Electrical Membrane Activity and Intracellular Calcium Buffering Control Exocytosis Efficiency in Xenopus Melanotrope Cells

Abstract

In neural and neuroendocrine cells, Ca²⁺ influx is essential for exocytosis. Ca²⁺ influx takes place through electrical membrane activity, which often occurs in bursts of action potentials that lead to intracellular Ca²⁺ oscillations. Cytoplasmic Ca²⁺ buffers and intracellular Ca²⁺ stores are involved in the propagation of the oscillations through the cell. Studies focused on action potential bursts with a high frequency up to 20 Hz indicate that, depending on the cell type under investigation, bursts either enhance or reduce exocytosis efficiency. In many cell types, the bursting frequency can be as low as 1 Hz, although no information is present on whether this influences exocytosis efficiency. The present study addresses the role of low-frequency bursts around 1 Hz and cytoplasmic Ca²⁺ buffering in the regulation of exocytosis efficiency, using neuroendocrine melanotrope cells of the amphibian Xenopus laevis. Exocytosis efficiency was determined by membrane capacitance measurements. Mimicking the bursting activity of 1 Hz (typical for this cell type) by repetitive depolarizing pulses enhanced exocytosis efficiency by 58% compared to application of only one single depolarizing pulse. This increase appears to be particularly due to a small number of distinct depolarizing pulses within a burst. Including the fast Ca²⁺ buffer BAPTA in the intracellular solution reduced exocytosis efficiency by 60% in the first part of a burst, whereas during the later part of the burst, stimulation (+50%) took place. We conclude that low-frequency bursting in the Xenopus melanotrope cell strongly promotes exocytosis efficiency and that this efficiency also depends on the capacity of the cytoplasm to buffer the intracellular Ca²⁺ signal; strong Ca²⁺ buffering during a short burst will decrease exocytosis efficiency, whereas with prolonged bursts, buffering capacity will be overcome, leading to Ca²⁺ accumulation and thus enhanced exocytosis efficiency.