BETA-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 beta-adrenergic modulation of membrane potential and of Na -K pump activity in adult (> 1 yr of age) and neonatal (2-10 days) can ine Purkinje fibers. Isoproterenol (10(-7) M) increased the rate of de velopment and magnitude of pacing-induced hyperpolarization of adult f ibers driven at a 1-s cycle length. This effect of isoproterenol was a ttenuated by treating dogs with pertussis toxin (PTX), (30 mu g/kg). O ther 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 pum p was studied by alternating solutions containing [K] at 0 mM (inhibit ing the pump) and 4 mM (reactivating the pump). Although the kinetics of the Na-K pump appeared faster in neonatal fibers than in adult fibe rs, measurement of cell surface-to-volume ratio compensated for the di fference. We therefore conclude that 1) the apparent age-related chang es in Na-K pump activity in canine Purkinje fibers in fact reflect cel l 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.