Intracellular pH during ischemia in skeletal muscle: relationship to membrane potential,extracellular pH,tissue lactic acid and ATP

Intracellular pH (pH_i) was measured with double-barrelled microelectrodes during 4 h of complete tourniquet ischemia in rabbit gastrocnemius muscle (group I). The pH_i was related to extracellular pH (pH_e), membrane potential (E_m), tissue lactic acid (LA) and ATP. A fall in pH_i from 7.00±0.03 to 6.60±0.05 occurred during 4 h of ischemia, with a slight pH-drop (0.07 pH units) during the initial hour and a more pronounced drop of 0.13 pH units during the last hour of ischemia. These changes were paralleled by a considerable decrease in pH_e from 7.30±0.01 to 6.36±0.05 and a sixfold increase of tissue LA. The buffering capacity during the 4 h of ischemia was estimated to 81.9±5.6 mmol H⁺/l×pH. In parallel with the reduction in pH_e, the resting membrane potential decreased from –90 mV and stabilised at around –60 mV after 2 h of ischemia. A less negative cell interior would favour H⁺ extrusion since the E_m-E_H ⁺ gradient was unchanged at about –70 mV during the entire period of ischemia. This could contribute to muscle fiber buffering during ischemia.In another set of experiments (group II) the muscular glycogen reserve was reduced 20 min prior to a 4 h period of ischemia. Thereby an ischemic state was created where ATP levels decreased to 30% of initial, in contrast to the unaltered ATP content in group I. In the low-ATP group II the E_m-E_H ⁺ gradient decreased to 58% of initial and the buffering capacity was slightly but significantly lower (64.6±4 mmol H⁺/l×pH) than in the high-ATP group I. Significant correlation coefficients were obtained between the ATP loss and the E_m-E_H ⁺ gradient (r=0.861,P<0.001). These data suggest that maintenance of the transmembrane H⁺ gradient and to a certain degree the buffer capacity depend on the energy-state of the muscle cell.