pH_i regulation in myocardium of the spontaneously hypertensive rat: Compensated enhanced activity of the Na⁺-H⁺ exchanger

To elucidate the mechanisms controlling pH_i in myocardium of the spontaneously hypertensive rat (SHR), experiments were performed in papillary muscles (isometrically contracting at 0.2 Hz) from SHR and age-matched normotensive Wistar-Kyoto (WKY) rats loaded with the pH-sensitive fluorescent probe BCECF-AM. An enhanced activity of the Na⁺-H⁺ exchanger was detected in the hypertrophic myocardium of SHR. This conclusion was based on the following: (1) The myocardial pH, was more alkaline in SHR (7.23±0.03) than in WKY rats (7.10±0.03) (P<.05) in HEPES buffer. (2) SITS (0.1 mmol/L in HEPES buffer) did not alter pH_i in the SHR (pH_i 7.26±0.03 and 7.28±0.03 before and after SITS, respectively). (3) The fall in pH, observed after 20 minutes of Na⁺-H⁺ exchanger inhibition [5 μmol/L 5-(N-ethyl-N-isopropyl)amiloride (EIPA)] was greater in SHR (-0.16±0.01) than in WKY rats (-0.09±0.02, P<.05). (4) The velocity of pH, recovery from an intracellular acid load was faster in SHR than in WKY rats (0.068±0.02 versus 0.014±0.002 pH units/min at pH_i 6.99, P<.05). (5) After EIPA inhibition, the rate of pH_i recovery from the same acid load decreased to a similar value in both rat strains (0.0032±0.002 pH units/min in SHR and 0.0032±0.002 pH units/min in WKY rats). Under the more physiological HCO₃^--CO₂ buffer, no significant difference in steady state myocardial pH_i was detected between rat strains (7.15±0.03 in SHR and 7.11±0.05 in WKY rats). This finding suggested that an acidifying bicarbonate-dependent mechanism was fully compensating for the hyperactivity of the Na⁺-H⁺ exchanger in SHR. The following pieces of evidence support an enhanced activity of the Na⁺-independent Cl^--HCO₃^- exchanger as the mechanism accounting for the compensation: (1) SITS (0.1 mmol/L) increased steady state pH, in the presence of HCO₃^--CO₂ buffer in SHR (+0.08±0.02, P<.05) but not in WKY rats (+0.04±0.04). (2) The rate of pH_i recovery from an alkaline load was faster in SHR than in WKY rats (0.075±0.028 versus 0.027±0.016 pH units per minute, respectively; P<.05). (3) The enhanced recovery from an alkaline load in the SHR was Na⁺ independent. (4) No difference in the rate of pH_i recovery was detected between SHR and WKY rats when the alkaline load was performed after SITS blockade. Comparison of net HCO₃^- efflux at a given pH_i suggests that an increased pH_i is not the cause of the hyperactivity of the anion exchanger. Since this anion exchanger is not driving Na⁺, the offset of the increase in pH_i induced by the antiport would not prevent an increase in intracellular Na⁺ mediated by the Na⁺-H⁺ exchanger.