Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.