Figure 4.

Cytoplasmic cAMP diffusivity is not changed by cytoplasmic acidification. (A) Experiment similar to that shown in Figure 1B repeated on myocytes exposed to 80 mM acetate to acidify cytoplasm uniformly (n = 7/3). 5-(N,N)-dimethylamiloride (DMA; 30 µM) was included in both microstreams to inhibit pH regulation by Na⁺/H⁺ exchanger 1. (B) Model fit to experimental data, with best-fit D_cAMP = 41 ± 12.2 µm²/s confirming slow diffusivity. (C) Experiment shown in Figure 4A repeated, but with acetate in the distal microstream only; DMA (30 µM) was included in both microstreams (n = 6/3). This protocol produces a pH-gradient overlying the gradient of β-adrenoceptor activation; higher pH in ISO-containing microstream favours greater cAMP production. (D) Model fit to experimental data with best-fit D_cAMP = 32 ± 4.8 µm²/s confirming slow diffusivity. To illustrate the goodness-of-fit data, longitudinal profiles in Bii and Dii were also simulated for a four-fold lower (dotted lines) and a four-fold higher (dashed lines) D_cAMP.