Effects of acidosis on mechanical function and Ca2+ exchange in rabbit myocardium

The effects of acidosis on myocardial function and calcium exchange have been studied in the isolated but arterially perfused interventricular septum of the rabbit. Temperature was 28 degrees C and stimulation rate 48 beats/min. Acidosis was induced either by increase of the perfusate PCO2 (pH reduced from 7.35 to 6.68) or by decrease of the bicarbonate-chloride ratio (pH 7.35 to 6.72). The effect on calcium efflux was assessed by introduction of acidosis at different times during the washout of 45Ca2+ from the muscle. The uptake of 47Ca2+ was recorded directly with a NaI crystal and counter. An increase of perfusate PCO2 caused a rapid fall in developed tension. The efflux of slowly exchanging 45Ca2+ and the uptake of 47Ca2+ were inhibited. There was no rapid displacement of calcium from the muscle. Decrease of the bicarbonate-chloride ratio caused a slower fall of developed tension and neither the efflux nor uptake of calcium were altered. These results suggest that developed tension and calcium exchange in the myocardium are more responsive to acidosis within the cell or cell membrane than to extracellular acidosis.