Potentiating effect of calmodulin and catalytic subunit of cyclic AMP‐dependent protein kinase on ATP‐dependent Ca2+‐transport by cardiac sarcolemma

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Abstract
Highly purified sarcolemmal membranes were prepared from pig heart homogenates by differential and density gradient centrifugations. The membrane fragments exhibit ATP‐dependent Ca2+‐transport and Na+/Ca2+‐exchange activities. ATP‐dependent Ca2+‐transport (K Ca2+ 0.5 = 0.3 μM; V max = 4.6 nmol Ca2+•mg protein−1 •min−1)_is not stimulated by oxalate. Ca2+‐uptake is also not supported by p‐nitrophenylphosphate. Preincubation of sarcolemma with MgATP, calmodulin and catalytic subunit of cyclic AMP‐dependent protein kinase stimulates active Ca2+‐transport 1.8‐fold. The effects of calmodulin and catalytic subunit are potentiating rather than additive. A large portion of the Ca2+ additionally accumulated after prephosphorylation of membranes is exchangable for Na+ via the Na+/Ca2+‐exchange system.