The cellular basis of intrinsic sinus node recovery time.
OBJECTIVE
The aim was to investigate the mechanism of the intrinsic sinus node recovery time.


METHODS
The effect of 2 min periods of 20% and 50% overdrive on the electrical activity of fibres in the sinoatrial node was studied in isolated atria of rabbits under complete autonomic blockade (atropine 3 x 10(-6) M and propranolol 3 x 10(-7) M).


METHODS
Rabbits (New Zealand white) of either sex up to 3 kg weight were used.


RESULTS
The first returning cycle after overdrive is prolonged not only by the time needed for retrograde plus antegrade conduction but also by a delay in impulse formation (overdrive suppression). During pacing, action potential duration, amplitude, maximum diastolic potential (only in primary pacemaker fibres), and diastolic depolarisation rate were all decreased. Action potential duration, amplitude and maximum diastolic potential returned to control value during the first cycle following a period of overdrive, but diastolic depolarisation remained depressed during many consecutive cycles. In primary pacemaker fibres, diastolic depolarisation appeared to be depressed throughout diastole. In latent pacemaker fibres diastolic depolarisation was depressed only in the second part of the diastole.


CONCLUSIONS
Sinus node recovery time has two components: (1) a conduction component of both retrograde and antegrade conduction, and (2) a depression of the automaticity (= overdrive suppression), which is only due to a slowing of diastolic depolarisation.
Prinsze
F J
FJ
Department of Physiology, University of Amsterdam, Academic Medical Centre, The Netherlands.
Bouman
L N
LN
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002304:Cardiac Pacing, Artificial; D005260:Female; D006339:Heart Rate / Q000502:physiology; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials; D011433:Propranolol / Q000494:pharmacology; D011817:Rabbits; D012849:Sinoatrial Node / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors
D001285:Atropine; D011433:Propranolol