Augmentation of calcium channel currents in response to G protein activation by GTPγS in chick sensory neurons

Abstract

G protein-mediated downregulation of current through neuronal voltage- gated Ca2+ channels is well known. We now report that G protein activation by GTPγS increases the Ba2+ conductance of high-voltage-activated Ca2+ channels of chick dorsal root ganglion (DRG) cells. This occurs with a delay of minutes during which the channels are inhibited by the activated G proteins. The Ba2+ current (I(Ba)) showed an absolute enhancement by a factor near 2, 15 min after GTPγS application. However, by utilizing prior observations of the voltage dependence of the inhibitory action we could demonstrate that the G protein-inhibited component of I(Ba) was still present. Moreover, the achieved amount of I(Ba) disinhibition showed little variation throughout the experiments. This indicates that the increase in I(Ba) is not due to a relief of the inhibitory action of activated G proteins but to the slow appearance of a distinct upregulating action, probably through a different pathway. Augmentation of I(Ba) was eliminated by pertussis toxin (PTX) infusion or pretreatment, but was also prevented by intracellularly infusing protein kinase C (PKC) inhibitors together with GTPγS. The upregulation of neuronal Ca2+ channels thus appears to be exerted through a messenger pathway upstream of PKC activation that involves G proteins. Augmentation of Ca2+ currents (I(Ca)) was observed only with strong intracellular [Ca2+] buffering, which suggests a control of the upregulating action by even moderate increase in intracellular [Ca2+].