Differential stimulation of noradrenaline release by reversal of the Na+/Ca2+ exchanger and depolarization in chromaffin cells.
We compared the effectiveness of Ca2+ entering by Na+/Ca2+ exchange with that of Ca2+ entering by channels produced by membrane depolarization with K+ in inducing catecholamine release from bovine adrenal chromaffin cells. The Ca2+ influx through the Na+/Ca2+ exchanger was promoted by reversing the normal inward gradient of Na+ by preincubating the cells with ouabain to increase the intracellular Na+ and then removing Na+ from the external medium. In this way we were able to increase the cytosolic free Ca2+ concentration ([Ca2+]c) by Na+/Ca2+ exchange to 325 +/- 14 nM, which was similar to the rise in [Ca2+]c observed upon depolarization with 35 mM K+ of cells not treated with ouabain. After incubating the cells with ouabain, K+ depolarization raised the [Ca2+]c to 398 +/- 31 nM, and the recovery of [Ca2+]c to resting levels was significantly slower. Reversal of the Na+ gradient caused an approximately 6-fold increase in the release of noradrenaline or adrenaline, whereas K+ depolarization induced a 12-fold increase in noradrenaline release but only a 9-fold increase in adrenaline release. The ratio of noradrenaline to adrenaline release was 1.24 +/- 0.23 upon reversal of the Na+/Ca2+ exchange, whereas it was 1.83 +/- 0.19 for K+ depolarization. Reversal of the Na+/Ca2+ exchange appeared to be as efficient as membrane depolarization in inducing adrenaline release, in that the relation of [Ca2+]c to adrenaline release was the same in both cases. In contrast, we found that for the same average [Ca2+]c, the Ca2+ influx through voltage-gated channels was much more efficient than the Ca2+ entering through the Na+/Ca2+ exchanger in inducing noradrenaline release from chromaffin cells.(ABSTRACT TRUNCATED AT 250 WORDS)
Duarte
E P
EP
Centre for Neurosciences of Coimbra, University of Coimbra, Portugal.
Baltazar
G
G
Carvalho
A P
AP
D000313:Adrenal Medulla / Q000166:cytology*; D000818:Animals; D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D002417:Cattle; D004837:Epinephrine / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects; D009638:Norepinephrine / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D017920:Antiporters; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D009638:Norepinephrine; D004837:Epinephrine