Title

Modulation of N-type calcium channels by intracellular pH in chick dorsal root ganglion neurons

Date of Completion

January 1996

Keywords

Biology, Neuroscience

Degree

Ph.D.

Abstract

Calcium currents were recorded from chick embryo dorsal root ganglion neurons in order to examine and characterize the influence of protons on the function of N-type calcium channels. In standard whole cell recordings, two different responses to changes in intracellular pH (pH$\sb{\rm i})$ were observed. In one population of cells, which exhibited low current density $(<$50 pA/pF), changes in pH$\sb{\rm I}$ between pH 7.1 and 7.6 resulted in a 2 fold change in calcium current magnitude. In cells that exhibited high current densities $(>$50 pA/pF), changes in pH$\sb{\rm i}$ in the same range did not influence calcium currents. Conversely, when cells were recorded with the perforated patch technique, which maintains intracellular biochemical integrity, calcium currents were insensitive to changes in pH$\sb{\rm i}$ between 7.1 and 7.6, regardless of the current density. This pH sensitivity was not affected by changes in intracellular calcium between 60 and 300 nM.^ The neuromodulator, somatostatin, reduced calcium current density and produced kinetic slowing via activation of G-proteins. Both somatostatin-induced inhibition and kinetic slowing were reversed by increases in pH$\sb{\rm i}$ between 7.2 and 7.6. In contrast, constitutive activation of G-proteins by the non-hydrolyzable GTP-analogue 5'-Guanylylimidodiphosphate (GppNHp), which also decreased calcium current density, was not influenced by similar changes in pH$\sb{\rm i}.$^ These data demonstrate that N-type calcium channel function can be modulated by changes in pH$\sb{\rm i}$ from 7.1 to 7.6. Calcium currents in biochemically intact cells are insensitive to changes in pH$\sb{\rm i}$ between 7.1 and 7.6, unless they are functioning under the influence of the inhibitory neuromodulator, somatostatin, in which case they then become quite sensitive to changes in pH$\sb{\rm i}$ above 7.1. It appears that the influence of pH$\sb{\rm i}$ on somatostatin-inhibited channels does not occur at the level of the G-protein coupling to the channel. These data suggest that subtle changes in pH$\sb{\rm i}$ near physiological values may provide an important mechanism by which N-type calcium channels and calcium-dependent functions are modulated. ^