Intracellular pH vs MgATP2- concentration: relative importance as determinants of Ca2+-activated force generation of disrupted rabbit cardiac cells

Early decline of cardiac contractility during ischaemia and hypoxia may be related in part to lowered concentrations of MgATP2− (magnesium salt of adenosine triphosphate) limiting the ability of the contractile proteins to generate force in response to Ca2+-activation. These experiments were designed to determine the relative importance of altered pH vs MgATP2− depletion on Ca2+-activated isometric force generation of mammalian cardiac fibres. The cardiac contractile apparatus was functionally isolated by disrupting cell membranes; small bundles of rabbit left ventricular cells with disrupted sarcolemmas were prepared by homogenisation and continuous exposure to 7 mmol·litre−1 EGTA (ethyleneglycolbis-(β-amino-ethyl ether)-N-N'-tetra-acetic acid). Removal of the major barrier to diffusion into the cells allowed control and alteration of the fluid surrounding the contractile proteins. Bathing solutions approximated intracellular conditions in important constituents and concentrations while providing adequate buffering of Ca2+ (EGTA buffer), H+ (imidazole buffer) and MgATP2− (creatine phosphate, creatine phosphokinase regenerating system). The magnitude of Ca2+-activated steady-state isometric forces were determined using a photodiode transducer. At each Ca2+ concentration, self-paired comparisons of force were made for selected combinations of bathing solution pH and MgATP2− concentration. In comparison to forces at normal conditions (2 mmol·litre−1 MgATP2−, pH 7.0) maximum forces and Ca2+-sensitivity were greatly depressed by acidosis (pH 6.5) alone. Combining lowered MgATP2− concentration (0.1 or 0.03 mmol·litre1) with pH 6.5 caused no additional change in Ca2+-sensitivity and force except that maximum force at 0.03 mmol·litre1 MgATP2− was enhanced rather than depressed. Thus, within the pH and MgATP2− concentration ranges thought to be characteristic of ischaemic intracellular conditions, only acidosis caused depression of contractile protein-Ca2+-activated force generating ability.