Effect of norepinephrine and heart rate on intracellular sodium activity and membrane potential in beating guinea pig ventricular muscle.

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Abstract
The effect of 3 microM norepinephrine (NE) on intracellular sodium activity (aiNa) and resting membrane potential was studied by continuous intracellular recordings with a conventional and an ion-selective microelectrode. The electrodes were impaled simultaneously in small (diameter, 0.3 mm) superfused trabeculae of the beating guinea pig ventricle at 37 degrees C. In the absence of NE, changes of the beating rate produced an increase of aiNa by 1.5 +/- 0.17 mM (from 0 to 1 Hz) and 1.9 +/- 0.47 mM (from 0 to 2 Hz). In the presence of NE, there was a very small significant increase of aiNa during constant stimulation (1 Hz) and at at [K+]o of 4.7 and 11.5 mM. After 7 minutes of exposure, aiNa increased by 0.5 +/- 0.19 mM (mean +/- SEM, n = 4) at [K+]o of 4.7 mM and by 0.5 +/- 0.22 (n = 6) at [K+]o of 11.5 mM. Resting membrane potential became more positive by 1 mV at both levels of [K+]o. The effect of NE became also clearly manifest from the configurational changes of action potentials (profound increase in plateau height and duration). Stimulation of the Na(+)-K+ pump by NE became manifest from the changes of resting membrane potential and aiNa after abrupt cessation of stimulation. The magnitude and the rate of the decrease in aiNa and the initial rate of hyperpolarization were significantly greater in the presence of NE than in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)