The effects of pH and P on tension and Ca sensitivity of ventricular myofilaments from the anoxia-tolerant painted turtle.

We aimed to determine how increases in intracellular H and inorganic phosphate (P) to levels observed during anoxic submergence affect contractility in ventricular muscle of the anoxia-tolerant Western painted turtle, Skinned multicellular preparations were exposed to six treatments with physiologically relevant levels of pH (7.4, 7.0, 6.6) and P (3 and 8 mmol l). 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 Ca sensitivity (pCa) and tension development (<0.001). Increasing [P] also decreased contractility by decreasing tension development at every pH level (<0.001) but, alone, did not affect Ca sensitivity (=0.689). Simultaneous increases in [H] and [P] interacted to attenuate the decreased tension development and Ca sensitivity (<0.001), possibly reflecting a decreased sensitivity to P 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 sensitivity, which is consistent with previous studies of ectothermic vertebrates.