The contribution of voltage‐gated Ca 2+ currents to K + channel activation during ovine adrenal chromaffin cell development

Hypoxia-elicited catecholamine release is controlled by L-type as well as N/PQ types of calcium channels in rat embryo chromaffin cells

At early life, the adrenal chromaffin cells respond with a catecholamine surge under hypoxic conditions. This response depends on Ca(2+) entry through voltage-activated calcium channels (VACCs). We have investigated here three unresolved questions that concern this response in rat embryo chromaffin cells (ECCs): 1) the relative contribution of L (α1D, Cav1.3), N (α1B, Cav2.2), and PQ (α1A, Cav2.1) to the whole cell Ca(2+) current (ICa); 2) the relative contribution of L and N/PQ channels to the cytosolic Ca(2+) elevations triggered by hypoxia (Δ[Ca(2+)]c); and 3) the role of L and non-L high-VACCs in the regulation of the catecholamine surge occurring during prolonged (1 min) hypoxia exposure of ECCs. Nimodipine halved peak ICa and blocked 60% the total Ca(2+) entry during a 50-ms depolarizing pulse to 0 mV (QCa). Combined ω-agatoxin IVA plus ω-conotoxin GVIA (Aga/GVIA) blocked 30% of both ICa peak and QCa. This relative proportion of L- and non-L VACCs was corroborated by Western blot that indicated 55, 23, and 25% relative expression of L, N, and PQ VACCs. Exposure of ECCs to hypoxia elicited a mild but sustained Δ[Ca(2+)]c; the area of Δ[Ca(2+)]c was blocked 50% by nifedipine and 10% by Aga/GVIA. Exposure of ECCs to 1-min hypoxia elicited an initial transient burst of amperometric secretory spikes followed by scattered spikes along the time of cell exposure to hypoxia. This bulk response was blocked 85% by nimodipine and 35% by Aga/GVIA. Histograms on secretory spike frequency vs. time indicated a faster initial inactivation when Ca(2+) entry took place through N/PQ channels; more sustained secretion but at a lower rate was associated to Ca(2+) entry through L channels. The results suggest that the HIS response may initially be controlled by L and P/Q channels, but later on, N/PQ channels inactivate and the delayed HIS response is maintained at lower rate by slow-inactivating L channels.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.