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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 muM, respectively. Mg2+ up to 5 mM significantly enhanced both activities but was inhibitory at higher concentrations (greater than 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 muM. 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.
Louis et al. (1976) studied this question.