Adrenergic response modulates guinea pig M cell action potential. At 1000 ms basic cycle length, isoproterenol (1 μM) shortens APD (A). (B) Action potential duration measured at 90% repolarization (APD90) under different BCLs: 300, 500, 800, 1000, and 2000 ms. Isoproterenol significantly decreased APD90 at 800 and 1000 ms. Phase 0 amplitude (APA) shows significant increment in the presence of isoproterenol (1 μM), under different BCLs (C). Values are mean ± SEM, n = 4. *P < 0.05 and **P < 0.005.