Doxorubicin has biphasic effects on cardiac sarcoplasmic reticulum calcium channels in planar lipid bilayers

J Mel

P85

Cell

Cardiol

22 (Supplement

I) (1990)

DOXORUBICIR BAB BIPRASIC KFFKCTB OR CABDIAC CJWJR’KIS I3 PIARAR LIPID BILAYRBS. Louis Borgatta, Karol Ondrias, Do Han Kim,

BARCOPIASRIC

RKTICDIJJR

CALCIDM

Barbara E. Ehrlich. Departments of Medicine/Cardiology and Physiology. University of Connecticut, Farmington, CT. Doxorubicin (DOX) is a widely used chemotherapeutic agent that causea a dilated cardiomyopathy after prolonged usage. To define the mechanism of DOX cardiotoxicity, the effects of DOX and Caffeine (CAF) vere examined on calcium channels from cardiac sarcoplasmic reticulum that were incorporated into planar lipid bilayers. Channel open probability increased from
p86

JUNCTIONAL Ca CHANNELS ASSOCIATED WITH THE RYANODINE RECEPTOR OF “FEET” STRUCTURE TRIGGER THE SARCOPLASMIC RETICULUM (SR) Ca RELEASE IN RAT MYOCARDIUM. Maya Saxon, Ravil Gainn1.li.n. Department of Pharmacology and Medicine, Colledge of Physician and Surgeon, Columbia University, 630 West 168th Street, New York, N.Y. 10032, Institute of Biological Physics, Acad. Sci. of the USSR, Pushchino, 142292. The dihydropiridine (DHP) Ca channel activators, CGP28392 and Bay K 8644 (1uM) caused two opposite inotropic effects in rat myocardium: a positive one in a polarized muscles and negative one in partially depolarized tissue. Opposite inotropic effects were associated with the action potential prolongation and were antagonized by nifedipine. The positive notropy was explained by a standard stimulation of Ca entry by the DHP activators. The depressive effect, resembling the ryanodine activity, was explained by the depletion of SR due to stimulation of Ca leakage from intracellular stores. The “RYANODINE-LIKE” effect is thought to be mediated by the “JUNCTIONAL” population of L-type Ca channels in the T-tubules. Ca coming through the "junctional" channels act as activator of Ca release and muscle contraction.

P87

CALCIUM RETICULUM

ACTIVATED PROTEIN

NEUTRAL STRUCTURE

PROTEASE EFFECTS AND FUNCTION.

UPON

CARDIAC

In

SARCOPLASMIC

J.S.C. Gilchrist, C. Machan, S. Katz and A. N. Belcastro. Laboratory of Cell Physiology and Exercise, 6081 University Boulevard, University of British Columbia, Vancouver, Canada, V6T lW5. The ryanodine receptor of cardiac and skeletal muscle is a key struchrre in excitation-contraction coupling and is synonymous with the high conductance Ca 2+ release channel of the Sarcoplasmic Reticulum (SR). The receptor exhibits extreme sensitivity to proteolysis although the effects of Ca2+-Activated Neutral Protease (CANP) upon cardiac SR structure and function are unclear. Purified Heavy SR (HSR) referable to the t-tubule/SR junction from rat and canine cardiac muscle was exposed to 2 forms of CANP activated by micromolar @CANI’) and millimolar (mCANP) concentrations of Ca2+. The major substrate for both forms of CANP was the receptor protein (-500 kDa) in rat and canine HSR. Limited proteolysis resulted in immediate production of a major peptide (-400 kDa) which was further degraded to a 300 kDa peptide. Additionally, a 250 kDa protein was also degraded by both forms of CANP. The 110 kDa Ca 2+-ATPase was not a substrate for either form of CANP. Degradation of the ryanodine receptor was associated with a progressive decline in passive Ca2+ loading and loss of sensitivity to the Ca2+ release channel inhibitors Mg2+ (1OmM) and ruthenium red (2m). These data indicate that CANP effects may be important during periods of intracellular Ca2+ overload associated with myopathic states. Supported byB.C.andYukonHeartandStrokeFoundation andNSERCCanada. s.29