Inhibition of the hyperpolarization-activated current (i f) of rabbit SA node myocytes by niflumic acid

The effects of the amphiphilic substance niflumic acid (NFA) were examined in myocytes isolated from the sino-atrial node of the rabbit heart. NFA (50 and 500 μM), for 30–60 s, produced a reversible negative chronotropic effect by reducing the rate of diastolic depolarization, suggesting an inhibitory effect on the hyperpolarization-activated “pacemaker” current (i _f). NFA (from 0.05 to 500 μM) inhibited i _f by modifying the current kinetics, without alteration of the conductance. This was shown by evidence indicating that: (1) NFA inhibited i _f during hyperpolarizing pulses to the mid-point of i _f activation but not at fully activating voltages; (2) the slope and reversal potential of the fully activated current/voltage (I/V) relation were not altered by NFA, indicating no change in slope conductance or ion selectivity; and (3) hyperpolarizing ramp protocols confirmed the lack of action of 50 μM NFA on the fully activated current and a shift of approximately −8 mV. Although similar to inhibition by acetylcholine (ACh), inhibition by NFA was only partly additive with the action of ACh and was not altered by atropine or pertussis toxin, both of which eliminated the action of ACh. The effect of NFA was present after stimulation of adenylate cyclase by forskolin and after inhibition of phosphodiesterase by isobutylmethylxanthine (IBMX). In cell-attached patch measurements, NFA applied externally did not affect i _f measured in the patch. Finally, application of NFA to the cytoplasmic side of excised patches did not alter the current in the absence or presence of adenosine 3′,5′-cyclic monophosphate (cAMP). These results suggest an external, membrane-delimited action of NFA on i _f. Received: 15 June 1995/Received after revision: 26 September 1995/Accepted: 28 September 1995