Figure 4.

Effect of PKA phosphorylation on ATP-dependent ⁴⁵Ca²⁺-transport into intact isolated nuclei. Isolated nuclei before (○) and after phosphorylation (•) were incubated at 37°C for 5 min in the presence of 1 mM ATP. Calcium chloride was added into the medium bathing nuclei so as to give the indicated free calcium concentration according to Fabiato (34). Traces of ⁴⁵Ca²⁺ were also present in the medium (2 μCi/ml; 1 Ci = 37 Gbq). Ca²⁺ uptake was terminated by filtering under vacuum over GF/B Whatman glass fiber filters, followed by scintillation counting of the ⁴⁵Ca²⁺ trapped on the filters. Values are expressed as means ± SEM, and statistical significance was evaluated by using the Mann–Whitney U test (∗, P < 0.05, control vs. PKA).