Figure 2. Effects on adenylyl cyclase activity of isoproterenol (n = 6), Gpp(NH)p (n = 6), and forskolin (n = 6) in the presence of MnCl²without and with halothane in human myocardial membranes. Ordinate: maximal increase in adenylyl cyclase activity in picomoles cyclic adenosine monophosphate (cAMP) per milligram protein x 20 min. Abscissa: studied conditions. Assays were performed in the absence of MgCl². Basal adenylyl cyclase activities in control (+ halothane) in picomoles cAMP per milligram protein x 20 min were as follows: isoproterenol: 1,158 plus/minus 190 (1,075 plus/minus 110); Gpp(NH)p: 586 plus/minus 78 (749 plus/minus 86); and forskolin: 745 plus/minus 88 (758 plus/minus 140)