Received September 10, 2001
Accepted after revision February 8, 2002

Intrinsic H⁺ ion mobility in the mammalian ventricular myocyte

¹ University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK
² Nora Eccles Harrison Cardiovascular Research & Training Institute, University of Utah, Salt Lake City, UT, USA
³ Department of Mathematics, University of Utah, Salt Lake City, UT, USA

^* To whom correspondence should be addressed.

The intrinsic mobility of intracellular H⁺ ions was investigated by confocally imaging the longitudinal movement of acid inside rabbit ventricular myocytes loaded with the acetoxymethyl ester (AM) form of carboxy-seminaphthorhodafluor-1 (carboxy-SNARF-1). Acid was diffused into one end of the cell through a patch pipette filled with an isotonic KCl solution of pH 3.0. Intracellular H⁺ mobility was low, acid taking 20-30 s to move 40 µm down the cell. Inhibiting sarcolemmal Na⁺-H⁺ exchange with 1 mM amiloride had no effect on this time delay. Net H⁺_i movement was associated with a longitudinal intracellular pH (pH_i) gradient of up to 0.4 pH units. H⁺_i movement could be modelled using the equations for diffusion, assuming an apparent diffusion coefficient for H⁺ ions (D^H_app) of 3.78 x 10^-7 cm² s^-1, a value more than 300-fold lower than the H⁺ diffusion coefficient in a dilute, unbuffered solution. Measurement of the intracellular concentration of SNARF (~400 µM) and its intracellular diffusion coefficient (0.9 x 10^-7 cm² s^-1) indicated that the fluorophore itself exerted an insignificant effect (between 0.6 and 3.3 %) on the longitudinal movement of H⁺ equivalents inside the cell. The longitudinal movement of intracellular H⁺ is discussed in terms of a diffusive shuttling of H⁺ equivalents on high capacity mobile buffers which comprise about half (~11 mM) of the total intrinsic buffering capacity within the myocyte (the other half being fixed buffer sites on low mobility, intracellular proteins). Intrinsic H⁺_i mobility is consistent with an average diffusion coefficient for the intracellular mobile buffers (D_mob) of ~9 x 10^-7 cm² s^-1.