Translation of flecainide- and mexiletine-induced cardiac sodium channel inhibition and ventricular conduction slowing from nonclinical models to clinical.

INTRODUCTION: Nonclinical in vivo models used for cardiovascular safety testing have not previously been studied for their sensitivity for detection of conduction slowing resulting from cardiac sodium channel block. The goal of this study was to examine the sensitivity of in vivo models to cardiac sodium channel block, and translation of the effect from in vitro to in vivo models using sodium channel inhibitors flecainide and mexiletine; flecainide, but not mexiletine is commonly associated with QRS complex prolongation in humans. METHODS: Inhibition of cloned cardiac sodium channels (hNav1.5) was studied using the IonWorks platform. Conduction slowing was measured in vitro in the rabbit isolated ventricular wedge (RVW) and in vivo in the conscious telemetered rat and dog, and anaesthetised dog. RESULTS: Flecainide and mexiletine inhibited hNav1.5 channels with IC50 values of 10.7 and 67.2 µM respectively. In the RVW, QRS was increased by flecainide at 60 bpm, and at 120bpm, there was an increased effect of both drugs. In conscious rats, flecainide significantly increased QRS complex duration; mexiletine had no significant effect, but there was an increase at the highest dose in 4/6 animals. QRS complex was increased by flecainide and mexiletine in anaesthetised dogs but this was not statistically significant; in conscious dog, only flecainide produced a significant increase in QRS complex. DISCUSSION: When compared to clinical data, effects of flecainide and mexiletine in RVW and conscious dog compared well with effects in patients and healthy volunteers in terms of sensitivity. The anaesthetised dog was least sensitive for detection of changes in QRS. All assays showed some differentiation between the expected conduction slowing activity of flecainide and mexiletine. Based on these data, RVW and conscious dog were most predictive for effects of compounds on QRS complex and cardiac conduction.

Tachycardia-induced primate model of heart failure in cardiovascular drug discovery.

Congestive heart failure (CHF) is a complex, multifactoral disease involving genetic and environmental factors that represents a large unmet medical need. There are currently many animal models of CHF that have provided some insight into the etiology of this disease. However, due to the complex interactions of environmental and genetic components of this disease most animal models are somewhat limited. Nonhuman primates offer a unique opportunity to investigate the genetic aspects of this complex disease due to their close genetic and phenotypic similarity to humans. Here we describe a novel tachycardia-induced primate model of CHF characterized by depressed global function that progresses to a symptomatic stage consistent with clinical data. No animal model, including this one, can exactly mimic the clinical pathophysiology of CHF. However, this tachycardia-induced primate model of CHF has similarities to the dynamic state of CHF in humans and affords the opportunity to evaluate changes in gene expression using genomic and proteomic technologies throughout the progression of the disease.

NO modulates myocardial O2 consumption in the nonhuman primate: an additional mechanism of action of amlodipine.

Recent evidence from our laboratory and others suggests that nitric oxide (NO) is a modulator of in vivo and in vitro oxygen consumption in the murine and canine heart. Therefore, the goal of our study was twofold: to determine whether NO modulates myocardial oxygen consumption in the nonhuman primate heart in vitro and to evaluate whether the seemingly cardioprotective actions of amlodipine may involve an NO-mediated mechanism. Using a Clark-type O2 electrode, we measured oxygen consumption in cynomologous monkey heart at baseline and after increasing doses of S-nitroso-N-acetylpenicillamine (SNAP; 10(-7)-10(-4) M), bradykinin (10(-7)-10(-4) M), ramiprilat (10(-7)-10(-4) M), and amlodipine (10(-7)-10(-5) M). SNAP (-38 +/- 5.8%), bradykinin (-19 +/- 3.9%), ramiprilat (-28 +/- 2.3%), and amlodipine (-23 +/- 4.5%) each caused significant (P < 0.05) reductions in myocardial oxygen consumption at their highest dose. Preincubation of tissue with nitro-L-arginine methyl ester (10(-4) M) blunted the effects of bradykinin (-5.4 +/- 3.2%), ramiprilat (-4.8 +/- 5.0%), and amlodipine (-5.3 +/- 5.0%) but had no effect on the tissue response to SNAP (-38 +/- 5.8%). Our results indicate that NO can reduce oxygen consumption in the primate myocardium in vitro, and they support a role for the calcium-channel blocker amlodipine as a modulator of myocardial oxygen consumption via a kinin-NO mediated mechanism.

Reflex vascular responses in the anesthetized dog to large rapid changes in carotid sinus pressure.

This study examined reflex vascular responses to large rapid increases and decreases in carotid sinus pressure to determine whether delayed or inappropriate vascular responses might be obtained that, if they occurred in people, could lead to hypotension during exposure to rapidly alternating gravitational forces. In chloralose-anesthetized open-chest dogs, a perfusion circuit controlled carotid sinus and thoracic aortic pressures and blood flows to both the vascularly isolated abdominal circulation and a hindlimb (perfusion pressure changes denoted resistance). When carotid pressure was increased and decreased over the range of 60-180 mmHg, the resulting reflex vasodilatation occurred significantly more rapidly than the vasoconstriction (P < 0.001). In the abdominal vascular bed, time constants for vasodilatation and vasoconstriction were 4.2 +/- 0.5 and 7.5 +/- 1.0 s, respectively. Decreases in carotid pressure in pulses of 10-s duration or less failed to elicit maximal vasoconstriction, whereas increases in carotid pressure lasting as little as 5 s did elicit maximal vasodilatation. "Square-wave" alternations in carotid pressure with periods of 10 s or less (5 s high, 5 s low) resulted in attenuation of the vasoconstriction, and at a 4-s period, both vascular beds remained almost maximally vasodilated throughout. The failure of vascular resistance to follow carotid pressure changes was not due to a failure of the response of sympathetic efferent activity, since the time constants for the reduction and increase in discharge were much shorter at 0.56 +/- 0.13 and 0.43 +/- 0.10 s, respectively. These results indicate that rapid changes in carotid pressure could result in inappropriate vasodilatation and hypotension and might, in some circumstances, such as in pilots flying high-performance aircraft, predispose to syncope.

Reflex vascular responses to alterations in abdominal arterial pressure and flow in anaesthetized dogs.

The existence of abdominal arterial baroreceptors has long been controversial. Previously difficulties have been encountered in localizing a stimulus to abdominal arteries without affecting reflexogenic areas elsewhere. In these experiments, using anaesthetized dogs, the abdomen was vascularly isolated at the level of the diaphragm, perfused through the aorta, and drained from the inferior vena cava to a reservoir. Changes in abdominal arterial pressure were effected by changing the perfusion pump speed. During this procedure the flow back to the animal from the venous outflow reservoir was held constant. Increases and decreases in abdominal arterial pressure resulted, respectively, in decreases and increases in perfusion pressure to a vascularly isolated hind-limb and in some dogs also a forelimb. Responses were significantly larger when carotid sinus pressure was high (120-180 mmHg) than when it was low (60 mmHg). Responses were still obtained after cutting vagus, phrenic and splanchnic nerves, but were abolished by spinal cord lesion at T12. These experiments provide evidence for the existence of abdominal arterial baroreceptors. The afferent pathway for the reflex vasodilatation appears to run in the spinal cord.

Reflex vascular responses to abdominal venous distension in the anesthetized dog.

This was undertaken to determine whether distension of the subdiaphragmatic veins results in reflex vasoconstriction and interacts with the carotid baroreflex. In alpha-chloralose-anesthetized open-chest dogs, a perfusion circuit controlled carotid and thoracic aortic pressures, splanchnic and limb blood flows, and cardiopulmonary blood flows. At carotid sinus pressures below approximately 90 mmHg, increases in splanchnic pressure of 7 mmHg or more resulted in increases in vascular resistance in both the splanchnic and limb circulations; there was no response at higher carotid pressures. At high venous pressures, the average maximum gains of the carotid baroreflex for splanchnic and limb resistance responses were increased by 106 and 67%, respectively. The responses were not abolished by cutting the vagal or phrenic nerves but were prevented by cutting the splanchnic nerves and, for the limb, the sciatic and femoral nerves. These results suggest that splanchnic congestion, by causing vasoconstriction and augmentation of the carotid baroreflex, may be important in the maintenance of blood pressure during gravitational stress.

An acute animal model that simulates the hemodynamic situations present during +Gz acceleration.

Air combat maneuver acceleration (G) profiles with onset/offset patterns that occur faster than the response characteristics of the human cardiovascular system may lead to regulatory instability and, ultimately, acceleration-induced loss of consciousness (G-LOC) incidents. We have developed an acute animal model that simulates the hemodynamic situations seen under acceleration to study the effects of complex G environments on individual reflexogenic areas. This preparation allowed us to individually isolate the effects of high gravity on venous return and cardiac preload, arterial baroreflexes and splanchnic capacity. This report describes the preparation and presents examples of the types of +Gz simulations possible and recordings of the responses of the animals. Further, we tested the hypothesis that the volume of blood displaced from the cephalic regions of the circulation and the rate of displacement into the splanchnic capacitance with G onset is affected by distending pressure at the carotid/aortic baroreceptor sites. Early results from 7 dogs show that resistance to flow into the splanchnic beds is affected by changes in distending pressure occurring at arterial baroreceptor sites. When pressure distending the carotid/aortic baroreceptors was increased, resistance to flow into the abdominal vascular beds was decreased. This result suggests that sudden increases in +Gz loads occurring during the overshoot phase from a previous G-peak may result in reduced tolerance.