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.
摘要
酸性物质引起的I型细胞内钙浓度（[Ca2+]i）升高被认为在颈动脉体的pH/PCO2化学感受中起关键作用。在本研究中，我们调查了酶分离的新生大鼠I型细胞中[Ca2+]i升高的原因。2. 无钙培养基和2 mM Ni2+可抑制高碳酸血症性酸中毒诱导的[Ca2+]i升高。酸中毒还增加了Mn2+通透性。因此，[Ca2+]i的升高依赖于细胞外介质中的Ca2+内流。3. 尼卡地平和甲氧基维拉帕米（D600）均可减弱酸性物质引起的[Ca2+]i升高，提示L型钙通道起作用。4. 酸中毒使I型细胞去极化，并经常（约50%的细胞）诱发电动作电位。这些效应与[Ca2+]i升高同时出现。当通过电压钳阻止膜去极化时，酸中毒未能引起[Ca2+]i升高。因此，酸性物质引起的[Ca2+]i升高是膜去极化的结果。5. 酸中毒降低了I型细胞的静息膜电导。酸敏感电流的反转电位约为 -75 mV。6. 去极化（30 mM [K+]o）诱导的[Ca2+]i升高可被去除细胞外Ca2+或存在2 mM Ni2+所阻断，也被尼卡地平显著抑制。在电压钳条件下，[Ca2+]i对膜电位呈钟形依赖性。因此，去极化通过电压门控钙通道升高[Ca2+]i。7. 咖啡因（10 mM）仅引起[Ca2+]i小幅升高（< 30 mM细胞外K+诱导升高的10%）。因此，钙诱导的钙释放不太可能对去极化诱导的[Ca2+]i升高有很大贡献。8. 虽然用N - 甲基 - D - 葡糖胺（NMG）而非Li+替代细胞外Na+可抑制酸性物质引起的[Ca2+]i升高，但这是由于膜超极化，而非抑制Na(+)-Ca2+交换或Na(+)依赖性动作电位。9. 去除细胞外Na+（用NMG替代）对静息[Ca2+]i无显著影响，且仅在从0复极化至 -60 mV后略微减慢[Ca2+]i恢复。因此，如果存在，Na(+)-Ca2+交换在[Ca2+]i稳态中仅起次要作用。10. 总之，在新生大鼠I型细胞中，高碳酸血症性酸中毒通过膜去极化和电压门控钙内流升高[Ca2+]i。
