EFFECTS OF ACIDOSIS ON MECHANICAL FUNCTION AND CA2+ EXCHANGE IN RABBIT MYOCARDIUM

The effects of acidosis on myocardial function and Ca exchange were studied in the isolated but arterially perfused interventricular septum of the rabbit. Temperature was 28.degree. C and stimulation rate was 48 beats/min. Acidosis was induced by increase of the perfusate PCO2 (CO2 partial pressure) (pH reduced from 7.35-6.68) or by decrease of the bicarbonate-chloride ratio (pH 7.35-6.72). The effect on Ca 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 Ca from the muscle. Decrease of the bicarbonate-chloride ratio caused a slower fall of developed tension and the efflux or uptake of Ca were not altered. Apparently developed tension and Ca exchange in the myocardium are more responsive to acidosis within the cell or cell membrane than to extracellular acidosis.