GTP modulates run-up of whole-cell Ca2+ channel current in a Ca(2+)-dependent manner

GTP-dependent run-up of Piezo2-type mechanically activated currents in rat dorsal root ganglion neurons

Rapidly adapting mechanically activated channels (RA) are expressed in primary afferent neurons and identified as Piezo2 ion channels. We made whole-cell voltage-clamp recordings from cultured dorsal root ganglion (DRG) neurons to study RA channel regulation. RA currents showed gradual increases in current amplitude (current "run-up") after establishing whole-cell mode when 0.33 mM GTP or 0.33 mM GTPγS was included in the patch pipette internal solution. RA current run-up was also observed in HEK293 cells that heterologously expressed Piezo2 ion channels. No significant RA current run-up was observed in DRG neurons when GTP was omitted from the patch pipette internal solution, when GTP was replaced with 0.33 mM GDP, or when recordings were made under the perforated patch-clamp recording configuration. Our findings revealed a GTP-dependent up-regulation of the function of piezo2 ion channels in DRG neurons.

[Ca2+]iregulates trafficking of Cav1.3 (α1DCa2+channel) in insulin-secreting cells

Chronic exposure of pancreatic β-cells to high concentrations of glucose impairs the insulin secretory response to further glucose stimulation. This phenomenon is referred to as glucose desensitization. It has been shown that glucose desensitization is associated with abnormal elevation of β-cell basal intracellular free Ca2+concentration ([Ca2+]i). We have investigated the relationship between the basal intracellular free Ca2+and the L-type (Cav1.3) Ca2+channel translocation in insulin-secreting cells. Glucose stimulation or membrane depolarization induced a nifedipine-sensitive Ca2+influx, which was attenuated when the basal [Ca2+]iwas elevated. Using voltage-clamp techniques, we found that changing [Ca2+]icould regulate the amplitude of the Ca2+current. This effect was attenuated by drugs that interfere with the cytoskeleton. Immunofluorescent labeling of Cav1.3 showed an increase in the cytoplasmic distribution of the channels under high [Ca2+]iconditions by deconvolution microscopy. The [Ca2+]i-dependent translocation of Cav1.3 channel was also demonstrated by Western blot analysis of biotinylation/NeutrAvidin-bead-eluted surface proteins in cells preincubated at various [Ca2+]i. These results suggest that Cav1.3 channel trafficking is involved in glucose desensitization of pancreatic β-cells.

Differential control of three after-hyperpolarizations in rat hippocampal neurones by intracellular calcium buffering

1. The whole-cell recording technique, combined with internal perfusion, was used to study the effects of intracellular Ca2+ buffering on fast, medium and slow after-hyperpolarizations (fAHP, mAHP and sAHP) in hippocampal CA1 pyramidal neurones in rat brain slices at room temperature. 2. The action potentials and the fAHP were unaffected by 100 microM to 3 mM concentrations of the internally applied fast Ca2+ chelator BAPTA. At higher (10-15 mM) concentrations, BAPTA inhibited the fAHP and prolonged the decay of the action potential, suggesting that the corresponding large-conductance Ca2+-activated K+ channels are located close to the sites of Ca2+ entry during an action potential. Addition of Ca2+ to the BAPTA-containing solution (at a ratio of 4.5 [Ca2+] : 10 [BAPTA]) to maintain the control level of [Ca2+]i did not prevent the effects of high concentrations of BAPTA. 3. The mAHP, activated by a train of action potentials, was inhibited by internally applied BAPTA within the range of concentrations used (100 microM to 15 mM), and this effect could not be reversed or prevented by addition of Ca2+ to the BAPTA-containing solution. The inhibition of the mAHP by BAPTA could also be observed after blockade of the hyperpolarization-activated IQ type mixed Na+-K+ current (also known as Ih) component of the mAHP by bath-applied 3-5 mM Cs+, suggesting that the inhibition of the mAHP by BAPTA is due to inhibition of the depolarization-activated IM (muscarinic) type K+ current. 4. The sAHP, activated by a train of action potentials, was potentiated by 100-300 microM internally applied BAPTA, both with and without added Ca2+. At 1-2 mM or higher concentrations, the potentiation of the sAHP by BAPTA without added Ca2+ was transient and was followed by a fast decrease. With added Ca2+, however, BAPTA caused a persistent potentiation of the sAHP with more than a 10-fold increase in duration for periods exceeding 1 h even at concentrations of the buffer as high as 10-15 mM. Earlier reports showing a blockade of the sAHP by BAPTA, based on experiments without added Ca2+, were apparently due to a sharp reduction in intracellular free [Ca2+] and to a high intracellular concentration of the free buffer. 5. Internally applied BAPTA caused a prolongation of the spike discharge during an 800 ms-long depolarizing current step. At 100-300 microM BAPTA, but not at 1-2 mM or higher concentrations, this effect could be reversed by addition of Ca2+. The effects of BAPTA on the spike discharge occurred in parallel with the changes in the sAHP time course, which was more prolonged at higher concentrations of the buffer. 6. The concentration-dependent differential control of the three types of AHP in hippocampal neurones by BAPTA is related to modulation of intracellular Ca2+ diffusion by a fast acting mobile Ca2+ buffer.

ATP and G proteins affect the runup of the Ca2+ current in bovine chromaffin cells

The Ca2+ current recorded by the whole-cell technique in chromaffin cells shows, before the often described rundown, a transient facilitation or runup. Initial current amplitude was 570 +/- 165 pA and then it increased by 49 +/- 23% (n = 19, SD) over 2 +/- 1 min in the absence of adenosine 5'-triphosphate (ATP). In the presence of ATP, this process occurred with the same magnitude but it was slowed in a dose-dependent manner, lasting 17 +/- 2 min with 2 mM ATP (n = 8). Since adenosine 5'-diphosphate (ADP) does not reproduce this ATP effect, a complex series of phosphorylations is likely to intervene and we show that, at least, a cAMP-dependent i.e., cyclic adenosine monophosphate) phosphorylation occurs. Pertussis toxin (PTX) pretreatment yielded an already maximal Ca2+ current (around 1000 pA) at the time of the patch rupture, which only slightly increased thereafter (10%, n = 11). Also, guanosine 5'-diphosphate (GDP) and guanosine 5'-O-(2-thiodiphosphate) (GDP[ beta s]), induced a fast runup, which was absent in the presence of GTP. Furthermore, we show that facilitation does not occur in the presence of dihydrophyridine (DHP) antagonists. Globally, our data suggest that an ATP-dependent phosphorylation stabilizes the inhibitory control exerted by a PTX-sensitive G protein and, as a result, slows down the facilitation of L-type Ca2+ channels. The recruitment of L-type channels can also be facilitated by the application of a DHP agonist or a depolarizing prepulse protocol.l We show that these processes are only effective over a period which parallels the runup and are not additive to it.(ABSTRACT TRUNCATED AT 250 WORDS)