| dc.description | In mammalian heart, contractions are initiated by release of intracellular Ca2+ stores from the sarcoplasmic reticulum (SR). Ca 2+ release from the SR is triggered by influx of L-type Ca 2+ current (ICa-L) across the sarcolemma. This process is called Ca2+-induced Ca2+ release (CICR). The gain of CICR (Ca2+ released per Ca2+ influx) varies with membrane potential. When cells are partially depolarized with voltage clamp techniques, CICR gain is low and contractions are proportional to the magnitude of ICa-L. However, in intact cells at normal resting potentials CICR exhibits high gain, and large responses can be elicited at potentials where ICa-L is very small. Ca2+ channel blockers inhibit ICa-L and exert a strong negative inotropic effect on the heart. However, it is uncertain whether this effect is mediated by inhibition of the trigger for CICR, depletion of SR Ca2+ stores, or both. Furthermore, the mechanisms by which organic and inorganic Ca 2+ channel blockers inhibit ICa-L differ. Thus, the mechanism by which organic and inorganic blockers exert their negative inotropic effects also may differ. Therefore, the objective of this study was to investigate and compare the cellular bases for the negative inotropic effects of the organic Ca2+ channel blocker, nifedipine, and the inorganic Ca 2+ channel blocker, cadmium (Cd2+), in isolated guinea pig ventricular myocytes. Cell shortening was measured with an edge detector and Ca2+ transients were measured with the Ca2+ sensitive fluorescent dye, fura-2 at 37°C. Sequential test steps from -65 to -40 mV and from -40 to 0 mV were used to activate contractions elicited by high and low gain CICR, respectively. In voltage clamped cells, block of ICa-L by 20 muM nifedipine strongly inhibited contractions elicited by high and low gain CICR. Surprisingly, this effect was accompanied by an increase in SR Ca2+ stores, assessed with 10 mM caffeine. As well, in cells activated by action potentials (AP), 20 muM nifedipine strongly inhibited contractions and Ca2+ transients without significantly changing SR Ca2+ stores. In cells activated by APs, 100 muM Cd2+ also rapidly abolished contractions and Ca2+ transients without significantly altering SR Ca2+ stores. Cd2+ also significantly shortened AP duration (measured at 90% repolarization) from 150 to 60 msec. In contrast, in voltage clamped myocytes, block of ICa-L by 100 muM Cd 2+ initially inhibited contractions and Ca2+ transients elicited by low gain CICR but not those elicited by high gain CICR. (Abstract shortened by UMI.) | en_US |
