SPECIFIC-INHIBITION OF CARDIAC AND SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM CA2-89( PUMPS BY H)

The isoquinolinesulfonamide H-89, an inhibitor of cyclic AMP-dependent protein kinases (EC 2.7.1.37, cAPrK), inhibited the Ca2+-ATPase activ ity of cardiac and skeletal muscle sarcoplasmic reticulum (SR) with co ncentrations giving half-maximal inhibition of 8.1 +/- 1.3 and 7.2 +/- 0.9 mu mol/L, respectively. The effect of H-89 on cardiac SR Ca2+-ATP ase (EC 3.6.1.38) was the same irrespective of the presence or absence of inhibitors of cAPrK and furthermore, was not affected by a neutral ising monoclonal antibody raised against phospholamban. Thus, the acti on of H-89 in inhibiting SR Ca2+-ATPase would not appear to be mediate d by inhibition of cAPrK to reduce the phosphorylation state of phosph olamban. In both cardiac and skeletal muscle SR, the inhibition by H-8 9 was noncompetitive with respect to ATP at a low concentration of ATP (<1 mmol/L) and of a mixed pattern at high concentrations of ATP. H-8 9 produced a decrease in affinity of the SR Ca2+ pump to Ca2+ with an increase in the K-m for Ca from 0.52 +/- 0.01 to 0.94 +/- 0.03 mu mol/ L (P < 0.05) in cardiac SR and from 0.39 +/- 0.01 to 0.79 +/- 0.02 mu mol/L (P < 0.05) in skeletal muscle SR. These results suggest that H-8 9 inhibits SR Ca2+-ATPase by a direct action on the SR Ca2+ pump to de crease its affinity to Ca2+. Such an action may contribute to the phar macological effect of H-89. (C) 1997 Elsevier Science Inc.