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(Circulation Research. 2003;93:1082.)
© 2003 American Heart Association, Inc.

Cellular Biology

Influence of Na⁺-Independent Cl^--HCO₃^- Exchange on the Slow Force Response to Myocardial Stretch

Horacio E. Cingolani, Gladys E. Chiappe, Irene L. Ennis, Patricio G. Morgan, Bernardo V. Alvarez, Joseph R. Casey, Raúl A. Dulce, Néstor G. Pérez, María C. Camilión de Hurtado

From the Centro de Investigaciones Cardiovasculares (H.E.C., G.E.C., I.L.E., R.A.D., N.G.P., M.C.C.d.H.), Facultad de Ciencias Médicas, Universidad Nacional de La Plata, La Plata, Argentina; and the Department of Physiology (P.G.M., B.V.A., J.R.C.), University of Alberta, Edmonton, Canada.

Correspondence to Horacio E. Cingolani, Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, Calle 60 y 120, 1900 La Plata, Argentina. E-mail cicmes{at}infovia.com.ar

Previous work demonstrated that the slow force response (SFR) to stretch is due to the increase in calcium transients (Ca²⁺T) produced by an autocrine-paracrine mechanism of locally produced angiotensin II/endothelin activating Na⁺-H⁺ exchange. Although a rise in pH_i is presumed to follow stretch, it was observed only in the absence of extracellular bicarbonate, suggesting pH_i compensation through the Na⁺-independent Cl^--HCO₃^- exchange (AE) mechanism. Because available AE inhibitors do not distinguish between different bicarbonate-dependent mechanisms or even between AE isoforms, we developed a functional inhibitory antibody against both the AE3c and AE3fl isoforms (anti-AE3Loop III) that was used to explore if pH_i would rise in stretched cat papillary muscles superfused with bicarbonate after AE3 inhibition. In addition, the influence of this potential increase in pH_i on the SFR was analyzed. In this study, we present evidence that cancellation of AE3 isoforms activity (either by superfusion with bicarbonate-free buffer or with anti-AE3Loop III) results in pH_i increase after stretch and the magnitude of the SFR was larger than when AE was operative, despite of similar increases in [Na⁺]_i and Ca²⁺T under both conditions. Inhibition of reverse mode Na⁺-Ca²⁺ exchange reduced the SFR to the half when the AE was inactive and totally suppressed it when AE3 was active. The difference in the SFR magnitude and response to inhibition of reverse mode Na⁺-Ca²⁺ exchange can be ascribed to a pH_i-induced increase in myofilament Ca²⁺ responsiveness.

Key Words: myocardial pH_i • Na⁺-H⁺ exchanger • AE3 • Na⁺-Ca²⁺ exchange • myocardial stretch

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