Basal responses of the L-type Ca2+ and hyperpolarization-activated currents to autonomic agonists in the rabbit sino-atrial node

1. The dose dependence of the cholinergic agonist acetylcholine (ACh) and the beta-adrenergic agonist isoprenaline (Iso) were determined for the hyperpolarization-activated current (I-f) and the L-type Ca2+ current (I-Ca,I-L) in single cells isolated from the rabbit sino-atrial (SA) node. 2. ACh inhibited I-f by a negative shift of its activation curve with a maximal effect of - 9.9 mV; half-maximal effect was produced by 0.019 mu M. ACh. High ACh concentrations were required to inhibit I-Ca,I-L only partially (31% inhibition at 300 mu M). 3. In contrast, I-f and I-Ca,I-L responded to Iso over a similar dose range, with concentrations for half-maximal enhancement of 0.0136 and 0.0070 mu M, respectively. 4. The effects on spontaneous activity of ACh (range 0.001-0.03 mu M) and Iso (range 0.001-1 mu M) were investigated. ACh decreased the slope of diastolic depolarization at concentrations similar to those inhibiting I-f (> 50% at 0.03 mu M). Iso enhanced diastolic depolarization at concentrations similar to those affecting both I-f and I-Ca,I-L (half-maximal effect at 0.027 mu M) 5. In a ramp-clamp protocol simulating diastolic depolarization, the threshold for activation of inward nifedipine-sensitive current was - 41.22 +/- 0.68 mV. Although enhancing I-Ca,I-L, Iso did not affect this threshold. 6. Half-maximal ACh h concentrations for inhibition of automaticity and I-f are similar and are lower than the threshold concentrations for modulation of I-Ca,I-L; this argues against a role of I-Ca,I-L in direct muscarinic modulation of pacemaking. In contrast, modulation of I-f, I-Ca,I-L and automaticity occur at similar Iso concentrations. The difference between maximum diastolic potential (- 61.95 +/- 0.93 mV) and the threshold for Iso-stimulated I-Ca,I-L (- 39.54 +/- 1.03 mV) suggests that this current plays a role only at later stages of diastolic depolarization.