Voltage-dependent potentiation of low-voltage-activated Ca2+ channel currents in cultured rat bone marrow cells

1. The whole-cell patch-clamp technique was used to study Ca2+ channel currents in stromal cells of 7-10 day dexamethasone-treated and control rat bone marrow cultures. In saline containing either 108 mM Ba2+ or a 2.5 mM Ca(2+)-1 mM Mg2+ mixture, most cells expressed both fast-inactivating, low-voltage-activated (LVA) and slow-inactivating, high-voltage-activated (HVA) currents. 2. Repeated application of 400 ms voltage steps to 60 mV above the holding potential (Vh, -90 mV in Ca(2+)-Mg2+ mixture and -60 mV in Ba2+) at a frequency > or = 0.1 Hz resulted in a potentiation of the LVA component of the 2nd and subsequent currents. 3. LVA current potentiation was examined using a two-pulse (prepulse-test pulse) method. Prepulses to Vh + 150 mV induced an 80-100% increase in the amplitude of the LVA component of Ca2+ channel currents in saline containing either Ba2+ or Ca(2+)-Mg2+. This effect was also seen in non-dexamethasone-treated cultures. 4. Potentiation was not modified by omission of ATP and GTP from the pipette saline, and was not inhibited by extracellular application of the broad spectrum kinase inhibitors H-7 or RK252-a. 5. Potentiation was dependent on the amplitude and duration of the prepulse. Using the standard protocol, the threshold for potentiation was approximately Vh + 45 mV and saturation occurred at Vh + 150-180 mV. Further increases in prepulse amplitude did not modify potentiation. With a prepulse to +10 mV (Ba2+ saline) potentiation was half-maximal with a prepulse duration of 250-300 ms duration and saturated at 750-1000 ms. 6. Peak potentiation occurred 1-2 s after the prepulse. The time for total decay of potentiation varied from 10 to 90 s. 7. Voltage dependency of prepulse-induced potentiation did not resemble that of inactivation induced by similar prepulses. 8. Current kinetics, I-V relationship and sensitivity to blockade by Ni2+ and diphenylhydantoin of prepulse-recruited current resembled those of control LVA current. 9. The amplitude of prepulse-recruited current was positively correlated with control LVA current amplitude. 10. LVA currents supported regenerative potentials under current clamp. Repeated activation reduced spike latency. 11. It is suggested that current potentiation may be recruited physiologically, possibly in association with activation of stretch-sensitive channels, causing enhanced activation of HVA Ca2+ currents.