Journal List > Korean J Physiol Pharmacol > v.12(5) > 1025561

Kang: PKC-Independent Stimulation of Cardiac Na+/Ca2+ Exchanger by Staurosporine

Abstract

[Ca2+]i transients by reverse mode of cardiac Na+/Ca2+ exchanger (NCX1) were recorded in fura-2 loaded BHK cells with stable expression of NCX1. Repeated stimulation of reverse NCX1 produced a long-lasting decrease of Ca2+ transients (‘rundown’). Rundown of NCX1 was independent of membrane PIP2 depletion. Although the activation of protein kinase C (PKC) was observed during the Ca2+ transients, neither a selective PKC inhibitor (calphostin C) nor a PKC activator (PMA) changed the degrees of rundown. By comparison, a non-specific PKC inhibitor, staurosporine (STS), reversed rundown in a dose-dependent and reversible manner. The action of STS was unaffected by pretreatment of the cells with calphostin C, PMA, or forskolin. Taken together, the results suggest that the stimulation of reverse NCX1 by STS is independent of PKC and/or PKA inhibition.

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Fig. 1.
Rundown of reverse mode of Na+/Ca2+ exchanger (NCX1). (A) When NCX1 was repeatedly stimulated with Na+-free, 1 mM Ca2+-containing solution, a progressive decrease of the Ca2+ transients was observed. A Na+-pump inhibitor, ouabain, did not enhance the transients. (B) Increased rising time of the Ca2+ transients was observed during rundown. The representative traces recorded from the same cell were drawn for comparison. An arrow indicates the starting time of stimulation. The mean values of the time-to-peak values are described in text. The peak ratio values of the 1st, 2nd, and 3rd Ca2+ transient were 3.1±0.44, 2.4±0.25, and 1.8±0.15, respectively (n=4). (C) Rundown of the outward whole-cell NCX1 current. (D) The degrees and the time courses of rundown obtained from the Ca2+ transients (n=13) and the whole-cell current (n=7) were compared. Amplitudes of the 2nd, 3rd, and 4th Ca2+ transient were reduced to 48±12, 35±9.7, and 28±8.4% of the initial (‘1st’) transient, respectively. Amplitudes of the 2nd, 3rd, and 4th current were reduced to 20±3.2, 17±2.8, and 16±2.9% of the initial one, respectively.
kjpp-12-259f1.tif
Fig. 2.
Using a confocal imaging, changes of the protein kinase C activity and the PIP2 concentration during the Ca2+ transients were monitored via PKCβ -II-GFP and PLCδ1-PH-GFP. Fluorescence intensity obtained along the horizontal axis was shown as histogram with corresponding confocal image. The normalized fluorescence intensity (Fc/Fm, %) was calculated at every 10 sec and plotted against time. Fc, fluorescence intensity of the cytosol; Fm, fluorescence intensity of the plasma membrane. (Upper panel) PKCβ-II-GFP, initially located in the cytosol, rapidly translocated to the plasma membrane upon Ca2+ influx via reverse NCX1. PKCβ-II-GFP returned to the cytosol when NCX1 was turned off. (Lower panel) No significant translocation of PH domain was monitored during the Ca2+ transient.
kjpp-12-259f2.tif
Fig. 3.
The effects of protein kinase C modulators on rundown. (A) Pretreatment of the cells with a selective PKC inhibitor, calphostin C (1μM), did not inhibit rundown of the Ca2+ transients. (B) Pretreatment of a strong PKC activator, PMA (0.1μ M), did not change rundown. (C) Normal rundown was recorded at the beginning of the recording (the first 3 repeats), and the 0.1μM STS was applied to the cells. The post-treatment of staurosporine (STS) recovered the Ca2+ transients and the final amplitudes were higher than the initial one. (D) Summarized data are illustrated. Control, rundown from control cells (n=8); Calp-C, pretreatment with calphostin C (n=6); PMA, pretreatment of phorbol ester (n=6); STS, post-treatment of STS after the 3rd transient (n=4). The black bars in C and D denote the period of time with STS.
kjpp-12-259f3.tif
Fig. 4.
Staurosporine (STS) stimulated Na+/Ca2+ exchanger in a reversible and a dose-dependent manner. (A) In this recording, the initial Ca2+ transient was followed by a complete rundown of NCX1 (i.e. the 2nd transient was completely disappeared). When STS (0.1μM) was briefly applied prior to the 3rd one, it rapidly restored the Ca2+ transients. After the washing of STS, the next rundown was processed. By the second application of STS, restoration of the transients started again. (B) A lower concentration of STS (50 nM), that was applied before the initial stimulation, significantly inhibited rundown (compare the amplitudes of the initial 3 Ca2+ transients). When the concentrations of STS were increased up to 0.3μM, the amplitudes of the transients were increased dose-dependently.
kjpp-12-259f4.tif
Fig. 5.
STS-increased Na+/Ca2+ exchanger is independent of protein kinase C and/or protein kinase A. (A~C) Even in the presence of calphostin C (1μM), PMA (0.1μM), and a PKA activator forskolin (FSK, 1μM), rundown of NCX1 was normally developed (i.e. the 2nd or 3rd transient was markedly decreased). However, the addition of STS (0.1μM) with each chemical almost completely restored the Ca2+ transients. STS was applied after the recording of the 2nd Ca2+ transient. (D) Summarized data were illustrated with time courses of rundown. Calp-C, calphostin C with STS (n=4); PMA, PMA with STS (n=7); FSK, forskolin with STS (n=5).
kjpp-12-259f5.tif
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