Effects of hypercapnia on membrane potential and intracellular calcium in rat carotid body type I cells.
1. An acid-induced rise in the intracellular calcium concentration ([Ca2+]i) of type I cells is thought to play a vital role in pH/PCO2 chemoreception by the carotid body. In this present study we have investigated the cause of this rise in [Ca2+]i in enzymatically isolated, neonatal rat type I cells. 2. The rise in [Ca2+]i induced by a hypercapnic acidosis was inhibited in Ca(2+)-free media, and by 2 mM Ni2+. Acidosis also increased Mn2+ permeability. The rise in [Ca2+]i is dependent, therefore, upon a Ca2+ influx from the external medium. 3. The acid-induced rise in [Ca2+]i was attenuated by both nicardipine and methoxyverapamil (D600), suggesting a role for L-type Ca2+ channels. 4. Acidosis depolarized type I cells and often (approximately 50% of cells) induced action potentials. These effects coincided with a rise in [Ca2+]i. When membrane depolarization was prevented by a voltage clamp, acidosis failed to evoke a rise in [Ca2+]i. The acid-induced rise in [Ca2+]i is a consequence, therefore, of membrane depolarization. 5. Acidosis decreased the resting membrane conductance of type I cells. The reversal potential of the acid-sensitive current was about -75 mV. 6. A depolarization (30 mM [K+]o)-induced rise in [Ca2+]i was blocked by either the removal of extracellular Ca2+ or the presence of 2 mM Ni2+, and was also substantially inhibited by nicardipine. Under voltage-clamp conditions, [Ca2+]i displayed a bell-shaped dependence on membrane potential. Depolarization raises [Ca2+]i, therefore, through voltage-operated Ca2+ channels. 7. Caffeine (10 mM) induced only a small rise in [Ca2+]i (< 10% of that induced by 30 mM extracellular K+). Ca(2+)-induced Ca2+ release is unlikely, therefore, to contribute greatly to the rise in [Ca2+]i induced by depolarization. 8. Although the replacement of extracellular Na+ with N-methyl-D-glucamine (NMG), but not Li+, inhibited the acid-induced rise in [Ca2+]i, this was due to membrane hyperpolarization and not to the inhibition of Na(+)-Ca2+ exchange or Na(+)-dependent action potentials. 9. The removal of extracellular Na+ (NMG substituted) did not have a significant effect upon the resting [Ca2+]i, and only slowed [Ca2+]i recovery slightly following repolarization from 0 to -60 mV. Therefore, if present, Na(+)-Ca2+ exchange plays only a minor role in [Ca2+]i homeostasis. 10. In summary, in the neonatal rat type I cell, hypercapnic acidosis raises [Ca2+]i through membrane depolarization and voltage-gated Ca2+ entry.
University Laboratory of Physiology, University of Oxford.
K J
KJ
Buckler
R D
RD
Vaughan-Jones
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002344:Carotid Body / Q000502:physiology*; D015687:Cell Hypoxia; D002463:Cell Membrane Permeability; D016257:Fura-2; D005711:Gallopamil / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008345:Manganese / Q000378:metabolism; D008536:Meglumine / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D009529:Nicardipine / Q000494:pharmacology; D009532:Nickel / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013327:Strophanthidin / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D005711:Gallopamil; D008345:Manganese; D013779:Tetrodotoxin; D013327:Strophanthidin; D008536:Meglumine; D009532:Nickel; D012964:Sodium; D009529:Nicardipine; D011188:Potassium; D002118:Calcium; D016257:Fura-2