Abstract

Na⁺ reabsorption is regulated in proximal tubules by hormones that stimulate protein kinase C (PKC). To determine whether stimulation of PKC causes a reduction in intracellular Na⁺concentration ([Na⁺]_i) that might link Na⁺ pump activation to increased Na⁺reabsorption, [Na⁺]_i was measured in kidney cells loaded with the Na⁺-sensitive fluorescent indicator SBFI. Rapid digital imaging fluorescence microscopy determinations were performed in epithelial kidney cells transfected with the rodent Na⁺ pump α1 cDNA. In 42 determinations, the basal [Na⁺]_i was 19.7 ± 2.4 mm. Stimulation of PKC reduced the [Na⁺]_i to 5.6 ± 0.6 mm in ∼10 sec. This drastic change in [Na⁺]_i requires a transient 74–120-fold increase in Na⁺ pump activity. After the new steady state [Na⁺]_i is reached, the Na⁺ pump is 58% activated. The entry of Na⁺ into the cells is not affected by stimulation of PKC; therefore, the reduction in [Na⁺]_i is exclusively dependent on activation of the Na⁺ pump. Accordingly, PKC stimulation does not affect the [Na⁺]_i of cells expressing a mutant Na⁺ pump that is not stimulated by PKC. The decrease in [Na⁺]_i observed in cells transfected with the rodent Na⁺ pump α1 cDNA is large and sufficiently fast that it is expected to stimulate rapidly passive Na⁺-influx into the cells, thereby accounting for the observed PKC-induced stimulation of Na⁺ reabsorption.