DIRECT MEASUREMENT OF INTRACELLULAR PH OF MAMMALIAN CARDIAC-MUSCLE

The intracellular pH (pHi) of sheep heart Purkinje fibers and rat, ferret and guinea-pig ventricle was measured using recessed-tip pH-sensitive microelectrodes. In the absence of CO2 the pHi was approximately 7.2 in all the preparations used. In 5% CO2 the mean pHi was 7.14 in rat and ferret ventricle and 7.02 in sheep Purkinje fibers. The pHi response to an increase or a decrease in the CO2 level (at constant external pH) was biphasic with a large transient change followed by a partial recovery to a new sustained pHi. The intracellular buffering capacity was 34.8 .+-. 2.7 meq H+/pH unit per 1 (.+-. standard error of mean) in sheep Purkinje fibers, 76.6 .+-. 13.6 in rat ventricle and approximately 69 in ferret ventricle. The pHi of all the preparations tested indicated that H+ ions were not passively distributed across the cell membrane. There was also little or no pHi change produced by depolarization with high K solutions. Short exposures to hypertonic solutions (100 mM sucrose or 50 mM-KCl) produced a decrease in pHi of approximately 0.1 pH units. Acetazolamide slowed the pHi response to CO2 changes. Restoration of the pHi after displacement by increasing the CO2 was not blocked by ouabain. The relationship between pHi and cardiac contractility was discussed.