics with a half-time of 1.11 min., whereas the disposition followed three-compartment kinetics with half-times for the relevant two redistributory and the terminal phases of 0.39, 1.51 and 5.44 min., respectively. At a steady-state drug concentration in the perfusate of 6.12 nmol ml-1, the average concentration of pinacidil in the myocardium was 20.6 nmol g-1. The accumulated amount could predictically be referred with 57% to a central and 31 and 12% to two peripheral (deeper) drug pools. The pharmacodynamic effects of pinacidil in the isolated perfused rabbit heart were studied at stepwise increasing concentrations from 0.15 to 100 microM. Coronary flowrate increased initially up to 24.5% at 1.5 microM pinacidil and then gradually decreased. Amplitude and velocity of contraction were both inhibited in a biphasic way up to 92.7 and 94.1%, respectively. Apparent dynamic steady states developed within 13-15 min. The computer-derived inhibitory Em-values related to the first phase were 49.2 and 52.4% and those related to the second phase were 111.7 and 108.3%, respectively. Heart frequency decreased monophasically and exhibited an inhibitory Em-value of 19.6%. Oxygen consumption decreased at pinacidil concentrations higher than 15 microM and the Em-value was 69.7%. The frequency-corrected QT-interval decreased biphasically and the related inhibitory Em-values were 8.6 and 58.7%. The QRS-interval did not change and the PQ-interval only showed a minor increase at the highest pinacidil concentration. Our findings are compatible with the concept of pinacidil being a potassium channel opener.

Hypotensive and peripheral vascular effects in healthy volunteers of repeated oral administration of pinacidil

The hypotensive and peripheral vascular effects of two different oral formulations of pinacidil were investigated in twelve healthy male volunteers. Arterial blood pressure, heart rate and calf blood flow were measured on Days 1 and 7. The vasodilator activity of both formulations was confirmed by a significant increase in calf blood flow on both days, which was correlated with a significant decrease in diastolic blood pressure. There was no differences between the tablet and the pellet.

The negative inotropic effect of nicorandil is independent of cyclic GMP changes: a comparison with pinacidil and cromakalim in canine atrial muscle

1. The negative inotropic effects of nicorandil, a nitrate with K-channel opening properties, have been compared with those of pinacidil, cromakalim and nifedipine, in canine right atrial muscle. 2. Cromakalim, pinacidil and nicorandil all produced a negative inotropic effect. However, even at their maximally effective concentrations, the force of contraction remained at about 10% of control levels, whereas contraction was abolished by nifedipine. 3. The pD2 values for the negative inotropic effects of cromakalim, pinacidil and nicorandil were 5.93, 5.37, and 4.35, respectively. 4. The negative inotropic effects of cromakalim (3 x 10(-5)M), pinacidil (3 x 10(-5) M and 3 x 10(-4) M) and nicorandil (3 x 10(-5) M) were not accompanied by changes in cyclic AMP and cyclic GMP levels, whereas that of 3 x 10(-4) M nicorandil was accompanied by an increase in cyclic GMP but not cyclic AMP concentrations. 5. The negative inotropic effect produced by 3 x 10(-4) M nicorandil was greatly reduced by 10(-2) M tetraethylammonium, whereas the increase in cyclic GMP produced by this concentration of nicorandil was not significantly changed. Sodium nitroprusside (10(-3) M) produced a large increase in cyclic GMP concentrations but had no significant negative inotropic effect. 6. It is concluded that the negative inotropic effects of nicorandil like those of cromakalim and pinacidil do not result from an increase in cyclic GMP concentrations. Instead these effects may be due to their action as K-channel openers.

Pinacidil opens K+-selective channels causing hyperpolarization and relaxation of noradrenaline contractions in rat mesenteric resistance vessels

1. The effects of pinacidil on noradrenaline-induced tone, smooth muscle membrane potential and 42K- and 86Rb-efflux from isolated mesenteric resistance vessels (internal diameter 200 microns) of the rat have been studied. 2. Pinacidil (0.3-10 microM) produced concentration-dependent suppression of noradrenaline-induced tone. 3. Pinacidil (0.3-10 microM) caused concentration-dependent hyperpolarization of the smooth muscle. 4. In rat resistance vessels loaded with 42K, pinacidil (1-10 microM) significantly increased the 42K-efflux rate constant. 5. With the use of 86Rb as a marker for K+, 1 microM pinacidil did not affect the 86Rb-efflux rate constant, while 10 microM pinacidil transiently increased the 86Rb rate constant. 6. The results indicate that the relaxant action of pinacidil in these vessels is due to the opening of K+-channels and consequent hyperpolarization. The K+-channels opened are selective for 42K over 86Rb.

Comparative effects of K+ channel blockade on the vasorelaxant activity of cromakalim, pinacidil and nicorandil

Three agents with K+ channel blocking activity, procaine, 4-aminopyridine (4-AP) and tetraethylammonium (TEA), were tested for inhibition of vasorelaxation and 86Rb+ efflux induced by cromakalim (BRL 34915), pinacidil and nicorandil in rabbit isolated mesenteric artery. The potency order for inhibition of vasorelaxation was procaine greater than 4-AP greater than TEA and for inhibition of efflux was procaine = 4-AP greater than TEA. The K+ channel blockers did not discriminate between cromakalim, pinacidil or nicorandil on efflux but demonstrated preferential inhibition of vasorelaxation to cromakalim greater than pinacidil greater than nicorandil. In addition, the maximum response to cromakalim was depressed but that to pinacidil and nicorandil was not. The results confirm the role of K+ channel activation in vasorelaxation to cromakalim, pinacidil and nicorandil, but suggest that additional mechanisms may be involved for pinacidil and, in particular, for nicorandil.

In vitro studies on the mode of action of pinacidil

(+/-) Pinacidil inhibited noradrenaline-induced contractions in rat aorta and portal vein. The spontaneous tone of guinea-pig bronchial and taenia caeci muscles was relaxed and the spontaneous mechanical activity of rat portal vein was abolished. (+/-) Pinacidil abolished contractions produced by low concentrations of KCl in rat aorta and portal vein, but had relatively little effect on responses to high KCl concentrations. The mechano-inhibitory effects of (+/-) pinacidil were antagonised by tetraethylammonium or procaine. In studies with the purified enantiomers of pinacidil, (-) pinacidil was approximately 20 times more potent that (+) pinacidil. Measurements of electrical activity showed that low concentrations of (+/-) pinacidil selectively inhibited spontaneous electrical discharges in rat portal vein. The duration of multispike electrical complexes was shortened, but spike frequency within a complex and the rate of spike rise and fall were unaffected. At higher concentrations, a dose-dependent hyperpolarisation was observed in both rat aorta and portal vein and the membrane potential approached EK. Using both 86Rb and 42K, (+/-) pinacidil produced a concentration-dependent increase in isotope exchange which correlated with those concentrations at which electrical and mechanical inhibitory effects were observed. Using radioimmunoassay, no pinacidil-induced changes in cyclic AMP or cyclic GMP concentrations were detected in rat aorta. These electrical, ion flux and biochemical measurements suggest that the in vitro mechano-inhibitory effects of pinacidil are associated with the opening of 86Rb-permeable K+ channels in smooth muscle. These effects were observed at concentrations of pinacidil similar to those found in vivo in the plasma of experimental animals and man. It is thus concluded that the hypotensive and antihypertensive effects of pinacidil are the consequence of the cessation of ongoing electrical activity and hyperpolarisation which follows the opening of K+ channels in vascular smooth muscle.

Long term haemodynamic effects of pinacidil and hydralazine in arterial hypertension

Eight patients with a diastolic blood pressure greater than or equal to 100mm Hg when treated with a diuretic and a beta-blocker participated in a randomised crossover study comparing the haemodynamic effects of adjunctive therapy with pinacidil or hydralazine. The vasodilator dose was increased until the diastolic blood pressure was less than 90mm Hg or the maximum dosage, hydralazine 100mg twice daily, or pinacidil 50mg twice daily, was reached. Treatment continued for 3 to 6 months and a haemodynamic study was performed. After washout, the patients received the alternative treatment. In the upright position, during supine rest and during isometric as well as dynamic exercise, pinacidil lowered blood pressure more effectively than hydralazine. No differences between the 2 treatments were found in heart rate, stroke index, cardiac index, end systolic wall stress or glomerular filtration rate. Pulmonary mean and wedge pressure were lower during treatment with pinacidil. Forearm blood flow was higher and forearm vascular resistance lower during treatment with pinacidil. Cardiac contractility, judged from the systolic time interval ratio PEP: LVET, was lower during treatment with pinacidil compared with hydralazine. The median daily dose of pinacidil was 50mg and that of hydralazine 200mg. It was also noted that during long term treatment, pinacidil seemed more effective in reducing blood pressure than hydralazine.

Antihypertensive effects of pinacidil in patients with and without indomethacin pretreatment

To determine the potential role of prostaglandins in mediating the hypotensive action of the new antihypertensive agent pinacidil, we measured the blood pressure, regional blood flow and neurohumoral responses to pinacidil in thirteen hypertensive patients randomly assigned to receive pretreatment with either indomethacin (75 mg) or placebo. After baseline measurements had been obtained, each patient received an oral dose of pinacidil to which he had previously demonstrated a therapeutic response. The doses of pinacidil administered between the two groups did not differ. Serial measurements of blood pressure and heart rate over two hours revealed no attenuation of the hypotensive effect of pinacidil in the indomethacin-pretreated patients (-12.7 +/- 4.1 mm Hg) compared to the placebo group (-9.3 +/- 3.2 mm Hg). While significant vasodilation was not observed in the forearm, renal vasodilation occurred and was not different between the two groups. Pinacidil had no effect on glomerular filtration rate. Neither did pinacidil significantly increase plasma catecholamines or renin activity. The results indicate that prostaglandins probably do not play a major role in the vasodilator action of pinacidil, and that therapeutic doses of the drug have a differential effect on regional blood flows that result in hypotension, but not significant neurohumoral stimulation, in patients with mild to moderate hypertension.

Effects of formulation on the pharmacokinetics of orally administered pinacidil in humans

Serum concentrations, relative bioavailability, and urinary excretion of pinacidil [(+/-)-2-cyano-1-(4-pyridyl)-3-(1,2,2-trimethylpropyl) guanidine monohydrate] from two sustained-release oral formulations (tablet and capsule) were compared in 12 healthy volunteers. Maximum measured serum concentrations (Cmax) from the sustained-release tablet and capsule did not differ significantly (75 +/- 17 versus 70 +/- 14 ng/mL, p greater than 0.05), but the time to achieve maximum concentration (tmax), was longer for the capsule (2.4 +/- 1.8 versus 0.98 +/- 0.5 h, p less than 0.05). There was no significant difference in bioavailability between the formulations, as measured by the area under the concentration-time curve (AUC0-8 h; 279 +/- 99 versus 311 +/- 85 ng.h/mL, p greater than 0.05). Twenty-four hour urinary excretion of both pinacidil and its major metabolite, pinacidil pyridine-N-oxide, was similar for both tablet and capsule preparations (3.9 +/- 1.2 and 55 +/- 19% versus 4.4 +/- 1.0 and 55 +/- 14%, respectively, of the administered dose, p greater than 0.05).

Pinacidil increases the background potassium current in single ventricular cells

Pinacidil, a new antihypertensive agent, increased the background (inward rectifier) potassium current and abolished its negative slope, and did not change the time-dependent component of the outward (delayed outward) current in single ventricular cells isolated from the guinea-pig heart. These effects must be responsible for the negative inotropic action of this drug.

Intravenous pinacidil in the acute treatment of hypertension

Pinacidil (N''-cyano-N-4-pyridyls-N'-1,2,2-trimethyl-propyl guanidine, monohydrate), a recently developed direct-acting vasodilator, was given intravenously in a dosage of 0.1-0.2 mg/kg body weight to ten untreated hypertensive patients. Pinacidil caused a fall of blood pressure from 170/108 +/- 6/3 to 156/80 +/- 5/4 mm Hg (mean +/- SE). The proportional decrease of mean arterial pressure (MAP) was 13.7 +/- 1.6%. Together with the decrease of blood pressure an increase of heart rate by 29.7 +/- 6.2% occurred. The heart rate increased by 13.6 beats/min per 10 mm Hg decrease of MAP. Pinacidil also caused significant rise of plasma noradrenaline and plasma renin activity, whereas plasma adrenaline and aldosterone remained unchanged. The serum concentrations of pinacidil and its major metabolite pinacidil-N-oxide were within the expected limits. The authors conclude that intravenously administered pinacidil causes a rapid decrease of blood pressure, but at the cost of a considerable increase of heart rate, and thus does not offer advantages over other vasodilators.

Evidence that the mechanism of the inhibitory action of pinacidil in rat and guinea-pig smooth muscle differs from that of glyceryl trinitrate

The effects of pinacidil have been compared with those of glyceryl trinitrate (GTN) using the aorta and portal vein of the rat and the trachealis and taenia caeci of the guinea-pig. In aorta, both pinacidil and GTN inhibited responses to noradrenaline and showed some selective inhibition of contractions to 20 mM K+. Responses to 80 mM K+ were little affected. In trachealis, both pinacidil and GTN inhibited spontaneous tone and selectively relaxed spasms to 20 mM K+. Responses to 80 mM K+ were unaffected. In portal vein, pinacidil completely inhibited spontaneous electrical and mechanical activity. GTN reduced the amplitude of tension waves and extracellularly-recorded discharges, but increased the frequency of spontaneous electrical and mechanical activity. In portal vein, pinacidil inhibited contractions to noradrenaline and selectively inhibited responses to 20 mM K+. GTN had little inhibitory effect on responses to either noradrenaline or K+. In portal veins loaded with 86Rb as a K+-marker, pinacidil significantly increased the 86Rb efflux rate coefficient whilst GTN had no effect on 86Rb exchange. In taenia caeci, both pinacidil and GTN inhibited the spontaneous tone of the preparation. These inhibitory effects were not antagonized by apamin. It is concluded that pinacidil and GTN do not share a common relaxant mechanism. Evidence has been obtained that pinacidil exerts its inhibitory effects by the opening of apamin-insensitive, 86Rb-permeable K+ channels.

Acute haemodynamic effects of pinacidil in man

The acute haemodynamic effects of i.v. pinacidil 0.2 mg kg-1 infused over 8 min were studied in 10 normotensive patients undergoing cardiac catheterisation. Mean arterial pressure fell from 94 +/- 3 mmHg (mean +/- s.e. mean) before infusion to 74 +/- 3 mmHg at 10 min after commencing infusion (P less than 0.001) and during this time heart rate increased from 75 +/- 4 to 106 +/- 7 beats min-1 (P less than 0.001). Significant changes were recorded until the end of the observation period (70 min after commencing infusion). Cardiac index increased from 3.2 +/- 0.2 to 4.0 +/- 0.2 l min-1 m-2 (P less than 0.001) and systemic vascular resistance fell from 16 +/- 1 to 10 +/- 1 units (P less than 0.001) at 10 min after commencing infusion. By the end of the observation period, the values had returned to pre-infusion levels. Only small changes in pulmonary haemodynamics were observed. These results indicate that pinacidil acts as a peripheral arteriolar vasodilator, and as such may have a role in the treatment of arterial hypertension and of cardiac failure.

Liquid chromatographic determination of pinacidil, a new antihypertensive drug, and its major metabolite, pinacidil N-oxide, in plasma

Two procedures are described, one for the determination of pinacidil, the other for the determination of both pinacidil and its metabolite, pinacidil N-oxide, in plasma. When only parent drug levels are required, the plasma proteins are precipitated with acetonitrile, the solids discarded and the supernatant is evaporated to dryness. The residue is then reconstituted for analysis. For the determination of both drug and metabolite, the analytes are selectively retained from plasma on a solid-phase extraction column and eluted with methanol. After evaporation to dryness, the residue is reconstituted in mobile phase. Both procedures utilize reversed-phase liquid chromatographic separations with ultraviolet detection. The limits of detection are 10 ng/ml pinacidil in plasma and 5 ng/ml each of pinacidil and pinacidil N-oxide in plasma for the two procedures, respectively.