AJP - Heart Calcium Transients and Cell-Sarcomere
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Am J Physiol Heart Circ Physiol 276: H1178-H1189, 1999;
0363-6135/99 $5.00
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Vol. 276, Issue 4, H1178-H1189, April 1999

BAY K 8644 modifies Ca2+ cross signaling between DHP and ryanodine receptors in rat ventricular myocytes

Satomi Adachi-Akahane, Lars Cleemann, and Martin Morad

Institute for Cardiovascular Sciences and Department of Pharmacology, Georgetown University Medical Center, Washington, District of Columbia 20007

The amplification factor of dihydropyridine (DHP)/ryanodine receptors was defined as the amount of Ca2+ released from the sarcoplasmic reticulum (SR) relative to the influx of Ca2+ through L-type Ca2+ channels in rat ventricular myocytes. The amplification factor showed steep voltage dependence at potentials negative to -10 mV but was less dependent on voltage at potentials positive to this value. In cells dialyzed with 0.2 mM cAMP in addition to 2 mM fura 2, the Ca2+-channel agonist (-)-BAY K 8644 enhanced Ca2+-channel current (ICa), shifted the activation curve by -10 mV, and significantly delayed its inactivation. Surprisingly, BAY K 8644 reduced the amplification factor by 50% at all potentials, even though the caffeine-releasable Ca2+ stores were mostly intact at holding potentials of -90 mV. In contrast, brief elevation of extracellular Ca2+ activity from 2 to 10 mM enhanced both ICa and intracellular Ca2+ transients in the absence or presence of BAY K 8644 but had no significant effect on the amplification factor. BAY K 8644 abolished the direct dependence of the rate of inactivation of ICa on the release of Ca2+ from the SR. These findings suggest that the gain of the Ca2+-induced Ca2+ release in cardiac myocytes is regulated by the gating kinetics of cardiac L-type Ca2+ channels via local exchange of Ca2+ signals between DHP and ryanodine receptors and that BAY K 8644 suppresses the amplification factor through attenuation of the Ca2+-dependent inactivation of Ca2+ channels.

dihydropyridine; amplification factor; calcium-induced calcium release; calcium channel; inactivation of calcium channel; cardiac excitation-contraction coupling


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