Effect of cAMP and cGMP on inactivation of cAS-PDE by 8-BDB-TcAMP. The enzyme was preincubated with 20 mmol/L cAMP (□) or cGMP (20 mmol/L) (▵) or with buffer (○) for 2 minutes, then 5 mmol/L 8-BDB-TcAMP was added and timed aliquots were taken, and diluted 1,000-fold in 50 mmol/L HEPES buffer pH 7.5, 5 mmol/L MgCl₂ . The diluted mixture (5 μL) was assayed for activity in triplicate. The data is presented as percent of control activity (buffer).