Regulation and deregulation of cardiac Na(+)-Ca2+ exchange in giant excised sarcolemmal membrane patches.
A plasmalemmal Na(+)-Ca2+ exchange mechanism is an important electrogenic determinant of contractility in cardiac cells. As in other cell types, calcium influx by Na(+)-Ca2+ exchange is secondarily activated by cytoplasmic calcium and probably ATP, but these modulatory mechanisms are either absent or altered in isolated cardiac sarcolemmal vesicles. Involvement of a calcium-dependent protein kinase in exchange regulation has been suggested but not verified. Here I describe measurements of outward Na(+)-Ca2+ exchange current, corresponding to calcium influx, in giant excised sarcolemmal patches from guinea pig myocytes. The exchange current is stimulated by both calcium and Mg-ATP from the cytoplasmic face, evidently through separate mechanisms. Activation by cytoplasmic calcium takes place within seconds, is reversible, and does not require ATP. Stimulation by Mg-ATP reverses only slowly over greater than 10 min, or not at all. Unexpectedly, a substantial decrease in exchange current occurs during activation by cytoplasmic sodium, which seems to reflect an inactivation process rather than ion concentration changes or a 'first pass' exchange cycle. This apparent inactivation, and the modulations by cytoplasmic calcium and Mg-ATP, are all abolished by brief treatment of the cytoplasmic surface with chymotrypsin, leaving the exchanger in a maintained state of high activity. Therefore, limited proteolysis deregulates Na(+)-Ca2+ exchange and could contribute to the loss of secondary regulation of the exchange in isolated sarcolemmal vesicles.
Hilgemann
D W
DW
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235.
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002352:Carrier Proteins / Q000502:physiology*; D002918:Chymotrypsin / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism; D004553:Electric Conductivity; D006168:Guinea Pigs; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D012508:Sarcolemma / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D011189:Potassium Chloride; D000255:Adenosine Triphosphate; D012964:Sodium; D002918:Chymotrypsin; D002118:Calcium