Two distinct modes of Ca2+ signalling by ACh in rat pancreatic beta-cells: concentration, glucose dependence and Ca2+ origin.

1. Calcium signalling by acetylcholine (ACh) in single rat pancreatic beta-cells was studied. The cytosolic free Ca2+ concentration ([Ca2+]i) was measured by dual-wavelength fura-2 microfluorometry. 2. In the presence of basal glucose (2.8 mM), 10(-6) to 10(-4) M ACh (high ACh) transiently increased [Ca2+]i. The [Ca2+]i response to 10(-5) M ACh was little altered under Ca(2+)-free conditions. Brief pulses of 10(-5) M ACh evoked successive [Ca2+]i responses, which were progressively inhibited by 0.2-0.5 microM thapsigargin, a specific inhibitor of the endoplasmic reticulum (ER) Ca2+ pump. 3. Elevation of glucose to 8.3 mM, a concentration which stimulates insulin release, increased [Ca2+]i to an initial peak followed by a sustained, moderate elevation. Addition of 10(-8) to 10(-7) M ACh (low ACh) evoked a further increase in [Ca2+]i. The [Ca2+]i response to 10(-7) M ACh was completely inhibited under Ca(2+)-free conditions by 1 microM nitrendipine, a blocker of L-type Ca2+ channels, and by 100 microM diazoxide, an opener of ATP-sensitive K+ channels. 4. In the presence of 8.3 mM glucose, [Ca2+]i responses to 10(-5) M ACh were reduced but not abolished by Ca(2+)-free conditions, nitrendipine and diazoxide. Successive [Ca2+]i transients induced by 10(-5) M ACh pulses in the presence of nitrendipine were progressively inhibited by thapsigargin. 5. The results revealed two distinct modes of Ca2+ signalling: low ACh increases [Ca2+]i by stimulating Ca2+ influx through voltage-dependent L-type Ca2+ channels only in the beta-cells in which glucose has already elevated [Ca2+]i, while high ACh increases [Ca2+]i at basal as well as stimulatory glucose concentrations by releasing Ca2+ from the ER. The former mechanism is likely to relate to the potentiator action and the latter to the initiator action of ACh on insulin release. High ACh and elevated glucose provoke both modes of Ca2+ signalling.