Comparison of subsarcolemmal and bulk calcium concentration during spontaneous calcium release in rat ventricular myocytes.

Abstract

1. The aim of these experiments was to compare the time course of changes in intracellular Ca2+ concentration ([Ca2+]i) measured in the bulk cytoplasm with those estimated to occur near the sarcolemma. Sarcolemmal Na(+)-Ca2+ exchange current and [Ca2+]i were measured in single, voltage-clamped ventricular myocytes. 2. Spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) resulted in a transient inward current. This current developed and decayed more quickly than the accompanying changes in [Ca2+]i (measured with indo-1) resulting in a hysteresis between [Ca2+]i and current. A similar hysteresis was also observed if [Ca2+]i was elevated with caffeine and was removed if the current was low pass filtered with a time constant of 132 ms. 3. Digital video imaging (using fluo-3 or calcium green-1 to measure [Ca2+]i) allowed measurement of [Ca2+]i at all points in the cell during the wave of spontaneous Ca2+ release. The hysteresis between [Ca2+]i and current remained, even after allowing for the spatial and temporal properties of this wave. 4. The hysteresis can be accounted for if there is a barrier to diffusion of Ca2+ ions separating the bulk cytoplasm from the space under the sarcolemma (into which Ca2+ is released from the sarcoplasmic reticulum). The calculated subsarcolemmal [Ca2+] rises and falls more quickly (and reaches a higher peak) than does the bulk [Ca2+]. The delay introduced by this barrier is equivalent to a time constant of 133 ms. 5. The subsarcolemmal space described in this paper may be equivalent to the 'fuzzy space' previously suggested to be important in controlling SR Ca2+ release.

Intracellular calcium and Na+‐Ca2+ exchange current in isolated toad pacemaker cells

“…Figure 9 shows a simultaneously recorded Ca¥ transient and inward current, both recorded with a 10 Hz low-pass filter. Note that the caffeine-induced inward current precedes the [Ca¥]é by about 100 ms as noted previously in other cell types (Callewaert et al 1989;Trafford, Diaz, O'Neill & Eisner, 1995). Callewaert et al (1989) suggested that the lag was caused by the slow kinetics of indo_1; however the delays due to indo_1 are only of the order of 5-10 ms (Westerblad & Allen, 1996) and we think it more likely that the delay is because the inward current is triggered by the rise of [Ca¥]é close to the surface membrane whereas it takes some time for caffeine to diffuse to the interior of the cell and trigger Ca¥ release there (Trafford et al 1995).…”

Functional analysis of Na+/K+-ATPase isoform distribution in rat ventricular myocytes

“…In cardiac myocytes, NCX is mostly located in the TT (6,10) and is preferentially modulated by Ca 2ϩ released from the SR, as indicated by functional measurements (23,26). We found that NKA-␣2 function is concentrated in the TT vs. external sarcolemma, with the amount of NKA-␣1 and NKA-␣2 in the TT being comparable.…”

Intracellular Na + Concentration Is Elevated in Heart Failure But Na/K Pump Function Is Unchanged

“…Actual Ca 2ϩ transient from control was used 19 ; for HF, the peak was reduced by 30%. 4 ] i , 23 we approximated [Ca 2ϩ ] sm by using the procedure described by Weber et al 24 (Figure 5B ] sm may be elevated transiently as the result of Na ϩ influx through Na ϩ channels or NCX, favoring more outward I NCX .…”