Calcium-independent potentiation of insulin release by cyclic AMP in single β-cells

Abstract

How does cyclic AMP potentiate insulin secretion from pancreatic islet β-cells? This question is fundamental to understanding how hormones such as glucagon, which elevates cAMP¹, stimulate insulin secretion and so contribute to the normal secretory response of the islet^2,3. It is well established that a rise in the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) is essential for insulin secretion⁴ and therefore cAMP has been proposed to act by elevating [Ca²⁺_i. But studies on permeabilized β-cells indicate that cAMP increases insulin release even when [Ca²⁺]_i is held constant^5,6. We have used microfluorimetry and the patch-clamp technique to measure changes simultaneously in Ca²⁺ currents, [Ca²⁺]_i and exocytosis^7–9 in a single β-cell in response to cAMP. We show here that cAMP, through activation of protein kinase A, increases Ca2+-influx through voltage-dependent L-type Ca2+ channels, thereby elevating [Ca²⁺]_i and accelerating exocytosis. More importantly, cAMP also promotes insulin release by a direct interaction with the secretory machinery, which accounts for as much as 80% of its effect.