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

We investigated the role of the fast sodium current (I-Na) in triggering Ca release from the sarcoplasmic reticulum (SR), using adult rat left ventricular myocytes, loaded with Fura-2 to measure intracellular Ca (Ca-i), 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 I-Na and a phasic Ca-i 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 mu M), the test pulse still activated I-Na and a phasic Ca-i transient, the amplitude of which was not significantly different from control (P > 0.05; t-test). When a rapid switch to 20 mu M nifedipine plus 30 mu M tetrodotoxin (TTX) was made 15 s before a test pulse, both I-Na and the Ca-i transient were completely abolished (n = 6). When a switch was made to Na-free (Li) solution, which contained 20 CIM nifedipine to block L-type Ca current, I-Ca,I-L, there was no significant difference in the Ca-i transient amplitude from that of control (P > 0.05; It 6). Brief depolarising test pulses (-80 mV to +20 mV, 10 ms duration) to simulate membrane potential escape also elicited a Ca-i transient which attained 90.0% (+/-2.8%; n = 7) of the Ca-i transient activated by a conditioning pulse to +10 mV. The Cai transient with a brief pulse was not significantly affected by application of 20 mu M nifedipine (P > 0.05), but adding TTX with nifedipine reduced the Ca-i transient amplitude to 76.9% (+/-6.8%, P < 0.02; n=8). In four cells, the Ca-i 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 I-Na 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 Ca-i 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 Ca-i transient which accompanies I-Na in rat myocytes, under these experimental recording conditions.