Stephen J. Humphrey

Pathogenesis of cardiovascular alterations in dogs treated with minoxidil.

Minoxidil and other potent vasodilators cause coronary arterial injury, right atrial hemorrhagic lesions, and subendocardial necrosis in dogs. This paper discusses the pathogenesis of coronary arterial and right atrial lesions associated with minoxidil in the dog. Acute coronary vascular injury characterized by segmental medial hemorrhage and necrosis and perivascular inflammation occurred only during the first few days of treatment, after which tolerance to further acute injury developed. At 30 d or more of treatment, coronary vascular injury was characterized by perivascular fibrosis rarely attended by medial distortion or hyperplasia and subintimal thickening, changes consistent with responses to previous injury. Right atrial hemorrhagic lesions, unlike coronary vascular injury, often became progressively more extensive with continued treatment. At 3 d, atrial hemorrhage and inflammation were confined to the subepicardium of the right atrium, evidently around affected subepicardial branches of the right coronary artery. At 30 d, fibrovascular proliferative right atrial lesions (granulation tissue with evidence of continual hemorrhage) extended from the epicardium to the myocardium, with eventual replacement of the atrial wall by mature connective tissue at 1 yr of treatment. Minoxidil-induced cardiovascular lesions were not prevented by treatment with a β-blocker (propranolol), or an α-blocker (dibenzylene), or by sympathetic neural activity suppression (surgical sympathectomy or constant carotid sinus nerve stimulation), suggesting that the sympathetic response to the pharmacologic activity of minoxidil was not responsible for the induction of the cardiovascular lesions. Minoxidil-related vascular lesions were confined to the most pharmacologically responsive segment of the arterial system, the coronary arteries, suggesting that medial injury may have been associated with tensile changes in the arterial wall.

Evidence for a central depressor action of postsynaptic α1-adrenergic receptor antagonists

Abstract The effects of α-adrenergic receptor antagonists on sympathetic nervous discharge (SND) recorded from the external carotid and splanchnic nerves were studied in baroreceptor intact and denervated cats. Prazosin (50 μg/kg, i.v.) produced a rapid fall in mean arterial pressure (MAP) and no significant change in heart rate (HR) in baroreceptor denervated cats. Prazosin administration was also associated with a prolonged inhibition of SND. Nerve activity was significantly reduced within 5 min of prazosin administration and remained depressed throughout the 2 h observation period. Like prazosin, WB-4101 (0.5 mg/kg, i.v.) also produced significant reductions in MAP and SND. In addition, WB-4101 produced a transient bradycardia. The decreases in MAP and SND were reversed by piperoxane (0.5 mg/kg, i.v.). Both prazosin and WB-4101 inhibited the pressor response to i.v. norepinephrine. In baroreceptor intact cats, prazosin decreased MAP and SND, but did not affect HR. In contrast, phentolamine (1 mg/kg, i.v.) decreased MAP but increased SND and HR. These data indicate that the sympatholytic action of WB-4101 and prazosin results from a centrally mediated reduction in SND as well as a peripheral blockade of α-adrenergic receptors. These data further suggest that noradrenergic neurons normally facilitate the outflow of sympathetic nerve activity from the central nervous system.

Whole body and regional hemodynamic effects of minoxidil in the conscious dog.

Microsphere estimates of whole body hemodynamics and tissue blood flow were made in conscious and pentobarbital-anesthetized dogs treated orally with the peripheral vasodilator minoxidil. Under both circumstances, 1.0-30 mg/kg minoxidil significantly reduced mean arterial pressure 21-41% and total peripheral resistance 52-75% 4 h after administration. Dose-dependent increases in heart rate and cardiac output were evident under conscious conditions, with both parameters approximately doubling at 1.0 mg/kg. The near maximal vasodilation achieved with this dose of minoxidil was due to diminished vascular resistance in all major tissue beds. The enhanced cardiac output was associated with significant 50-87% increases in blood flow to the skin, skeletal muscle, bone, stomach, large intestine, and pancreas. Far more dramatic six- to 10-fold increases in regional myocardial blood flow were seen at this dose, which appeared to be only partially dependent on increased cardiac work. Comparable blood flow patterns were seen with acute minoxidil at doses greater than 1.0 mg/kg, and with chronic minoxidil at 1.0 and 30 mg/kg/day. These experiments establish minoxidils relative vasodilation in the major tissue beds of the dog which contributes to its hypotensive activity.

A rat model for predicting orthostatic hypotension during acute and chronic antihypertensive drug therapy.

An experimental model has been developed to determine postural hypotension in chloralose-urethane-pentobarbital (CUP) anesthetized rats. Using this technique, mean arterial pressure, heart rate, and blood pressure compensation to 60 degree tilt were examined in rats treated acutely and chronically with a spectrum of standard antihypertensive agents. The results closely parallel the orthostatic profiles seen clinically with these drugs. Significant orthostatic hypotension was seen with the acute intravenous administration of the alpha-adrenergic antagonists phenoxybenzamine, phentolamine, and prazosin, the neuronal suppressant guanethidine, and the ganglionic blockers hexamethonium and chlorisondamine. The central antihypertensive clonidine displayed mild, acute orthostasis that dissipated by 1 hr. The vasodilator minoxidil was entirely free of postural effects. Chronically, guanethidine, the ganglionic blocker mecamylamine, and a high dose of reserpine all resulted in significant postural hypotension after 4 days of oral administration. Prazosins acute orthostasis had largely dissipated by this time. Chronic minoxidil resulted in slight overcompensation to tilt. Based on the consistency to these data relative to the clinical profiles of these standard antihypertensive agents, it would appear that the CUP anesthetized rat is an accurate, efficient test model for identifying orthostasis in novel hypotensive agents under acute and chronic drug treatment conditions.

Evidence for GABA mediation of sympathetic inhibition evoked from midline medullary depressor sites

GABA antagonists blocked, and diazepam potentiated, inhibition of spontaneous sympathetic activity elicited by electrical stimulation of classic midline medullary depressor sites. Picrotoxin often converted inhibitory effects of raphe stimulation into sympathoexcitatory responses. Serotonin antagonists blocked these sympathoexcitatory responses. The midline medullary raphe complex is heterogeneous in respect to autonomic function with sympathoinhibitory elements mediated at least in part by GABA and sympathoexcitatory pathways mediated by serotonin.

Evidence that L-Glutamic Acid Mediates Baroreceptor Function in the Cat

The possible role of L-glutamic acid (L-glu) as a neurotransmitter of baroreceptor afferent neurons was investigated in the cat by monitoring the changes in three indices of baroreceptor function seen with the L-glu antagonists L-glutamic acid diethyl ester (GDEE) and 1-hydroxy-3-amino-pyrrolidone-2-(HA-966). Baroreceptor function was determined from a) the computer summed inhibition of sympathetic nerve discharge (SND) evoked by electrical stimulation of vagal baroreceptor afferent pathways, b) the locking of SND to the cardiac cycle, and c) the sympathoinhibitory response to i.v. pressor doses of phenylephrine. Direct bilateral microinjections of GDEE (20 micrograms) and HA-966 (4 micrograms) into the region of the nucleus tractus solitarii (NTS) resulted in immediate, marked reductions in the SND inhibitory response to vagal stimulation, a loss in SND locking to the cardiac cycle, a shift in the arterial pulse/SND phase relation, and a diminished sympathoinhibitory response to phenylephrine. Control microinjections of isotonic saline (1 mu 1/NTS) were devoid of these effects. The vagal induced sympathoinhibitory response was restored after NTS microinjections of GDEE by increasing the intensity of the vagal stimulus, or by directly stimulating the NTS injection site, suggesting that the impairment in baroreceptor function seen with this L-glu antagonist was independent of mechanical or local anesthetic effects. These data strongly suggest that L-glu may act as a neurotransmitter of baroreceptor afferent neurons in the NTS of the cat.

Cardiovascular sympathomimetic amine interactions in rats treated with monoamine oxidase inhibitors and the novel oxazolidinone antibiotic linezolid.

Linezolid (PNU-100766) is a new gram-positive oxazolidinone antibiotic that is effective at in vitro concentrations ≤4 &mgr;g/ml and in vivo doses ≤10 mg/kg. Because linezolid also competitively inhibits human monoamine oxidase-A (MAO-A; Ki = 55 &mgr;M), we monitored its effects on the cardiovascular responses to tyramine and amine cold remedies in comparison with standard MAO inhibitors. In anesthetized rats, the pressor response to 16 &mgr;g i.v. tyramine was potentiated by the MAO-A inhibitors clorgyline (0.1–1.0 mg/kg i.v.) and moclobemide (5.0–50 mg/kg p.o.), but not by the MAO-B inhibitor selegiline (0.15–15 mg/kg p.o.). Fifteen milligrams per kilogram intravenous linezolid weakly potentiated i.v. tyramine independent of changes in &agr;-adrenoceptor reactivity, but this effect was not enhanced chronically (90–100 mg/kg/day). In conscious rats, 30 mg/kg/day oral linezolid (8 &mgr;g/ml plasma concentration) minimally affected the pressor response to 20 mg/kg oral tyramine, whereas 100 mg/kg/day linezolid (20 &mgr;g/ml plasma concentration) moderately potentiated this response similar to 3 mg/kg per day moclobemide. Linezolids tyramine potentiation was reversible, attenuated by food, and independent of pseudoephedrine, phenylpropanolamine, and dextromethorphan interactions. These studies demonstrate that high-dose linezolid only moderately potentiates the cardiovascular effects of tyramine and validate these models for evaluating such MAO inhibitory interactions.

Renal and vascular effects of chemically distinct ATP-sensitive K+ channel blockers in rats

Summary The ATP-sensitive K+ channel Mocker U-37883A was given intravenously (i.v.) to rats to determine if pharmacologic diversity among ATP-sensitive K+ channels observed in vitro is also apparent in vivo. Vascular K+ channel blockade was quantified as inhibition of the decrease in blood pressure (BP) produced by a standard dose of the K+channel opener pinacidil. Renal natriuretic effects were evaluated by an increase in urinary Na+ excretion. In both instances, effects of U-37883A. a guanidine, were compared with those of the sulfonylurea glyburide. In addition, the ability of these K+ channel blockers to lower plasma glucose levels was compared. U-37883A was nine times more potent than glyburide as a natriuretic and a comparable six times more potent than glyburide in blocking pinacidil. suggesting common features between ATP-sensitive K+ channels in vascular smooth muscle (VSM) and renal tubules. In contrast, a dose of U-37883A that blocked pinacidil and increased Na+ excretion had no effect on plasma glucose, whereas a dose of glyburide that was natriuretic and equally as effective against pinacidil as U-37883 A decreased plasma glucose, suggesting that ATP-sensitive K+ channels in pancreatic β cells and renal tubules have dissimilar binding sites and/or features. U-37883A appeared to be more renal/vascular selective, an observation entirely consistent with previous findings in vitro. Our results represent the first in vivo suggestion of structural differences among the channel and/or accessory proteins from this family of K+-selective channels.

Cardiovascular interactions characterizing the vasodilator components of nicorandil in conscious dogs.

We conducted cardiovascular interaction studies in conscious dogs to characterize the cyclic GMP and KATP channel opener (PCO)-dependent peripheral vasodilator components of nicorandil (NIC). Intravenously (i.v.) administered NIC (6-24 microg/kg/min for 6 h) reduced arterial blood pressure (BP) and increased heart rate (HR), and 15 mug/kg/min NIC was simulated by combining the cyclic GMP vasodilator nitroglycerin (NTG 1-5 microg/kg/min i.v.) with the PCO minoxidil (MNX 0.23 microg/kg i.v.). Tolerance to NTG-induced vasodilation was accelerated by oral isosorbide dinitrate (ISD 400 mg/2.5 days), and MNX was antagonized by the KATP blocker U-37883A (6 microg/kg/min i.v.). U-37883A attenuated NIC-induced hypotension with a mild tachycardia, whereas chronic ISD attenuated and delayed NIC-induced hypotension, with a pronounced tachycardia. Chronic ISD plus U-37883A completely blocked NIC-induced hypotension, although NIC-induced tachycardia persisted. These studies demonstrate that the cyclic GMP component of NIC induces a minimally tachycardiac acute vasodilation at lower drug concentrations resistant to K ATP blockade but cross-tolerant to ISD. Conversely, the PCO component of NIC induces a more tachycardiac vasodilation at higher concentrations free of ISD cross-tolerance but sensitive to KATP blockade. Therefore, low-dose NIC free of adjunctive nitrates should optimize its cyclic GMP vasodilator component for clinical indications.

K-atp-blocking Diuretic Pnu-37883a Reduces Plasma Renin Activity in Dogs

We determined the effects of the K-adenosine triphosphate (ATP)-blocking diuretic PNU-37883A on plasma renin activity (PRA) in conscious and anesthetized dogs. In conscious dogs, oral PNU-37883A (6-60 mg/kg) was less potent than hydrochlorothiazide (0.15-1.5 mg/kg) and furosemide (FURO; 0.3-3.0 mg/kg) but exhibited high natriuretic efficacy with little kaliuresis. Unlike the standard diuretics, PNU-37883A reduced PRA by 46-76%, and its high dose minimally affected 24-h urinary aldosterone excretion. PNU-37883A, 1 mg/kg i.v., also blunted the hyperreninemia induced by 1 mg/kg i.v. FURO. In cannulated dogs, 10 mg/kg i.v. PNU-37883A maximally increased fractional Na+ clearance 140% and reduced PRA 76%, but these effects were accompanied by a mean 13 mm Hg pressor effect. In anesthetized dogs, renal artery-infused PNU-37883A (3 mg/kg/h i.r.a.) increased Na+ excretion and reduced renal venous PRA independent of hemodynamics, whereas half this dosage selectively reduced renal venous PRA and renin release, independent of hemodynamics and natriuresis. These data demonstrate that the K-ATP blocker diuretic PNU-37883A reduces PRA in dogs after oral, i.v., and i.r.a. administration and could be a useful pharmacologic agent for exploring the role of K-ATP channels in regulating renin release.