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|Title:||Reduction of Intrinsic Sinoatrial Frequency and Chronotropic Responsiveness in the Exercised Rat|
|Authors:||Hughson, Lee Richard|
|Keywords:||Medical Sciences;Medical Sciences|
|Abstract:||<p>The changes in the physiological properties of the sinoatrial node which follow a programme of exercise training have been examined in the rat. The influence on the intrinsic sinoatrial frequency (ISF) of altered levels of autonomic nervous activity accompanying the physical training has been studied in two series of experiments combining daily pharmacological agents with ten weeks of treadmill exercise. The first series (Study I) examined the influence on the ISF of the increased parasympathetic activity reported in the trained state by blocking with atropine or stimulating with carbachol the receptors of the parasympathetic system in the heart. The second series (Study 2) examined the influence on the ISF of the increased sympathetic activity which occurs during exercise by stimulating with noradrenaline or isoprenaline, or blocking with propranolol the receptors of the sympathetic system in the heart. The content of potassium (K) and sodium (Na) in the plasma and myocardium were measured in all animals at the completion of the ten week period, and an estimate of electrophysiological changes was derived from the plasma to myocardial K⁺ ratio (K⁺o/K⁺i). In addition, the chronotropic response of the sinoatrial node to acetylcholine and noradrenaline was examined in vitro for all animals in Studies 1 and 2. The chronotropic responses of the sinoatrial node to exercise and noradrenaline were further explored in the in vivo experiments of Study 3.</p> <p>In Study 1, ISF was reduced with training. Blockade of the parasympathetic receptors with atropine also reduced ISF, and the effects of exercise and atropine appeared to be additive. Stimulation of the parasympathetic receptors with carbachol had little effect on ISF. In Study 2, stimulation of the sympathetic receptors with either noradrenaline or isoprenaline caused a reduction in ISF, but when combined with exercise, no reduction occurred. Sympathetic receptor blockade with propranolol reduced ISF, and propranolol plus exercise also reduced ISF. It appears that the exercise-induced reduction of ISF was not a consequence of increased parasympathetic activity in the trained state, or of increased sympathetic activity during exercise. The K⁺o/K⁺i was reduced with training. This was consistent with a more negative maximum diastolic repolarization, and a reduced ISF. The K⁺o/K⁺i of the drug treated groups indicated that ISF was probably reduced through factors other than a more negative maximum diastolic repolarization. No difference was found in the sensitivity of the atria of any group to acetylcholine in vitro. Increased chronotropic sensitivity to noradrenaline was found in vitro in the two chronically noradrenaline treated groups. The maximum response to noradrenaline was reduced in vitro. The maximum cardiac frequency in response to either exercise or noradrenaline infusion in vivo was also reduced. This reduction of maximum frequency in both the atria and the intact heart was observed to be positively correlated with the ISF, thus those hearts with a low ISF obtained a low maximum frequency. This observation led to the proposal that electrophysiological properties of the sinoatrial node which establish the ISF are also involved in imposing a limit on the maximum sinoatrial frequency.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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