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1 Institute of Molecular Physiology and Genetics, Slovak Academy of Sciences, Bratislava, Slovakia (Slovak Republic)
2 Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, TX, USA
* To whom correspondence should be addressed. E-mail: ivan.zahradnik{at}savba.sk.
In mammalian cardiac myocytes, calcium released into the dyadic space rapidly inactivates ICa. We used this Ca2+ release-dependent inactivation of ICa (RDI) as a local probe of SR Ca release activation. In whole-cell patch clamped rat ventricular myocytes, Ca2+ entry induced by short prepulses from -50 mV to positive voltages caused suppression of peak ICa during a test pulse. The negative correlation between peak ICa suppression and ICa inactivation during the test pulse indicated that RDI evoked by the prepulse affected only calcium channels in those dyads, in which calcium release was activated. Ca2+ ions injected during the prepulse and during the subsequent tail current suppressed peak ICa in the test pulse to a different extent. Quantitative analysis indicated that equal Ca2+ charge was 3.5x less effective in inducing release when entering during the prepulse than when entering during the tail. Tail Ca2+ charge injected by the first DHPR openings was 3x less effective than that injected by DHPR reopenings. These findings suggest that calcium release activation can be profoundly influenced by the recent history of L-type Ca channel activity due to potentiation of ryanodine receptors (RyRs) by previous calcium influx. This conclusion was confirmed at the level of single RyRs in planar lipid bilayers: using flash photolysis of the calcium cage NP-EGTA to generate two sequential calcium stimuli, we showed that RyR activation in response to the second stimulus was 4x higher than that in response to the first stimulus.
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