Effects of methoxamine and phenylephrine on the action potential and the membrane currents in spontaneously beating rabbit sino-atrial node cells were examined by means of a two-microelectrode voltage-clamp technique. Both methoxamine and phenylephrine (10(-4) mol/l) prolonged the cycle length (CL) and the action potential duration (APD), significantly. At concentrations higher than 3 x 10(-4) mol/l, phenylephrine increased the maximum rate of rise of action potential (Vmax) but methoxamine reduced it. Both agents depolarized the maximum diastolic potential (MDP). These changes in the action potential parameters occurred in a concentration-dependent manner. In the presence of phentolamine (10(-5) mol/l), methoxamine (3 x 10(-4) mol/l) did not modify the action potential parameters. Also, phenylephrine did not affect them during exposure to phentolamine (10(-5) mol/l) and pindolol (10(-7) mol/l). In voltage-clamp experiments, at 10(-3) mol/l both methoxamine and phenylephrine slightly increased the slow inward current (Isi), but decreased the time-dependent outward current (Ik). The steady-state activation variable of Ik (p infinity) was unaffected by these agents. The hyperpolarization-activated current (Ih) was suppressed in the presence of methoxamine, but enhanced in the presence of phenylephrine. An additional application of pindolol (10(-7) mol/l) during exposure to phenylephrine (10(-3) mol/l) depressed the action potential amplitude (APA) and Vmax, and prolonged CL slightly. Under the same condition, all the membrane currents (Isi, Ik and Ih) were decreased. In addition, the time courses of decay for Isi were not modified in the absence and the presence of phenylephrine (10(-3) mol/l) and phenylephrine plus pindolol (10(-7) mol/l).(ABSTRACT TRUNCATED AT 250 WORDS)