title: Passive calcium-buffering capacity of a rabbit ventricular homogenate preparation.
abstract: This study characterizes the passive Ca2+-binding capacity of a crude homogenate fraction prepared from rabbit ventricular tissue. The soluble and particulate fractions of the homogenate were separated by centrifugation, and the Ca2+-binding capacity of each fraction was measured separately with Ca2+-selective electrodes and then summed to obtain the Ca2+-binding capacity of the total tissue homogenate. Qualitatively, the Ca2+ bound to the homogenate exhibited both relatively high- (Km less than 1 microM) and low-affinity (Km greater than 10 microM) sites over the physiological range of free Ca2+ concentrations. The total homogenate bound 72, 128, and 308 mumol Ca2+/kg wet wt tissue at 1, 10, and 50 microM free Ca2+ concentrations, respectively. The Ca2+ bound to the homogenate was mostly La3+ displaceable, which suggests that it represents a largely noncompartmentalized, rapidly exchangeable fraction of Ca2+. Myofibrillar ATPase activity was examined as a function of free Ca2+ concentration in an ionic medium similar to that used in the Ca2+-binding experiments. At free Ca2+ concentrations that stimulated myofibrillar ATPase activity half maximally, the measured rise in Ca2+ bound to the homogenate above resting values was greater than 100 mumol/kg wet wt. These data provide base-line values for the passive Ca2+-buffering capacity of the myocardium, although relevance to the physiological setting must be interpreted with caution.
authors.forename: G N
authors.initials: GN
authors.lastname: Pierce
authors.forename: K D
authors.initials: KD
authors.lastname: Philipson
authors.forename: G A
authors.initials: GA
authors.lastname: Langer
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D006352:Heart Ventricles / Q000378:metabolism; D007700:Kinetics; D007811:Lanthanum / Q000494:pharmacology; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D009210:Myofibrils / Q000201:enzymology; D011817:Rabbits
chemical_list: D002021:Buffers; D004533:Egtazic Acid; D007811:Lanthanum; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D002118:Calcium