Enhancement of Ca2+ channel currents in human neuroblastoma (SH-SY5Y) cells by phorbol esters with and without activation of protein kinase C

The effects of phorbol esters on Ca²⁺ channel currents in human neuroblastoma SH-SY5Y cells were studied using whole-cell patch-clamp recordings. Bath application of 12-O-tetradecanoylphorbol-13-acetate (TPA) or phorbol 12,13-dibutyrate (PDBu; 100 nM to 1 M), known activators of protein kinase C (PKC), enhanced Ca²⁺ channel currents in a voltage-dependent manner similar to that of Bay K 8644. TPA also enhanced Ca²⁺ channel currents during cell dialysis with the PKC pseudosubstrate, PKC(19–36), and in cells which had been pre-incubated with 500 nM staurosporine, and which were exposed to staurosporine during recordings. Application of 4-phorbol12, 13-didecanoate (4-PDD; 100 nM), which does not activate PKC, caused current enhancement similar to the effects of TPA. However, intracellular dialysis of TPA was without effect on Ca²⁺ channel currents. Residual Ca²⁺ channel currents recorded after exposure to 1 M -conotoxin GVIA were still enhanced by TPA, but in the presence of either Bay K 8644 (5 M) or nifedipine (5 M), TPA was without effect. When cells were pre-incubated for 10 min at 37° C with 100 nM TPA, currents subsequently recorded in its absence were enhanced as compared to untreated cells; 5 M nifedipine still inhibited currents to the same degree. This enhancement was not mimicked by 4PDD, and was inhibited by staurosporine. Our results indicate that acute applications of phorbol esters (at concentrations commonly used to activate PKC) enhance L-type Ca²⁺ channel currents in SH-SY5Y cells via a PKC-independent mechanism which appears similar to that induced by Bay K 8644. By contrast, pre-incubation with TPA enhances both L- and N-type currents via activation of PKC.