Effect on the indo‐1 transient of applying Ca2+ channel blocker for a single beat in voltage‐clamped guinea‐pig cardiac myocytes.

1. We used rapid solution changes to investigate the mechanisms which trigger Ca2+ release from the sarcoplasmic reticulum (SR) in guinea‐pig ventricular myocytes. We patch‐clamped myocytes at 36 degrees C and used indo‐1 to monitor intracellular Ca2+. Before each test pulse, we established a standard level of SR Ca2+ load by applying a train of conditioning pulses. 2. We switched rapidly to 32 microM nifedipine (an L‐type Ca2+ current (ICa,L) blocker) 8 s before a test pulse, and just after applying nifedipine we applied a ramp depolarization to pre‐block Ca2+ channels. We found that ICa,L elicited by the following test pulse was inhibited almost completely (98‐99% inhibition). 3. The indo‐1 transient elicited by an 800 ms depolarizing pulse showed a rapid initial rise which was inhibited by ryanodine‐thapsigargin. This indicated that the rapid rise was due to Ca2+ release from the SR, and therefore provides an index of SR Ca2+ release. 4. In cells dialysed internally with 10 mM Na(+)‐containing solution, nifedipine application before a +10 mV test pulse blocked 62% of the rapid initial phase of the indo‐1 transient. Calibration curves of indo‐1 for intracellular Ca2+ (using a KD of indo‐1 for Ca2+ of either 250 or 850 nM, the reported range) indicated that between 67 and 76% of the Ca2+i transient was inhibited by nifedipine. Thus, in cells dialysed with 10 mM Na+ and depolarized to +10 mV, and in the absence of ICa,L, this suggests that another trigger mechanism for SR release is able to trigger between 33 and 24% of the Ca2+i transient. 5. For a given dialysing Na+ concentration, the fraction of indo‐1 transient which was inhibited by nifedipine decreased as test potential became more positive. In cells dialysed with 10 mM Na+ and pulsed to +110 mV, 24% of the rapid phase of the indo‐1 transient was inhibited by nifedipine (equivalent to between 27 and 37% of the Ca2+i transient). 6. For a given test potential, the fraction of the indo‐1 transient which was inhibited by nifedipine decreased as dialysing Na+ concentration increased. In cells dialysed with Na(+)‐free solution and pulsed to +10 mV, 84% of the indo‐1 transient was inhibited by nifedipine (equivalent to between 88 and 91% of the Ca2+i transient). In contrast, in cells dialysed with 20 mM Na+ and pulsed to +10 mV, 41% of the indo‐1 transient was inhibited by nifedipine (equivalent to between 47 and 57% of the Ca2+i transient). 7. Dialysing cells with different Na+ concentrations could lead to a different SR Ca2+ content. We therefore manipulated the conditioning train before each test pulse to change the extent of SR loading. For each dialysing Na+ concentration, we found no change in the degree to which nifedipine blocked the indo‐1 transient when SR content was either increased or decreased. 8. The results support the idea that both ICa, L and a second mechanism are able to trigger SR release and the resulting Ca2+i transient. When ICa, L was blocked with nifedipine, the fraction of Ca2+i transient which remained increased with more positive test potential and higher internal Na+. This is consistent with the hypothesis that the second SR trigger mechanism is Ca2+ entry via reverse Na(+)‐Ca2+ exchange, elicited by a step change in membrane potential.