[An observation on the pacemaker current I(f) in sheep cardiac Purkinje fibers under ischemia-mimic condition]

Effects of "ischemia" and adrenergic agonists on pacemaker current I(f) were observed in sheep cardiac Purkinje fibers. After perfusion with ischemia-mimic solution for 15, 30 and 60 min, the amplitude of I(f) current were decreased at all membrane potential levels between Ec-60 mV to -120 mV (n = 7, P < 0.05), activation time and half activation time to a steady state value of I(f) current were prolonged (n = 7, P < 0.05), activation curve of I(f) shifted to a more hyperpolarized position. Isoproterenol 1 x 10(-6) mol/L increased amplitude of I(f) current (n = 10, P < 0.05), shortened the activation time and half activation time (n = 10, P > 0.05), and shifted the activation curve to a more depolarized position; but isoproterenol of 1 x 10(-6) mol/L could not completely reverse the inhibitory effects of "ischemia" on I(f) current. In the presence of 5 x 10(-7) mol/L propranolol the effects of phenylephrine 5 x 10(-5) mol/L on I(f) current was variable; but aggravated the inhibitory effects of I(f) current due to "ischemia". The above results indicate that normal pacemaker activity of ventricular Purkinje fibers does not increase but rather decrease during myocardial ischemia-mimic condition, an observation suggesting that acute ischemic ventricular arrhythmia may not be due to abnormal strengthening of normal ventricular pacemaker activity.