L‐type Ca²⁺ channels in inspiratory neurones of mice and their modulation by hypoxia

Abstract

Whole‐cell (ICa) and single Ca2+ channel currents were measured in inspiratory neurones of neonatal mice (4–12 days old). During whole‐cell recordings, ICa slowly declined and disappeared within 10–20 min. The run‐down was delayed during hypoxia, indicating ICa potentiation. Ca2+ channels were recorded in cell‐attached patches using pipettes which contained 110 mm Ba2+. L‐type Ca2+ channels exhibited a non‐ohmic I–V relationship. The slope conductance was 24 pS below and 50 pS above their null potential. The open probability of the channels increased during oxygen depletion, reaching a maximum 2 min after the onset of hypoxia. Restoration of the oxygen supply brought the channel activity back to initial levels. The channel activity was enhanced by 3–30 μmS(–)Bay K 8644, an agonist of L‐type Ca2+ channels. The open probability was increased about 3‐fold and the activation curve was shifted by 20 mV in the hyperpolarizing direction. In the presence of the agonist, channel open time increased and long openings appeared. Agonist‐modulated channels were also potentiated during oxygen depletion. The effect was due to an increase in open time and a decrease in closed time. The channels were inhibited by bath application of nifedipine (10 μm) and nitrendipine (20 μm). Weak bases such as NH4Cl and TMA increased and weak acids such as sodium acetate and propionate decreased activity of the channels, indicating that they are modulated by intracellular pH. Bath application of 1 μm forskolin enhanced the channel activity, whereas 500 μm NaF suppressed it. L‐type Ca2+ channels were modulated by an agonist for mGluR1/5 receptors, (S)‐3,5‐dihydrophenylglycine (DHPG, 5 μm). In its presence, the hypoxic facilitation of channels was abolished. After blockade of L‐type Ca2+ channels, the respiratory response to hypoxia was modified. The transient enhancement of the respiratory rhythm (augmentation) was no longer evident and the secondary depression occurred earlier. We suggest that L‐type Ca2+ channels contribute to the early hypoxic response of the respiratory centre. Glutamate release during hypoxia stimulates postsynaptic metabotropic glutamate receptors, which activate the Ca2+ channels.

High Sensitivity to Neuromodulator-Activated Signaling Pathways at Physiological [K+] of Confocally Imaged Respiratory Center Neurons in On-Line-Calibrated Newborn Rat Brainstem Slices

“…Locomotor networks in neonatal rats (Nistri et al, 2006) and respiratory networks in lampreys (Bongianni et al, 2002) are similarly sensitive to the agent. Furthermore, DHPG augments L-type Ca 2ϩ channel currents in unidentified VRC/PBC neurons (Mironov and Richter, 1998) and enhances the excitability of respiratory motoneurons (Dong and Feldman, 1999;Nistri et al, 2006). Our data suggest that glutamate may act endogenously through group-I metabotropic receptors coupled to phospholipase-C inositol-1,4,5-triphosphate signaling (Conn and Pin, 1997) to stimulate breathing.…”

Histaminergic modulation of the intact respiratory network of adult mice

“…The H-1 receptor antagonist largely blocked the hypoxic depression, while hypoxic augmentation remained unaffected. Hypoxic augmentation is mediated by activation of peripheral chemoreceptors [17,23], leading to glutamate release [17,23,31] and activation of L-type calcium channels [20,21]. However, specific blockade of H-1 receptors failed to suppress hypoxic augmentation, which appears to be mediated also by metabotropic glutamate receptors [20].…”

Respiratory Rhythm Generation: Plasticity of a Neuronal Network

“…They seem to decrease glycine-and GABA A receptor-regulated Cl -currents and to increase currents flowing through ionotropic glutamate receptor-channels as well as Ca 2+ channels (26,36,(58)(59)(60)(61)(62). In essence, both PKA and PKC activation lead to increased neuronal excitability.…”

“…Once a new respiratory phase is started, sodium action potentials themselves bring the membrane potential into the voltage range where an intermediate-voltage activated Ca P current (29,35) and the high-voltage activated Ca L current (11,36,37) are activated. These currents allow prominent Ca 2+ influxes into the cell, leading to a three-to fourfold increase of intracellular Ca 2+ concentration (11).…”