AJP - Heart and Circulatory Physiology, Vol 271, Issue 2 706-H712, Copyright © 1996 by American Physiological Society
Beta-adrenergic modulation of Na-K pump activity in young and adult canine cardiac Purkinje fibers
F. Charpentier, M. J. Legato, S. F. Steinberg, I. S. Cohen and M. R. Rosen
Department of Pharmacology, College of Physicians and Surgeons of Columbia University, New York 10032, USA.
We used standard microelectrode techniques to study the developmental
changes and beta-adrenergic modulation of membrane potential and of Na-K
pump activity in adult (> 1 yr of age) and neonatal (2-10 days) canine
Purkinje fibers. Isoproterenol (10(-7) M) increased the rate of development
and magnitude of pacing-induced hyperpolarization of adult fibers driven at
a 1-s cycle length. This effect of isoproterenol was attenuated by treating
dogs with pertussis toxin (PTX), (30 micrograms/kg). Other adult and
neonatal fibers were superfused with a Tyrode solution containing Ba2+ 0.2
mM, Cs+ 2 mM, and 10(-6) M verapamil, thus leading to depolarization and
cessation of spontaneous activity. The Na-K pump was studied by alternating
solutions containing [K] at 0 mM (inhibiting the pump) and 4 mM
(reactivating the pump). Although the kinetics of the Na-K pump appeared
faster in neonatal fibers than in adult fibers, measurement of cell
surface-to-volume ratio compensated for the difference. We therefore
conclude that 1) the apparent age-related changes in Na-K pump activity in
canine Purkinje fibers in fact reflect cell surface-to-volume ratio and, 2)
the beta-adrenergic agonist-induced hyperpolarization in adults requires
the presence of a PTX-sensitive G protein for its occurrence.
