Figure 4. The effects of OAG on the activity and Ser23 phosphorylation of NKA at both low and high [Ca²⁺]_i.

Confluent cultures of COS cells expressing rat renal NKA were treated with OAG (10⁻⁸ M) for 15 min at 37 °C under low and high [Ca²⁺]_i conditions. Following drug treatment, both the activity and Ser23 phosphorylation of NKA were assayed. A, the effect of OAG on the activity of NKA. Values are means ±s.e.m. of 5 separate experiments. The basal NKA activity (in nmol mg⁻¹ min⁻¹) was 18.75 ± 2.10 at low [Ca²⁺]_i and 20.62 ± 2.53 at high [Ca²⁺]_i. B, the effect of OAG on the state of Ser23 phosphorylation, which was determined by Western blot analysis with an antibody specifically recognizing the Ser23-dephosphorylated, but not the phosphorylated, form of NKA. A blot representative of 3 experiments is shown. *P < 0.01.