Transport and binding of Ca by a plasma membrane-enriched fraction from rat gastric fundus smooth muscle

The plasma membrane-enriched fraction F2 from rat gastric fundus smooth muscle can take up Ca actively from media containing submicromolar concentrations of Ca²⁺. The uptake changes only marginally in the pH range 6.27-7.47 and is not activated by oxalate. For the apparent initial velocity of the ATP-dependent Ca-uptake Hill coefficient ≃ 1 and K(m) for Ca²⁺=0.3-0.5 μM. The fraction F2 also shows passive Ca uptake which is pH dependent. At pH 6.27 the passive Ca uptake is a nonsaturable process. The passive Ca uptake increases with pH with a pK of approx 6.8. Therefore, the passive Ca uptake data are treated as the sum of the Ca uptake at pH 6.27 and the pH-dependent Ca-uptake (difference between uptake at pH 7.07 and 6.27). The pH-dependent Ca uptake occurs with a K(m) for Ca²⁺ of approx. 0.4 μM and is saturated at 1 μM Ca²⁺ concentration. The passively taken up ⁴⁵Ca, at pH 7.07 is not released by the Ca ionophore A23187 alone but is released very rapidly on exposure to H⁺ or ⁴⁰Ca in the presence of A23187. The distribution of the pH-dependent Ca uptake in the subcellular fractions correlates very well with the distribution of the plasma membrane markers but not with the mitochondrial or the putative endoplasmic reticulum markers. Physiological implications of the two types of plasma membranes - Ca²⁺ interactions are discussed.