Calcium modulation of exocytosis-linked plasma membrane potential oscillations in INS-1 832/13 cells

In the presence of high glucose or pyruvate, INS-1 832/13 insulinoma cells undergo stochastic oscillations in plasma membrane potential (Δψ_p) leading to associated fluctuations in cytosolic free Ca²⁺ concentration ([Ca²⁺]_c). Oscillations are not driven by upstream metabolic fluctuations, but rather by autonomous ionic mechanisms, the details of which are unclear. We have investigated the nature of the oscillator, with simultaneous fluorescence monitoring of Δψ_p, [Ca²⁺]_c and exocytosis at single-cell resolution, combined with analysis of the occurrence, frequency and amplitude of Δψ_p oscillations. Oscillations were closely coupled to exocytosis, indicated by coincident synaptopHluorin fluorescence enhancement. L-type Ca²⁺ channel inhibitors enhanced Δψ_p and [Ca²⁺]_c oscillation frequency in the presence of pyruvate, but abolished the sustained [Ca²⁺]_c response following KCl depolarization. The L-type Ca²⁺ channel inhibitor isradipine did not inhibit oscillation-linked exocytosis. The T-type Ca²⁺ channel inhibitor NNC 55-0396 inhibited Δψ_p and [Ca²⁺]_c oscillations, implying that T-type Ca²⁺ channels trigger oscillations and consequent exocytosis. Since distinct ion channels operate in oscillating and non-oscillating cells, quantitative analysis of Δψ_p and [Ca²⁺]_c oscillations in a β-cell population may help to improve our understanding of the link between metabolism and insulin secretion.