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.