HIGHLY SUPRALINEAR FEEDBACK INHIBITION OF CA2+ UPTAKE BY THE CA2+ LOAD OF INTRACELLULAR STORES

Net Ca2+ uptake into intracellular Ca2+ stores of homogenized cells is transient, even when the extravesicular Ca2+ concentration is kept co nstant. To study the mechanism underlying the phenomenon, we have inve stigated Ca-45(2+) uptake by HL-60 cell homogenates. The initial rate of Ca2+ uptake as well as the final amount of stored Ca2+ were a funct ion of the extravesicular Ca2+ concentration. However, Ca2+ uptake sto pped independently of the extravesicular Ca2+ concentration after appr oximately 10 min. Studies using Ca2+-ATPase inhibitors demonstrated th at the transient nature of the net uptake was not due to Ca2+ efflux. Monovalent cat ion ionophores did not influence the Ca2+ uptake curves , excluding a relevant involvement of pH and membrane potential. Toget her with the observation of a continued Ca2+ uptake in the presence of the intralumenal Ca2+ chelator oxalate, these results strongly sugges t a feedback inhibition of Ca2+ uptake by the Ca2+ load of intracellul ar stores. The concentration-inhibition relationship between the Ca2load and the rate of Ca2+ uptake was highly supralinear (slope factor greater than or equal to 4). IC50 and maximum of the dose-inhibition c urve, but not the slope factor were a function of the extravesicular f ree Ca2+ concentration. A series of three logistic equations derived f rom our data allowed an appropriate description of the behavior of Ca2 + uptake. Our results suggest, in addition to its well known activatio n by cytosolic Ca2+ concentration, a highly supralinear feedback inhib ition of Ca2+ uptake by the Ca2+ load of intracellular stores. The ste epness of the feedback inhibition might have a profound effect on spat ial and temporal behavior of the Ca2+ signal.