Key pointsr A locally generating, angiotensin II (ANG II) system is present in the rat carotid body (CB) and up-regulation of this system occurs in certain pathophysiological situations, enhancing sympathetic activity. ] i persisted in Ca 2+ -free medium but was sensitive to store depletion with cyclopiazonic acid (1 μM). Similar to P2Y2 receptor agonists, ANG II (20-1000 nM) activated pannexin-1 (Panx-1) current that was reversibly abolished by carbenoxolone (5 μM). This current arose with a variable delay and was reversibly inhibited by losartan. Repeated application of ANG II often led to current run-down, attributable to AT 1 R desensitization. When applied to the same cell the combined actions of ANG II and ATP on Panx-1 current were synergistic. Current induced by either ligand was inhibited by BAPTA-AM (1 μM), suggesting that intracellular Ca 2+ signalling contributed to Panx-1 channel activation. Because open Panx-1 channels release ATP, a key CB excitatory neurotransmitter, it is plausible that paracrine stimulation of type II cells by ANG II contributes to enhanced CB excitability, especially in pathophysiological conditions such as CHF and sleep apnoea.

Key pointsr Carotid body chemoreceptors are organized in clusters containing receptor type I and contiguous glial-like type II cells.r While type I cells depolarize and release ATP during chemostimulation, the role of type II cells which express purinergic P2Y2 receptors (P2Y2Rs) and ATP-permeable pannexin-1 (Panx-1) channels, is unclear. r We propose that reciprocal crosstalk between type I and type II cells contributes to sensory processing in the carotid body via purinergic signalling pathways. AbstractThe mammalian carotid body (CB) is excited by blood-borne stimuli including hypoxia and acid hypercapnia, leading to respiratory and cardiovascular reflex responses. This chemosensory organ consists of innervated clusters of receptor type I cells, ensheathed by processes of adjacent glial-like type II cells. ATP is a major excitatory neurotransmitter released from type I cells and type II cells express purinergic P2Y2 receptors (P2Y2Rs), the activation of which leads to the opening of ATP-permeable, pannexin-1 (Panx-1) channels. While these properties support crosstalk between type I and type II cells during chemotransduction, direct evidence is lacking. To address this, we first exposed isolated rat chemoreceptor clusters to acute hypoxia, isohydric hypercapnia, or the depolarizing stimulus high K + , and monitored intracellular [Ca 2+ ] using Fura-2. As expected, these stimuli induced intracellular [Ca 2+ ] elevations ( [Ca 2+ ] i ) in type I cells. Interestingly, however, there was often a delayed, secondary [Ca 2+ ] i in nearby type II cells that was reversibly inhibited by the P2Y2R antagonist suramin, or by the nucleoside hydrolase apyrase. By contrast, type II cell stimulation with the P2Y2R agonist uridine-5 -triphosphate (100 μM) often led to a delayed, secondary [Ca 2+ ] i response in nearby type I cells that was reversibly inhibited by the Panx-1 blocker carbenoxolone (5 μM). This [Ca 2+ ] i response was also strongly inhibited by blockers of either the adenosine A 2A receptor (SCH 58261) or of the 5 -ectonucleotidase (AOPCP), suggesting it was due to adenosine arising from breakdown of ATP released through Panx-1 channels. Collectively, these data strongly suggest that purinergic signalling mechanisms mediate crosstalk between CB chemoreceptor and glial cells during chemotransduction.

Carotid body (CB) chemoreceptor (type I) cells can synthesize and release 5-HT and increased autocrine-paracrine 5-HT receptor signalling contributes to sensory long-term facilitation during chronic intermittent hypoxia (CIH). However, recent studies suggest that adjacent glial-like type II cells can respond to CB paracrine signals by elevating intracellular calcium (Δ[Ca ] ) and activating carbenoxolone-sensitive, ATP-permeable, pannexin (Panx)-1-like channels. In the present study, using dissociated rat CB cultures, we found that 5-HT induced Δ[Ca ] responses in a subpopulation of type I cells, as well as in most (∼67%) type II cells identified by their sensitivity to the P2Y2 receptor agonist, UTP. The 5-HT-induced Ca response in type II cells was dose-dependent (EC ∼183 nm) and largely inhibited by the 5-HT receptor blocker, ketanserin (1 μm), and also arose mainly from intracellular stores. 5-HT also activated an inward current (I ) in type II cells (EC ∼200 nm) that was reversibly inhibited by ketanserin (1-10 nm), the Ca chelator BAPTA-AM (5 μm), and low concentrations of carbenoxolone (5 μm), a putative Panx-1 channel blocker. I reversed direction at approximately -11 mV and was indistinguishable from the UTP-activated current (I ). Consistent with a role for Panx-1 channels, I was reversibly inhibited by the specific Panx-1 mimetic peptide blocker Panx (100 μm), although not by its scrambled control peptide ( Panx). Because ATP is an excitatory CB neurotransmitter, it is possible that the contribution of enhanced 5-HT signalling to the increased sensory discharge during CIH may occur, in part, by a boosting of ATP release from type II cells via Panx-1 channels.

The carotid body (CB) chemosensory complex uses ATP as a key excitatory neurotransmitter that is the main contributor to the sensory discharge during acute hypoxia. The complex includes receptor type I cells, which depolarize and release various neurochemicals including ATP during hypoxia, and contiguous glial-like type II cells which express purinergic P2Y2 receptors (P2Y2R). We previously showed that activation of P2Y2R on rat type II cells led to the opening of pannexin-1 (Panx-1) channels, which acted as conduits for the further release of ATP. More recently, we considered the possibility that other CB neuromodulators may have a similar paracrine role, leading to the activation of type II cells. Here, we examine the evidence that angiotensin II (ANG II), endothelin- (ET-1), and muscarinic agonists (e.g. acetylcholine, ACh) may activate intracellular Ca(2+) signals in type II cells and, in the case of ANG II and ACh, Panx-1 currents as well. Using ratiometric Ca(2+) imaging, we found that a substantial population of type II cells responded to 100 nM ANG II with a robust rise in intracellular Ca(2+) and activation of Panx-1 current. Both effects of ANG II were mediated via AT(1) receptors (AT(1)Rs) and current activation could be inhibited by the Panx-1 channel blocker, carbenoxolone (CBX; 5 μM). Additionally, low concentrations of ET-1 (1 nM) evoked robust intracellular Ca(2+) responses in subpopulations of type II cells. The mAChR agonist muscarine (10 μM) also induced a rise in intracellular Ca(2+) in some type II cells, and preliminary perforated-patch, whole-cell recordings revealed that ACh (10 μM) may activate Panx-1-like currents. These data suggest that paracrine activation of type II cells by endogenous neuromodulators may be a common feature of signal processing in the rat CB.

Hypercalcemic crisis due to severe decompensated hypercalcemia represents a life-threatening medical emergency leading to renal failure and altered mental status. Hypercalcemic crisis most commonly results from hypercalcemia of malignancy, undiagnosed primary hyperparathyroidism, granulomatous diseases, and medication-induced hypercalcemia. Commonly prescribed medications like thiazide diuretics, lithium therapy, and teriparatide are among the medications known to cause hypercalcemia. Here, we describe a case of the hypercalcemic crisis caused by excessive calcium carbonate ingestion emphasizing physicians to be aware of this potentially life-threatening adverse effect of a widely available over-the-counter acid reflux medication and educate patients regarding the same.