Regulation of Ca2+ signals in rat carotid glomus cells
Glomus cells of the carotid body are peripheral chemoreceptors that detect changes in arterial oxygen levels. Hypoxia suppresses oxygen-sensitive K + channels in glomus cells, resulting in cytosolic [Ca 2+ ] ([Ca 2+ ] i ) elevation in glomus cells via the activation of voltage-gated Ca 2+ channels. The resultant transmitter release stimulates the carotid sinus nerve (CSN) and the triggering of respiratory and cardiovascular reflexes. Hypoxia also causes mitochondrial depolarization and
... zation and mitochondrial inhibitors have been shown to cause depolarization in glomus cells via the inhibition of oxygen-sensitive K + channels. In the first project, with the patch clamp technique in conjunction with [Ca 2+ ] i measurement (with indo-1), I found that mitochondrial Ca 2+ uptake played a dominant role in cytosolic Ca 2+ clearance in rat glomus cells. Importantly, mitochondrial inhibition increased the duration of the Ca 2+ signal triggered by a voltage-clamped depolarization, which contributed to an enhancement of exocytotic response. Under hypoxic conditions, there was a slowing in cytosolic Ca 2+ clearance, consistent with the scenario that hypoxia caused mitochondrial depolarization and thus reduced mitochondrial Ca 2+ uptake. It has been reported that the hypoxia-triggered CSN discharge is enhanced in the presence of extracellular bicarbonate ion (HCO 3 -). Therefore, in the second project, I investigated the role of HCO 3 in the regulation of Ca 2+ dynamics in glomus cells. Extracellular HCO 3 slowed the rate of cytosolic Ca 2+ clearance in a concentration-dependent manner. Measurement of the mitochondrial Ca 2+ signal with rhod-2 shows that HCO 3 reduced mitochondrial Ca 2+ uptake and this inhibition was abolished in cells treated with scavengers of reactive oxygen species (ROS). Thus, HCO 3 reduced mitochondrial Ca 2+ uptake via a mechanism that was dependent on ROS. Overall, my results show that mitochondrial Ca 2+ uptake in glomus cells could be reduced by hypoxia or by the presence of a physiological concentration of extracellular HCO 3 -. This effect resulted in a slowing in cytosolic Ca 2+ clearance and more transmitter release. The multiplicity of the influences of mitochondria on glomus cell Ca 2+ signaling and exocytosis underscores the importance of mitochondria in hypoxic chemotransduction in the carotid bodies.