A, action potentials of a representative cell at 0·1 Hz before and after exposure to 10 μM phenylephrine (in the presence of 1 μM propranolol). B, action potentials of a different cell in the presence of 1 mM TEA, before and after exposure to 10 μM phenylephrine (in the presence of 1 μM propranolol). C, action potentials obtained from one cell before and after 1 μM isoproterenol (Iso). D, action potentials obtained from a cell exposed first to 1 mM TEA and then 1 mM TEA plus 1 μM isoproterenol.