AJP - Cell Physiology, Vol 233, Issue 3 99-103, Copyright © 1977 by American Physiological Society
Blockade of myocardial slow inward current at low pH
S. Vogel and N. Sperelakis
The effect of low pH on the slow cationic inward current was studied in
isolated perfused embryonic chick ventricles (16-21 days old). In order to
study the slow current, the fast Na+ current was inactivated by partial
depolarization to about -40 mV by elevation of K+ (25 mM). Subsequent
exposure of the tissue to catecholamines or methylxanthines allowed slowly
rising overshooting electrical responses (the "slow response") with with
accompanying contractions to be elicited by electrical stimulation. These
slow responses are insensitive to tetrodotoxin and are Na+- and
Ca2+-dependent. It was found that the isoproterenol- and caffeine-induced
slow responses were abolished at about pH 6.1; 50% inhibition occurred at
about pH 6.5. The rate of rise of the normal action potential, which is
dependent on a fast Na+ current, was only slightly affected at these same
pH levels; however, electromechanical uncoupling occurred, as expected from
inhibition of the slow current. Therefore, the slow current was blocked at
an acid pH that did not block the fast Na+ current.
