Modulation of Ca2+ influx by protein phosphorylation in single intact clonal pituitary cells

In pituitary cells, electrical activity generates characteristic oscillations of the cytosolic free Ca2+ concentration, [Ca2+]i. These oscillations are controlled by activators as well as by inhibitors of secretion. We studied, in single fura-2-loaded cells, the role of protein phosphorylation in modulating [Ca2+]i oscillations, using either okadaic acid, an inhibitor of protein phosphatases, or activators of protein kinases A and C. Okadaic acid always increased rapidly both the frequency and amplitude of [Ca2+]i oscillations. In contrast, activation of protein kinases A or C generated more complex kinetic [Ca2+]i patterns: phosphorylation due to both kinases resulted in a sustained activation of [Ca2+]i oscillations in about one-third of the cells, whereas two-thirds of the cells responded by an arrest of [Ca2+]i oscillations. This transient phase of arrest was followed, after a few minutes, by a recovery of [Ca2+]i oscillations, often with enhanced frequency. During the arrest, depolarizing the cells with an external microelectrode could not trigger an increase in [Ca2+]i. We conclude that: (i) the fine regulation between phosphorylation/dephosphorylation events is crucial for the modulation of [Ca2+]i oscillations, and (ii) protein kinases A and C can control Ca2+ influx bidirectionally.