Minimal Model for Intracellular Calcium Oscillations and Electrical Bursting in Melanotrope Cells of Xenopus Laevis

A minimal model is presented to explain changes in frequency, shape, and amplitude of Ca²⁺ oscillations in the neuroendocrine melanotrope cell of Xenopus Laevis. It describes the cell as a plasma membrane oscillator with influx of extracellular Ca²⁺ via voltage-gated Ca²⁺ channels in the plasma membrane. The Ca²⁺ oscillations in the Xenopus melanotrope show specific features that cannot be explained by previous models for electrically bursting cells using one set of parameters. The model assumes a K_Ca-channel with slow Ca²⁺ -dependent gating kinetics that initiates and terminates the bursts. The slow kinetics of this channel cause an activation of the K_Ca-channel with a phase shift relative to the intracellular Ca²⁺ concentration. The phase shift, together with the presence of a Na⁺ channel that has a lower threshold than the Ca²⁺ channel, generate the characteristic features of the Ca²⁺ oscillations in the Xenopus melanotrope cell.