ALPHA-CHYMOTRYPSIN DEREGULATION OF THE SODIUM-CALCIUM EXCHANGER IN BARNACLE MUSCLE-CELLS

To gain insight into the mechanism by which the protease alpha-chymotr ypsin (alpha-chym) activates the Na-Ca exchanger in muscle cells we st udied 1) the ability of this enzyme to remove the intracellular ''cata lytic'' Ca2+ requirement for activation of all the modes of exchange m ediated by the Na-Ca exchanger (i.e., Na(o)-Ca(i), Na(i)-Ca(o), Na(o)- Na(i), and Ca(o)-Ca(i), where the subscripts o and i represent extrace llular and intracellular, respectively), and 2) the ability of certain monovalent cations to stimulate Ca(o)-Ca(i) exchange after activation of the exchanger by alpha-chym. Barnacle muscle cells were used as mo dels because these cells are so large that they can be internally perf used and voltage clamped. The results show that alpha-chym produces a highly activated Na-Ca exchanger able to operate in all its modes of e xchange independently of catalytic Ca(i). The concentration-dependent effect of alpha-chym was biphasic; maximal activation occurred at 0.5 mg alpha-chym/ml perfusate for 20 min of perfusion at a perfusion rate of 2.5 mul/min. The results are discussed in terms of the possible ef fects of a-chym on the kinetic modulation of the exchanger.