Mechanisms of calcium transport across the basolateral membrane of the rabbit cortical thick ascending limb of Henle's loop.
Although net Ca2+ absorption takes place in the thick ascending limb of Henle's loop, detailed mechanisms are unknown. Because it has been reported that the Ca2+ entry step across the luminal membrane is mediated by Ca2+ channels inserted by stimulation with parathyroid hormone, we studied the mechanism of Ca2+ transport across the basolateral membrane of rabbit cortical thick ascending limb (CTAL) perfused in vitro by using microscopic fluorometry of cytosolic Ca2+ ([Ca2+]i) with fura-2. The resting [Ca2+]i in this segment was 49.8 +/- 4.5 nmol/l. Neither Na+ removal from the bathing solution nor addition of ouabain (0.1 mmol/l) to the bath increased [Ca2+]i, indicating that a Na+/Ca2+ exchanger in the basolateral membrane may not contribute in any major way to [Ca2+]i of CTAL. To confirm our technical accuracy, similar protocols were conducted in the connecting tubule, where the existence of a Na+/Ca2+ exchanger has been reported. In this segment, Na+ removal from the bath increased cell Ca2+ from 148.6 +/- 6.4 nmol/l to 647.6 +/- 132.0 nmol/l, confirming the documented fact. [Ca2+]i in the CTAL was markedly increased when 1 mmol/l NaCN was added to the bath in the absence of glucose. Calmodulin inhibitors (trifluoperazine or W-7) increased [Ca2+]i. When the bath pH was made alkaline, [Ca2+]i was also increased. This response was abolished when Ca2+ was eliminated from the bath, indicating that the Ca2+ entry across the basolateral membrane is dependent on bath pH. Increase in [Ca2+]i induced by an alkaline bath was inhibited by increased the bath K+ from 5 nmol/l to 50 mmol/l, suggesting that the Ca2+ entry system is voltage-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)
Second Department of Internal Medicine, Jikei University School of Medicine, Tokyo, Japan.
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Hanaoka
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Sakai
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Imai
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Yoshitomi
D000818:Animals; D001485:Basement Membrane / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D005260:Female; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects; D008138:Loop of Henle / Q000187:drug effects / Q000502:physiology*; D009529:Nicardipine / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D010281:Parathyroid Hormone / Q000494:pharmacology*; D011817:Rabbits; D012966:Sodium Cyanide / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects
D015220:Calcium Channels; D002352:Carrier Proteins; D010281:Parathyroid Hormone; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D009529:Nicardipine; D000254:Sodium-Potassium-Exchanging ATPase; D012966:Sodium Cyanide; D002118:Calcium