INHIBITION OF PACEMAKER CURRENT BY THE BRADYCARDIAC AGENT ZD-7288 IS LOST USE-DEPENDENTLY IN SHEEP CARDIAC PURKINJE-FIBERS

The inhibition of the pacemaker current (i(f)) in sheep cardiac Purkin je fibres by ZD 7288 nylamino)-1,2-dimethyl-6-(methylamino)pyrimidiniu m chloride] is lost use-dependently. This disinhibition of i(f) was in vestigated by using the two-microelectrode voltage-clamp technique. Th e pulse protocol consisted of a rest period (holding potential of abou t -50 mV, 1-10 mu mol/l ZD 7288) followed by a train of test pulses (p otential negative to -100 mV, stimulation frequency 0.05 Hz). At the b eginning of the first test pulse there was an immediate reduction of i (f) but inhibition was lost during continued stimulation. Activation o f i(f) is sigmoidal and the early delay in current activation was prol onged from 33 ms (no ZD 7288) to 424 ms (10 mu mol/l ZD 7288). Therefo re hardly any disinhibition occurred during short test pulses (0.5 s). During longer test pulses (5 s, -120 mV, 10 mu mol/l) disinhibition d eveloped with a time constant of about 2 s. The inhibition of i(f) by ZD 7288 was lost voltage-dependently. With 10 mu mol/l ZD 7288 the hal f-maximal disinhibition occurred at -92 mV and the slope factor of the disinhibition/voltage curve (Boltzmann relation) was 4.8 mV. The volt age-dependent disinhibition could be abolished largely by extracellula r application of protease (0.5 mg/ml, 7 min). After prior disinhibitio n, reinhibition at the holding potential (about -50 mV) followed a bi- exponential time course indicating that inhibition may be produced by a fast (tau=0.7 min) and a slow component (tau=20-30 min). Increasing ZD 7288 concentration from 1 to 10 mu mol/l accelerated reinhibition, mainly by an increase of the amplitude (A) of the fast component. The ratio A(fast)/A(slow) was 0.399 at 1 mu mol/l and 2.65 at 10 mu mol/l ZD 7288. The reinhibition of i(f) was unchanged by shifting the holdin g potential from -50 mV to -20 mV. Trials to wash out the effects of 1 0 mu mol/l ZD 7288 gave two results. The inhibition of i(f) was slight ly reversed after a wash-out of 1.5 h with drug-free solution. A secon d effect of the drug, the fast reinhibition, could be completely remov ed by washout. In summary if is inhibited by ZD 7288 at membrane poten tials at which the virtual i(f) gate is closed. Disinhibition occurs d uring long-lasting hyperpolarization but will hardly be operative in u nclamped fibres under physiological conditions.