Dysfunctional Electrogenic Sodium/Bicarbonate Cotransport in Cardiomyocytes of Spontaneously Hypertensive Rats

ALEJANDRO ORLOWSKI; DE GIUSTI VC; AIELLO EA

Congreso; High Blood Pressure Research 2011 Scientific Sessions; 2011

DYSFUNCTIONAL ELECTROGENIC Na+/HCO3- COTRANSPORT IN CARDIOMYOCYTES OF SPONTANEOUSLY HYPERTENSIVE RATS.  Orlowski A, De Giusti VC, Aiello EA. Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, UNLP, La Plata, Argentina (control tracking number 11-HPBR-L-941-AHA).   The Na+/HCO3- cotransport (NBC) is an important cardiac acid extruder. Electroneutral (NBCn1, 1 Na+:1 HCO3-) and electrogenic (NBCe1, 1 Na+:2 HCO3-) isoforms have been characterized in the heart. We studied the expression and function of NBCe1 in myocardium of normal Wistar (W) and spontaneously hypertensive rats (SHR). In western-blot assays we found an over-expression of NBCe1 in SHR compared to W (145.5±8.3 %, n=10 vs. 98.4±3.7 %, n=10, p<0.05). Intracellular pH was monitored by epifluorescence in isolated cardiomyocytes. Depolarization of membrane potential evoked by high extracellular K+ (45 mmol/L K+o) induced a voltage-dependent alkalization that was due to selective activation of NBCe1. The initial rate of alkalization (pH units/min) was significantly lower in SHR than in W (0.054±0.007, n=22 vs. 0.081±0.009, n=23, p<0.05), indicating that, despite the increase in expression, the NBCe1 function appears to be diminished in the hypertrophic hearts. Total NBC (NBCn1+NBCe1) activity (net H+ efflux, JH in mmol/l/min, at 6.8) was recorded during recovery from acidosis induced by an ammonium pulse. Total JH was similar in both strains (W: 3.29±0.67, n=8; SHR: 3.14±0.6, n=6). However, in the presence of a-L3, an specific antibody against NBCe1 extracellular loop 3, which cancels its activity, JH decreased by 11% in SHR (2.8±0.23, n=4) and 50% in W (1.71±0.38, n=4), suggesting that the deteriorated cardiac NBCe1 function observed in SHR is compensated by an enhanced activity of NBCn1. We used confocal microscopy and immunofluorescence to determine the localization of NBCe1 in the cardiomyocytes. Although NBCe1 was detected in T tubules of both strains, a substantial amount of the protein was also found in endosomes near the nucleus only in SHR, suggesting a dysfunctional NBCe1 trafficking in these rats. We conclude that, despite the fact that NBCe1 expression is increased in hypertrophied hearts, its activity is decreased, possibly due to retained transporter protein units in early endosomes. Moreover, NBCn1 activity seems to be increased in SHR, compensating impaired function of NBCe1. Considering the different stoichiometry of the NBC isosforms, this upregulation of NBCn1 could lead to sodium and/or calcium overload in hypertrophic hearts.