Amplification of exocytosis by Ca2+-induced Ca2+ release in INS-1 pancreatic beta cells.
Functional coupling between Ca(2+)-induced Ca(2+) release (CICR) and quantal exocytosis in 5-hydroxytryptamine-loaded INS-1 beta cells was assessed through the use of carbon fibre amperometry in combination with Fura-2. CICR was evoked by the glucagon-like-peptide-1 (GLP-1) receptor agonist exendin-4 (Ex-4) and was accompanied by quantal secretory events appearing as amperometric current spikes time-locked to the increase of [Ca(2+)](i). The action of Ex-4 was reproduced by treatment with caffeine, and the source of Ca(2+) serving as a stimulus for exocytosis originated from ryanodine and thapsigargin-sensitive Ca(2+) stores. Two distinct patterns of exocytosis occurred within 5 s following the initiation of CICR. Non-summating exocytosis (NS-type) was defined as multiple asynchronous current spikes, and the half-height duration of each spike was 12-48 ms. Summating exocytosis (S-type) was defined as a cluster of spikes. It generated a macroscopic current, the half-height duration of which was 243-682 ms. The release charge of S-type exocytosis was 3.2-fold greater than that of NS-type when measured 2 s following the initiation of secretion. NS-type exocytosis was observed frequently under conditions in which the basal Ca(2+) concentration ([Ca(2+)](B)) was low (75-150 nM), whereas S-type exocytosis predominated under conditions in which the [Ca(2+)](B) was elevated (200-275 nM). Depolarization-induced Ca(2+) influx triggered NS-type exocytosis in most cells tested, irrespective of [Ca(2+)](B). It is concluded that CICR is a highly effective stimulus for exocytosis in INS-1 cells. The increase of [Ca(2+)](i) that accompanies CICR stimulates the asynchronous release of a small number of secretory granules under conditions of low [Ca(2+)](B). When [Ca(2+)](B) is slightly elevated, CICR targets a much larger pool of secretory granules that undergo summating exocytosis. The transition from NS-type to S-type exocytosis may represent an amplification mechanism for Ca(2+)-dependent exocytosis.
Kang
Guoxin
G
Department of Physiology and Neuroscience, New York University School of Medicine, New York, NY 10016, USA.
Holz
George G
GG
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D007515:Islets of Langerhans / Q000502:physiology*; D007700:Kinetics; D051381:Rats; D012433:Ryanodine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D012433:Ryanodine; D015124:8-Bromo Cyclic Adenosine Monophosphate; D002110:Caffeine; D019284:Thapsigargin; D002118:Calcium