Mechanisms by which discharge modulates diastolic depolarization in sheep and dog Purkinje fibers.

For reasons unknown, a fast drive is prone to induce overdrive excitation in sheep Purkinje fibers under conditions that still induce overdrive suppression in dog Purkinje fibers. Our aim was to study by means of a microelectrode technique diastolic depolarization (DD) and its changes with overdrive in sheep and dog Purkinje fibers perfused in vitro under identical conditions. The major results are: (a) At a slow rate, diastolic depolarization is much faster and larger in sheep than in dog Purkinje fibers. (b) Faster rates increase DD slope and amplitude in sheep and decrease them in dog Purkinje fibers. (c) DD slope and amplitude increase in sheep and decrease in dog if the same number of action potentials are separated by a shorter diastole. (d) The change in DD slope and amplitude induced by a fast drive persists after a subsequent slow drive of approximately 20 s. (e) The fastest drives can induce an oscillatory potential superimposed on early DD in sheep. (f) In both species, high Ca(2+) increases and low Ca(2+) decreases DD slope and amplitude. (g) Neither high nor low Ca(2+) change the DD rate-dependence patterns peculiar to either species. (h) DD amplitude in dog in high Ca(2+) is still smaller than that in sheep in Tyrode solution. (i) Caffeine prevents the steepening of early DD by drive, but not the subsequent increase which can lead to overdrive excitation in both species. (j) TTX decreases DD slope and amplitude in both species. (k) Cs(+) markedly reduces DD slope and amplitude and more so at faster rates, especially in the sheep. We conclude that the differences in diastolic depolarization and the different behavior of DD with overdrive in the two species account for the propensity of sheep Purkinje fibers to develop overdrive excitation and for that of dog Purkinje fibers to develop overdrive suppression.