BETA-ADRENERGIC STIMULATION DOES NOT REGULATE NA PUMP FUNCTION IN VOLTAGE-CLAMPED VENTRICULAR MYOCYTES OF THE RAT-HEART

Na pump current was measured in rat ventricular myocytes to determine if beta-adrenergic stimulation can directly modulate Na,K-ATPase activ ity. Enzymatically-isolated heart cells were voltage-clamped with a si ngle patch electrode and Na pump current was briefly activated by rapi dly increasing extracellular K+! from 0 to 15 mM for 3-5 s after othe r ionic currents were blocked or inactivated. The salt solution in the voltage-clamping electrode included (in mM): (1) 100 Na+, 10 EGTA, (2 ) 5 Na+, 10 EGTA, or (3) 100 Na+, 7.5 Ca2+, 10 EGTA (free Ca2+! = 480 nM). With all three electrode solutions, Na pump current was not sign ificantly changed after 2-4 min in the presence of 10 muM isoprenaline . beta-adrenergic pathways were still intact as evidenced by the two-f old increase in Cd2+-sensitive Ba2+ current through calcium channels t hat was observed in the presence of isoprenaline. Thus, beta-adrenergi c stimulation does not appear to directly regulate Na,K-ATPase activit y in rat ventricular myocytes.