Hypoxic modulation of L-type Ca 2+ channels in inspiratory brainstem neurones: Intracellular signalling pathways and metabotropic glutamate receptors

Abstract

Brief hypoxia (2 min) enhances the activity of L-type Ca 2+ (Ca L) channels. The effect is due to glutamate release and concomitant stimulation of metabotropic glutamate receptors of the mGLUR1/5 type [22] [S.L. Mironov, D.W. Richter, L-type Ca 2+ channels in inspiratory neurones and their modulation by hypoxia, J. Physiol. 512 (1998) 75–87.]. Besides increasing single channel activity, hypoxia induces a negative shift of the activation curve and slows down the inactivation of the Ca L current. In the present study we investigated these effects further, aiming to reveal intracellular signalling pathways that mediate the coupling between mGLURs and Ca L channels. Channel activity was recorded in cell-attached patches from inspiratory brainstem neurones of neonatal mice (P6–11). Ca L channels were inhibited by the mGluR2/3 agonists. mGluR1/5 agonists accelerated and mGluR2/3 agonists suppressed the respiratory output, and correspondingly modified the hypoxic response of the respiratory center. Ca L channels were also modulated by protein kinase C, but this did not prevent the hypoxic modification of channel activity. G-protein activators enhanced and G-protein inhibitors suppressed the Ca L channel activity, and in the presence of these agents the effects of hypoxia were abolished. Ryanodine but not thapsigargin inhibited the channel activity and occluded the hypoxic potentiation. Only G-protein-specific agents and ryanodine prevented the slowing down of inactivation induced by hypoxia. Our data indicate that coupling between mGluR1/5 and Ca L channels is mediated by pathways that utilize G-proteins and ryanodine receptors. Glutamate release and concomitant activation of Ca L channels are responsible for accelerating of respiratory rhythm during early hypoxia.