The effects of pH and Pi 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 Pi (3 mM and 8 mM). Each preparation was tested in a range of calcium concentrations (pCa 9.0-pCa 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 [Pi] 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 [Pi] interacted to attenuate the decreased tension development and Ca2+ sensitivity (p<0.001), possibly reflecting a decreased sensitivity to Pi when it is present as the dihydrogen phosphate form, which increases as pH decreases. Compared to mammals, the ventricle of turtles exhibits higher Ca2+ sensitivity, which is consistent with previous studies of ectothermic vertebrates.