Blockade of the inotropic effect of Bay K 8644 by cytochalasin-B and phloretin.

1. The positive inotropic effect in rabbit atria and papillary muscles of Bay K 8644 is blocked by cytochalasin-B (Cyto-B) and phloretin, two compounds known to block the facilitated diffusion of glucose. These compounds do not change the concentration-response curve of calcium. 2. Cyto-B is more potent in atria than in papillary muscles, 10(-7) M having a maximal effect in atria whereas 2 x 10(-5) M was required for a maximal effect in papillary muscles. Phloretin was fully effective at 10(-4) M, the only concentration tested. 3. The inotropic effect of Bay K 8644 was virtually abolished in atria bathed in a glucose-free medium or one containing 5 mM pyruvate. The contractile response to Bay K 8644 of papillary muscles was not changed significantly in glucose-free or in pyruvate-containing medium. 4. Cyto-B (2 x 10(-5) M) caused a slight but significant increase in the KD for the binding of nitrendipine to a crude sarcolemnal preparation from rabbit ventricles. The Bmax was unchanged. 5. These results may best be explained by the hypothesis that there is a metabolic requirement for the inotropic effect of Bay K 8644.