If-dependent modulation of pacemaker rate mediated by cAMP in the presence of ryanodine in rabbit sino-atrial node cells

I-f contributes to generation and autonomic control of spontaneous activity of cardiac pacemaker cells through a cAMP-dependent, Ca2+-independent mechanism of rate regulation. However, disruption of Ca2+ release from sarcoplasmic reticulum (SR) by ryanodine (Ry) has been recently shown to slow spontaneous rate and inhibit beta-adrenergic receptor (betaAR)-induced rate acceleration, leading to the suggestion that the target of betaAR modulation of pacemaking is the intracellular Ca2+-regulatory process. We have investigated whether the Ry-induced decrease of betaAR rate modulation alternatively involves disruption of the betaAR-adenylate-cyclase-cAMP-I-f mechanism. Prolonged exposure to Ry (3 muM, >2 min) slowed spontaneous rate of pacemaker cells by 29.8% via a depolarizing shift of take-off potential (TOP) without significantly changing early diastolic depolarization rate. Ry depressed rate acceleration caused by isoproterenol (Iso) (1 muM, 23.6% in control vs. 8.0%), but did not modify that caused by two membrane-permeable cAMP analogs, CPT-cAMP (300 muM, 17.7% vs. 17.3%) and Rp-cAMPs (50 muM 18.0% vs. 20.6%). Consistent with the rate effect, exposure to Ry decreased the shift induced by Iso, but not that induced by either cAMP analog on the I-f-activation curve. We conclude that disruption of Ry receptor function and SR Ca2+ release depresses PAR-induced modulation of heart rate, but does not impair cAMP-dependent rate acceleration mediated by I-f. However, abolishment of normal Ca2+ homeostasis may result in the failure of betaAR agonists to sufficiently elevate cAMP near f-channels. The molecular basis for Ca2+-dependent interference in beta-adrenergic signaling remains to be determined. (C) 2003 Elsevier Ltd. All rights reserved.