Role of the Sympathetic Nervous System in the Sinus Node Resistance to High Potassium

Abstract

The resistance of sinus node automaticity to high potassium (K) levels was studied in anesthetized dogs by perfusing the sinus node artery with oxygenated Tyrode's solution containing different K concentrations. The following results were obtained.(1) Sinus node automaticity persisted at a K concentration of 21.6 DIM, although the pacemaker site tended to shift in and out of the node at this concentration. (2) Sinus node dominance at high K concentrations was progressively lost as sympathetic influences were progressively eliminated. (3) Sinus node pacemaker activity was suppressed at a K concentration lower than 21.6 mM when calcium was omitted from the perfusion fluid. (4) The pacemaker site shifted consistently from the sinus node towards the atrioventricular node at high K concentrations after the elimination of sympathetic activity (5) The transient sinus tachycardia caused by high K concentrations persisted after bilateral vagotomy. It is concluded that catecholamines participate in the resistance of the sinus node to K, suggesting that the difference in resistance to high K concentrations between the sinus node and Purkinje fibers may, in part, result from the more abundant innervation of the sinus node. KEY WORDScatecholamines reserpine sympathectomy propranolol calcium dog• Ventricular Purkinje fibers perfused in vitro usually become quiescent when the potassium (K) content of Tyrode's solution is increased from 2.7 mM to 5.4 mM (1). In contrast, the sinus node continues to discharge spontaneously when K is increased to a concentration at which atrial fibers become inexcitable (2-6). The reason for the resistance of sinus node pacemaker activity to high K concentrations is not understood. It seems likely that this resistance to K reflects membrane characteristics peculiar to sinus node cells, but other factors cannot be excluded. One of these factors could be sympathetic innervation, since it has been shown that catecholamines counteract the depressant effect of K on cardiac excitability (4, 7). It does not seem unreasonable, therefore, to propose that catecholamines also counteract the depressant effect of K on automaticity. Sympathetic innervation of ventricular Purkinje fibers is sparse and that of the sinus node abundant (8) This work was supported in part by grants from the U. S. Public Health Service and the New York Heart Association. The work was carried out while Dr. Vassalle was the recipient of a Sinsheimer Award.A preliminary report has appeared in abstract form (Am J Cardiol 26:636, 1970).Received September 22, 1972. Accepted for publication January 11, 1973. reinforces the possibility that catecholamines play an important role in the difference in sensitivity between atrial and ventricular pacemakers to K.The aim of the present investigation was to study the effect of the sympathetic nervous system on the sinus node resistance to high K concentrations by perfusing the sinus node artery with a solution in which K content was varied. Autonomic denervation, catecholamine depletion...

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