Effects of hemodynamic variables on myocardial K+ balance during and after shortlasting ischemia

Reperfusion arrhythmias in isolated perfused pig hearts. Inhomogeneities in extracellular potassium, ST and TQ potentials, and transmembrane action potentials

We recorded direct current electrograms and local [K+]o at multiple sites and transmembrane potentials at selected sites during reperfusion after 5 minutes and 10 minutes of regional ischemia in isolated perfused pig hearts. After 10 minutes of ischemia, the incidence of ventricular fibrillation (VF) was 38%. At 80-90 seconds after reperfusion, [K+]o was 0.8 mM less than in normal tissue in half of the reperfused tissue, especially in the border zone. This was associated with TQ elevation of +4.5 mV and large peaked T waves. The latter was caused by an abrupt decrease of action potential duration in reperfused tissue, leading to a difference of up to 165 msec with normal tissue. Reperfusion VF started with a closely coupled ventricular premature beat. Activation block between reperfused and normal tissue permitted reentrant activation, leading to VF. Pretreatment with ryanodine (10(-6) M) and reperfusion with elevated [K+] (both of which prevent delayed afterdepolarizations) did not prevent closely coupled ventricular premature beats or VF. Five minutes of ischemia never caused VF. K+ depletion and TQ elevation in the reperfused zone was less frequent and smaller (-0.4 mM and 1.8 mV, respectively). Peaked T waves did not occur, and shortening of the action potential duration was less. We conclude that extracellular K+ depletion and marked action potential duration shortening in the reperfused tissue play a role in the genesis of reperfusion VF, which is caused by reentry. The closely coupled ventricular premature beat that initiates reentry is not caused by delayed afterdepolarizations but most likely by intramural reentry.

Myocardial potassium balance associated with regional ischaemia in the pig: effects of β-adrenoceptor blockade, duration of ischaemia and preceding ischaemic periods

The effects of beta-adrenoceptor blockade, duration of ischaemia and preceding ischaemic periods on ischaemia-induced changes in myocardial K+ balance were studied in 12 open-chest pigs. Coronary venous blood was directed through a shunt from the coronary sinus to the right atrium. Continuous recordings of arterial, shunt blood [K+] and shunt flow enabled us to compute myocardial K+ balance during and after consecutive 2-, 2-, 5-, 10- and 2-min periods of regional ischaemia separated by 30 min of reperfusion. beta-adrenoceptor blockade (propranolol 1 mg kg-1 i.v.) given between the first and second ischaemic period did not alter the effects of 2 min ischaemia on myocardial K+ balance. Total K+ losses induced by 2, 5 and 10 min of ischaemia were 67.1 (40.6-93.3), 106.7 (69.4-176.8) and 192.2 (117.7-332.6) mumol 100 g-1, respectively. Thus, the plateau observed in extracellular [K+] between 2 and 10 min of regional ischaemia could, at least partly, be explained by continuous drainage of K+ from ischaemic myocardium into the surrounding normally perfused tissue. The total K+ loss induced by the second and last 2-min ischaemic period were 67.1 (40.6-93.3) and 35.6 (23.1-53.6) mumol 100 g-1 (P less than 0.001), respectively. This reduction shows that ion homeostasis during ischaemia was greatly changed in myocardium which had been 'preconditioned' and 'stunned' by 5 plus 10 min of ischaemia. Total amount, maximal rate and duration of post-ischaemic K+ reuptake increased with the duration of the preceding ischaemia. Moreover, K+ re-uptake after 2 min of ischaemia and the number of sarcolemmal Na/K pumps ([3H]ouabain binding), were normal in stunned myocardium. From these observations we conclude that progressive stimulation of the Na/K-pump occurred when ischaemia was prolonged from 2 to 10 min, and that Na/K-pump function was preserved in stunned myocardium.