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This study investigated developmental changes in the effect of acidosis on intracellular pH (pHi) and [Ca]i in the isolated heart and isolated myocyte preparations. The whole heart or myocytes of newborn (5-7 days old) and adult rabbits were loaded with the fluorescent pH indicator 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) or calcium indicator fura-2. Left ventricular pressure in the isolated heart preparation and the magnitude of cell contraction in the single-cell preparation were monitored. The heart and single cell were illuminated with excitation lights (340 and 380 nm, respectively, for fura-2 and 438 and 490 nm for BCECF). The intensity of fluorescence from the ventricular surface or from the cell was detected. [Ca]i was estimated from the following ratio: fluorescence at 505 nm during excitation at 340 nm/fluorescence at 505 nm during excitation at 380 nm. pHi was estimated from the following ratio: fluorescence at 530 nm during excitation at 490 nm/fluorescence at 530 nm during excitation at 438 nm. In the newborn, depression of contractile function during respiratory acidosis or metabolic acidosis was less than in the adult. Diastolic and systolic [Ca]i increased during respiratory acidosis in both the newborn and adult, and the net changes in [Ca]i were similar in the two age groups. During respiratory or metabolic acidosis, pHi decreased, but the decrease in the newborn was significantly less than in the adult. These data suggest that the greater resistance of the newborn myocardium to acidosis is due to the smaller change of pHi in this age group and not due to the difference in [Ca]i alteration.

Effect of acidosis on intracellular pH and calcium concentration in the newborn and adult rabbit myocardium.