Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca2+ cycling and IKACh

Lyashkov AE, Vinogradova TM, Zahanich I, Li Y, Younes A, Nuss HB, Spurgeon HA, Maltsev VA, Lakatta EG. Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca2+ cycling and I-KACh. Am J Physiol Heart Circ Physiol 297: H949-H959, 2009. First published June 19, 2009; doi: 10.1152/ajpheart.01340.2008.-Prior studies indicate that cholinergic receptor (ChR) activation is linked to beating rate reduction (BRR) in sinoatrial nodal cells (SANC) via 1) a G(i)-coupled reduction in adenylyl cyclase (AC) activity, leading to a reduction of cAMP or protein kinase A (PKA) modulation of hyperpolarization-activated current (I-f) or L-type Ca2+ currents (I-Ca,I- L), respectively; and 2) direct G(i)-coupled activation of ACh-activated potassium current (I-KACh). More recent studies, however, have indicated that Ca2+ cycling by the sarcoplasmic reticulum within SANC (referred to as a Ca2+ clock) generates rhythmic, spontaneous local Ca2+ releases (LCR) that are AC-PKA dependent. LCRs activate Na+-Ca2+ exchange (NCX) current, which ignites the surface membrane ion channels to effect an AP. The purpose of the present study was to determine how ChR signaling initiated by a cholinergic agonist, carbachol (CCh), affects AC, cAMP, and PKA or sarcolemmal ion channels and LCRs and how these effects become integrated to generate the net response to a given intensity of ChR stimulation in single, isolated rabbit SANC. The threshold CCh concentration ([CCh]) for BRR was similar to 10 nM, half maximal inhibition (IC50) was achieved at 100 nM, and 1,000 nM stopped spontaneous beating. G(i) inhibition by pertussis toxin blocked all CCh effects on BRR. Using specific ion channel blockers, we established that I-f blockade did not affect BRR at any [CCh] and that I-KACh activation, evidenced by hyperpolarization, first became apparent at [CCh] > 30 nM. At IC50, CCh reduced cAMP and reduced PKA-dependent phospholamban (PLB) phosphorylation by similar to 50%. The dose response of BRR to CCh in the presence of I-KACh blockade by a specific inhibitor, tertiapin Q, mirrored that of CCh to reduced PLB phosphorylation. At IC50, CCh caused a time-dependent reduction in the number and size of LCRs and a time dependent increase in LCR period that paralleled coincident BRR. The phosphatase inhibitor calyculin A reversed the effect of IC50 CCh on SANC LCRs and BRR. Numerical model simulations demonstrated that Ca2+ cycling is integrated into the cholinergic modulation of BRR via LCR-induced activation of NCX current, providing theoretical support for the experimental findings. Thus ChR stimulation-induced BRR is entirely dependent on G(i) activation and the extent of G(i) coupling to Ca2+ cycling via PKA signaling or to I-KACh: at low [CCh], I-KACh activation is not evident and BRR is attributable to a suppression of cAMP-mediated, PKA-dependent Ca2+ signaling; as [CCh] increases beyond 30 nM, a tight coupling between suppression of PKA-dependent Ca2+ signaling and I-KACh activation underlies a more pronounced BRR.