pH i Regulation in Myocardium of the Spontaneously Hypertensive Rat

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 i 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 i 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 i 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 HCO 3 − -CO 2 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 − -HCO 3 − exchanger as the mechanism accounting for the compensation: (1) SITS (0.1 mmol/L) increased steady state pH i in the presence of HCO 3 − -CO 2 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 HCO 3 − 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.