Calcium transport and contractile activity in dissociated mammalian heart cells

A homogeneous population of dissociated myocytes can be obtained by enzymic perfusion of adult rabbit hearts. The external membrane of the dissociated cells is freely permeable to electrolytes and is electrochemically shunted. Nevertheless, in the presence of 0.2--0.6 muM Ca2+, the myocytes undergo phasic contractions. Within the range of Ca2+ concentrations eliciting phasic contractions, the dissociated myocytes exhibit Ca2+ uptake activity that is sustained by endogenous ATP in the presence of oxygen and metabolites, or by hexogenous ATP in the presence of oligomycin and antimycin. The uptake is greatly enhanced by oxalate, an effect that is attributed to the activity of sarcoplasmic reticulum. Addition of the calcium ionophore A23187 prevents the occurrence of both Ca2+ accumulation and phasic contractions. In the presence of the ionophore the myocytes remain relaxed at Ca2+ concentrations (less than 1.0 mM) normally sustaining phasic contractile activity, and undergo irreversible contracture at Ca2+ concentrations (greater than 1.0 muM) sufficient to produce direct activation of the myofibrils. It is concluded that Ca2+ transport by sarcoplasmic reticulum is directly related to the occurrence of phasic contractions in dissociated myocytes prepared from rabbit heart. On the other hand, metabolic activity is required for ATP regeneration.