Na+Ca2+ exchange and calcium permeability in canine basolateral membrane vesicles: The effects of dibutyryl cAMP and specific inhibitors

John E. Scoble, Edward J. Cragoe, Keith A. Hruska

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8 Scopus citations

Abstract

The role of dibutyryl 3′,5′-cyclic adenosine monophosphate (dibutyryl cAMP) as putative second messenger for parathyroid hormone (PTH) in regulating canine proximal tubular basolateral membrane Na+Ca2+ exchange and passive calcium permeability was assessed, as was the nature of this passive calcium permeability. Dibutyryl cAMP (50 mg) infused in vivo over 30 min increased fractional phosphate excretion from 4.9 ± 1.8% to 20.5 ± 4.6%, P < 0.05, n = 6, but had no effect on either passive Ca2+ efflux or sodium-stimulated Ca2+ efflux from Ca2+-preloaded basolateral membrane vesicles (BLMV). Both of these mechanisms have been previously shown to be stimulated by PTH. Further studies were performed to investigate the mechanism of the passive calcium flux. Calcium uptake by BLMV was blocked by lanthanum (La3+) but not by the calcium-channel blocker verapamil. La3+ blocked efflux of Ca2+ from preloaded vesicles when it was placed in the external solution. This La3+-blockable efflux was larger in potassium equivalent BLMV prepared from normal dogs than in the BLMV prepared from thyroparathyroidectomized dogs. Benzamil produced 50% inhibition of sodium-stimulated Ca2+ uptake at 250 μM whereas neither amiloride nor diltiazem achieved 50% inhibition at the maximal doses studied. Benzamil, 1 mM, had no effect on passive calcium efflux and neither did the substitution of sucrose for potassium, which has been shown to affect Ca2+Ca2+ exchange by the Na2+Ca2+ exchanger. This suggests that the calcium flux under potassium equivalent conditions was not mediated by Ca2+Ca2+ exchange by the Na2+Ca2+ exchanger. These results demonstrate that the basolateral membrane of proximal tubular cells possesses both a Na2+Ca2+ exchanger inhibitable by benzamil and a passive calcium permeability not inhibited by benzamil nor by verapamil but by La3+. Neither of these two mechanisms of calcium flux was affected by dibutyryl cAMP whereas both have been shown to be stimulated by PTH.

Original languageEnglish
Pages (from-to)233-241
Number of pages9
JournalBBA - Biomembranes
Volume944
Issue number2
DOIs
StatePublished - Oct 6 1988

Keywords

  • (Dog)
  • Basolateral membrane vesicle
  • Calcium permeability
  • Parathyroid hormone
  • Sodium-calcium ion exchange
  • cyclic AMP

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