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


RESULTS
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.


CONCLUSIONS
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.
Laboratory of Cardiovascular Science, National Institute on Aging, Baltimore, MD.
M C
MC
Haigney
E G
EG
Lakatta
M D
MD
Stern
H S
HS
Silverman
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001572:Benzofurans; D052160:Benzothiazoles; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002469:Cell Separation; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D000860:Hypoxia / Q000378:metabolism*; D008012:Lidocaine / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010880:Piperidines / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology; D013844:Thiazoles / Q000494:pharmacology
D001572:Benzofurans; D052160:Benzothiazoles; D002121:Calcium Channel Blockers; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D010880:Piperidines; D026941:Sodium Channel Blockers; D017923:Sodium-Hydrogen Exchangers; D013844:Thiazoles; C061665:sodium-binding benzofuran isophthalate; D013779:Tetrodotoxin; D000584:Amiloride; C054551:R 56865; D008012:Lidocaine; D012964:Sodium; D002118:Calcium; C039614:ethylisopropylamiloride