Regulation of L- and N-types of Ca²⁺ channels by intracellular ATP^4– in frog dorsal root ganglion neurons

Abstract 

The roles of free Mg²⁺ ions, ATP^4– ions and Mg-ATP complexes in the regulation of N- and L-types of Ca²⁺ channels were studied in frog dorsal root ganglion (DRG) neurons using the whole-cell patch-clamp technique. Because Mg²⁺ ions interact with ATP^4– ions to form Mg-ATP complexes, addition of one species can influence the concentrations of the other two. In this study their concentrations were carefully controlled by varying the concentrations of two constituents at a time while keeping the third constant. The effects of each of the three species on barium currents through L-type (I _BaL) and N-type (I _BaN) Ca²⁺ channels were plotted against its concentrations. The dose-response curves for ATP^4– show that I _BaL and I _BaN proportionally increased with ATP^4– concentrations up to 1 mM at three different Mg²⁺ concentrations. At a fixed concentration of ATP^4–, I _BaL and I _BaN remained unchanged even when pMg changed from 3 to 5. Dose-response curves for I _BaL and I _BaN plotted against Mg-ATP concentration did not show a consistent pattern. H-7 and Mg²⁺ ions did not exert any blocking effect on the activity of either Ca²⁺ channel type, and neither dibutyryl-cAMP nor NKH-477 had any stimulating effect, suggesting that phosphorylation is not likely to be involved in ATP-induced potentiation. From these observations, it is concluded that L-type and N-type Ca²⁺ channels in frog DRG neurons are regulated by ATP^4– ions alone, and that the neuronal Ca²⁺ channels are regulated by mechanisms that are different from those regulating the cardiac Ca²⁺ channels.