Mechanisms of adrenergic control of sino-atrial node discharge

Abstract

Among the mechanisms proposed for the increase in discharge of sino-atrial node (SAN) by norepinephrine (NE) are an increase in the hyperpolarization-activated current I_f and in the slow inward current I_Ca,L. If I_f is the primary mechanism, cesium (a blocker of I_f) should eliminate the positive chronotropic effect of NE. If I_Ca,L, is involved, [Ca²⁺]_o should condition NE effects. We studied the electrophysiological changes induced by NE in isolated guinea pig SAN superfused in vitro with Tyrode solution (both SAN dominant and subsidiary pacemaker mechanisms are present) as well as with high [K⁺]_o, higher Cs⁺ or Ba²⁺ (only the dominant pacemaker mechanism is present). In Tyrode solution, NE (0.5-1µM) increased the SAN rate and adding Cs⁺ (∼ 12 mM) caused a decaying voltage tail during diastole in subsidiary pacemakers. NE enhanced the Cs⁺-induced tail, and increased the rate but less than in Tyrode solution. In higher [Cs⁺]_o (15–18 mM), Ba²⁺ (1 mM) or Ba²⁺ plus Cs⁺ (10 mM) dominant action potentials (not followed by a tail) were present and NE accelerated them as in Tyrode solution. In high [K⁺]_o, NE increased the rate in the absence and presence of Cs⁺, Ba²⁺ or Ba²⁺ plus Cs⁺. In these solutions, NE increased the overshoot and maximum diastolic potential of dominant action potentials (APs) and increased the rate by steepening diastolic depolarization and shifting the threshold for upstroke to more negative values. High [Ca²⁺]_o alone increased the rate and NE enhanced this action, whereas low [Ca²⁺]_o reduced or abolished the increase in rate by NE. In SAN quiescent in high [K⁺]_o plus indapamide, NE induced spontaneous discharge by decreasing the resting potential and initiating progressively larger voltage oscillations. Thus, NE increases the SAN rate by acting primarily on dominant APs in a manner consistent with an increase of I_Ca,L and I_K and under conditions where I_f is either blocked or not activated. NEinitiates spontaneous discharge by inducing voltage oscillations unrelated to I_f.