β-Adrenergic modulation of Na-K pump activity in young and adult canine cardiac Purkinje fibers

We used standard microelectrode techniques to study the developmental changes and β-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 μg/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 β-adrenergic agonist-induced hyperpolarization in adults requires the presence of a PTX-sensitive G protein for its occurrence.