Figure 3. The relationship between the levels of [Ca²⁺]_i and changes in NKA activity induced by forskolin and IBMX.

The percentage changes in NKA activity induced by forskolin (10⁻⁵ M) and IBMX (5 × 10⁻⁴ M) (mean values of 2-4 separate experiments) were plotted as a function of the levels of [Ca²⁺]_i at which the regulation of NKA activity was performed. The activity of NKA in both control and forskolin- and IBMX-incubated cells was measured as ouabain-sensitive ⁸⁶Rb⁺ uptake. The different levels of [Ca²⁺]_i were produced by the application of different concentrations of A23187 and were measured in a SPEX dual-beam excitation spectrofluometer as described in the Methods. The [Ca²⁺]_i values shown here represent the mean values of the [Ca²⁺]_i from 16-21 individual cells in 4 separate experiments. These five [Ca²⁺]_i values are (mean ±s.e.m.) 120 ± 14, 151 ± 15, 188 ± 23, 331 ± 24 and 420 ± 32 nM, which represent the levels of [Ca²⁺]_i of control cells and cells treated with A23187 at 10⁻⁸, 10⁻⁷, 10⁻⁶ and 10⁻⁵ M, respectively.