H+ permeation through the cardiac gap junction

Although H+ ions modulate junctional conductance, their junctional permeabilità has not been measured. We investigated this in end-to-end pairs of guinea pig ventricular myocytes superfused with 20 mM HEPES-buffered Tyrode (pH 7.4 37°C). HCl(pH 3 in isotonic KCl) was diffused into cell-1 from patch pipette while confocal imaging pHi (AM-loaded SNARF). Acidification above an arbitrary threshold of 20 nM Delta[H+]I was used to record longitudinal acid movement between 2 downstream regions of interest (ROIs, 60 um apart) in cell-1 and two similarly spaced ROIs in cell-2. Intracellular delay between ROIs was 18s and 21s in cell-1 and 2 respectively (n=7). In contrast, delay over an intercellular distance of 10-20 um was about 75s. 40uM Beta-glycyrrhetinic acid, a junctional uncoupler, blocked junctional H+ permeation (n=2) and greatly increased resting intercellular resistance (from 6 MOhms to 1.7 GOhms; n=2, double patch experiments). In contrast, 30 uM Hoe 642 (a Na-H exchange inhibitor) had no effect on junctional H+ delay. Measurements of junctional SNARF flux (n=3) indicated it accounted for <0.7% of junctional acid flux. We conclude that acid permeates gap junctions, but that the rate of movement along a cell pair is limited by the junctional transfer. This will help to restrict the spread of an intracellular acid insult within the heart, such as may occur during myocardial ischaemia.