Adenosine triphosphate - dependent calcium binding and accumulation by guinea pig cardiac sarcolemma

Abstract

Sarcolemma isolated from guinea pig heart ventricles possessed ATP-dependent Ca2+ binding and accumulation (+ oxalate) activities which were not inhibited by sodium azide, oligomycin, or ruthenium red. Ca2+ binding and accumulation by sarcolemma were sensitive to pH, the optimum being about pH 6.8. The concentrations of ATP required for half-maximal binding and accumulation were 94.3 and 172 μM, respectively. Mg2+ up to 5 mM significantly enhanced both activities but was inhibitory at higher concentrations (> 10 mM). Sarcolemmal Ca2+ binding and accumulation were stimulated 100% by K+, half-maximal enhancement occurring at 5–10 mM K+. Ca2+ binding and accumulation were both saturable processes and the respective apparent Km values for Ca2+ were 16.4 and 14.3 μM. Ca2+ binding by sarcolemma was a rapid process and the bound Ca2+ was released upon depletion of ATP in the medium. It is suggested that the sarcolemmal Ca2+ transport system may well be of significance in regulation of the contraction–relaxation cycle of cardiac muscle.