PKG, and not PKA, mediates heparin inhibition of DNA synthesis. A7r5 (A, n = 3) and porcine (B, n = 3) VSMC were treated as in Figure 1. In these assays, some cells were treated with 100 μM 8-Br-cAMP, 100 μM IBMX, and/or Rp-8-pCPT-cAMS at 2 μM. Heparin, 8-Br-cGMP, 8-Br-cAMP and IBMX blocked PDGF-stimulated BrdU incorporation (p < 0.0001). Inhibition of PKA with Rp-8-pCPT-cAMS blocked the effects of 8-Br-cAMP (p < 0.0001). However, only Rp-8-pCPT-cGMS completely blocked the effects of heparin and 8-Br-cGMP (p < 0.0001). Interestingly, Rp-8-pCPT-cAMS did have a partial effect on 8-Br-cGMP in the porcine cells (-*). Both inhibitors decreased the effects of IBMX (p < 0.0001).