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

If 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 β-adrenergic receptor (βAR)-induced rate acceleration, leading to the suggestion that the target of βAR modulation of pacemaking is the intracellular Ca2+-regulatory process. We have investigated whether the Ry-induced decrease of βAR rate modulation alternatively involves disruption of the βAR-adenylate-cyclase-cAMP-If mechanism. Prolonged exposure to Ry (3 μM, >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 μM, 23.6% in control vs. 8.0%), but did not modify that caused by two membrane-permeable cAMP analogs, CPT-cAMP (300 μM, 17.7% vs. 17.3%) and Rp-cAMPs (50 μM, 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 If-activation curve. We conclude that disruption of Ry receptor function and SR Ca2+ release depresses βAR-induced modulation of heart rate, but does not impair cAMP-dependent rate acceleration mediated by If. However, abolishment of normal Ca2+ homeostasis may result in the failure of βAR agonists to sufficiently elevate cAMP near f-channels. The molecular basis for Ca2+-dependent interference in β-adrenergic signaling remains to be determined.