Effects of hypoxia or low pH on the alternation of canine ventricular action potentials following an abrupt increase in driving rate

Abstract

Effects of hypoxia or low extracellular pH on the alternation of ventricular action potentials occurring after an abrupt increase in driving rate (rate change induced alternation — RCI alternation) were studied using standard microelectrode methods in canine papillary muscle preparations. Under the control conditions the alternation always occurred after a rate change from 10 to 100 beats·min⁻¹ or 60 to 200 beats·min⁻¹, but it diminished rapidly during the faster rates. Under the hypoxic condition the degree of the RCI alternation gradually increased to the peak 20 to 60 min after the onset of the hypoxic perfusion and then decreased. The hypoxic perfusion caused an increase in beat-to-beat alternating change of the action potential configuration and a marked persistence of the phenomenon. In the initial stage of reoxygenation after 2 hours of the hypoxic perfusion, the RCI alternation transiently increased again. During hypoxia in six out of 15 preparations an unusual alternation of action potentials with an inverted phase occasionally occurred after the rate change from 60 to 200 beats·min⁻¹. Acidic perfusion (pH = 6.0) had similar effects on the RCI alternation. It also caused an increase in beat-to-beat alternating change in the action potential configuration and a prolongation of the phenomenon. In the period when the RCI alternation was markedly increased, a steady-state alternation of action potentials spontaneously occurred at a constant drive rate under hypoxia or low pH. The mechanisms responsible for the RCI alternation of action potentials and the possible role of the phenomenon in the genesis of cardiac arrhythmias in the ischaemic heart are discussed.