Modulatory effects of FMRF-NH sub(2) on outward currents and oscillatory activity in heart interneurons of the medicinal leech

Using single-electrode voltage clamp, heart interneurons of the medicinal leech were shown to possess both a rapidly inactivating outward current, I sub(A), and a more slowly inactivating outward current, I sub(K) I sub(A) and I sub(K) could be separated by their voltage sensitivity and kinetic properties. FMRF-NH sub(2) (Phe-Met-Arg-Phe-NH sub(2)) modulates I sub(K) by shifting both steady state activation and inactivation to more hyperpolarized potentials, but it does not affect the time constants. FMRF-NH sub(2) produces two apparently dissimilar effects on the heartbeat rhythm--acceleration and disruption. We suggest that both effects could result from the hyperpolarizing shifts in steady state activation and inactivation of I sub(K).