Acetylcholine elevates intracellular Ca2+ via muscarinic and nicotinic receptors in rat carotid body type I cells

Oxygen and carotid body chemotransduction: the cholinergic hypothesis - a brief history and new evaluation

Oxygen can be said to be the most fundamentally necessary substrate for life. In those organisms having a cardiopulmonary system for delivering it in blood to the tissues the carotid body functions as the principal detector of decreases in arterial oxygen. Such a decrease stimulates an increase in neural output from the carotid body to the nucleus tractus solitarii, and this can precipitate a wide array of systemic reflex responses. The neural mechanisms involved in the genesis of increased signal from the carotid body remain unclear. But a current model of carotid body chemotransduction postulates that transmitter-laden glomus cells initiate the neural activity by being depolarized by hypoxemia and releasing an excitatory transmitter which binds to postsynaptic receptors of the adjacent sensory afferent fibers as well as to presynaptic glomus cell autoreceptors. This Frontiers Review evaluates anew the data supporting the hypothesis that acetylcholine (ACh) is an (the) essential excitatory transmitter in this process by examining ACh's fulfillment of criteria required to establish a substance as a synaptic transmitter. All eight criteria are fulfilled in the case of ACh. Indeed, additional data further support the Cholinergic Hypothesis.

Chemosensory response to high pCO is blocked by cadmium, a voltage-sensitive calcium channel blocker

In the dark, during normocapnic (pCO2=35 Torr, pHo=7.4) normoxia (pO2=100 Torr), high pCO (>300 Torr) causes Ca2+-dependent photolabile excitation of chemosensors in the carotid body (CB). We previously proposed that the source of this Ca2+ was the [Ca2+]i stores because CO would react only intracellularly. However, influx of extracellular Ca2+ was not excluded. Now, using perfused rat CB (n=6) in the presence of normal extracellular [Ca2+] we show that chemosensory response to CO (pCO approximately 550 Torr) in normoxic (pO2 approximately 100 Torr) normocapnia (pCO2 approximately 30 Torr, pH approximately 7.4) is completely but reversibly inhibited by Cd2+ (200 microM), a voltage-gated Ca2+ channel blocker. Thus, extracellular Ca2+ is necessary for excitatory chemosensory response to high pCO. Cd2+ block occurs in spite of an enhanced [Ca2+]i rise. This shows that Ca2+ rise alone is unable to release neurotransmitter and to elicit a chemosensory response. Therefore, as a corollary, we conclude that Cd2+ blocks the Ca2+ flux that is needed for vesicle-membrane fusion for neurotransmitter release and neural discharge.

Sodium nitroprusside blocks the cat carotid chemosensory inhibition induced by dopamine, but not that by hyperoxia

We studied the effects of the nitric oxide (NO) synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME), and the NO donor, sodium nitroprusside (SNP) on cat chemosensory responses to intravenous injections of NaCN (0.1-100 microg/kg) and dopamine (0. 1-20 microg/kg), and to hyperoxic ventilation (100% O2, 60-120 s). Cats were anesthetized with sodium pentobarbitone, paralyzed and artificially ventilated to prevent secondary ventilatory effects. The frequency of chemosensory discharges (fx) was recorded from one sectioned carotid sinus nerve. L-NAME (50 mg/kg i.v.) increased basal fx and slightly potentiated the responses to NaCN and dopamine. SNP (1-2 mg/kg i.v.) increased basal fx, but reduced the NaCN-induced increases of fx over baseline and the transient fx inhibitions induced by dopamine, but not those produced by hyperoxia. Present results indicate that besides the known inhibitory effect of NO on chemosensory responses to low PO2, NO also blocks the chemosensory response to dopamine, leaving hyperoxic responses largely unchanged.