THE EFFECTS OF BETA-STIMULATION ON THE NA-K+ PUMP CURRENT-VOLTAGE RELATIONSHIP IN GUINEA-PIG VENTRICULAR MYOCYTES()

1. The whole cell patch clamp technique was used to study effects of t he beta agonist isoprenaline (Iso) on the current-voltage (I-V) relati onship of the Na+-K+ pump current (I-p) in acutely isolated guinea-pig ventricular myocytes. 2. The effect of Iso on I-p at high [Ca2+](i) ( 1.4 mu M) was voltage dependent. The I-V relationship of I-p in Iso sh ifted by approximately 30 mV in the negative direction on the voltage axis, increasing I-p at negative voltages but leaving I-p unchanged at positive voltages. 3. Intracellular application of the calmodulin ant agonist, calmodulin-dependent protein kinase II fragment 290-309, did not eliminate or reduce the Iso-induced voltage shift, suggesting calm odulin-dependent protein kinase II was not involved. 4. The Iso inhibi tion of I-p at low [Ca2+](i) (15 nM) was not voltage dependent. I-p wa s reduced by 20 to 30% in the presence of Iso at each holding potentia l. 5. When the voltage dependence of I-p was largely reduced by substi tution of N-methyl-D-glucamine(+) for external Na+, the magnitude of t he low [Ca2+](i), Iso-induced inhibition of I-p was progressively elim inated by increasing the [Ca2+](i). At a [Ca2+](i) of 1.4 mu M, this i nhibition disappeared. 6. At intermediate values of [Ca2+](i), the I-V curves in Na+-containing solution in the presence and the absence of Iso crossed over. The higher the [Ca2+](i), the more positive the volt age at which the two I-V curves intersected. 7. During beta-adrenergic activation our results suggest intracellular Ca2+ has two effects: (a ) It prevents protein kinase A (PKA) phosphorylation-induced inhibitio n of I-p. (b) It causes a PKA phosphorylation-induced shift of the pum p I-V relationship in the negative direction on the voltage axis. Thes e effects may have important physiological significance in the regulat ion of heart rate and cardiac contractility.