Cytoplasmic calcium transients due to single action potentials and voltage-clamp depolarizations in mouse pancreatic B-cells.

Abstract

Changes in the cytoplasmic free calcium concentration ([Ca2+]i) in pancreatic B‐cells play an important role in the regulation of insulin secretion. We have recorded [Ca2+]i transients evoked by single action potentials and voltage‐clamp Ca2+ currents in isolated B‐cells by the combination of dual wavelength emission spectrofluorimetry and the patch‐clamp technique. A 500–1000 ms depolarization of the B‐cell from −70 to −10 mV evoked a transient rise in [Ca2+]i from a resting value of approximately 100 nM to a peak concentration of 550 nM. Similar [Ca2+]i changes were associated with individual action potentials. The depolarization‐induced [Ca2+]i transients were abolished by application of nifedipine, a blocker of L‐type Ca2+ channels, indicating their dependence on influx of extracellular Ca2+. Following the voltage‐clamp step, [Ca2+]i decayed with a time constant of approximately 2.5 s and summation of [Ca2+]i occurred whenever depolarizations were applied with an interval of less than 2 s. The importance of the Na(+)‐Ca2+ exchange for B‐cell [Ca2+]i maintenance was evidenced by the demonstration that basal [Ca2+]i rose to 200 nM and the magnitude of the depolarization‐evoked [Ca2+]i transients was markedly increased after omission of extracellular Na+. However, the rate by which [Ca2+]i returned to basal was not affected, suggesting the existence of additional [Ca2+]i buffering processes.

Characterization of a Ca2+ Release-activated Nonselective Cation Current Regulating Membrane Potential and [Ca2+] Oscillations in Transgenically Derived β-Cells

“…MTX also stimulated insulin secretion in the presence of glucose alone, to the same extent as that produced by the combined administration of glucose, TEA and thapsigargin. Interestingly, in the absence of glucose, MTX was without effect on insulin secretion, 3 consistent with the proposal that the MTX-activated current is of small magnitude and unable to override I KATP , which would be activated under these conditions. Suppression of current flow through I CRAN had the opposite effect on the glucose-stimulated [Ca 2ϩ ] i oscillations in ␤TC3-neo cells.…”

“…Together, two voltage-dependent processes have been proposed to constitute the glucose-stimulated rise in [Ca 2ϩ ] i : the influx of extracellular Ca 2ϩ through Ca 2ϩ channels (2)(3)(4) and the release of intracellular Ca 2ϩ from the endoplasmic reticulum (ER) (5)(6)(7). Glucose initiates changes in membrane potential via an increase in cytosolic ATP, derived from the glycolytic reduction of NAD ϩ , that results in block of ATP-sensitive K ϩ channels (KATP) (8,9).…”

Protein kinase A‐dependent and ‐independent stimulation of exocytosis by cAMP in mouse pancreatic B‐cells

“…The peak Ca¥ current evoked by the first depolarization that was not associated with exocytosis was only reduced by 12 ± 9% (n = 5) with respect to the first depolarization. Parallel recordings of Ca¥ currents and changes in [Ca¥]é have also revealed that despite the inactivation of the Ca¥ current, [Ca¥]é remains elevated or even continues to increase during the train of depolarizations (Rorsman, Amm al a, Berggren, Bokvist & Larsson, 1992). The 'exhaustion' of exocytosis we therefore interpret as the depletion of the readily releasable pool of granules.…”

Tricyclic antidepressant imipramine reduces the insulin secretory rate in islet cells of Wistar albino rats through a calcium antagonistic action

“…We investigated the dependence of the Ca 2+ current on the voltage using 450 ms depolarisation pulses. Each depolarising pulse was separated from the following by 20 seconds to allow return to the basal calcium level [19]. Depolarisations from a holding potential of −70 mV to voltages between −40 mV and +30 mV evoked an inward current that reached a maximum amplitude around 0 mV, with gating properties consistent with the slowly deactivating Ca 2+ current (L-type Ca 2+ channels) described in rat beta cells [20].…”