RELATIONSHIP BETWEEN CA2+-TRANSPORT AND ATP HYDROLYTIC ACTIVITIES IN GUINEA-PIG PANCREATIC ACINAR PLASMA-MEMBRANES

Partially purified plasma membrane fractions were prepared from guinea-pig pancreatic acini. These membrane preparations were found to contain an ATP-dependent Ca2+-transporter as well as a heterogenous ATP-hydrolytic activity. The Ca2+-transporter showed high affinity for Ca2+ (K(Ca2+) = 0.04 +/- 0.01-mu-M), an apparent requirement for Mg2+ and high substrate specificity. The major component of ATPase activity could be stimulated by either Ca2+ or Mg2+ but showed a low affinity for these cations. At low concentrations, Mg2+ appeared to inhibit the Ca2+-dependent ATPase activity expressed by these membranes. However, in the presence of high Mg2+ concentration (0.5-1 mM), a high affinity Ca2+-dependent ATPase activity was observed (K(Ca2+) = 0.08 +/- 0.02-mu-M). The hydrolytic activity showed little specificity towards ATP. Neither the Ca2+-transport nor high affinity Ca2+-ATPase activity were stimulated by calmodulin. The results demonstrate, in addition to a low affinity Ca2+(or Mg2+)-ATPase activity, the presence of both a high affinity Ca2+-pump and high affinity Ca2+-dependent ATPase. However, the high affinity Ca2+-ATPase activity does not appear to be the biochemical expression of the Ca2+-pump.