Table 1. Concentration and properties of the major cellular buffers in ventricular myocytes.
| Concentration | Kd | On rate | |
|---|---|---|---|
| μmol/L cell cytoplasm | μM | M-1.s-1 | |
| Ca buffers | |||
| Troponin C (regulatory) 9, 10 | 70 | 0.6 | 3.27x107 |
| SERCA11 | 47 | 0.6 | †1.00 x108 |
| Calmodulin3 | 24 | 7 | 3.4 x107 |
| Sarcolemma (low)12 | 42 | 13 | †1.0 x108 |
| Sarcolemma (high) 12 | 15 | 0.3 | †1.0 x108 |
| Ca/Mg buffers | |||
| Troponin C (Mg) 9, 13 | 140 | 0.0195 | 3.3 x103 |
| Myosin9 | 140 | 4.62 | 9.6 x104 |
| ATP14 | 5000 | 1200 | 1.9 x106 |
| Histidyl dipeptides15, 16 | 20000 | 1000 | 6.1 x106 |
| Chemical probes | |||
| BAPTA17 | 0.178 | 5.0 x108 | |
| EGTA18 (pH7.2) | 0.180 | 2.3 x106 | |
The cellular buffers are organized in two groups: top, Ca buffers; middle Ca/Mg buffers. The properties of two chemical agents (BAPTA & EGTA) are shown below. For the cellular buffers, the first two columns show the buffer concentration and its Kd. The third column gives the value of the on rate constant for Ca2+ binding to the buffer. This has been calculated under standard conditions (initial free [Ca2+] 100 nM, addition of 10 μM Ca2+ to 10 μM buffer). The value for calmodulin is an approximation to one binding site 11. For the Mg/Ca buffers the Kd and rate constant have been corrected for the apparent value in the presence of a cellular Mg2+ of 0.5 mM.
†
Indicates rate constant was estimated based on diffusion limit.