© 1995 American Heart Association, Inc.
Articles |
Na+-Ca2+ Exchange in Intact Endothelium of Rabbit Cardiac Valve
From the Department of Pharmacology and Therapeutics, The University of British Columbia, Vancouver, Canada.
Correspondence to Cornelis van Breemen, Department of Pharmacology and Therapeutics, Faculty of Medicine, 2176 Health Sciences Mall, The University of British Columbia, Vancouver, BC V6T 1Z3, Canada.
Abstract A new method of measuring cytoplasmic free Ca2+ ([Ca2+]i) of individual intact cardiovascular endothelial cells by using imaging fluorescence microscopy was designed. Application of agonist to the aortic or pulmonary valve of the rabbit triggered an increase in [Ca2+]i, which depended on the existence of endothelium on the surface of the valve. Under resting conditions, sudden reversal of the Na+ gradient by substituting external Na+ with N-methyl D-glucamine (NMDG) resulted in a [Ca2+]i spike, which then returned toward the resting level. Increasing intracellular Na+ concentration ([Na+]i) by application of ouabain or monensin induced a sustained [Ca2+]i increase. Na+ substitution by NMDG during the agonist- or monensin-induced [Ca2+]i increase gave rise to a further [Ca2+]i spike, which subsequently declined to a level higher than that before removal of external Na+. A selective inhibitor of Na+-Ca2+ exchange, 3',4'-dichlorobenzamyl (DCB), abolished the transient [Ca2+]i increase induced by Na+ substitution, and Mg2+, an inorganic inhibitor of Na+-Ca2+ exchanger, markedly reduced this transient [Ca2+]i increase. On the other hand, the selective Na+-H+ exchanger blocker 5-(N,N-hexamethylene)amiloride (HMA) did not abolish the transient [Ca2+]i increase caused by Na+ substitution. In summary, decreasing the Na+ gradient of the endothelial cells through either receptor stimulation (agonist), Na+-K+ pump inhibition (ouabain), pretreatment with Na+ ionophore (monensin), or reversing the Na+ gradient through Na+ substitution (NMDG) all increased [Ca2+]i. This raised [Ca2+]i was antagonized by agents such as DCB or Mg2+, which are thought to inhibit Na+-Ca2+ exchange, but not by HMA, an inhibitor of Na+-H+ exchange. Taken together, these results strongly imply the presence of Na+-Ca2+ exchange as a viable mechanism for Ca2+ transport in intact cardiovascular endothelium and that the Ca2+ entry component is enhanced when [Na+]i is elevated.
Key Words: endothelium • Na+-Ca2+ exchange • cytoplasmic Ca2+ • Na+ • cardiac valves
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