A study of temporal relation between intracellular pH and contractile performance in toad ventricular strips during hypercapnic acidosis

Experiments were performed on toad (Bufo marinus) ventricular strips to determine the temporal relation between the decrease in intracellular pH (pH_i) and the changes in mechanical performance accompanying increases in P_CO2. Changes in pH_i were measured with a dual wavelength spectrophotometer using the pH sensitive dye neutral red. Regardless of whether the superfusate pH (pH₀) or HCO₃^?] was held constant, changes in P_CO2 were accompanied by a monotonic, biphasic change in pH_i. A plot of 1n ΔpH_i v. t consistently yielded a curvilinear line which was well fit by two straight lines. The developed tension also exhibited a biphasic response to ΔP_CO2: an initial decrease followed by a rebound which returned τ to control values (98.4 ± 1.6%) after 30 min. The onset of the second, slow phase of acidification correlated well (r = 0.83) with the onset of the mechanical rebound. The time of this onset was affected by HCO₃^?]. Further experiments showed that with P_CO2 held constant, a decrease in HCO₃^?] was accompanied by a slow decrease in pH_i. The data from these experiments suggest that: (1) changes in P_CO2 affect the pH of at least two intracellular compartments and these effects are temporally dissociated; (2) the response of the faster compartment correlates with the initial decrease of τ; the response of the slower with the rebound of τ; (3) when P_CO2 is held constant, decreases in pH₀ are accompanied by a slow decrease in pH_i suggesting a possible H⁺ or HCO₃^?] leak.