Calcium transients which accompany the activation of sodium current in rat ventricular myocytes at 37 degrees C: a trigger role for reverse Na-Ca exchange activated by membrane potential?

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We investigated the role of the fast sodium current (INa) in triggering Ca release from the sarcoplasmic reticulum (SR), using adult rat left ventricular myocytes, loaded with Fura-2 to measure intracellular Ca (Cai), which were whole-cell patch-clamped at 35-37 degrees C. Before each test pulse, a series of 400-ms conditioning pulses to +10 mV were applied to establish a constant level of SR Ca load. Pulses were applied every 15 s. A test pulse from -80 mV to -50 mV elicited a rapid INa and a phasic Cai transient. When the solution perfusing a myocyte was rapidly switched for 15 s before a test pulse to one containing the L-type Ca channel blocker nifedipine (20 microM), the test pulse still activated INa and a phasic Cai transient, the amplitude of which was not significantly different from control (P > 0.05; t-test). When a rapid switch to 20 microM nifedipine plus 30 microM tetrodotoxin (TTX) was made 15 s before a test pulse, both INa and the Cai transient were completely abolished (n = 6). When a switch was made to Na-free (Li) solution, which contained 20 microM nifedipine to block L-type Ca current, ICa,L, there was no significant difference in the Cai transient amplitude from that of control (P > 0.05; n = 6). Brief depolarising test pulses (-80 mV to +20 mV, 10 ms duration) to simulate membrane potential escape also elicited a Cai transient which attained 90.0% (+/-2.8%; n = 7) of the Cai transient activated by a conditioning pulse to +10 mV. The Cai transient with a brief pulse was not significantly affected by application of 20 microM nifedipine (P > 0.05), but adding TTX with nifedipine reduced the Cai transient amplitude to 76.9% (+/-6.8%; P < 0.02; n = 8). In four cells, the Cai transient remaining in the presence of nifedipine plus TTX was abolished by adding 5 mM Ni. These data are consistent with voltage escape during activation of INa leading to a trigger Ca entry via a mechanism other than L-type Ca channels or subsarcolemmal Na accumulation with reverse Na-Ca exchange. The block by Ni of the Cai transient suggests that a brief membrane potential escape might directly activate reverse mode Na-Ca exchange to trigger SR release, and this mechanism would seem to account largely for the Cai transient which accompanies INa in rat myocytes, under these experimental recording conditions.