|
|
Summers BA; Overholt JL; Prabhakar NR. 2002. CO2 and pH independently modulate L-type Ca2+ current in rabbit carotid body glomus cells. JOURNAL OF NEUROPHYSIOLOGY. 88(2):604-612. Address: Prabhakar NR, Case Western Reserve Univ, Sch Med, Dept Physiol & Biophys, 10900 Euclid Ave, Cleveland, OH 44106
The carotid bodies respond to changes in arterial O-2,CO2, and pH, and Ca2+ influx via voltage-gated Ca2+ channels is an important step in the chemoreception process. The objectives of the present study were as follows: 1) to determine whether hypercapnia modulates Ca2+ current in glomus cells, and if so, to determine if this modulation is secondary to changes in pH; 2) to examine the mechanism of CO2 modulation of the Ca2+ current; and 3) to determine whether the effects of hypercapnia and hypoxia on Ca2+ channel activity in glomus cells are synergistic. The effects of CO2 on Ca2+ current were monitored in glomus cells isolated from rabbit carotid bodies using both perforated and conventional patch-clamp techniques. Raising CO2 in the extracellular solution from 5 to 10% (hypercapnia) reversibly augmented the whole-cell Ca2+ current. This augmentation was rapid and increased the whole-cell Ca2+ current similarly in both the perforated and the conventional patch configurations by 16+/-2% (n=5) and 15+/-1% (n=32), respectively. The following observations suggest that the effects of CO2 are not secondary to changes in pH: 1) isohydric hypercapnia (pH maintained at 7.4) augmented the Ca2+ current by 24+/-2% (n=6); 2) decreasing the pH of the extra- or intracellular solutions decreased the Ca2+ current by 43+/-4% (n=8) and 13+/-1% (n=5), respectively; and 3) hypercapnia did not shift the half-maximal activation voltage (V-1/2), whereas intracellular and extracellular acidosis alone caused shifts in V-1/2. Furthermore, 100 nM of a membrane-permeable protein kinase A inhibitor prevented the augmentation by CO2, and 500 muM 8-Br-cAMP mimicked the effect of CO2 by augmenting the Ca2+ current by 10+/-2% (n=6). Also, cyclic AMP levels in carotid bodies increased from 1.98+/-0.6 to 9.0+/-2 pmol/mug protein in response to hypercapnia. In contrast, decreasing pH in the nominal absence of CO2 did not affect cAMP levels in rabbit carotid bodies. Further, nisoldipine, but not omega-conotoxin MVIIC, prevented augmentation of the Ca2+ current by CO2. In addition, when combined, hypercapnia and hypoxia augmented the Ca2+ current by 26+/-4% (n=7), which is greater than either stimulus alone, suggesting the effects are additive. Taken together, these results indicate that L-type Ca2+ current is augmented by hypercapnia. The effect of CO2 is not secondary to changes in pH and seems to be mediated by a protein kinase A-dependent mechanism. Furthermore, hypercapnia and hypoxia act additively in stimulating Ca2+ current in glomus cells.
|
