Presynaptic antisympathetic action of amiodarone and its metabolite desethylamiodarone

Measurement of sympathetic nervous system activity in heart failure: the role of norepinephrine kinetics

Recent demonstration that the level of sympathetic nervous drive to the failing heart in patients with severe heart failure is a major determinant of prognosis, and that mortality in heart failure is reduced by beta-adrenergic blockade, indicate the clinical relevance of heart failure neuroscience research. The cardiac sympathetic nerves are preferentially stimulated in severe heart failure, with the application of isotope dilution methods for measuring cardiac norepinephrine release to plasma indicating that in untreated patients cardiac norepinephrine spillover is increased as much as 50-fold, similar to levels of release seen in the healthy heart during near maximal exercise. This preferential activation of the cardiac sympathetic outflow contributes to arrhythmia development and to progressive deterioration of the myocardium, and has been linked to mortality in both mild and severe cardiac failure. Although the central nervous system mechanisms involved in the sympathetic nervous activation at present remain uncertain, increased intracardiac diastolic pressure seems to be one peripheral reflex stimulus, and increased forebrain norepinephrine turnover an important central mechanism.Additional neurophysiological abnormalities present in the failing human heart include release of the sympathetic cotransmitters, epinephrine and neuropeptide Y, at high levels more typical of their release during exercise in healthy subjects, and the possible presynaptic augmentation of norepinephrine release from the cardiac sympathetic nerves by the regionally released epinephrine. Following on the demonstrable benefit of beta-adrenergic blockade in heart failure, additional antiadrenergic measures (central suppression of sympathetic outflow with imidazoline binding agents such as clonidine, blocking of norepinephrine synthesis by dopamine-beta-hydroxylase inhibition, antagonism of neuropeptide Y) are now under active investigation.

Amiodarone Decreases Heat, Cold, and Mechanical Hyperalgesia in a Rat Model of Neuropathic Pain

Lidocaine is effective in controlling ventricular dysrhythmia and neuropathic pain. Amiodarone, like lidocaine, has sodium channel blocking properties. In the present study we explore whether amiodarone has a similar effect as lidocaine on the heat, cold, and mechanical hyperalgesia seen in the rat model of neuropathic pain. Ten male Sprague-Dawley rats were anesthetized. Four loose ligatures were placed on the sciatic nerve of the right hindpaw. A sham operation was performed on the contralateral hindpaw (control). Heat hyperalgesia was determined by comparing each paw withdrawal latency to heat stimulation (radiant heat source, 50 degrees C). Cold hyperalgesia was assessed with acetone application. Mechanical hyperalgesia was determined by comparing the mechanical threshold in the ligated and control hind paws using calibrated von Frey filaments. Amiodarone was intraperitoneally administered at doses of 1, 5, 10, 20, 50, and 100 mg/kg after the development of hyperalgesia. The animals were tested for hyperalgesia before and 1, 3, and 24 h after the administration of a single dose of amiodarone. Intrathecal catheters were implanted in 5 new rats, and amiodarone 5 mg/kg was injected. Testing for heat, mechanical, and cold hyperalgesia was performed similarly in the intrathecal amiodarone administration group. Amiodarone produces statistically significant decreases of heat, cold, and mechanical hyperalgesia after intraperitoneal administration. Results are statistically significant at 10 mg/kg (heat hyperalgesia), 20 mg/kg (mechanical hyperalgesia), and 100 mg/kg (cold hyperalgesia) intraperitoneally. Hyperalgesia returns 24 h after a dose. The intrathecal administration of amiodarone produces a nonstatistically significant reduction of hyperalgesia. Amiodarone seems to have a similar effect as lidocaine on the hyperalgesia seen in the rat model of neuropathic pain. As the half-life of amiodarone is significantly longer that that of lidocaine (mean, 53 days versus 90 min) in humans, it may have the potential to provide a longer lasting (and perhaps more effective) effect than lidocaine on neuropathic pain states.

IMPLICATIONS

Amiodarone was found to produce a statistically significant decrease in heat, cold, and mechanical hyperalgesia in a rat model of neuropathic pain after intraperitoneal injection. Considering its long half-life in humans, amiodarone has the potential to provide long lasting pain relief in neuropathic pain states.

Acute electrophysiologic effects of intravenous amiodarone are independent of a sympatholytic action in humans

Previous experiments in animals demonstrated a novel sympatholytic action of acute intravascular amiodarone (AM). It is not known if this action also occurs in humans. Twelve male volunteers performed handgrip for 10 min before and after 300 mg intravenous (IV) AM over 60 min. The effect of handgrip was determined from changes in blood pressure (BP), heart rate (HR), and cardiac noradrenaline (NA) spillover. Changes in cardiac spillover of dihydroxyphenylglycol (DHPG), the metabolite of NA, were measured during AM infusion. The electrophysiological effects of AM were determined from changes to the A-H intervals during right atrial stimulation (100 beats/min). Handgrip increased HR (63 +/- 2 to 84 +/- 5 beats/min and 65 +/- 3 to 84 +/- 4 beats/min), systolic BP (141 +/- 4 to 179 +/- 6 mm Hg and 140 +/- 4 to 179 +/- 7 mm Hg), and cardiac NA spillover (11.9 +/- 4 to 44.3 +/- 13 ng/min and 17.3 +/- 4 to 55.5 +/- 11 ng/min) before and after AM, respectively (P < 0.02 in all groups). There was good correlation between increases in cardiac NA spillover and HR (r2 = 0.86) and systolic BP (r2 = 0.87). AM increased the A-H interval (95.5 +/- 18 to 107.8 +/- 20 ms, P < 0.02). There was no difference in hemodynamic or NA response to handgrip before or after the AM infusion. There was also no change in DHPG cardiac spillover during AM infusion. Acute IV AM did not exert a sympatholytic action in humans, with no attenuation in hemodynamic or NA response to handgrip or increase in DHPG production, despite producing an electrophysiologic response.