Key numbers for cAMP compartmentation
| Biological parameters | Values |
| Length scales | Adult cardiac myocyte, 120 µm; HEK293, 16 µm; endothelial cell, 40 µm; neuronal dendrite length/width, 100/3 µm; cAMP hydrodynamic radius, 1.4 nm |
| Concentrations | Free cAMP, 0.1–5 µM; type I PKA, 0.59 µM; type II PKA, 25 nM; Saucerman et al., 2003; CNG channels, 40 nM; Rich et al., 2001 |
| Diffusion coefficients for cAMP | Aqueous solution, 444 µm2 s−1; Dworkin and Keller, 1977; CHO cell, 487 µm2 s−1 (v = 40 µm s−1); Nikolaev et al., 2004; Cardiac myocyte, 136 µm2 s−1 (v = 16 µm s−1); Nikolaev et al., 2006. Physical barriers between membrane and cytosol: 0.5 µm2 s−1 over 1 µm; Rich et al., 2000; 2 µm2 s−1 over 2.5 µm; Saucerman et al., 2006 |
| Rate constants | |
| AC: | kcat-AC, 12–60 s−1; Rich et al., 2000; 8.5 s−1; Saucerman et al., 2003 |
| 20 s−1; Feinstein et al., 2012; | |
| Vmax-AC: cardiac myocyte, 0.1–0.4 µM s−1; Saucerman et al., 2003, 2006; endothelial cell, 0.14 µM s−1; | |
| Feinstein et al., 2012; HEK293, 0.13 µM s−1; Rich et al., 2001 | |
| PDE: | kcat-PDE3: 3.5 s−1; kcat-PDE4: 5 s−1; Saucerman et al., 2004; Vmax-PDE: cardiac myocyte, 0.2–0.4 µM s−1; |
| Saucerman et al., 2003; endothelial cell, 0.3 µM s−1; Feinstein et al., 2012; neuron, 3.4 µM s−1; Neves et al., 2008 | |
| MRP4: | k-MRP4: 10−4 to 10−3 s−1; Cheepala et al., 2013 |
| Back-of-the-envelope calculations | Time for cAMP to diffuse across a cell: |
| Cardiac myocyte: | |
| Aqueous diffusion rate: 4 s | |
| Cytosolic diffusion rate: 13 s | |
| HEK293: | |
| Aqueous diffusion rate: 0.07 s | |
| Time for cAMP to diffuse from membrane to cytosol in cardiac myocyte: 1.6 s | |
| Time to degradation of cAMP by PDE: 0.5–50 s | |
| Distance for cAMP diffusion before degradation by PDE: | |
| Aqueous diffusion rate: sqrt(2*444*tD) = 21–210 µm | |
| Cardiac myocyte cytosol: 12–117 µm | |
| Restricted diffusion cardiac myocyte = 1–10 µm | |
| CHO cytosol: 22–220 µm |
| Biological parameters | Values |
| Length scales | Adult cardiac myocyte, 120 µm; HEK293, 16 µm; endothelial cell, 40 µm; neuronal dendrite length/width, 100/3 µm; cAMP hydrodynamic radius, 1.4 nm |
| Concentrations | Free cAMP, 0.1–5 µM; type I PKA, 0.59 µM; type II PKA, 25 nM; Saucerman et al., 2003; CNG channels, 40 nM; Rich et al., 2001 |
| Diffusion coefficients for cAMP | Aqueous solution, 444 µm2 s−1; Dworkin and Keller, 1977; CHO cell, 487 µm2 s−1 (v = 40 µm s−1); Nikolaev et al., 2004; Cardiac myocyte, 136 µm2 s−1 (v = 16 µm s−1); Nikolaev et al., 2006. Physical barriers between membrane and cytosol: 0.5 µm2 s−1 over 1 µm; Rich et al., 2000; 2 µm2 s−1 over 2.5 µm; Saucerman et al., 2006 |
| Rate constants | |
| AC: | kcat-AC, 12–60 s−1; Rich et al., 2000; 8.5 s−1; Saucerman et al., 2003 |
| 20 s−1; Feinstein et al., 2012; | |
| Vmax-AC: cardiac myocyte, 0.1–0.4 µM s−1; Saucerman et al., 2003, 2006; endothelial cell, 0.14 µM s−1; | |
| Feinstein et al., 2012; HEK293, 0.13 µM s−1; Rich et al., 2001 | |
| PDE: | kcat-PDE3: 3.5 s−1; kcat-PDE4: 5 s−1; Saucerman et al., 2004; Vmax-PDE: cardiac myocyte, 0.2–0.4 µM s−1; |
| Saucerman et al., 2003; endothelial cell, 0.3 µM s−1; Feinstein et al., 2012; neuron, 3.4 µM s−1; Neves et al., 2008 | |
| MRP4: | k-MRP4: 10−4 to 10−3 s−1; Cheepala et al., 2013 |
| Back-of-the-envelope calculations | Time for cAMP to diffuse across a cell: |
| Cardiac myocyte: | |
| Aqueous diffusion rate: 4 s | |
| Cytosolic diffusion rate: 13 s | |
| HEK293: | |
| Aqueous diffusion rate: 0.07 s | |
| Time for cAMP to diffuse from membrane to cytosol in cardiac myocyte: 1.6 s | |
| Time to degradation of cAMP by PDE: 0.5–50 s | |
| Distance for cAMP diffusion before degradation by PDE: | |
| Aqueous diffusion rate: sqrt(2*444*tD) = 21–210 µm | |
| Cardiac myocyte cytosol: 12–117 µm | |
| Restricted diffusion cardiac myocyte = 1–10 µm | |
| CHO cytosol: 22–220 µm |
Rate constants were obtained from example models as determined from biochemical experiments. Diffusion calculations performed using the solution to the 1-D diffusion equation, <x2> = 2Dt, where <x2> is the mean-squared distance traveled, D is the diffusion coefficient, and t is time (Codling et al., 2008). Using a linear approximation of cAMP degradation by PDE, the time to degradation was calculated by tdeg = [cAMP]/Vmax − PDE. The time to degradation was then used in the 1-D diffusion equation to determine the diffusion distance before degradation. Note that the range of action of cAMP is predicted to increase with increasing [cAMP] and decrease with increasing [PDE].