Chiral recognition of pinacidil and its 3-pyridyl isomer by canine cardiac and smooth muscle: antagonism by sulfonylureas.

Pinacidil, a potassium channel opener (PCO), relaxes vascular smooth muscle by increasing potassium ion membrane conductance, thereby causing membrane hyperpolarization. PCOs also act on cardiac muscle to decrease action potential duration (APD) selectively. To examine the enantiomeric selectivity of pinacidil, the stereoisomers of pinacidil (a 4-pyridylcyanoguanidine) and its 3-pyridyl isomer (LY222675) were synthesized and studied in canine Purkinje fibers and cephalic veins. The (-)-enantiomers of both pinacidil and LY222675 were more potent in relaxing phenylephrine-contracted cephalic veins and decreasing APD than were their corresponding (+)-enantiomers. The EC50 values for (-)-pinacidil and (-)-LY222675 in relaxing cephalic veins were 0.44 and 0.09 microM, respectively. In decreasing APD, the EC50 values were 3.2 microM for (-)-pinacidil and 0.43 microM for (-)-LY222675. The eudismic ratio was greater for the 3-pyridyl isomer than for pinacidil in both cardiac (71 vs. 22) and vascular (53 vs. 17) tissues. (-)-LY222675 and (-)-pinacidil (0.1-30 microM) also increased 86Rb efflux from cephalic veins to a greater extent than did their respective optical antipodes. The antidiabetic sulfonylurea, glyburide (1-30 microM), shifted the vascular concentration-response curve of (-)-pinacidil to the right by a similar extent at each inhibitor concentration. Glipizide also antagonized the response to (-)-pinacidil, but was about 1/10 as potent with a maximal shift occurring at 10 and 30 microM. Glyburide antagonized the vascular relaxant effects of 0.3 microM (-)-LY222675 (EC50, 2.3 microM) and reversed the decrease in APD caused by 3 microM (-)-LY222675 (EC50, 1.9 microM). Nitroprusside did not alter 86Rb efflux, and vascular relaxation induced by sodium nitroprusside was unaffected by sulfonylureas. Thus, the enantiomers of the 3-pyridyl isomer of pinacidil demonstrate enhanced stereospecificity in both canine cardiac and vascular tissues compared to the enantiomers of pinacidil. However, the relative selectivity of pinacidil and its 3-pyridyl isomer for cardiac and vascular smooth muscle remains unaltered. Sulfonylureas antagonize the more potent enantiomers in both tissues, supporting the involvement of an ATP-sensitive potassium channel in the action of PCOs; however, antagonism in canine vascular smooth muscle by sulfonylureas does not resemble classical competitive antagonism.

Modification by glibenclamide of the electrophysiological consequences of myocardial ischaemia in dogs and rabbits.

Glibenclamide has been shown to block ATP-dependent K+ channels in the heart and prevent the shortening of cardiac action potentials caused by hypoxia in vitro. The present study examines the ability of glibenclamide to modify the effect of acute ischaemia on monophasic action potential duration in pentobarbital-anaesthetized rabbits, and on monophasic action potential duration and ventricular fibrillation threshold in pentobarbital-anaesthetized dogs. Left ventricular endocardial monophasic action potential duration was measured using a contact electrode catheter, and ventricular fibrillation threshold was measured by the single pulse method. Ischaemia was produced in rabbits by occluding the circumflex coronary for 5 min and in dogs by occluding the left anterior descending coronary artery for 40 min. In rabbits, glibenclamide (0.3-3 mg/kg, i.v.) had no effect on baseline monophasic action potential duration, but attenuated action potential shortening during ischaemia in a dose-related manner. In dogs, monophasic action potential duration did not shorten during ischaemia in the vehicle group, but tended to increase in the glibenclamide group (0.5 mg/kg, i.v.) both before and during ischaemia (7 +/- 5% and 14 +/- 8%, respectively, NS). Likewise, ventricular effective refractory period was significantly increased by glibenclamide prior to ischaemia (5 +/- 1%). Ventricular fibrillation threshold tended to increase during 40 min of ischaemia in vehicle-treated dogs (40 +/- 29%, NS), but was unchanged during ischaemia in the glibenclamide-treated dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

The relation between vascular relaxant and cardiac electrophysiological effects of pinacidil.

Pinacidil may represent an example of a new class of vasodilators that act by increasing membrane permeability to potassium ions. In the present study, the cardiac electrophysiological and venorelaxant effects of a series of pinacidil analogs in canine tissues in vitro were examined. Piacidil (3 x 10(-5) M) markedly reduced action potential duration in Purkinje fibers (82 +/- 3% decrease) and ventricular muscle (54 +/- 2% decrease) without significantly affecting maximal upstroke velocity of the action potential or conduction time. The EC50 for the reduction in Purkinje fiber action potential duration was 2.6 +/- 0.5 microM. Pinacidil also decreased barium-induced automaticity in Purkinje fibers; the concentration that decreased the rate of firing by 50% was identical to the EC50 for decreasing action potential duration. In some preparations, high concentrations of pinacidil (greater than or equal to 3 x 10(-5) M) were associated with the appearance of spontaneous action potentials that were closely coupled to the preceding driven action potential. The EC50 for pinacidil in relaxing phenylephrine-contracted cephalic veins was 0.43 +/- 0.09 microM, and in isolated cat papillary muscle, pinacidil had a direct negative inotropic effect with an EC50 of 4.1 +/- 0.7 microM. Thus, pinacidil was 6 and 10 times more potent in relaxing phenylephrine-contracted veins than in shortening action potential or decreasing cardiac contractility. There was an excellent correlation (r = 0.933, p = 0.002) between decreases in action potential duration and venorelaxation for all pinacidil analogs, as well as for BRL 34915 and nicorandil, two purported potassium channel openers. Significant correlations were also obtained between negative inotropic effects and reductions in action potential duration for the pinacidil series. Pinacidil (10(-5) M) also inhibited the venoconstrictor responses to the selective alpha 2 agonist, B-HT 920, to a greater extent than the alpha 1 agonist, methoxamine. Since a good correlation exists in vitro among all the compounds studied in reducing action potential duration, relaxing vascular tissue, and decreasing cardiac contractility, it is concluded that pinacidil as well as nicorandil and BRL 34915 affect vascular and cardiac tissues by similar mechanisms, possibly by increases in potassium ion permeability, although other mechanisms may also play a role.

Depressant effects of fast sodium channel blockade on the electrical activity of ischaemic canine ventricle: mediation by the sympathetic nervous system.

In this study we examined the possibility that local anaesthetic agents such as tetrodotoxin may exacerbate electrical changes during acute myocardial ischaemia by inhibiting fast sodium channels, both in cardiac cells and in sympathetic nerve terminals. Bipolar electrograms were recorded during serial 2 to 5 min occlusions of the left anterior descending coronary artery in open-chest, anaesthetised dogs. Tetrodotoxin (1 or 2 micrograms X kg-1 iv) given prior to occlusion did not affect activation times or electrograms in normal myocardium but exacerbated activation delay and loss of electrogram amplitude during ischaemia. Bilateral stellectomy reversed the effects of tetrodotoxin during ischaemia. Tetrodotoxin (1 microgram X kg-1 iv) reduced changes in heart rate and mean arterial blood pressure produced by stellate ganglia stimulation. Intracoronary infusion of tetrodotoxin (10(-5) mol X litre-1) during normal perfusion lengthened mean effective ventricular refractory periods and propranolol (0.5 mg X kg-1 iv) or bilateral stellectomy prevented this effect. Thus, tetrodotoxin appeared to increase ventricular refractoriness and exacerbate ischaemia-induced activation delay by inhibiting sympathetic nerve activity. Other agents with local anaesthetic properties may have similar effects.

The chronic influence of sympathetic nerves on cerebral vessels is age-related.

The purpose of this study was to determine the effect of chronic sympathetic denervation on cerebral vessels of the spontaneously hypertensive rat when superior cervical ganglionectomy was performed in adulthood. In a previous study, we have demonstrated increased protein transfer across the cerebral vessels of the chronically denervated hemisphere when superior cervical ganglionectomy was performed in adolescent spontaneously hypertensive rats. After four weeks of sympathetic denervation, the adult rats in this study did not demonstrate increased protein transfer in the denervated compared to the innervated cerebral hemisphere. Thus, the "trophic" effect of sympathetic nerves on cerebral vessels appears to be age-related.

Association between sympathetic nerve activity and cerebrovascular protection in young spontaneously hypertensive rats.

The purpose of this study was to determine resting and maximal superior cervical sympathetic nerve activity in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) at five and ten weeks of age as hypertension was developing. Basal cervical sympathetic nerve activity (SNA) of five week SHR was 58 +/- 3 muv* which was significantly elevated over age-matched WKY (SNA = 30 +/- 4 muv, *p less than 0.001) and ten week SHR (SNA = 30 +/- 4 muv, *p less than 0.001) as well as ten week WKY (SNA = 24 +/- 4 muv, *p less than 0.001). Thus, during basal conditions five week SHR nerve traffic was approximately two times that found in age-matched WKY as well as in ten week SHR and WKY. The peak sympathetic nerve activity in response to rapid hemorrhage in five week SHR (215 +/- 16 muv*) was significantly elevated over the maximal response of WKY (140 +/- 23 muv) (*p less than 0.02). Ten week SHR also reached a maximal sympathetic nerve activity (187 +/- 28 muv*) that was significantly elevated over WKY (100 +/- 15 muv) (*p less than 0.02). Thus, both five and ten week SHR had a greater capacity for elevated nerve activity following rapid hemorrhage than age-matched WKY. The elevation in resting cervical sympathetic activity in five week SHR, and the elevated capacity for sympathetic neural response in both five as well as ten week SHR, are consistent with a central nervous system abnormality in SHR that could relate to the previously described protective influence of sympathetic nerves on SHR cerebral blood vessels as hypertension is developing.