Modulation of sodium-hydrogen exchange activity in cardiac myocytes during acidosis and realkalinisation: effects on calcium, pHi, and cell shortening.
OBJECTIVE
The aim was to examine the effects of the Na+/H+ exchange inhibitor methylisobutylamiloride (MIA) as well as protein kinase C, a putative regulator of Na+/H+ exchange, on intracellular calcium, intracellular pH, and unloaded cell shortening in isolated guinea pig cardiac myocytes subjected to lactic acid induced acidosis followed by realkalinisation.


METHODS
Calcium transient amplitude and cell shortening were measured simultaneously in single isolated myocytes loaded with fura2-AM. Intracellular pH was measured in cells loaded with BCECF-AM.


RESULTS
Exposure of cells to 5 min of lactate (20 mM) acidosis (pH 6.8) caused an increase in calcium transient amplitude and a decrease in cell shortening and intracellular pH. During realkalinisation (pH 7.3), the calcium transient gradually decreased while intracellular pH became more alkaline than pre-acidosis values. The cells underwent transient hypercontractility as evidenced by a marked increase in systolic cell shortening and a decrease in diastolic cell length. Inhibition of sodium/hydrogen exchange with MIA (1 microM) caused a significant attenuation of the increase in calcium transient amplitude during acidosis and further depressed cell shortening as well as intracellular pH. In addition, MIA significantly attenuated hypercontractility and abolished cell contracture upon realkalinisation. In contrast, phorbol 12-myristate 13-acetate (10(-12) M) exerted no effects on the response to acidosis; however, this treatment exacerbated cell hypercontractility and reduced functional recovery upon realkalinisation.


CONCLUSIONS
Inhibition of Na+/H+ exchange activity during acidosis/realkalinisation enhances recovery of cell function.
Ward
C A
CA
Department of Pharmacology and Toxicology, University of Western Ontario, London, Canada.
Moffat
M P
MP
D000138:Acidosis / Q000378:metabolism*; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D048429:Cell Size / Q000187:drug effects; D002478:Cells, Cultured; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007773:Lactates / Q000494:pharmacology; D019344:Lactic Acid; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D011493:Protein Kinase C / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers / Q000187:drug effects / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D007773:Lactates; D017923:Sodium-Hydrogen Exchangers; D019344:Lactic Acid; D000584:Amiloride; C056369:5-(N-methyl-N-isobutyl)amiloride; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium