Dihydropyridine-induced Ca2+ release from ryanodine-sensitive Ca2+ pools in human skeletal muscle cells

1. Dihydropyridines (DHPs) are widely used antihypertensive drugs and inhib it excitation-contraction (E-C) coupling in vascular smooth muscle and in m yocardial cells by antagonizing L-type Ca2+ channels (DHP receptors). Howev er, contradictory reports exist about the interaction of the DHP with the s keletal muscle isoform of the DHP receptor and E-C coupling in skeletal mus cle cells. 2. Using the intracellular fluorescent Ca2+ indicator fura-2, an increase i n [Ca2+](i) was observed after extracellular application of nifedipine to c ultured human skeletal muscle cells. The rise in [Ca2+](i) was dose depende nt with a calculated EC50 of 614 +/- 96 nM nifedipine and a maximum increme nt in [Ca2+](i) of 80 +/- 3.2 nM. Similar values were obtained with nitrend ipine. 3. This effect of DHPs was restricted to differentiated skeletal muscle cel ls and was not seen in non-differentiated cells or in PC12 cells. In spite of the observed increase in [Ca2+](i), whole-cell. patch clamp experiments revealed that 10 mu M nifedipine abolished inward Ba2+ currents through L-t ype Ca2+ channels completely. 4. Similar to nifedipine, (+/-)Bay K 8644, an agonist of the L-type Ca2+ ch annel, also increased [Ca2+](i). This effect could not be enhanced by furth er addition of nifedipine, suggesting that both DHPs act via a common signa lling pathway. 5. Based on the specific mechanism of the skeletal muscle E-C coupling, we propose the stabilization of a conformational state of the DHP receptor by DHPs, which is sufficient to activate the ryanodine receptor.