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Hanaoka K, Sakai O, Imai M, Yoshitomi K: Mechanisms of calcium transport across the basolateral membrane of the rabbit cortical thick ascending limb of Henle's loop. Pflugers Arch. 1993 Jan;422(4):339-46.
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) |
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