Noradrenaline inhibits exocytosis via the G protein βγ subunit and refilling of the readily releasable granule pool via the αi1/2 subunit

The molecular mechanisms responsible for the ‘distal’ effect by which noradrenaline (NA) blocks exocytosis in the β‐cell were examined by whole‐cell and cell‐attached patch clamp capacitance measurements in INS 832/13 β‐cells. NA inhibited Ca2+‐evoked exocytosis by reducing the number of exocytotic events, without modifying vesicle size. Fusion pore properties also were unaffected. NA‐induced inhibition of exocytosis was abolished by a high level of Ca2+ influx, by intracellular application of antibodies against the G protein subunit Gβ and was mimicked by the myristoylated βγ‐binding/activating peptide mSIRK. NA‐induced inhibition was also abolished by treatment with BoNT/A, which cleaves the C‐terminal nine amino acids of SNAP‐25, and also by a SNAP‐25 C‐terminal‐blocking peptide containing the BoNT/A cleavage site. These data indicate that inhibition of exocytosis by NA is downstream of increased [Ca2+]i and is mediated by an interaction between Gβγ and the C‐terminus of SNAP‐25, as is the case for inhibition of neurotransmitter release. Remarkably, in the course of this work, a novel effect of NA was discovered. NA induced a marked retardation of the rate of refilling of the readily releasable pool (RRP) of secretory granules. This retardation was specifically abolished by a Gαi1/2 blocking peptide demonstrating that the effect is mediated via activation of Gαi1 and/or Gαi2. The secretion of insulin from granules in the β‐cells in the pancreas is carefully controlled by agents in the blood that stimulate secretion, such as glucose, and others that inhibit secretion, such as noradrenaline (NA). Insulin is released as needed by the β‐cell when the insulin‐containing granules fuse with the outer membrane of the cell. We studied the mechanisms by which NA inhibits secretion. NA activates a receptor on the β‐cell that then releases messenger proteins in the cell (called G proteins) that have two subunits (α and βγ). It was found that the α subunits slow the refilling of a readily releasable pool of granules, thus decreasing the number of granules available for release, and that the βγ subunits inhibit release by directly blocking the fusion of the granules with the cell membrane. The findings are of relevance to type 2 diabetes.