Tight coupling between electrical activity and exocytosis in mouse glucagon-secreting alpha-cells.
alpha-Cells were identified in preparations of dispersed mouse islets by immunofluorescence microscopy. A high fraction of alpha-cells correlated with a small cell size measured as the average cell diameter (10 microm) and whole-cell capacitance (<4 pF). The alpha-cells generated action potentials at a low frequency (1 Hz) in the absence of glucose. These action potentials were reversibly inhibited by elevation of the glucose concentration to 20 mmol/l. The action potentials originated from a membrane potential more negative than -50 mV, had a maximal upstroke velocity of 5 V/s, and peaked at +1 mV. Voltage-clamp experiments revealed the ionic conductances underlying the generation of action potentials. alpha-Cells are equipped with a delayed tetraethyl-ammonium-blockable outward current (activating at voltages above -20 mV), a large tetrodotoxin-sensitive Na+ current (above -30 mV; peak current 200 pA at +10 mV), and a small Ca2+ current (above -50 mV; peak current 30 pA at +10 mV). The latter flowed through omega-conotoxin GVIA (25%)- and nifedipine-sensitive (50%) Ca(2+)-channels. Mouse alpha-cells contained, on average, 7,300 granules, which undergo Ca(2+)-induced exocytosis when the alpha-cell is depolarized. Three functional subsets of granules were identified, and the size of the immediately releasable pool was estimated as 80 granules, or 1% of the total granule number. The maximal rate of exocytosis (1.5 pF/s) was observed 21 ms after the onset of the voltage-clamp depolarization, which is precisely the duration of Ca(2+)-influx during an action potential. Our results suggest that the secretory machinery of the alpha-cell is optimized for maximal efficiency in the use of Ca2+ for exocytosis.
Barg
S
S
Department of Molecular and Cellular Physiology, Institute of Physiological Sciences, University of Lund, Sweden.
Galvanovskis
J
J
Göpel
S O
SO
Rorsman
P
P
Eliasson
L
L
D000818:Animals; D002478:Cells, Cultured; D003594:Cytoplasmic Granules / Q000502:physiology; D003981:Diazoxide / Q000494:pharmacology; D005089:Exocytosis; D005934:Glucagon / Q000032:analysis / Q000378:metabolism*; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008815:Mice, Inbred Strains; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology; D014044:Tolbutamide / Q000494:pharmacology
D015222:Sodium Channels; D013779:Tetrodotoxin; D005934:Glucagon; D014044:Tolbutamide; D003981:Diazoxide