The effects of pH and P_i on tension and Ca²⁺ sensitivity of ventricular myofilaments from the anoxia-tolerant painted turtle

We aimed to determine how increases in intracellular H+ and inorganic phosphate (Pi) to levels observed during anoxic submergence affect contractility in ventricular muscle of the anoxia-tolerant Western painted turtle, Chrysemys picta bellii. Skinned multicellular preparations were exposed to six treatments with physiologically relevant levels of pH (7.4, 7.0, 6.6) and P_i (3 and 8 mmol l^-1). Each preparation was tested in a range of calcium concentrations (PCA 9.0-4.5) to determine the PCA-tension relationship for each treatment. Acidosis significantly decreased contractility by decreasing Ca²⁺ sensitivity (PCA₅₀) and tension development (P<0.001). Increasing [P_i] also decreased contractility by decreasing tension development at every pH level (P<0.001) but, alone, did not affect Ca²⁺ sensitivity (P=0.689). Simultaneous increases in [H⁺] and [P_i] interacted to attenuate the decreased tension development and Ca²⁺ sensitivity (P<0.001), possibly reflecting a decreased sensitivity to P_i when it is present as the dihydrogen phosphate form, which increases as pH decreases. Compared with that of mammals, the ventricle of turtles exhibits higher Ca^2+# sensitivity, which is consistent with previous studies of ectothermic vertebrates.