Sodium channel blockade reduces hypoxic sodium loading and sodium-dependent calcium loading

• Published in: Circulation (New York, N.Y.) Vol. 90; no. 1; pp. 391 - 399
• Main Authors: HAIGNEY, M. C. P, LAKATTA, E. G, STERN, M. D, SILVERMAN, H. S
• Format: Journal Article
• Language: English
• Published: Hagerstown, MD Lippincott Williams & Wilkins 01.07.1994; American Heart Association, Inc
• Subjects: Amiloride - analogs & derivatives; Amiloride - pharmacology; Animals; Benzofurans; Benzothiazoles; Biological and medical sciences; Calcium - metabolism; Calcium Channel Blockers - pharmacology; Cell Separation; Ethers, Cyclic; Fluorescent Dyes; Fundamental and applied biological sciences. Psychology; Heart; Hypoxia - metabolism; Lidocaine - pharmacology; Myocardial Contraction - drug effects; Myocardium - cytology; Myocardium - metabolism; Piperidines - pharmacology; Rats; Sodium - metabolism; Sodium Channel Blockers; Sodium-Hydrogen Exchangers - metabolism; Tetrodotoxin - pharmacology; Thiazoles - pharmacology; Vertebrates: cardiovascular system; Heart; Oxygen; Calcium; Rat; Rodentia; Exploration; Ionic channel; Experimental study; Myocyte; Isolated organ; Vertebrata; Mammalia; Sodium; Animal; Hypoxia; Circulatory system
• ISSN: 0009-7322; 1524-4539
• DOI: 10.1161/01.cir.90.1.391

Studies have shown that the rise in intracellular ionized calcium, [Ca2+]i, in hypoxic myocardium is driven by an increase in sodium, [Na+]i, but the source of Na+ is not known. Inhibitors of the voltage-gated Na+ channel were used to investigate the effect of Na+ channel blockade on hypoxic Na+ loading, Na(+)-dependent Ca2+ loading, and reoxygenation hypercontracture in isolated adult rat cardiac myocytes. Single electrically stimulated (0.2 Hz) cells were loaded with either SBFI (to index [Na+]i) or indo-1 (to index [Ca2+]i) and exposed to glucose-free hypoxia (PO2 < 0.02 mm Hg). Both [Na+]i and [Ca2+]i increased during hypoxia when cells became inexcitable following ATP-depletion contracture. The hypoxic rise in [Na+]i and [Ca2+]i was significantly attenuated by 1 mumol/L R 56865. Tetrodotoxin (60 mumol/L), a selective Na(+)-channel blocker, also markedly reduced the rise in [Ca2+]i during hypoxia and reoxygenation. Reoxygenation-induced cellular hypercontracture was reduced from 83% (45 of 54 cells) under control conditions to 12% (4 of 32) in the presence of R 56865 (P < .05). Lidocaine reduced hypercontracture dose dependently with 13% of cells hypercontracting in 100 mumol/L lidocaine, 42% in 50 mumol/L lidocaine, and 93% in 25 mumol/L lidocaine. The Na(+)-H+ exchange blocker, ethylisopropylamiloride (10 mumol/L) was also effective, limiting hypercontracture to 12%. R 56865, lidocaine, and ethylisopropylamiloride were also effective in preventing hypercontracture in normoxic myocytes induced by 75 mumol/L veratridine, an agent that impairs Na+ channel inactivation. Ethylisopropylamiloride prevented the veratridine-induced rise in [Ca2+]i without affecting Na(+)-Ca2+ exchange, suggesting that amiloride derivatives can reduce Ca2+ loading by blocking Na+ entry through Na+ channels, an action that may in part underlie their ability to prevent hypoxic Na+ and Ca2+ loading. Na+ influx through the voltage-gated Na+ channel is an important route of hypoxic Na+ loading, Na(+)-dependent Ca2+ loading, and reoxygenation hypercontracture in isolated rat cardiac myocytes. Importantly, the Na+ channel appears to serve as a route for hypoxic Na+ influx after myocytes become inexcitable.